DE102012111066A1 - Coated steel sheet comprises zinc-based coating, organic coating, iron-containing layer and non-crystalline conversion layer, which is arranged between the iron-containing layer and the organic coating - Google Patents

Abstract

Coated steel sheet comprises zinc-based coating, which is applied to the steel sheet in one or both sides, an organic coating, an iron-containing layer (7) with a coating weight of less than 1 g/m 2>, which is applied electrolytically on the zinc-based coating and a non-crystalline conversion layer (8), which is arranged between the iron-containing layer and the organic coating. An independent claim is also included for producing the coated steel sheet, comprising initially providing a steel band with one- or two-sided zinc based coating made of zinc or zinc alloy and organic coating, subsequently cutting the steel sheet from the steel band, where the iron-containing layer with coating weight of less than 1 g/m 2> is applied on the zinc-based coating made of the steel band and applying the non-crystalline layer to the iron-containing layer using a conversion agent.

Description

The invention relates to a coated steel sheet with a steel sheet, a single or double-sided zinc-based coating applied to the steel sheet and an organic coating.

Furthermore, the invention relates to a method for producing a coated steel sheet, wherein first a steel strip is provided with a one- or two-sided zinc-based coating of zinc or a zinc alloy and then an organic coating, wherein subsequently the steel sheets are cut from the steel strip.

Surface finishing of steel materials has become increasingly important in recent years. For reasons of corrosion protection in the automotive industry, the galvanizing of steel strips as a starting material for the production of body parts for automobiles has become of paramount importance. But also in the construction industry or architecture as well as brown (consumer electronics) or white goods (household appliances) provided with metallic coatings and occasionally provided with a subsequent painting steel sheets have now prevailed. The paint is usually applied as a wet coating on the component. However, it is also possible to provide the metal-coated steel strip at the factory with an organic coating, for example in a so-called coil coating process (continuous metal strip coating process), and then to process it into a component.

For the galvanizing of steel strips, various methods are used. On the one hand, this is the molten hot-dip galvanizing, primarily for non-visible body parts, on the other hand, the electrolytic galvanizing can apply with the more uniform zinc layers on the steel strip, which for further processing, eg. B. in subsequent painting, is more advantageous.

For the coil coating process, the galvanized metal strips are delivered to the coating system in the rolled-up state. There, they are unwound and fed via a tape storage of the coating system. In the first step, the tapes are alkaline cleaned to remove fats and oils that have been applied to protect the surface after metal strip production. Subsequently, the tape is pretreated by a suitable method. Now, a first coating layer, the primer, is applied in a rolling process and baked at about 240 ° C. Subsequently, a second coating layer, the topcoat or the topcoat, in turn applied by rolling and baked again at about 240 ° C. Then the paint can be protected by a laminating film. Finally, the metal strip is again passed over a memory and rolled up into a coil.

The two layers of lacquer that are applied perform different functions. The primer is important for adhesion and corrosion protection and the topcoat for the decorative appearance. Almost all color wishes of the customer can be considered.

In the automotive industry mainly galvanized strip material is processed. In today's conventional design for self-supporting bodies deep-drawn sheet metal shells by resistance spot welding, z.T. in combination with other procedures, linked together. In this construction, so-called flange areas arise in which a plurality of superimposed sheet metal layers are connected by welding points. The interstices of these sheet metal layers are insufficiently protectable by a cathodic dip painting, which is the central element of corrosion protection in the automotive industry. Therefore, such areas are sealed by subsequent secondary corrosion protection measures. In some cases, instead of secondary measures, weldable corrosion protection primers (CSPs) already applied in the steelworks by coil coating are used. It is advantageous if the primer is suitable for the resistance spot welding mainly used in the automotive industry. This KSP has the complex task of forming underlay-stable layers both uncoated - in the flange - and under later topcoats in adjoining areas.

Galvanization is understood below to mean all possible zinc-based coatings and alloys, such as zinc. Alloys with iron, aluminum, magnesium, nickel.

It is true that a metallic zinc coating which is applied to the steel strip by electrolytic or hot dipping methods has a cathodic protective effect which prevents active dissolution of the nobler core material at cut edges and at mechanically induced damage to the zinc coating. However, the high reactivity of the galvanizing is unfavorable for the formation of immigration-stable boundary layers. Therefore, zinc coatings are passivated prior to coating with primers with one or more conversion treatments to achieve a substantial increase in undercounter stability.

