DE102015118869A1 - Method for producing a corrosion protection coating for hardenable steel sheets and corrosion protection layer for hardenable steel sheets - Google Patents

Abstract

The invention relates to a method for producing a corrosion protection coating for hardenable steel sheets, wherein at least two metal layers are sequentially deposited on the steel substrate, wherein the one metal layer is a layer of zinc or based on zinc and the other layer is a layer of a metal forms with Zn or Fe intermetallic less noble phases and has a higher oxidation potential than Zn, namely Ni, Cu, Co, Mn or Mo, or is a layer based on these metals, as well as a corrosion protection layer for hardenable steel sheets.

Description

The invention relates to a method for producing a corrosion protection coating for hardenable steel sheets and a corrosion protection layer for hardenable steel sheets.

For the hardening of steel sheets or the production of sheet steel components made of sheet steel, which are hardened, and in particular Karossieriebauteile, there are currently two common methods.

Common to both methods is that a steel strip is produced from a steel material by hot rolling and usually also subsequent cold rolling and the steel strip is subsequently continuously galvanized. The usual galvanizing in this case is the hot dip galvanizing, in which the steel strip is passed through a trough with liquid zinc, wherein the liquid zinc adheres to the steel, the galvanized steel strip is usually conveyed vertically from the trough and then superfluous zinc is stripped with Abstreiferdüsen and the tape then optionally subjected to a heat treatment. The galvanized steel strip thus produced is then usually placed in coils, i. wound up.

In order to produce hardened steel sheet components from this steel strip, blanks of a desired size are punched out of the steel strip and these blanks are then further processed in two different ways.

In a first method, the boards are formed in a conventional manner in a multi-stage process and in particular deep-drawn until the component is formed in its final appearance. In this case, however, usually the component is formed smaller in all three spatial directions about 2% smaller, to take into account a subsequent thermal expansion. Subsequently, this sheet metal component is heated to an austenitizing temperature, ie a temperature above Ac ₃ and optionally held until the steel material is present in the austenitic phase. Subsequently, the heated sheet steel component is transferred to a mold hardening tool and held in the mold hardening tool, in which the heated sheet steel component is usually used form-fitting, pressed by a die and a male part, but not substantially reshaped. Due to the abutment of the die and the male part, which may also be cooled, the steel component is cooled at a speed above the critical hardening rate, resulting in a transformation of the austenite substantially to martensite and results in a high hardness of the component.

In a second known method, the board is heated directly to a temperature required for curing above Ac ₃ and optionally held and then formed in a tool consisting of female and male in a single-stage stroke and cooled simultaneously by the concern of the tool on the workpiece so quickly that the curing outlined above occurs. This process is called press hardening.

Form hardening is superior to press hardening in terms of the possible geometries of a component, since more complicated or more complex spatial forms can be realized in a multi-stage forming process, wherein only comparatively simple geometries can be produced during single-stage forming press hardening.

However, the end result of both processes is a hardened sheet steel component.

Common materials for these sheet steel components are so-called boron-manganese steels, in particular the 22MnB5 most widely used for this purpose.

It is known that, in particular in the press hardening process, problems may arise in that liquid zinc interacts with the austenite present in the steel material at high temperatures which are not yet completely explainable, but cause cracks to form in the strongly deformed areas. This phenomenon is referred to as so-called "liquid metal embrittlement".

It has already been attempted to counteract this phenomenon by using conversion-retarded steel grades which are austenitized at high temperature, then intercooled and, as a result of this intermediate cooling, reach temperatures below the melting point of the zinc phases in the coating, and only then carry out the transformation. Due to the conversion delay, even at these relatively low temperatures, the iron is still present as austenite, so that reliable quench hardening can be achieved.

From the DE 10 2010 030 465 A1 is a method for producing a provided with a corrosion protection coating and formed of a high-strength sheet steel material formed sheet metal part known. This method comprises the steps of forming a provided starting sheet material to a sheet metal part, forming the corrosion protection coating by electrolytic application of a zinc-nickel coating on the sheet metal part, wherein at the beginning of the coating process, first a thin nickel layer is deposited, which further prevents hydrogen embrittlement of the steel sheet material. Furthermore, a hot-formed and in particular press-hardened sheet metal part made of a high-strength sheet steel material with an electrolytically applied zinc-nickel coating is known from this. The purpose of this is to provide the nickel layer as a barrier to hydrogen typically introduced into the steel sheet material during electrolytic plating.

