DE102020203421A1 - Flat steel product with a ZnCu layer system - Google Patents

Abstract

The invention relates to a flat steel product (13) for producing a steel component comprising a steel substrate (15) with a corrosion protection coating (16) present on at least one side of the steel substrate (15). The corrosion protection coating (16) comprises a layer system (17). The layer system (17) here comprises: between 25 and 40% by weight of copper, a maximum of 5% by weight of other additional alloys, the remainder of zinc and unavoidable impurities.

Description

The invention relates to a flat steel product with an anti-corrosion coating which has a ZnCu layer system, a corresponding steel component and a method for producing a flat steel product and a steel component.

When "flat steel products" are mentioned here, they mean steel strips, steel sheets or blanks and the like obtained from them.

In order to offer the combination of low weight, maximum strength and protective effect required in modern body construction, hot-pressed components made of high-strength steels are used today in areas of the body that could be exposed to particularly high loads in the event of a crash.

In hot press hardening, also known as hot forming, steel blanks, which are separated from cold or hot rolled steel strip, are heated to a deformation temperature that is usually above the austenitizing temperature of the respective steel and placed in the heated state in the tool of a forming press. In the course of the subsequent reshaping, the sheet metal blank or the component formed from it undergoes rapid cooling through contact with the cool tool. The cooling rates are set so that a hardness structure results in the component. The structure is converted into a martensitic structure.

Typical steels that are suitable for hot press hardening are steels A-E, the chemical composition of which is listed in Table 4.

The advantages of the manganese-boron steels, which are particularly suitable for hot press hardening, are offset in practice by the disadvantage that steels containing manganese are generally not resistant to wet corrosion. This strong tendency to locally limited, but intense corrosion when exposed to increased chloride ion concentrations in comparison to lower alloy steels makes the use of steels belonging to the material group of high-alloy steel sheets difficult, especially in bodywork. In addition, steels containing manganese have a tendency to surface corrosion, which also limits their range of uses.

Furthermore, it is known from studies that with Mn-B quenched and tempered steels for complex crash-relevant structural components in vehicle bodies under unfavorable conditions, e.g. with increased hydrogen input and with the presence of increased tensile stresses, during the production or further processing of these steels there is a potential risk of hydrogen embrittlement or the There is a risk of hydrogen-induced delayed crack formation. The introduction of hydrogen is favored by the higher absorption capacity of the steel substrate in the austenitic structure state during the annealing treatment.

In the prior art, there are various proposals which aim to reduce the hydrogen uptake of manganese-containing steels during the tempered state or to provide such steels with a metallic coating that protects the steel from corrosive attack. A distinction is made between active and passive corrosion protection systems.

Active corrosion protection systems are usually produced by continuously applying a zinc-containing corrosion protection coating. Passive corrosion protection systems, on the other hand, are typically produced by applying an aluminum-containing coating, in particular an aluminum-silicon coating (AISi), which offers a good barrier effect with regard to corrosive attacks.

With previous aluminum-containing anti-corrosion coatings, there are several negative aspects. The energy consumption of a hot-dip coating system for the production of AlSi coatings is relatively high due to the high melting temperature of the coating material. In addition, these coatings on manganese-boron steels can only be cold formed to a certain extent. Due to a hard intermetallic Fe-Al-Si phase, the coating flakes off during the cold forming process. This limits the degree of deformation. The AISi coatings therefore regularly require direct hot forming. In combination with cathodic dip painting, which enables the paint layer to adhere well to the surface of the AISi coating, a good barrier effect with regard to corrosive attacks can be achieved. Furthermore, with this coating variant, the entry of hydrogen into the steel material must be taken into account, which in the case of unfavorable process conditions, the use of a May make dew point control on the continuous furnace necessary for the press hardening process. The energy consumption associated with dew point control causes additional costs in component manufacture.

The alternative use of zinc-containing anti-corrosion coatings, however, has the disadvantage that zinc has a relatively low melting point of approx. 419 ° C. During hot forming, the liquid, zinc-containing coating then penetrates the base material and leads to severe cracking (so-called liquid metal embrittlement).

