EP3882374A1 - Method for producing areas with different optical properties on galvanized steel strips and galvanized steel strips with areas with different optical properties - Google Patents

Abstract

The invention relates to a method for producing areas with different optical properties on galvanized steel surfaces, in particular steel strips, a metal salt solution, in particular an inorganic metal salt solution, being applied to the steel surface and the steel surface then being electrolytically galvanized.

Description

The invention relates to a method for producing areas with different optical properties on galvanized steel strips with the features of claim 1.

The invention also relates, in particular, to galvanized steel strips or steel plates with areas with different optical properties according to the preamble of claim 11, which are produced according to the above-mentioned method.

Optical effects or modification of optical properties on galvanized steel strips or plates have become increasingly important in recent years. On the one hand, for the traceability of product-relevant information along the production chain as part of quality assurance; on the other hand, it is about the provision of product-specific information for the user.

Optical properties or a marking on a steel surface can be created by various methods. Often, optical properties are modified by mechanical and / or chemical processing.

During mechanical processing, a change in topography is induced, ie depressions and / or elevations are produced on the surface. Depressions can be created by removing material, for example by laser embossing. Elevations are mostly created by applying structuring elements, for example by means of printing. Nowadays, different printing processes are used for printing, such as screen printing, indirect gravure printing or ink jet.

Screen printing is a printing process in which the material to be applied is applied with a squeegee through a fine-meshed fabric onto the surface to be printed. The mesh openings of the fabric are made impermeable at those points of the fabric where no material is to be applied in accordance with the printed image.

Indirect gravure printing is a printing process in which the elements to be imaged are present as depressions in the printing matrix. The entire printing matrix is dipped into the material to be applied before printing and the excess material is removed with a doctor blade so that the material to be applied is only located in the depressions. The material transfer takes place through high contact pressure and adhesive forces between the surface to be printed and the material.

Ink-Jet is a matrix printing process in which the material to be applied is in liquid form and is applied by targeted shooting or deflection of small drops of material. The drops of material are generated using a piezoelectric transducer and then electrostatically charged via a charging electrode. The droplets of material are then accelerated and their trajectory is controlled by means of a deflection electrode. After it hits the surface to be printed, a print image is generated.

In the processes mentioned, structuring elements are applied to the surface to be marked. These are mostly foreign substances that have a different chemical composition and therefore different physical properties than the coated surface. This can possibly make further surface treatment or processing of the workpiece more difficult.

Depressions or embossments on metallic surfaces are now often produced using laser technology.

A change in topography can also be produced by chemical means. It is known to produce depressions on the surface by etching with acids. For this purpose, the metal surface to be treated is brought into contact with, for example, hydrochloric or sulfuric acid. The depth of the marking is directly related to the treatment time.

When the desired depth is reached, the surface is treated with a basic solution, for example caustic soda, in order to prevent possible corrosion from remaining acid residues.

Optical properties or a marking can also be generated by means of a coating. It can be an organic or an inorganic coating.

An organic coating can be organic lacquers. Optical effects are often created using organic multilayer systems, such as the colofer®vario process. A steel surface is pretreated free of chromate. A chromate-free primer is then applied. The visual effect is defined by a basecoat and a texture layer on top. Finally, a top coat is applied. Several system runs are required for this. This increases the manufacturing cost. In addition, further processing of the workpiece coated in this way is made more difficult.

An inorganic coating can be a metallic coating, such as a zinc coating. A zinc coating is usually applied for corrosion protection purposes.

Various galvanizing processes are known. A common galvanizing process is so-called hot-dip galvanizing (also known as hot-dip galvanizing). Steel is immersed continuously (e.g. strip or wire) or piece by piece (e.g. components) at temperatures of around 450 ° C to 600 ° C in a melt of liquid zinc (the melting point of zinc is 419.5 ° C). The zinc melt conventionally has a zinc content of at least 98.0% by weight in accordance with DIN EN ISO 1461. In the case of a continuously galvanized strip, the zinc layer has a thickness of 5 µm to 40 µm. In the case of a piece-wise galvanized component, the zinc layer can have a thickness of 50 µm to 150 µm.

In the case of electrolytic galvanizing (galvanic galvanizing), steel strips or steel plates are not immersed in a zinc melt, but in a zinc electrolyte.