From the EP 2 290 133 B1 For example, there is known a method for producing a steel component provided with a metallic, corrosion-protective coating and the steel component itself. This is to provide a simple to implement in practice method is created, which allows to produce a steel component with comparatively little effort, which is provided with a well-adhering and safe from corrosion protective metallic coating, since it is stated that zinc coatings on the steel sheet for hot press hardening not adhere well. Furthermore, known coatings by oxidation of the surface should have a poor paint adhesion. According to this document, the applied anticorrosion layer should be an electrolytically applied γ-ZnNi phase, which should then well tolerate heating operations carried out for the purpose of austenitizing.

The EP 0 364 596 B1 relates to a process for the production of zinc-nickel alloy-coated thin sheets with good Pressverformeigenschaften, wherein the formability of such sheets is to be improved by a zinc-nickel alloy coating. The layer is to be deposited with about 30 g / m ² and a nickel content of 12.5%.

The object of the invention is to provide a method for producing hardened sheet steel components.

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

Advantageous developments are characterized in the subclaims.

It is a further object of the invention to provide a corrosion protection layer for hardenable steel sheets, which reduces or even prevents the liquid metal embrittlement with good cathodic protection against corrosion.

The object is achieved with a corrosion protection layer having the features of claim 11.

Advantageous developments are characterized in dependent claims.

According to the invention, an at least two-layer corrosion protection layer is produced on a steel sheet, wherein either a very thin nickel layer of 1 .mu.m electrolytically deposited on the steel and then a zinc layer is also deposited electrolytically on the nickel layer, or the thin nickel layer via an electrodeposition on the steel sheet is formed and then a zinc layer is applied via hot dip galvanizing. Another possibility is to apply a nickel-containing layer to a normal hot-dip galvanized sheet steel strip via a corresponding after-treatment (coater).

According to the invention, the nickel layer has a thickness of approximately 1 μm when applied as the first layer by means of electrolytic deposition.

When a galvanized zinc layer is post-treated, the outer nickel-containing layer is about 250 nm to 700 nm thick.

According to the invention, it has surprisingly been found that in the coating according to the invention or the layer structure according to the invention the nickel in no form forms a barrier against the ingress of liquid zinc to the steel, but rather the nickel reacts very quickly with the zinc and also with iron. that the melting point of the entire corrosion protection layer increases sharply, since instead of zinc-iron-Γ-phases zinc-nickel-iron phases are increasingly formed, which have a much higher melting point. This ensures that at the temperatures at which thermoformed and quench hardened, there are no liquid phases that could interact with the austenite. This is also the reason why, according to the invention, an externally applied nickel layer acts in a comparable manner, with the nickel which is deposited on the outermost surface diffusing into the corrosion protection layer so rapidly that the increase in the melting point is ensured.

According to the invention, instead of nickel, or layers based on nickel, other elements which form intermetallic-less noble phases with Zn or Fe and have a higher oxidation potential than Zn, such as Cu, Co, Mn or Mo, may be used, since manganese, Molybdenum, cobalt and copper the same effects can be achieved. "Based on" here means that these elements predominantly (> 50 wt .-%) are included, but other elements are present as alloying elements.

Both nickel and cobalt as well as manganese or copper do not act as a physical barrier against the diffusion of zinc and iron, but are dissolved and incorporated into the molten zinc and zinc-iron phases. In the case of a previously applied nickel layer and subsequent hot-dip galvanizing, the nickel is at least already dissolved by the zinc melt during galvanizing.

In a standard annealing for the purpose of austenitizing and subsequent forming could be found that forms a similar phase structure of the layer as in pure hot-dip galvanized layers (phs-Ultraform), but this phase structure is zinc-rich or has a greater proportion of Γ-phases. That these phases are more zinc-rich, is advantageous for the cathodic corrosion protection performance of the layer.

This is further supported by the fact that a very zinc-rich near-surface layer is formed, which is not observed in standard hot-dip galvanizing in the standard annealing without nickel interlayer.

The positive effect of the nickel in the layer or as a separately applied electrolytic layer results in the consideration of bending samples. The formation of cracks by the liquid metal embrittlement is impressively reduced.