Since the steel strip producers are increasingly turning to surface coating with metallic coatings to incorporate further corrosion coatings, in particular coil coatings, in strip production plants, there is an increased demand for anti-corrosion treatments there as well as in the processing industry for cutting edges and contact corrosion effectively prevent problems associated with paint adhesion.

The prior art describes various pretreatments that address the problem of edge protection. As an essential strategy, the improvement of the paint adhesion of the organic coating on the metallic surface-treated steel strip is pursued.

In the published patent application DE 10 2007 021 364 A1 discloses a metallizing pretreatment of zinc surfaces which produces in an aqueous medium metallic film deposits of not more than 100 mg / m ² of molybdenum, tungsten, cobalt, nickel, lead, tin and / or iron on the treated zinc surfaces. This method of metallizing pretreatment is very complicated and expensive to implement and thus often uneconomical in industrial production.

In order to set a predictable and defined reactivity, metal surfaces are usually provided with a conversion layer before painting. By the action of a treatment agent atoms from near-surface areas of the metal are dissolved out and converted into non-metallic compounds. In fast reaction with small amounts of conversion usually thin non-crystalline, i. X-ray amorphous layers with variable phase composition.

The term "conversion treatment" is generally understood to mean that components of the treatment solution chemically react with the metal surface to form a corrosion protection layer incorporating both components of the treatment solution and metal atoms from the metal surface.

At slower reaction with larger amounts of conversion agent higher ordered, thicker, crystalline layers can often be produced with homogeneous phase inventory. From a process engineering point of view, crystalline layers usually have to be rinsed off in order to remove unreacted treatment agent ("rinse" method), while in the case of non-crystalline layers, the entire amount of the treatment agent is dried ("no-rinse" method).

From the publication DE 32 26 239 A1 is known a steel sheet, which is provided electrolytically with a zinc-nickel coating. In order to improve the wet adhesion of the coating, an iron or iron-zinc layer is applied electrolytically to this coating and then phosphating as a conversion layer. This layer has a coating weight of 1 to 15 g / m ² with an iron content of at least 60%.

As the purpose of this iron layer is stated that in the subsequent phosphating iron from this layer is to be dissolved out to form phosphophyllite, which is to improve the wet adhesion of the subsequent coat. Phosphating is a conversion process that produces a crystalline conversion layer. This is done first a pickling attack on the base material, go in the metal cations with evolution of hydrogen in solution. Then the layer formation takes place by precipitation of crystalline, sparingly soluble phosphates. In this method, therefore, the original thickness of the iron layer is significantly reduced by the dissolution of iron and integration into the phosphate layer. So that after the phosphating a "residual thickness" of the iron layer is present, so before a correspondingly thick coating must be applied.

A disadvantage of this steel sheet is the complex phosphating as a crystalline conversion layer and the high coating weight of the iron layer required thereby, which can be produced only with great effort in this large thickness.

The object of the invention is to provide a coated steel sheet, which is compared to the known steel sheets with at least equally good corrosion properties with significantly lower iron layer thickness and less expensive to produce. Furthermore, a method for producing such a sheet is to be specified.

For the steel sheet, this object is achieved with the features of claim 1 and for the method with the features of claim 10. Advantageous developments are the subject of dependent claims.

The teaching of the invention comprises a coated steel sheet with improved corrosion properties, with a steel substrate, a one or two-sided metal coating of zinc or a zinc alloy and an organic coating, which is characterized in that the steel sheet on the metallic coating with an electrolytically applied iron-containing layer is provided with a coating weight of less than 1 g / m ² and arranged between the iron-containing layer and the organic coating, non-crystalline conversion layer.

The inventive method for producing a coated steel sheet with improved corrosion properties comprises a steel strip which is provided with a one- or two-sided zinc-based coating of zinc or a zinc alloy and then with an organic topcoat, wherein subsequently the steel sheets are cut from the steel strip, which thereby characterized in that on the metallic coating an iron-containing layer with a coating weight of less than 1 g / m ² and then by means of a conversion agent, a non-crystalline conversion layer is applied before or simultaneously with the application of the organic coating on the iron-containing layer.