The steel to be galvanized is introduced into the solution as a cathode and a dimensionally stable electrode is used as the anode. Electricity is passed through the electrolyte solution. The zinc present in ionic form (oxidation level + II) is reduced to metallic zinc and deposited on the steel surface. Compared to hot-dip galvanizing, electrolytic zinc plating can be used to apply thinner zinc layers. The zinc layer thickness is proportional to the current strength and duration of the current flow and thus the amount of charge.

Careful surface pretreatment is required to ensure the adhesion and uniformity of the zinc layer. This can be, for example, degreasing, alkaline cleaning, pickling, rinsing and / or pickling. After galvanizing, one or more subsequent treatments can be carried out, such as phosphating, oiling, passivating, applying organic coatings (KTL - cathodic dip painting).

It is also known to define the optical properties of electrolytically deposited zinc layers by changing parameters. This can, for example, be a changed electrolyte composition. The pre-treatment of the steel surface also plays a major role. Optical properties can be varied, for example, by changing the pickling or cleaning parameters. It is disadvantageous here that the change in the optical properties relates to the entire bandwidth and cannot be controlled locally. This means that no local identification patterns can be generated with this method.

From the DE 10 2017 106 672 A1 a marking and / or marking by means of laser of hot-dip galvanized metal components is known. Laser marking is carried out in such a way that the protective properties of the galvanized layer are fully retained. To generate the permanent marking, the zinc layer is selectively removed in a specified area by means of a laser and then subjected to a chemical reaction with an ambient gas (reaction gas). The chemical conversion of the zinc layer leads, among other things, to oxides that differ optically from the surrounding zinc layer and in this way create an optical effect or a pattern.

A comparable procedure is from the DE 10 2007 010 932 A1 known. The markings on the steel surface are also removed by laser. The zinc layer can be completely removed in order to increase the legibility of the marking. The subsequent oxidation of the base material increases the optical contrast.

the DE 40 33 230 A1 describes a combined marking process in which a metal strip is mechanically and chemically processed in order to create a surface structure. Initially, embossing engravings are made using a laser. The exposed areas are then etched by an acid to a specified depth, so that after the etching process, which can be repeated several times for the purpose of obtaining superimposed structures, a desired engraved profile is created.

From the DE 103 20 237 A1 a structuring process for electroplated thermoplastics is known. The thermoplastic is removed by a burning laser beam in the area of the desired symbols or structural elements. A galvanic layer is applied to the thermoplastic surface structured in this way. Since there is no thermoplastic layer in the structuring area, no electroplated layer can form there. As a result, the laser-generated structuring is also visible on the top electroplated layer.

From the DE 10 2011 051 266 A1 a structuring process is known in which structuring elements are applied to the surface to be structured by means of ink printing. The elevations produced by the ink pressure are hardened through. The surface is then coated with an electroplating layer.

In the case of the mentioned labeling methods, it is disadvantageous that the labeling can only be achieved with foreign substances. This can lead to problems with follow-up treatment. In addition, it is not possible to influence the appearance locally. As a result, the design options for the processes mentioned are limited. Furthermore, the named labeling procedures are not suitable for in-line use.

The object of the invention is to create a method for producing areas with different optical properties on galvanized steel strips, which method does not impair the zinc layer and can be controlled locally in a targeted manner.

The object is achieved with the method having the features of claim 1. Advantageous further developments are characterized in the dependent claims.

Another object of the invention is to create a steel strip produced by the above-mentioned method with areas with different optical properties.

The object is achieved with the product having the features of claim 11.

Advantageous further developments are characterized in the dependent claims.

According to the invention, areas with different optical properties are produced on galvanized steel strip or galvanized steel plates without changing the topography of the zinc layer. In addition, a physically and largely chemically homogeneous zinc layer is guaranteed. The changed optical properties are based on the generation of crystallographically different areas.

Gloss is defined as an optical property in the context of the invention. Shine is the optical property of a surface to reflect light completely or partially in a specular way. Gloss is a property, in particular also differences in gloss, which can be technically recorded and measured, in particular with the help of reflectometers.

The invention provides that a salt solution is applied locally in a targeted manner to a steel strip. The solution can be applied using any transfer method. This can be, for example, ink jet, stamping or indirect gravure printing. The steel surface is then electrolytically galvanized.

Inorganic and / or organic salt solutions are suitable for this process; metal salt solutions are particularly well suited. The metal salt solutions can contain, for example, metal ions of the 4th, 5th main group and / or 7th, 11th subgroup. It is advantageous if the following metals are included as cations: Mn, Sn, Pb, Bi, Cu, Au, Ag. It is particularly advantageous if the metal salt solution used comprises bismuth ions.