Zinc-containing anti-corrosion coatings for hot forming are, for example, from DE20122563U1 and the W02016 / 071399A1 known. The W02016 / 071399A1 the use of a zinc layer with an additional copper layer. However, only very broad ranges are disclosed for the copper and zinc content.

Proceeding from this, the invention was based on the object of providing a specific anti-corrosion coating that is suitable for hot forming, adequately protects the hot-formed steel component from corrosion and enables good processing of the steel component.

This object is achieved by a flat steel product for producing a steel component comprising a steel substrate with an anti-corrosion coating present on at least one side of the steel substrate. The structure of the steel substrate can be converted into a martensitic structure by hot forming. In addition, the anti-corrosion coating comprises a layer system, the layer system comprising between 25% by weight and 40% by weight of copper, a maximum of 5% by weight of other additional alloys, the remainder being zinc and unavoidable impurities. The other additional alloys can be formed by iron, for example.

In a further developed variant, the layer system comprises at least 26% by weight, preferably at least 27% by weight, particularly preferably at least 29% by weight copper. Regardless of this, the layer system preferably comprises a maximum of 35% by weight copper, particularly preferably a maximum of 34% by weight copper.

In particular, in addition to the copper contents mentioned, the layer system consists only of zinc, iron and unavoidable impurities or only of zinc and unavoidable impurities. In the first case iron forms the other additional alloys and in the second case no other additional alloys occur. For the purposes of this application, an element is an unavoidable contamination of the layer system if the mass fraction in the layer system is less than 1.0% by weight. This definition only relates to unavoidable contamination of the layer system. For the unavoidable impurities in the steel substrate mentioned later, the customary limit values apply.

A copper content of at least 25% by weight has several advantages. On the one hand, the melting point is sufficiently high that the proportion of liquid phases is reduced during hot forming. This can both reduce the liquid metal embrittlement described and also reduce the adhesion of the liquefied layer to tools. On the other hand, measurements have shown that from a copper content of 25% by weight, the maximum crack length is reduced during forming. The maximum crack length was determined using a light microscope on a metallographic cross-section, the material for this being taken from the crack-critical areas of an exemplary component (omega profile). In each case, the crack length within the steel substrate was evaluated, i.e. without taking into account the crack proportions within the coating.

There is also another advantage with increasing copper content. Studies have shown that further processing is facilitated by means of resistance welding. While resistance welding is possible with a copper content of 25% by weight, the process window with a welding area according to EN ISO 14327: 2004 is relatively small at 0.2 kA. In contrast, with a copper content of 30% by weight, the welding area is already 0.6kA, which is a good process window. Slight increases in the copper content in this area thus result in a significant improvement in weldability.

However, too high a copper content has an impact on the active corrosion protection, as the layer system becomes more and more noble due to the addition of copper. The copper content is therefore a maximum of 40% by weight, preferably a maximum of 35% by weight copper, particularly preferably a maximum of 34% by weight copper.

The steel substrate is typically a steel, in particular a manganese-boron steel, the structure of which can be converted into a martensitic structure by hot forming.

The steel substrate is particularly preferably a steel from the group of steels AE, the chemical analysis of which is given in Table 5. Table 5 is to be understood in such a way that the element proportions are given in percent by weight for each steel from the group of steels AE.

A minimum and a maximum weight fraction is specified here. For example, steel A thus has a carbon content C: 0.05% by weight - 0.10% by weight. If the lower limit is 0, the element is to be understood as optional. No entry in the table means that there is no limit for the element. For the elements chromium and molybdenum in steels C-E there is only an upper limit for the sum of the element contents of chromium and molybdenum. In addition to the elements listed in the table, steels A-E can contain other optional elements, e.g. Cu, N, Ni, V, Sn, Ca.The remainder consists of iron and unavoidable impurities.

The above explanations on preferred steel substrates naturally also apply to the steel substrate of the steel component described below, as well as the two methods described.