In comparison to DE 32 26 239 A1 In addition, a significantly lower iron coverage is required on the zinc surface in order to improve the immigration resistance. Investigations have shown that in a non-crystalline conversion treatment on the zinc surface significantly lower coating weights of iron are necessary to achieve a very good paint adhesion and thus very good corrosion properties of the coated steel sheet.

In contrast to applying a crystalline conversion treatment by means of phosphating to improve the paint adhesion, in which iron atoms are leached out of the applied iron layer to form phosphophyllite and thus the thickness of the iron layer is reduced again, the application of a non-crystalline conversion layer, the thickness of the applied Iron layer completely preserved so that it can be applied much thinner.

In the following, non-crystalline conversion layers essentially mean inorganic, very thin layers on a metal surface, which are produced by chemical reaction of an aqueous treatment solution with the metallic substrate. The conversion layers are a very good primer for subsequent organic coatings and also have anti-corrosive effects.

The non-crystalline conversion layer can in a first embodiment of the invention consist of a separately applied to the iron-containing layer layer, according to an advantageous embodiment of the invention, it is possible to apply this layer integrated into the organic coating (coating or coil coating), wherein the Conversion agent is part of the organic topcoat. In addition, the omitted operation of applying the conversion layer separately reduces the manufacturing cost of the steel sheet and enables functional extensions during the coating processes (e.g., 3-layer systems in one pass).

While in the known steel sheet iron deposits of 1 to 15 g / m ^{2 are} necessary in order to have sufficient iron available for phosphophyllite formation, in the invention less than 1 g / m ^2, preferably max. 0.5 or less than 0.25 and optimally at most 0.1 g / m ² iron coating on the zinc coating necessary. An iron deposit of 0.1 g / m ² corresponds to a layer thickness of about 0.01 microns.

Tests have shown that these very thin iron layers are sufficient to ensure excellent properties (adhesion and corrosion resistance) for the subsequent non-crystalline conversion layer and the organic topcoat.

Accordingly, the iron content in the layer applied to the surface of the zinc or zinc alloy coating should be at least more than 60% by weight, more preferably more than 90% by weight.

In principle, the zinc or zinc alloy coating of the bare steel sheet can be applied both by the hot dip process and electrolytically. It is advantageous, however, this coating be applied electrolytically, since the subsequent electrolytic application of the iron layer can then be advantageously coupled with this process step, so that a separate operation is eliminated.

In the case of a zinc alloy, the coating may consist, for example, of alloys containing one or more of the elements iron, aluminum, magnesium, and nickel in addition to the main constituent zinc.

Further features, advantages and details of the invention will become apparent from the following description of an embodiment.

Show it:

1 a sequence of a layer structure of a steel sheet according to the prior art,

2 a sequence of a layer structure of a steel sheet according to the invention,

3 a first alternative sequence of a layer structure of a steel sheet according to the invention,

4 a second alternative sequence of a layer structure of a steel sheet according to the invention and

5 Results of investigations in accelerated corrosion test "Fast Corrosion Test" (FCT).

The 1 shows a sequence of states I to V of a steel sheet 1 in the production of a layer structure on a steel sheet 1 According to the state of the art.

Starting from an uncoated sheet steel 1 According to the state I, which may be referred to as a steel substrate other than a coated steel sheet, is applied to the steel substrate of the steel sheet 1 in a first process step, a zinc-nickel coating 2 Applied (see Condition II). Subsequently, in a further process step, the zinc-nickel coating 2 an iron-zinc coating 3 with a minimum coating weight of 1 g / m ² by electrolytic deposition (see Condition III). By means of a subsequent phosphating is used to form a phosphating 4 Iron from the iron-zinc coating 3 dissolved and phosphophyllite formed (see Condition IV), with the thickness of the iron-zinc coating 3 decreases. Slimming the iron-zinc coating 3 is shown schematically in the region of the circle X between the states III and IV. Subsequently, in a further process step, the coating of the phosphating layer takes place 4 with an organic topcoat 5 , The layer structure is shown only schematically and not to scale. This also applies to the following 2 to 4 ,