Organic, inorganic ions or complexing agents can be used as anions. Inorganic anions can be monovalent, such as, for example, F ^- , Cl ^- , I ^- , Br ^- , bivalent, such as, for example, O ^2- , S ^2- , or have a complex structure, such as, for example, NO ₃ ^2- , SO ₄ ^2- , OH ^- . It is advantageous if chloride, nitrate, sulfate ions, carbonates, hydroxide or oxide ions are present as inorganic anions. It is particularly advantageous if nitrate ions are used. Basically, salt solutions with low pH values are to be preferred.

Anions derived from organic acids can be used as organic anions. It can be, for example, alcoholates, organic sulfates, organic nitrates. It is advantageous if acetate, citrate and / or oxalate ions are present in the salt solution used.

Thus, within the scope of the invention, the solution used can be selected from the group of Mn, Sn, Pb, Bi, Cu, Au, Ag and nitrate, chloride, hydroxide, oxide, sulfate, acetate, citrate, oxalate and mixtures and combinations thereof.

After the salt solution has been applied, a cementation reaction sets in, especially at low pH values. A metallic intermediate layer is formed on the steel surface. The reaction can be exemplified by the following equation:

2M ^{x +} + xFe → 2M ↓ + xFe ²⁺ (1)

It can be seen that the metal ions are reduced from the salt solution and deposited on the steel surface. The deposited intermediate layer can have a layer thickness which is in the nanometer range.

The saline solution is applied in-line. It is advantageous that the desired pattern is not impaired by the entire system cycle.

It is also advantageous that further processing steps are not necessary.

After the salt solution has been applied, the steel strip or steel plate is electrolytically galvanized. For this purpose, the steel plate and / or the steel strip is brought into contact with a zinc electrolyte solution. The zinc ions present in the electrolyte solution are reduced by the current passed through and crystallize on the surface to be coated. This process is known as electrocrystallization. The zinc layer produced can have a layer thickness that is in the micrometer range.

The layer thickness ratio of the metallic intermediate layer and the zinc layer can be defined as 1: 1000, for example.

The electrocrystallization of the zinc is influenced by the underlying metallic intermediate layer in such a way that the crystallization in this area (wide area of the first type) takes place in a much more orderly manner. This leads to an increased symmetry of the zinc grid. This changes the optical properties of the zinc in the pretreated areas in such a way that the pretreated areas have a higher gloss. Further follow-up treatment is not necessary here.

It is advantageous that there is no topography change, such as the layer thickness or the sheet thickness. If, for example, a marking or identification is required for the production process, this is no longer visible after a final painting due to the uniform topography.

It is also advantageous that the modification of optical properties according to the invention offers great design freedom. For example, logos or patterns for an attractive visual design of the galvanized surface as well as other markings can be applied without changing the topography of the coating.

It is also advantageous that no foreign substances or foreign elements are applied to the zinc surface. As a result, the marked surface is fully compatible with the other typical subsequent processes, such as phosphating, passivating, oiling, cleaning, and applying organic coatings.

The invention thus relates to a method for producing areas with different optical properties, specifically the gloss on galvanized steel strips or steel plates, a metal salt solution being applied to the steel surface and the steel surface then being galvanized.

In one embodiment of the invention, an inorganic and / or organic metal salt solution is used.

Advantageously, a metal salt solution is used whose cations are from the group of Mn, Sn, Pb, Bi, Cu, Au, Ag and their anions are from the group of nitrate, chloride, hydroxide, oxide, sulfate, acetate, citrate, oxalate and mixtures and combinations thereof are selected.

In another advantageous embodiment, a metal salt solution is used which contains bismuth ions.

It is also advantageous if a particularly acidic metal salt solution is used which comprises bismuth (III) nitrate pentahydrate and dilute nitric acid.

In a further advantageous embodiment, the applied metal salt solution induces a cementation reaction with the steel surface, a metallic intermediate layer being deposited on the steel surface. In particular, a rapidly occurring cementation reaction can simplify the application of the metallic intermediate layer in-line in one system run, since complex drying steps for the application of the intermediate layer can be dispensed with.

The metallic intermediate layer can advantageously have a layer thickness in the nanometer range. Layer thicknesses in this area have hardly any effects on the topographical surface profile and are accordingly no longer recognizable in the height profile, especially during subsequent galvanizing.