In a special configuration, the layer system consists of exactly one alloy layer with a zinc-copper alloy. This has the advantage that the layer system can be applied in a single coating step. A PVD process is then used in particular as the coating process, since an electrolytic deposition of a ZnCu alloy can be carried out efficiently and economically, if at all only from a cyanide electrolyte. However, due to its dangers to human health and the environment, this can only be used on an industrial scale with considerable additional effort.

In an alternative special embodiment, the layer system comprises at least two alternating layers of copper and zinc, an intermediate layer consisting of a zinc-copper alloy being arranged between adjacent layers of copper and zinc. In particular, the zinc layer is applied in the electrolytic process or by hot-dip coating, which can be achieved in an energy-efficient manner due to the low melting point, and the copper layer by electrolytic coating. Surprisingly, it has been shown that copper and zinc diffuse into one another very strongly even at room temperature, so that an intermediate layer consisting of a zinc-copper alloy is formed between adjacent layers of copper and zinc. Therefore, applying copper and zinc separately is a very practical way of applying a layer of zinc-copper alloy. As already explained, the simultaneous application of a zinc-copper alloy is possible, but associated with technical difficulties. In particular, not all coating processes are suitable for a zinc-copper alloy. In addition, it has been shown that by means of the separate coating with copper and zinc, special layer arrangements of the copper layer can be realized in a simple manner at the same time, which have certain advantages.

Thus, the layer system of the flat steel product has in particular a layer closest to the substrate, the layer closest to the substrate being a copper layer. This arrangement can be achieved by first applying a copper layer to the steel substrate and then applying a zinc layer. The advantage of this arrangement is that during hot forming, the liquid zinc cannot penetrate the base material as much and contribute to crack formation, as there is a copper layer with a higher melting point that acts as a barrier between the liquid zinc and the steel substrate. This arrangement thus reduces the liquid metal embrittlement and thus the formation of cracks.

In another variant, the layer system has a layer closest to the surface, the layer closest to the surface being a copper layer. This arrangement can be achieved in that when the layer system is applied to the steel substrate, a copper layer is applied last. It has been shown that even after hot forming, due to this arrangement, an area close to the surface with an increased copper content remains. The layer system is therefore not completely homogenized during hot forming into a steel component. This near-surface area with an increased copper content has better electrical conductivity (due to modified oxide formation), so that the steel component is better suited for resistance welding.

The object according to the invention is also achieved by a steel component, in particular manufactured by a flat steel product described above. In this case, the steel component comprises a steel substrate with a martensitic structure and with an anti-corrosion coating that is present on at least one side of the steel substrate. Here, the anti-corrosion coating comprises a layer system and an adjacent one Oxide layer. In addition, the mass fraction of copper in relation to the total mass of copper and zinc in the layer system and oxide layer is between 25 and 40%.

Since significant proportions of iron diffuse from the steel substrate into the corrosion protection coating during hot forming, when describing the hot-formed steel component, it is more practical to relate the element content of copper to the total mass of copper and zinc and not to the total mass of all elements. The element content of copper in relation to the total mass of copper and zinc after hot forming essentially corresponds to the proportion of copper in relation to the total mass before hot forming, since the total mass before hot forming is essentially made up of copper and zinc.

In a further developed variant, the mass fraction of copper based on the total mass of copper and zinc in the layer system and oxide layer is at least 26%, preferably at least 27%, particularly preferably at least 29%. Regardless of this, the mass fraction of copper based on the total mass of copper and zinc in the layer system and oxide layer is preferably a maximum of 35%, particularly preferably a maximum of 34%. The advantages of these values are explained above with reference to the flat steel product.

In a special development, the mass fraction of copper in the oxide layer is less than 5% by weight. It has been shown that the passivating oxide layer is essentially formed by zinc oxides despite a high copper content close to the surface.

In a special development of the steel component, the mass fraction of copper in relation to the total mass of copper and zinc in the layer system has a local maximum in the area of the layer system close to the surface. The area of the layer system adjoining the oxide layer and having a thickness of 300 nm is understood as the area close to the surface. This arrangement can be achieved by the above-described alternating coating with copper and zinc, with a copper layer being applied last when the layer system is applied to the steel substrate. It has been shown that even after hot forming, due to this arrangement, an area close to the surface with an increased copper content remains. The layer system is therefore not completely homogenized during hot forming into a steel component. This near-surface area with an increased copper content has better electrical conductivity, so that the steel component is better suited for resistance welding.