In the 2 is an inventive sequence of states I to V of a steel sheet 1 in the production of a layer structure on a steel sheet 1 Shown until the electrolytic deposition of an iron-containing layer 7 according to state III, the layer structure is similar to that 1 described layer structure. Instead of a zinc-nickel coating 2 comes in the state II, a zinc-based coating 6 (Zinc or zinc alloy) for use in which instead of or in addition to nickel, other alloying elements can be alloyed. This zinc based coating 6 could also be described as a metallic coating of zinc or a zinc alloy. Regarding the iron-containing layer 7 in the state III can in addition to the iron-zinc coating 3 According to the prior art, other compositions are used. The coating weight of the iron-containing layer 7 is less than 1 g / m ² . Preferably, the coating weight of the iron-containing layer 7 Max. 0.5 g / m ² or less than 0.25 g / m ² and optimally at most 0.1 g / m ² iron coating. An iron deposit of 0.1 g / m ² corresponds to a layer thickness of approximately 0.01 μm. In the case of the steel sheet according to the invention, instead of phosphating, a very thin noncrystalline conversion layer is subsequently added 8th Applied (see Condition IV) and then the paint or the paint 5 applied (see state V). In the area of the circle Y is shown schematically that the thickness of the iron-containing layer 7 between states III and IV does not decrease.

In 3 is a possible process route for manufacturing a steel sheet 1 according to the invention for use in the automotive industry. In a first step, a zinc-based coating 6 as a corrosion protection layer on the steel sheet 1 isolated (see state II). The zinc-based coating 6 is preferably an electrodeposited zinc layer or zinc alloy layer. On this zinc layer or zinc alloy layer is electrolytically an iron-containing layer 7 with a coating weight of less than 1 g / m ² , preferably by 0.1 g / m ² applied (see Condition III). In the case of electrolytically galvanized steel sheet, the galvanic deposition of iron takes place immediately after zinc deposition, ie in one process step. After application of the iron-containing layer 7 becomes a noncrystalline conversion layer 8th applied (see state IV). This is followed, for example, by the application of a resistance-weldable anticorrosive primer 9 in the coil coating process (see state IV). The noncrystalline conversion layer 8th can before the anti-corrosive primer 9 or with the anticorrosive primer 9 be applied. States II and III are assigned to a galvanizing line and state IV to a strip coating line.

In 4 is another possible process route for making a steel sheet 1 represented according to the invention for use in the construction industry or architecture or for white or brown goods. Again, in a first step, a zinc-based coating 6 as a corrosion protection layer on the steel sheet 1 isolated (see state II). The zinc-based coating 6 (Zinc or zinc alloy) is preferably applied in a hot dip process within a hot dip galvanizing line. On this zinc-based coating 6 becomes an iron-containing layer within the hot-dip galvanizing line 7 applied via a galvanic deposition of iron. This is advantageously done after solidification and cooling of the zinc-based coating 6 , The iron-containing layer 7 has a coating weight of less than 1 g / m ² , preferably about 0.1 g / m ² (see Condition III). It is exploited that an interfering for the electrodeposition aluminum oxide layer of the hot dipped steel strip 1 not immediately arises, but requires a certain amount of time for education. Subsequently, an organic coating 5 applied in the form of a paint system primer / topcoat in the coil coating process. States II and III are assigned to a galvanizing line and state IV to a strip coating line.

The 5 shows results of investigations in an accelerated corrosion test "Fast Corrosion Test" (FCT). For this test were two types of samples of coated steel sheets 1 manufactured with a different layer structure. The automobiltypische layer structure of the first type of sample consists of a steel substrate (steel 1 ), on both sides of an electrodeposited zinc-based layer 2 (abbreviated: ZE), each on a non-crystalline conversion layer 8th , a resistance weldable anticorrosive primer and then a cathodic dip paint. The electrodeposited zinc-based layer 2 has a layer thickness of 75/75, which corresponds to about 7.5 microns on each side. The second sample according to the invention additionally has between the zinc-based layer 7 and the non-crystalline conversion layer 8th an electrodeposited iron-containing layer 7 with a coating weight of less than 1 g / m ² . Several samples of these two types of specimens were provided with a scribe deep down to the steel substrate and then subjected to an accelerated corrosion test (FCT) at 40 ° C for 240 hours and evaluated after tape removal. In particular, the FCT investigates underwater corrosion in the long-term wetland at the phase boundary of a saline solution saturated with oxygen and water. The samples obtained after the FCT are in 5 shown. In the upper part of the picture are two samples of the first sample type without the additional iron-containing layer 7 and in the lower part of the picture two samples of the second type with the additional iron-containing layer 7 shown.

