METHOD FOR PRODUCING AN ENAMELED STEEL COMPONENT, ENAMELED STEEL COMPONENT, AND USE THEREOF

Abstract

In a method for producing an enameled steel component, a steel component is coated with an adhesion promoter layer which comprises a bonding metal or a bonding metal oxide or a salt thereof, and a binder, and the coated steel component is enameled.

Description

The present invention relates to a method for producing an enameled steel component, and an enameled steel component which is obtainable by the method.

PRIOR ART

Enameled steel components and methods for producing enameled steel components have been known for many years. An essential factor here is to ensure a sufficient adhesion of the steel substrate to the enamel. It is known that the presence of different metal oxides, such as for example nickel oxide, on the boundary surface between the steel and enamel improves the adhesion. In the prior art, in particular, the following technologies are used in order to form a sufficient adhesion of the enamel to the steel substrate.

Firstly, methods for producing an enameled steel component are known in which a metallic primer coat is applied before the application of the enamel layer in order to improve the adhesion. A corresponding method is disclosed, for example, in CH 409573. The base layer is deposited electrochemically by a galvanizing process. The metallic base layer is at least partially converted into an oxide layer in the subsequent enameling process, which increases the adhesion of the enamel to the steel substrate. However, the galvanizing process is associated with a high outlay in terms of energy and equipment and is very harmful to the environment and health. The adhesion of the enamel to some types of steel can also be insufficient.

Secondly, methods are known in which a plurality of enamel layers are applied, wherein metal oxides which increase the adhesion to the substrate are added to at least the first enamel layer. The drawback of these conventional methods is that a plurality of enamel layers are required, which is a drawback in terms of cost and from a process engineering perspective. Moreover, the application of enamel powders with metal oxides, which increase the adhesion, is very difficult since they are harmful to health and the environment.

Thirdly, single-layer direct enameling methods are known in which only a single enamel layer is applied to the substrate surface. Metal oxides which increase the adhesion are added to this enamel layer. Such methods have the drawback, however, that the metal oxide is not only present on the boundary layer but over the entire enamel layer and this is why a relatively large quantity of metal oxide is required in order to ensure a sufficient adhesion. The presence of metal oxides on the enamel surface can also result in further drawbacks such as reduced chemical and mechanical stability, such as resistance to chemicals, water or steam, corrosion resistance, scratch resistance or cleanability. Moreover, the application of such enameled steel components can be limited, in particular in food contact applications, since the metal oxides, which are potentially harmful to health, are present on the enamel surface.

For these reasons, the methods for producing enameled steel objects known from the prior art are capable of improvement.

OBJECT OF THE INVENTION

An object of the present invention is the provision of a method for producing an enameled steel component which remedies the drawbacks associated with the prior art. In particular, the object is to provide a method in which an excellent adhesion of the enamel to the steel substrate is achieved even with a small overall quantity of bonding metal and in which a plurality of types of steel can be used as the substrate. A further object of the method is to reduce the outlay in terms of energy and equipment and to reduce the risk to the environment and health. A further object is to provide a method for producing an enameled steel component which has excellent chemical and mechanical stability, such as resistance to chemicals, water or steam, corrosion resistance, scratch resistance and cleanability, and which is completely suitable for food contact applications.

A further object of the present invention is the provision of an enameled steel component which has the corresponding properties. In particular, one object is the provision of an enameled steel component which has an excellent adhesion of the enamel to the steel substrate, even with a small overall quantity of bonding metal, and in which a plurality of types of steel can be used as the substrate. A further object is to provide an enameled steel component which has excellent chemical and mechanical stability, such as resistance to chemicals, water and steam, corrosion resistance, scratch resistance and cleanability. A further object is the provision of an enameled steel component which is suitable for a plurality of uses, in particular for uses in the food contact sector.

BRIEF DESCRIPTION OF THE INVENTION

This object is achieved by the method as claimed in claim 1. Preferred embodiments of the method are defined in the dependent claims 2 to 13 which are also encompassed in combination with one another. The object is also achieved by the enameled steel component as claimed in claim 14. The object is further achieved by the use of the enameled steel component as claimed in claim 15.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing an enameled steel component, comprising the steps: providing a steel component; coating the steel component with an adhesion promoter layer which comprises at least one bonding metal, bonding metal oxide or a salt thereof, and a binder; and enameling the coated steel component.