In a further variant of the steel component, the mass fraction of copper in relation to the total mass of copper and zinc in the layer system has a local maximum in the region of the layer system close to the substrate. The region of the layer system adjoining the substrate and having a thickness of 300 nm is understood as the region close to the substrate. This arrangement can be achieved by the above-described alternating coating with copper and zinc, with a copper layer being applied first when the layer system is applied to the steel substrate. The advantage of this arrangement is that during hot forming, the liquid zinc cannot penetrate the base material as much and contribute to crack formation, as there is a copper layer with a higher melting point that acts as a barrier between the liquid zinc and the steel substrate. This arrangement thus reduces the liquid metal embrittlement and thus the formation of cracks.

• - Herstellen oder Bereitstellen eines Stahlsubstrats, wobei das Gefüge des Stahlsubstrats durch ein Warmumformen in ein martensitisches Gefüge umwandelbar ist,
• - Applizieren eines Schichtsystems zur Bildung eines Korrosionsschutzüberzugs, wobei das Schichtsystem zwischen 25 und 40 Gew.-% Kupfer, maximal 5 Gew.-% sonstiger Beilegierungen, Rest Zink und unvermeidbare Verunreinigungen umfasst.

The object according to the invention is also achieved by a method for producing a flat steel product described above. The process comprises at least the following steps:

• - Production or provision of a steel substrate, the structure of the steel substrate being convertible into a martensitic structure by hot forming,
• - Application of a layer system to form a corrosion protection coating, the layer system comprising between 25 and 40% by weight of copper, a maximum of 5% by weight of other additional alloys, the remainder being zinc and unavoidable impurities.

The method has the same advantages that were explained above with regard to the flat steel product. The same preferred numerical values and intervals that have been explained above with reference to the flat steel product also apply to the element contents of the corrosion protection coating.

• - Alternierendes Abscheiden von mindestens zwei aneinander angrenzenden Kupfer- und Zinkschichten, so dass sich durch Diffusion zwischen den aneinander angrenzenden Kupfer- und Zinkschichten jeweils eine Zwischenschicht ausbildet, die aus einer Zink-Kupfer-Legierung besteht.

In particular, the method is developed in such a way that the application of a layer system comprises at least the following steps:

• - Alternating deposition of at least two adjoining copper and zinc layers, so that an intermediate layer consisting of a zinc-copper alloy is formed between the adjoining copper and zinc layers by diffusion.

Surprisingly, it has been shown that copper and zinc diffuse into one another very strongly even at room temperature, so that an intermediate layer consisting of a zinc-copper alloy is formed between adjacent layers of copper and zinc. Therefore, applying copper and zinc separately is a very practical way of applying a layer of zinc-copper alloy. As already explained, the simultaneous application of a zinc-copper alloy is possible, but associated with technical difficulties. In particular, not all coating processes are suitable for a zinc-copper alloy. In addition, it has been shown that by means of the separate coating with copper and zinc, special layer arrangements of the copper layer can be realized in a simple manner at the same time, which have certain advantages.

When applying the layer system, a copper layer is preferably deposited as the first layer, so that the resulting layer system has a layer next to the substrate, the layer next to the substrate being a copper layer. The advantages of such a copper layer closest to the substrate are explained above with reference to the flat steel product and the steel component.

In a further variant, when the layer system is applied, a copper layer is deposited as the last layer, so that the resulting layer system has a layer closest to the surface, the layer closest to the surface being a copper layer. The advantage of such a copper layer closest to the surface is explained above with reference to the flat steel product and the steel component.

• - Gleichzeitiges Abscheiden von Kupfer und Zink, bevorzugt mittels PVD, zur Bildung genau einer Legierungsschicht mit einer Kupfer-Zink-Legierung.

Alternatively, the method is developed in such a way that the application of a layer system comprises at least the following steps:

• - Simultaneous deposition of copper and zinc, preferably by means of PVD, to form exactly one alloy layer with a copper-zinc alloy.