The invention will be explained by way of example with reference to a drawing. It shows:

1 : a light-microscopic etched grinding admission of a steel sheet with the coating according to the invention, wherein on a 1 micron thick nickel intermediate layer, a hot-dip galvanized layer was applied;

2 : the layer after 1 in an enlarged view;

3 : one layer after 1 , wherein by EDX element mapping the elements iron, zinc, nickel and aluminum are shown in their distribution in a 1 micron thick applied nickel intermediate layer;

4 : a micrograph of the coating with a 0.5 micron thick nickel layer and a 10 micron thick zinc layer annealed at 800 ° C;

5 : the layer after 4 with a 1 micron thick nickel layer;

6 a coating according to the invention after annealing, a holding time, a transfer time and a subsequent cooling for press hardening;

7 : an X-ray micrograph of a corrosion protection layer according to the invention after austenitizing annealing at 870 ° C;

8th : the layer after 7 with the distribution of iron;

9 : the layer after 7 with the distribution of zinc;

10 : the layer after 7 with the distribution of the nickel, wherein a nickel support layer is applied to the surface as a preparation aid;

11 : the layer after 7 with the distribution of aluminum;

12 : the layer after 7 with the distribution of manganese;

13 a coating according to the invention after austenitizing and quenching with an indicated EDX scan line;

14 : the coating after 13 with the scan profile for the elements iron, nickel and zinc;

15 : four V samples with bending radius 1.5 mm.

The method according to the invention for producing sheet steel components may be either a press hardening method or a shape hardening method, that is, a method in which a sheet steel member is heated and then quench hardened in a tool (mold hardening), or a method in which a board is single stage formed and quench hardened (press hardening).

According to the invention, a boron-manganese steel is used as the steel material for press-hardening or mold-hardening, in which, with regard to the transformation of the austenite into other phases, the transformation can shift into deeper regions and martensite is formed.

As a transformation retarder in such steels, i. H. As an element which shifts the phase transformation of austenite to martensite to lower temperatures, in particular the alloying elements boron, manganese, carbon and optionally chromium and molybdenum are present.

Steels of the general alloy composition are suitable for the invention (all figures in percent by weight): Carbon (C) 0.08 to 0.6 Manganese (Mn) 0.8-3.0 Aluminum (Al) 0.01-0.07 Silicon (Si) 0.01-0.5 Chrome (Cr) 0.02-0.6 Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 Remaining iron and impurities caused by melting.

In particular, steel assemblies have proved to be suitable as follows (all figures in weight percent): Carbon (C) from 0.08 to 0.34 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chrome (Cr) 0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 Remaining iron and impurities caused by melting.

Thus, in particular, the conventional steels 22MnB5 or 20MnB8 are suitable.

By adjusting the alloying elements acting as conversion retarders, quench hardening, i. H. rapid cooling with a cooling rate above the critical hardening speed, even at temperatures below 780 ° C. This means that in this case, below the peritectic system of the zinc-iron system is used, i. H. only below the peritectic mechanical stresses are applied. This also means that the moment in which mechanical stresses are applied, there are no longer any liquid zinc phases which can come into contact with austenite.

A corrosion protection layer according to the invention is a corrosion protection layer applied at least in two layers, wherein at least one nickel layer and one zinc layer are applied to a substrate of a hardenable steel material. Instead of a nickel layer, a manganese or copper layer can also be applied.

In this case, the nickel, copper or manganese layer is preferably applied electrolytically. The zinc layer can be applied electrolytically or by a hot dip process.

In principle, it is possible first to apply the nickel layer and then a zinc layer, wherein the subsequently applied zinc layer is applied electrolytically or by hot dip.

Another possibility is to apply the zinc layer as a first layer electrolytically or by hot dip method and then apply a nickel layer thereon to the outermost surface and in particular to deposit it electrolytically.

As used herein, nickel refers to other elements that form intermetallic-less noble phases with Zn or Fe and have a higher oxidation potential than Zn, such as Cu, Co, Mn, or Mo.

The element nickel is hereby also used for copper and manganese.

Surprisingly, it has been found that correspondingly applied metallic layers obviously intervene in the phase structure of a corrosion protection layer, but themselves do not constitute a diffusion barrier. Therefore, the invention is effective even when the thin nickel, copper or manganese layer is applied to a hot dip galvanizing layer.

In 1 One recognizes a light-microscopic etched cut representation of the layer according to the invention on a steel substrate. In 2 this is shown enlarged again.

In this layer, a 1 micron thick nickel intermediate layer is first applied to the steel substrate and then hot-dip galvanized, wherein the hot dip galvanizing the nickel intermediate layer was dissolved in the zinc bath.

When such a layer is measured with an EDX element distribution, it can be seen in FIG 3 in that an iron uniform distribution is present in the region of the steel, whereby the iron content decreases at the boundary layer to the layers applied thereon. The distribution of zinc shows that the zinc content increases at the boundary to higher layers.