The cathodic dip coating (KT painting) of the additional iron-containing but very thin layer 7 (abbreviated: FE) coated steel sheets 1 according to the second type of sample has a significantly better immigration resistance than those on the steel sheets with purely zinc-based coatings 2 according to the first type of sample (see Table 1 below). The infiltration (UW) of the paint in the Ritz was evaluated on the upper images of the 5 can be seen. sample UW Ritz gas phase / mm UW Ritz, liquid phase / mm ZE 75 / 75-1 0.5 4.3 ZE 75 / 75-2 0.8 2.8 ZE 75/75 + FE-1 0.0 1.5 ZE 75/75 + FE-2 0.0 0.9

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 102007021364 A1 [0012]
• DE 3226239 A1 [0016, 0023]

Claims (13)

Coated steel sheet comprising a steel sheet, a single or double-sided zinc-based coating applied to the steel sheet and an organic coating, characterized in that on the zinc-based coating ( 2 ) an iron-containing layer ( 7 ) is applied electrolytically with a coating weight of less than 1 g / m ² and between the iron-containing layer ( 7 ) and the organic coating ( 5 ) a non-crystalline conversion layer ( 8th ) is arranged. Coated steel sheet according to claim 1, characterized in that the non-crystalline conversion layer ( 8th ) separately on the iron-containing layer ( 7 ) is applied. Coated steel sheet according to claims 1 and 2, characterized in that the organic coating ( 5 ) comprises a corrosion inhibitor primer. Coated steel sheet according to claim 3, characterized in that the anticorrosion primer is weldable, preferably resistance spot weldable. Coated steel sheet according to one of claims 1 to 4, characterized in that the organic coating ( 5 ) comprises a corrosion inhibiting primer and a topcoat. Coated steel sheet according to one of claims 1 to 6, characterized in that the coating weight of the iron-containing layer ( 7 ) is less than 1 g / m ² , preferably less than 0.5 g / m ^2, and more preferably less than 0.25 g / m ² . Coated steel sheet according to one of claims 1 to 6, characterized in that the iron-containing layer ( 7 ) has an iron content of more than 60% by weight, preferably more than 90% by weight. Coated steel sheet according to one of claims 1 to 7, characterized in that the zinc-based or a zinc alloy existing zinc-based coating ( 2 ) is a layer applied electrolytically or by hot dipping. Coated steel sheet according to one of claims 1 to 8, characterized in that in the case of a zinc alloy as a zinc-based coating ( 2 ) in addition to zinc as the main constituent of one or more of the elements aluminum, iron, nickel, magnesium are contained in the zinc alloy. A method for producing a coated steel sheet, wherein first a steel strip is provided with a one- or two-sided zinc-based coating of zinc or a zinc alloy and then an organic coating, wherein subsequently the steel sheets are cut from the steel strip, characterized in that on the zinc-based coating ( 2 ) of the steel strip ( 1 ) an iron-containing layer ( 7 coated with a coating weight of less than 1 g / m ² and then by means of a conversion agent, a non-crystalline conversion layer ( 8th ) on the iron-containing layer ( 7 ) is applied. A method according to claim 10, characterized in that in a two-stage step, first the non-crystalline conversion layer ( 8th ) as a separate layer on the iron-containing layer ( 7 ) and then the organic coating ( 5 ) is applied. A method according to claim 10, characterized in that in a one-step step, the non-crystalline conversion layer ( 8th ) as a constituent of the organic coating ( 8th ) on the iron-containing layer ( 7 ) is applied. Method according to one of claims 10 to 12, characterized in that the steel strip ( 1 ) is electrolytically coated with zinc or with a zinc alloy and subsequently in the same step the iron-containing layer ( 7 ) is applied electrolytically.

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