In particular, the manufacturing process is developed in such a way that at least one coating process is used when applying the layer system, which is selected from the group consisting of electrolytic deposition, physical vapor deposition (PVD), hot-dip process, other immersion processes, chemical vapor deposition, slurry process, thermal spraying, roll cladding, and combinations thereof.

In the variant in which a zinc-copper alloy is applied directly as exactly one alloy layer, a PVD process is preferably used, since an electrolytic deposition of a ZnCu alloy is efficient and economical, if only from one cyanide electrolytes can be done. However, due to its dangers to human health and the environment, this can only be used on an industrial scale with considerable additional effort. A hot dip coating is also not advisable due to the higher melting point.

When copper layers and zinc layers are applied separately, the zinc layers are preferably applied in an electrolytic process or by hot dip coating, which can be achieved in an energy-efficient manner due to the low melting point. In contrast, the copper layers are preferably applied by electrolytic coating.

• - Bereitstellen oder Herstellen eines zuvor erläuterten Stahlflachproduktes oder Herstellen eines Stahlflachprodukts durch Anwendung eines zuvor erläuterten Verfahrens;
• - Erwärmen des Stahlflachprodukts auf eine Erwärmungstemperatur für das Warmumformen, wobei die Erwärmung unter Umgebungsatmosphäre, unter H2O-reduzierter Atmosphäre oder unter einer Schutzgasatmosphäre erfolgt;
• - Warmumformen des bereitgestellten oder hergestellten Stahlflachprodukts, so dass das Stahlbauteil resultiert.

The object according to the invention is also achieved by a method for producing an aforementioned steel component. The method for manufacturing a steel component comprises at least the following steps:

• - Provision or manufacture of a previously explained flat steel product or manufacture of a flat steel product by using a previously explained method;
• - Heating the flat steel product to a heating temperature for the hot forming, the heating taking place under an ambient atmosphere, under a H2O-reduced atmosphere or under a protective gas atmosphere;
• - Hot forming of the provided or manufactured flat steel product, so that the steel component results.

Heating the flat steel product to a heating temperature for hot forming means, in particular, heating to a temperature above Ac3, so that there is essentially an austenitic structure in the steel substrate. The subsequent hot forming includes deformation in a forming tool and rapid cooling of the flat steel product. This can in particular be carried out at the same time in that the cooling is achieved through contact with the forming tool.

The oxidation process is influenced by the use of a H2O-reduced atmosphere or a protective gas atmosphere during heating, so that the thickness of the oxide layer can be adjusted in a targeted manner.

The invention is explained in more detail with reference to the following exemplary embodiments.

The 1 an exemplary metallographic section of a flat steel product.

A steel sheet with a thickness of 1.5 mm made of a 22MnB5 steel and having a composition according to steel D of Table 4 was used as the steel substrate. A layer system was applied to this steel substrate to form an anti-corrosion coating. First, a copper layer was deposited electrolytically as the first layer. The copper layer is thus the layer closest to the substrate in the layer system. A zinc layer was then deposited electrolytically and then another copper layer as the last layer. The layer system thus has a layer closest to the surface, which is a copper layer. Three variants with different layer thicknesses were produced, so that different copper proportions result in the layer system. The layer structures of the three variants are given in Table 1 below. Table 1: Details of the two sample variants produced option A Variant B Variant C Cu content 25 wt% 30% by weight 10% by weight Substrate Substrate Substrate 1.0 µm Cu 1.3 µm Cu 0.4 µm Cu Layer structure 7.9 µm Zn 7.4 µm Zn 9.2 µm Zn 1.1 µm Cu 1.3 µm Cu 0.4 µm Cu surface surface surface

The layer systems therefore have a copper content that is specified in the table, and otherwise consist of zinc and unavoidable impurities.