The shape and type of the steel components or the steel substrates is not limited. In particular, flat components such as metal sheets, plates or foils can be used, but also components with three-dimensional geometries such as deep-drawn or cast molded parts. All types of steel can be used, such as alloyed or non-alloyed types of steel, for example IF steel or EK steel (see EN10209, for example DC0xEK) steel. According to the invention, the layers are deposited on the finished steel component, i.e. the resulting enameled steel component is no longer subjected to forming.

In particular, the step of coating the steel component with a specific adhesion promoter layer before the enameling is essential to the invention for the method. The adhesion promoter layer according to the present invention comprises at least one bonding metal, bonding metal oxide, or a salt thereof, and a binder. The adhesion promoter layer serves as a primer and is also denoted as the sacrificial layer since in embodiments it is at least partially dissolved in the subsequent enameling process. The presence of the binder and the bonding metal, bonding metal oxide or a salt thereof in the adhesion promoter layer is required in order to achieve the advantages according to the invention which are described in detail below.

The bonding metal can be used in elemental form, as an oxide or as a salt thereof. If the bonding metal is not present as an oxide, it is oxidized in the subsequent enameling process. The resulting bonding oxide ensures the excellent adhesion of the enamel to the steel substrate.

In embodiments, the resulting bonding metal oxide can at least partially diffuse into the enamel layer. Thus in embodiments a concentration gradient of the bonding metal oxide is produced in the resulting enamel layer in such a manner that the concentration of the bonding metal is at its greatest on the boundary surface with the steel substrate and reduces in the direction of the surface. According to the invention, no bonding metal oxide is present on the surface of the enamel layer. The distribution of the bonding metal oxide in the enamel layer can be established by means of REM/EXD.

The binder ensures a uniform distribution of the bonding metal, bonding metal oxide or a salt thereof in the adhesion promoter layer. The presence of the binder also ensures that the adhesion promoter layer can be applied in a simple and uniform manner. The binder is configured such that it does not interfere with the enameling process and that, in embodiments, either it thermally decomposes during the subsequent enameling step and/or it bonds with the enamel layer.

It is further ensured by the method according to the invention that the bonding metal contained in the bonding metal layer, the bonding metal oxide or a salt thereof, is not present on the surface of the enameled steel component but is merely located on the boundary layer between the steel substrate and enamel. This in turn results in an improvement in the chemical and mechanical stability of the coating, in particular an improvement in the stability relative to water, steam, food and corrosion in comparison with conventional direct enameling methods. It is further ensured by the absence of bonding metal, which is harmful to health and the environment, on the surface, that the enameled steel component is completely suitable for applications with food contact.

Relative to the method it is also advantageous that a deposition of the bonding metal via a chemical or electrochemical process is avoided, a chemical or electrochemical process being a drawback relative to cost and environmental considerations. The enameling can also be carried out with enamel powders without bonding metal components which are very expensive and harmful to the environment and health. At the same time, the method according to the invention has the advantage relative to conventional direct enameling methods that the bonding metals can be used in smaller quantities, but at the same time an excellent adhesion is achieved, i.e. the method according to the invention is advantageous in terms of cost and relative to environmental and health considerations.

An improved enamel adhesion can be achieved in spite of the use of smaller quantities of bonding metal. Moreover, cheaper and more readily available types of steel (for example IF rather than ED) can be used due to the improved enamel adhesion. In comparison with conventional processes, the application of the adhesion promoter layer can be carried out as a whole more easily and less critically and less energy-intensively.

In a preferred embodiment, the adhesion promoter layer is a sol-gel based system, i.e. it can be selected from a plurality of coating methods which are suitable for sol-gel materials, such as for example dipping, flow coating or spraying.

As a result, the adhesion promoter layer can be applied uniformly, effectively, cost-efficiently and in smaller layer thicknesses. Furthermore, an excellent adhesion of the enamel to the steel substrate can be achieved thereby. Sol-gel-based systems are known in the prior art. For producing the layer, the sols which are used as coating solutions are initially applied to the substrate surface. During the coating process and the drying of the sol, hydrolysis and condensation reactions of the utilized precursors take place until the aggregation of the sol particles leads to a solid gel film. The precursors comprise the educt materials for forming the layer and, in particular, the educt materials for forming the binder. Suitable precursors are known and not limited according to the invention.