With regard to the nickel one recognizes that there must be an equal distribution within the layer, since there is no clear color statement regarding the nickel. The same applies to aluminum, which is included in the zinc coating for hot dip galvanizing.

In 4 and 5 In each case layers are compared, in which once the nickel intermediate layer has a thickness of 0.5 microns ( 4 ) and once a layer thickness of 1 micron ( 5 ) would have. On each of these a hot dip galvanized layer of 10 microns is deposited. Both layer samples were then heated to 800 ° C. The uppermost light coat does not belong to the anticorrosive layers, it is a preparation auxiliary layer of nickel, which was applied before the sample preparation, ie after heating and cooling.

In the case of coatings according to the invention, a two-phase structure with a light phase which is interspersed with dark areas forms on the surface of the steel substrate ( 6 ). This was annealed at 870 ° C in a 1 micron thick intermediate nickel layer, then was waited for 45 s, added a transfer time of 5 s and then a cooling in a press.

One shift after 6 was measured with an EDX element distribution, whereby here too a nickel support layer is present as a preparation aid on the sample. The slice cut that was measured is in 7 shown.

In the distribution of iron ( 8th ) it can be seen that relatively little iron is present in the light phase, while the dark phase shows significant iron contents.

Zinc shows that it is highly enriched in the light phase, while it is present in much lower concentrations in the dark areas, so that apparently there is an iron-rich phase as nodules or roundish accumulation in a zinc matrix.

The nickel ( 10 ) is still very weakly recognizable in the bright zinc matrix, but is obviously not present in the iron-rich nodules, whereas aluminum ( 11 ) is distributed relatively evenly throughout the layer, albeit with enrichments in the iron-rich phases.

Manganese, which is present in the base steel material, is scarcely present in the entire layer and can only be detected in the substrate.

In a comparable layer, the element distribution was measured in depth with a so-called EDX line scan ( 13 ). In this case, the scan already starts in the nickel backing layer and goes deep into the steel base material.

For a coating that was annealed at 800 ° C, had 5 s transfer time, and then was press-cooled, the result is as in 14 you can see a corresponding distribution of the elements. The nickel peak is initially close to 100%, which is because the scan already starts in the nickel preparation layer. Subsequently, the nickel content drops, and it can be seen that the nickel content is significantly lower in the dark iron-rich zones than in the bright zinc-rich phases. Accordingly, starting from the outer surface, the iron content is very low and is about 10% and increases significantly in the dark iron-rich phase to then reach its maximum in the steel matrix. The content of iron is inversely related to the zinc content, which was to be expected according to the two-phase formation.

From an originally existing nickel layer is no longer visible in the phase structure.

The positive effect of nickel in the layer can be seen in bending samples with a radius of 1.5 mm ( 15 ).

While with a nickel interlayer of only 0.5 .mu.m (with the complete dissolution of the nickel in the anticorrosion coating, the thickness of the nickel layer has an effect only on the amount of nickel in the layer). At 0.5 μm nickel and 10 μm hot-dip galvanized layer, this results in a crack pattern at 870 ° C., 45 s holding time, 6 s transfer time and corresponding cooling in the press, as shown in FIG 15 can be seen in the two left representations.

In contrast, with a 1 micron thick nickel layer and otherwise the same conditions a considerably finer crack pattern ( 15 , the two right representations) achieved.

The invention thus makes it possible to influence the corrosion protection layer based on zinc via an additional nickel layer, such that this layer evidently forms faster solid phases during cooling, which then do not react with the austenite of the steel substrate during forming.

In particular, in the zinc-rich bright phases of the coating, more nickel dissolves than in the dark iron-rich phases, which by themselves have a higher melting point.

Compared with a zinc-nickel layer deposited uniformly via electrolysis, the invention has the advantage that it allows a mixed application in electrolytic and hot-dip coating processes. Furthermore, the nickel layer can be easily applied to conventional, already hot-dip galvanized sheets, for which purpose both electrolytic coatings can be used as well as other coating methods, for. B. roll application, i. a roll coating method, such as a coil coating method, in which a nickel-containing layer having a thickness of 250 nm to 700 nm is applied.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• DE 102010030465 A1 [0012]
• EP 2290133 B1 [0013]
• EP 0364596 B1 [0014]

Claims (18)