1 shows an example of a metallographic micrograph of a flat steel product 13th , specifically a sample of variant A. On the steel substrate 15th is a corrosion protection coating 16 comprising a layer system 17th arranged. The shift system 17th comprises a first copper layer 19th , which is the layer closest to the substrate, an intermediate layer 21 made of a copper-zinc alloy, a zinc layer 23 , another intermediate layer 25th made of a copper-zinc alloy and a second copper layer 27 , which forms the layer of the layer system that is closest to the surface. The two intermediate layers made of a copper-zinc alloy 21 and 25th have formed through diffusion between the copper layers and the zinc layer. Even at room temperature, the diffusion of copper and zinc is strong enough to form significant intermediate layers.

In a subsequent step, the coated steel sheets were heated to a heating temperature for hot forming, i.e. to a temperature above Ac3. In this specific case, the samples were annealed at 880 ° C. for an annealing period of between 30 seconds and 5 minutes in an ambient atmosphere. The specimens were then quenched and formed into omega profiles. The Omega profiles are examples of typical steel components. Finally, the maximum crack length and Vickers hardness (according to DIN EN ISO 13925: 2003-07 in 1/3 position). The maximum crack length was determined using a light microscope on a metallographic cross-section as described above, the material for this being taken from the crack-critical areas of the omega profiles. In each case, the crack length within the steel substrate was evaluated, i.e. without taking into account the crack proportions within the coating. In addition, the retained austenite content was determined in accordance with DIN EN 13925: 2003-07 . The results are shown in Table 2 below.

In the initial state and with annealing times of 1 minute and less, the structure of the steel substrate consists of ferrite, cementite and granular carbides. Only with longer annealing times does the transformation to austenite occur during annealing and martensite is formed during quenching. It can be clearly seen that the hardness in the substrate only increases after an annealing time of 2 minutes. The maximum possible hardness in the range of 500 HV5 is reached after 3 minutes of annealing. The structure is then completely converted into austenite during annealing and almost completely converted into martensite during quenching. Only a small proportion of retained austenite remains. At the same time, it can be seen that with variants A and B, acceptable maximum crack lengths occur on the omega profiles, while with variant C, which has a Cu content of 10%, the crack lengths are significantly longer.

In a further exemplary embodiment, a steel sheet with a thickness of 1.5 mm made of a 22MnB5 steel, which has a composition according to steel D of Table 5, was used as the steel substrate. A layer system was applied to this steel substrate to form an anti-corrosion coating. The layer system consisted of exactly one alloy layer with a copper-zinc alloy (variant D). Specifically, copper and zinc were deposited simultaneously using PVD, so that a copper-zinc alloy was formed on the substrate. In the present case, the alloy layer had a thickness of 10 μm and comprised 34% by weight of copper, the remainder being zinc and unavoidable impurities. In a subsequent step, the coated steel sheet was heated to a heating temperature for hot forming, i.e. to a temperature above Ac3. In this specific case, the samples were annealed at 880 ° C. for an annealing period of 5 minutes under ambient atmosphere. The sample was then shaped into an omega profile with quenching. Finally, the maximum crack length was determined to be 13 µm.

In a further exemplary embodiment, a steel sheet with a thickness of 1.5 mm made of a 22MnB5 steel, which has a composition according to steel D of Table 5, was used as the steel substrate. A layer system was applied to this steel substrate to form an anti-corrosion coating. First, a copper layer was deposited electrolytically as the first layer. The copper layer is thus the layer closest to the substrate in the layer system. A zinc layer was then deposited electrolytically and a copper layer on top. This middle copper layer was followed by another zinc layer and finally another copper layer as the last layer. The layer system thus has a layer closest to the surface, which is a copper layer. Overall, the layer system consisted of three copper layers and two zinc layers with the following thicknesses: Table 3: Layer structure of a further variant Variant E Cu content 30% by weight Substrate 0.9 µm Cu 3.7 µm Zn Layer structure 0.8 µm Cu 3.7 µm Zn 0.9 µm Cu surface

The layer system therefore includes a copper content, which is given in Table 3, and otherwise consists of zinc and unavoidable impurities.

In this case, too, metallographic investigations have confirmed that an intermediate layer of a copper-zinc alloy is formed immediately between adjacent layers of copper and zinc.