Another advantage is that the steel surface is galvanized after the application of the metal salt solution, the galvanizing being carried out electrolytically.

The metallic intermediate layer and the upper zinc layer advantageously have layer thicknesses which are in a ratio of 1: 1000 to 1: 20000 to one another. Changes in the height profile due to the metallic intermediate layer can therefore be neglected.

In addition, it is advantageous if the metallic intermediate layer influences the zinc electrocrystallization in such a way that the crystallization takes place in this area in a significantly more orderly manner.

A continuously rolled cold wide strip, in particular an annealed continuously rolled cold wide strip, can advantageously be used as a template.

With a further advantage, the zinc layer is deposited over the areas treated with the salt solution and untreated, with both areas having an unchanged topography after galvanizing.

In a further advantageous embodiment, the zinc layer is deposited over the areas of the first type and untreated areas of the second type treated with the salt solution, the areas of the first type having different optical properties, in particular a higher gloss.

It is advantageous if the steel strip or the steel plate has areas of the first type and areas of the second type, both areas having a uniform topography, the areas of the first type having a thin metallic intermediate layer and having different optical properties.

The metallic intermediate layer can advantageously comprise metal or metals which can be selected from the group of Mn, Sn, Pb, Bi, Cu, Au, Ag.

It is particularly advantageous if the metallic intermediate layer comprises bismuth.

It is also advantageous if the steel strip or the steel plate has an electrolytically deposited zinc layer.

In another advantageous embodiment, the steel strip or the steel plate is continuously cold-rolled, in particular continuously annealed and cold-rolled.

Advantageously, the areas of the first type carrying the metallic intermediate layer can have a significantly more ordered zinc crystal structure.

Figur 1:

Verändertes Erscheinungsbild des Zinks über der metallischen Zwischenschicht.

Figur 2:

Lichtmikroskop- und Topographieaufnahme einer veränderten Zinkschicht.

The invention is explained by way of example with the aid of a drawing. It shows:

Figure 1:

Changed appearance of the zinc over the metallic intermediate layer.

Figure 2:

Light microscope and topography image of a modified zinc layer.

According to the invention, a salt solution is applied to a steel surface. The steel surface can be a steel strip surface. The steel strip provided with the salt solution is then electrolytic in-line in a system run in a manner known per se galvanized. The areas treated according to the invention with the salt solution are still visible after galvanizing. In Figure 1 one recognizes an identification pattern generated with the method according to the invention. The left picture shows the Figure 1 the cold broadband (KBB) with an applied metallic intermediate layer in the form of a regular pattern. The picture on the right shows the same cold wide strip after electrolytic galvanizing, i.e. an electrolytically galvanized strip (EVB). The applied pattern is still recognizable on the electrolytically galvanized surface, shown in dotted lines in the right picture, since the electro-crystallization in the area of the metallic intermediate layer is more orderly and therefore differences in the optical properties, namely in the gloss, are generated. Due to the different optical properties of the areas of the first and second type, a defined visual appearance is produced according to the invention.

The generated visual appearance, which can contain identification patterns or any representations such as logos, does not show any change in topography in terms of layer thickness or sheet thickness change. This is in Figure 2 evident. In the Figure 2 Shown on the left is an enlarged transition from an area of the second type on the left to an area of the first type on the right, which is shown lighter due to its higher gloss.

The greatly enlarged light microscope image of the section of the galvanized steel strip shown in the left picture is in Figure 2 shown in the upper right picture. This shows the enlarged transition from an area of the second type to an area of the first type. The area shown dark on the right-hand side is the area of the first type which has a metallic intermediate layer. The zinc atoms are deposited on this metallic intermediate layer in a more ordered crystal structure, which creates areas with different optical properties and, in the present case, with a higher gloss. The fact that this area appears darker is due to the lighting, in this case a ring light, which is hardly reflected by the relatively flat area (evenly arranged "zinc shingles"). The lighter area in the left part of the upper picture was not treated with the salt solution and therefore only has an electrolytically deposited zinc layer. In this disordered area there are accordingly areas that reflect the ring light and therefore this area appears overall brighter. The generated gloss effect therefore depends on the lighting angle. It is essential that gloss differences are created, i.e. relative differences between the areas.

The second shot in Figure 2 The bottom right is a topographical image of the same sample. It can be seen that both areas have an identical topography. There are no elevations or depressions in the area of the marking.