A method for producing hardened sheet steel components, wherein deposited on a strip of a quench hardenable steel alloy as an anticorrosive coating at least two metal layers successively on the steel substrate, wherein the one metal layer is a layer of zinc or based on zinc and the other layer is a layer of a metal which forms intermetallic-less noble phases with Zn or Fe and has a higher oxidation potential than Zn, namely Ni, Cu, Co, Mn or Mo, or a layer based on these metals and wherein steels of the general alloy composition, in each case in percent by weight: Carbon (C) 0.08 to 0.6 Manganese (Mn) 0.8-3.0 Aluminum (Al) 0.01-0.07 Silicon (Si) 0.01-0.5 Chrome (Cr) 0.02-0.6 Titanium (Ti) 0.01-0.08 Nitrogen (N) <0.02 Boron (B) 0.002-0.02 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 Residual iron and melting impurities are used, wherein from the provided with the anti-corrosion coating tape boards are punched and the boards either - heated to a temperature> Ac ₃ and optionally held and then formed in a press hardening tool and quench hardened to produce the sheet steel component , or - the blanks are cold formed into a sheet steel component and the sheet steel component is subsequently heated to a temperature> Ac ₃ and quench hardened in a mold hardening tool. A method according to claim 1, characterized in that a material with the following alloy composition, in each case the indication in weight percent is used as steel material: Carbon (C) from 0.08 to 0.34 Manganese (Mn) 1.00-3.00 Aluminum (Al) 0.03-0.06 Silicon (Si) 0.01-0.20 Chrome (Cr) 0.02-0.3 Titanium (Ti) 0.03-0.04 Nitrogen (N) <0.007 Boron (B) 0.002-0.006 Phosphorus (P) <0.01 Sulfur (S) <0.01 Molybdenum (Mo) <1 Remaining iron and impurities caused by melting. A method according to claim 1 or 2, characterized in that the layer of zinc or based on zinc is applied electrolytically or by a hot dip process. A method according to claim 1 to 3, characterized in that the nickel, copper or manganese layer is applied electrolytically or by a roll coating method, for example a coil coating method. Method according to one of the preceding claims, characterized in that the nickel, copper or manganese layer is applied in a thickness of 0.5 .mu.m to 2 .mu.m in electrolytic deposition or 250 nm to 700 nm in roll application, in particular coil coating process. Method according to one of the preceding claims, characterized in that the zinc layer or layer based on zinc with a thickness of 6 microns to 30 microns is deposited. Method according to one of the preceding claims, characterized in that the layer of nickel, copper or manganese is first deposited on the steel substrate and thereon the zinc layer or coating based on zinc is deposited. Method according to one of the preceding claims, characterized in that the coating of zinc or based on zinc on the layer of nickel, copper or manganese is applied electrolytically or by hot-dip galvanizing. Method according to one of the preceding claims, characterized in that first the layer of zinc or on the basis of zinc is applied electrolytically or by hot dip coating method on the steel substrate and then the nickel layer electrolytically on the zinc layer or with a roller application method, eg. Coil-coating process, is applied to the zinc layer. Method according to one of the preceding claims, characterized in that the layer sequence between nickel, copper and manganese on the one hand and zinc or based on zinc on the other hand repeatedly applied.  An anticorrosion layer for use in a method according to any one of claims 1 to 10, wherein the anticorrosive layer has at least two layers, wherein a layer of nickel, copper or manganese is present and thereon or below there is a zinc or zinc based layer. Corrosion protection layer according to claim 11, characterized in that the layer of zinc or based on zinc is deposited electrolytically or by a hot dip process. Corrosion protection layer according to claim 11 or 12, characterized in that the nickel, copper or manganese layer is applied electrolytically or by means of a roller application method, for example coil coating method. Corrosion protection layer according to one of claims 11 to 13, characterized in that the nickel, copper or manganese layer has a thickness of 0.5 microns to 2 microns with electrolytic deposition or 250 nm to 700 nm in a roll coating method, in particular coil coating process, has. Corrosion protection layer according to one of claims 11 to 14, characterized in that the zinc layer or the layer based on zinc has a thickness of 6 microns to 30 microns. Corrosion protection layer according to one of claims 11 to 15, characterized in that the layer of nickel, copper or manganese is arranged on the steel substrate and above the zinc layer or the coating based on zinc. Corrosion protection layer according to one of claims 11 to 16, characterized in that on the steel substrate, a zinc layer or coating based on zinc, which has been deposited electrolytically or by hot dip coating, applied, and disposed on the zinc layer, the nickel layer, wherein the nickel layer electrolytically or by a roll coating method, in particular coil coating method, is applied. Corrosion protection layer according to one of claims 11 to 17, characterized in that a multiple sequence of the layers nickel, copper and manganese on the one hand and zinc or layers based on zinc on the other hand is present on the steel substrate.

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