In a subsequent step, the coated steel sheet was heated to a heating temperature for hot forming, i.e. to a temperature above Ac3. In this specific case, the samples were annealed at 880 ° C. for an annealing period of 5 minutes under ambient atmosphere. The sample was then shaped into an omega profile with quenching. Finally, the maximum crack length was determined to be 10 µm.

The welding area for resistance welding according to EN ISO 14327: 2004 was also determined on the previously described Omega profiles according to variants A, B and D. This resulted in a welding range of 0.2 kA for variant A with a copper content of 25% by weight, a welding range of 0.6 kA for variant B with a copper content of 30% by weight and for variant D with a copper content of 34% by weight, a welding range of 0.7 kA. Increasing the copper content therefore improves the weldability, which results in a significantly better process window for further processing by means of resistance welding. However, while the welding area increases significantly from variant A to variant B, the improvement from variant B to variant D is not so great, although the total copper content in the layer system increases almost as much. This is because the mass fraction of copper based on the total mass of copper and zinc in the layer system in variants A and B has a local maximum in the area of the layer system close to the surface. This local maximum results from the alternating coating with copper and zinc, with a copper layer being applied last. It has been shown that even after hot forming, due to this arrangement, an area close to the surface with an increased copper content remains. The increased copper content near the surface leads to two effects. On the one hand, the higher the copper content, the smaller the thickness of the oxide layer, since the oxide layer is essentially formed from zinc oxide. With a smaller oxide layer thickness, the heat development during resistance spot welding is also reduced due to the higher electrical conductivity. The occurrence of spatter during resistance spot welding is contained and wear of the welding caps due to excessive heat generation is avoided. In addition, the high melting point of the zinc-copper alloy in the area near the surface leads to a significantly reduced element diffusion between the coating material and the welding caps, which also significantly reduces wear on the caps. Both effects are positively influenced by the copper content in the near-surface area, so that variants A and B, due to the local maximum of the copper content in the near-surface area, show better welding behavior than would be expected based on the average copper content in the layer system.

A corresponding system is also observed for the maximum crack length of variants A, B and D. In principle, one would expect the smallest maximum crack length for variant D, since it has the highest copper content. Instead, variants A and B have lower values of 10 µm and 11 µm. This is because the mass fraction of copper in relation to the total mass of copper and zinc in the layer system in variants A and B has a local maximum in the area of the layer system close to the substrate. This local maximum results from the alternating coating with copper and zinc, with a copper layer being applied first. It has been shown that even after hot forming, due to this arrangement, an area close to the substrate remains with an increased copper content. The increased copper content close to the substrate means that the then liquid zinc cannot penetrate the base material as strongly and contribute to crack formation during hot forming, as there is a copper layer with a higher melting point that acts as a barrier between the liquid zinc and the steel substrate. This arrangement thus reduces the liquid metal embrittlement and thus the formation of cracks. Table 4 Steel grade min / max C. Si Mn P. S. Al Nb Ti Cr + Mo B. A. min 0.05 0.05 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 Max 0.10 0.35 1.00 0.030 0.025 0.075 0.100 0.150 0.0050 B. min 0.05 0.03 0.50 0.000 0.000 0.015 0.005 0.000 0.0000 Max 0.10 0.50 2.00 0.030 0.025 0.075 0.100 0.150 0.0050 C. min 0.05 0.05 1.00 0.000 0.000 0.015 0.005 0.000 0.00 0.0010 Max 0.16 0.40 1.40 0.025 0.010 0.150 0.050 0.050 0.50 0.0050 D. min 0.10 0.05 1.00 0.000 0.000 0.005 0.000 0.00 0.0010 Max 0.30 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0050 E. min 0.250 0.10 1.00 0.000 0.000 0.015 0.000 0.00 0.0010 Max 0.380 0.40 1.40 0.025 0.010 0.050 0.050 0.50 0.0500

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 20122563 U1 [0012]
• WO 2016/071399 A1 [0012]

Non-patent literature cited

• DIN EN ISO 13925: 2003-07 [0052]
• DIN EN 13925: 2003-07 [0052]

Claims (16)