The invention is explained by way of example on the basis of an exemplary embodiment.

An inorganic aqueous metal salt solution is prepared by adding 50 mL HNO ₃ (1N) to _{a spatula tip of Bi (NO 3} ) ₃ · 5H _{2 O.} The solution is stirred at room temperature until the salt is completely dissolved. The stock solution prepared in this way is then diluted with deionized water in a ratio of 1: 9. The diluted salt solution is applied in-line to a KBB (continuously rolled cold wide strip) by means of ink-jet or indirect gravure printing. The steel strip pretreated in this way is then electrolytically galvanized. In-line means in this context that the diluted salt solution is applied in the electrolytic galvanizing plant.

In the case of the invention, it is advantageous that the salt solution is applied locally in a targeted manner. As a result, the areas that arise later with different optical properties or patterns can be defined locally in a targeted manner.

It is advantageous here that the marking produced according to the invention is no longer visible after a further coating, for example a cathodic dip coating, due to the unchanged topography.

The marking according to the invention can thus be used for markings of any kind. It can be used, for example, for material description and / or data backup, such as for batch number, coil number, manufacturer information or brands, samples and the like.

In addition, barcodes or 3D codes can be generated with the method according to the invention.

If, on the other hand, there is no further coating, the appearance produced according to the invention can also be used to form any graphic representations, for example logos, which should be visible on the final product.

Claims (15)

Method for producing areas with different optical properties on galvanized steel surfaces, in particular steel strips, wherein a metal salt solution, in particular an inorganic metal salt solution, is applied to the steel surface and the steel surface is then electrolytically galvanized. Method according to one of the preceding claims, characterized in that a metal salt solution is used whose cations are from the group of Mn, Sn, Pb, Bi, Cu, Au, Ag and whose anions are from the group of nitrate, chloride, sulfate, hydroxide, Oxide, acetate, citrate, oxalate and mixtures and combinations thereof can be selected. Method according to one of the preceding claims, characterized in that a metal salt solution is used, the solution containing bismuth ions. Method according to one of the preceding claims, characterized in that a metal salt solution is used, the solution comprising bismuth (III) nitrate pentahydrate and dilute nitric acid. Method according to one of the preceding claims, characterized in that the applied metal salt solution induces a cementation reaction with the steel surface, a metallic intermediate layer being deposited on the steel surface. Method according to Claim 5, characterized in that a metallic intermediate layer is deposited, the metallic intermediate layer having a layer thickness in the nanometer range. Method according to Claim 5 or 6, characterized in that the metallic intermediate layer and the upper zinc layer have layer thicknesses which are in a ratio of 1: 1000 to 1: 20000 to one another. Process according to Claims 5 to 7, characterized in that the metallic intermediate layer influences the zinc electrocrystallization, the crystallization taking place in this area in a more orderly manner than in areas without the metallic intermediate layer. Method according to one of the preceding claims, characterized in that the zinc layer is deposited over the areas treated with the salt solution and untreated, both areas having an unchanged topography, in particular zinc layer thickness, after galvanizing. Method according to one of the preceding claims, characterized in that the zinc layer is deposited over the areas of the first type treated with the salt solution and untreated areas of the second type, the areas of the first type having different optical properties, in particular a higher gloss. Electrolytically galvanized steel strip, in particular produced by a method according to one of the preceding claims, characterized in that the electrolytically galvanized steel strip has areas of the first type and areas of the second type, both areas having a uniform topography, in particular zinc layer thickness, the areas of the first type being a have thin metallic intermediate layer between steel surface and zinc layer and have different optical properties and in particular have a higher gloss. Steel strip or steel plate according to claim 11, characterized in that the metallic intermediate layer comprises metal or metals which can be selected from the group of Mn, Sn, Pb, Bi, Cu, Au, Ag. Steel strip according to Claims 11 to 12, characterized in that the metallic intermediate layer comprises bismuth, the metallic intermediate layer having a layer thickness in the nanometer range. Steel strip according to Claims 11 to 13, characterized in that the metallic intermediate layer and the zinc layer have layer thicknesses which are in a ratio of 1: 1000 to 1: 20000 to one another. Steel strip according to claims 11 to 14, characterized in that the areas of the first type bearing the metallic intermediate layer have a clearly more ordered zinc crystal structure, the zinc layer overlying the metallic intermediate layer having different optical properties, in particular a higher gloss.

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