Flat steel product (13) for producing a steel component comprising a steel substrate (15) with a corrosion protection coating (16) present on at least one side of the steel substrate (15), characterized in that the corrosion protection coating (16) comprises a layer system (17), the layer system (17) comprises: between 25 and 40% by weight of copper, a maximum of 5% by weight of other additional alloys, the remainder being zinc and unavoidable impurities. Flat steel product according to Claim 1 , characterized in that the layer system consists of 25-40% by weight copper, preferably 26-35% by weight copper, particularly preferably 27-34% by weight copper, the remainder being zinc and unavoidable impurities. Flat steel product according to one of the Claims 1 - 2 , characterized in that the layer system (17) consists of exactly one alloy layer with a zinc-copper alloy. Flat steel product according to one of the Claims 1 - 2 , characterized in that the layer system (17) comprises at least two alternating layers of copper and zinc, with an intermediate layer (21, 25) being arranged between adjacent layers of copper (19, 27) and zinc (23), which consists of one Zinc-copper alloy. Flat steel product according to Claim 4 , characterized in that the layer system (17) has a layer (19) next to the substrate, the layer next to the substrate being a copper layer. Flat steel product according to one of the Claims 4 - 5 , characterized in that the layer system has a layer (27) closest to the surface, the layer closest to the surface being a copper layer. Steel component, in particular produced by hot forming a flat steel product (13) provided according to one of the preceding claims, comprising a steel substrate with a martensitic structure and with an anti-corrosion coating (16) present on at least one side of the steel substrate (15), the anti-corrosion coating being a layer system (17 ) and an adjacent oxide layer and wherein the mass fraction of copper based on the total mass of copper and zinc in the layer system and oxide layer is between 25 and 40%. Steel component according to Claim 7 , characterized in that the mass fraction of copper in the oxide layer is less than 5 wt .-%. Steel component according to one of the Claims 7 - 8th , characterized in that the mass fraction of copper based on the total mass of copper and zinc in the layer system has a local maximum in the area of the layer system (17) close to the surface. Steel component according to one of the Claims 7 - 9 , characterized in that the mass fraction of copper based on the total mass of copper and zinc in the layer system has a local maximum in the region of the layer system close to the substrate. Method for producing a flat steel product according to one of the Claims 1 until 6th , with the following steps: - producing or providing a steel substrate (15), wherein the structure of the steel substrate (15) can be converted into a martensitic structure by hot forming, - applying a layer system to form a corrosion protection coating (17), the layer system between 25 and 40% by weight of copper, a maximum of 5% by weight of other additional alloys, the remainder being zinc and unavoidable impurities. Procedure according to Claim 11 , characterized in that the application of a layer system comprises at least the following steps - alternating deposition of at least two adjoining copper and zinc layers, so that an intermediate layer is formed by diffusion between the adjoining copper and zinc layers, which consists of a zinc layer. Made of copper alloy. Procedure according to Claim 12 , characterized in that when the layer system is applied, a copper layer is deposited as the first layer, so that the resulting layer system has a layer next to the substrate, the layer next to the substrate being a copper layer. Method according to one of the Claims 12 - 13th , characterized in that when the layer system is applied, a copper layer is deposited as the last layer, so that the resulting layer system has a layer closest to the surface, the layer closest to the surface being a copper layer. Method according to one of the Claims 11 - 14th , characterized in that at least one coating process is used when applying the layer system, which is selected from the group consisting of electrolytic deposition, physical vapor deposition (PVD), hot-dip process, other immersion processes, chemical vapor deposition, slurry process, thermal spraying, Roll cladding and combinations thereof. Method for producing a according to one of the Claims 7 until 10 procured steel component, comprising the following work steps: - Provision or manufacture of a flat steel product according to one of the Claims 1 - 6th or manufacturing a flat steel product by using one of the Claims 11 until 15th specified procedure; - Heating the flat steel product to a heating temperature for hot forming, the heating taking place under an ambient atmosphere, under a H2O-reduced atmosphere or under a protective gas atmosphere; - Hot forming of the provided or manufactured flat steel product, so that the steel component results.

Images