The type of bonding metal is not limited. One, two or more bonding metals, bonding metal oxides or salts thereof can be used, i.e. the bonding metal can be used both in elemental form and in cationic form. In a preferred embodiment, the bonding metal is selected from the group comprising nickel, cobalt, copper, tin, iron, molybdenum or arsenic, preferably nickel, copper, iron or molybdenum. The use of these bonding metals results in a particularly good adhesion of the enamel layer to the steel substrate. In embodiments, if the bonding metal is used in elemental form, an oxidization takes place with the bonding metal oxide during the subsequent enameling. The type of oxide is not limited. All types of oxides or salts thereof can be used. Preferred inorganic salts are, for example, silicates, aluminates or halides, such as chlorides. In an alternative embodiment, organic anions such as for example polymer organic anions can be used. In particular, it is preferred that the bonding metal is already used as an oxide, i.e. no oxidation of the elemental bonding metal has to take place, such as is required, for example, in a conventional method with a metallic base layer. The method according to the invention relative to the conventional method for producing an enameled steel component, in which a metallic primer coat is applied before the application of the enamel layer, thus has the advantage of greater flexibility, for example relative to the bonding metals and types of enamel which can be used.

The concentration of the bonding metal or the salt thereof in the adhesion promoter layer is not limited. Suitable concentrations of the bonding metal or the salt thereof, for example, range from 1 to 95% by weight, preferably 20 to 90% by weight, more preferably 40 to 80% by weight, further preferably 50 to 70% by weight, relative to the total weight of the adhesion promoter.

The type of binder is not limited. One, two or more different binders can be used. The binder represents the main component, i.e. the matrix of the adhesion promoter during the application. The binder ensures a good applicability and a uniform distribution of the bonding metal or the salt thereof in the adhesion promoter layer. The main object thereof: it fixes the adhesion promoter layer to the component until the enameling.

In embodiments, the binder is an inorganic binder. Preferably the inorganic binder is formed via a sol-gel process (sol-gel is partially organic). Inorganic binders have the advantage that a good compatibility of the adhesion promoter layer is present with the enamel layer which generally consists of silicate-based or oxide-based systems. Thus a particularly good adhesion can be achieved. A good compatibility is also present, in particular with the bonding metals, in particular bonding metal oxides.

The type of inorganic binder is not limited. Preferably, the inorganic binder is an oxide-based binder which is in particular preferably selected from aluminum oxide-based binders, titanium oxide-based binders or silicate-based binders. Such types of binder are generally known and are not explained in more detail here. These systems have the advantage that they are available easily and cost-effectively and are compatible with conventional enamel layers. Moreover, the adhesion promoter system can be easily adjusted in a variable manner and also results in excellent adhesion. There is also a high level of compatibility with a plurality of bonding metals or bonding metal oxides.

In alternative embodiments, the binder is an organic binder. The type of organic binder is not limited. For the subsequent enameling process, however, it is necessary for the organic binder to be thermally decomposed during the enameling step. The binder is preferably a polymer-based binder. Organic binders and, in particular, polymer organic binders are able to be obtained and applied easily and cost-efficiently.

It is preferred that the polymer-based binder is selected from the group comprising epoxy-based binders, acrylate-based binders, such as poly(meth)acrylate binders, polyester-based binders, polyurethane-based binders, polycarbonate-based binders, gum-based binders, such as gum arabic binders, or combinations thereof. These binders are able to be thermally decomposed easily and fully, and thus result in particularly advantageous bonding properties of the resulting enameled steel component. There is also a high level of compatibility with a plurality of bonding metals or bonding metal oxides. In embodiments, the organic binder is a polymer anionic binder. This has the advantage that adhesion promoter systems can be produced thereby with a high proportion of bonding metal cations which can be easily applied.

According to the invention, all other types of binder systems are encompassed, such as for example organic-inorganic hybrid systems. Furthermore, in some embodiments inorganic particles such as silicon oxide, aluminum oxide, titanium oxide or zirconium oxide particles can be added to the binder. In particular, binder systems containing nanoparticles are encompassed, which result in particularly good adhesion properties.

In embodiments, the adhesion promoter comprises further components or additives such as for example fillers, compatibilizers, stabilizers, dyes, antioxidants, thixotropic agents, etc. These are commonly known and are not explained in more detail here. In embodiments, the further components or additives are present in an overall quantity ranging from 10% by weight or less, preferably 1 to 0.001% by weight, more preferably 0.1 to 0.01% by weight, relative to the total weight of the adhesion promoter.

The type of application of the adhesion promoter layer according to the invention is not limited. All conventional coating processes can be used.

In embodiments, the coating is implemented via a wet coating process. The adhesion promoter is applied in liquid form, for example as a solution, dispersion, suspension or emulsion. The concentration of the adhesion promoter is not limited and, for example, ranges from 1 to 90% by weight, preferably 10 to 70% by weight, more preferably 20 to 50% by weight. The type of wet coating process according to the invention is not limited. Suitable wet coating processes are known in the prior art. Preferably, the application is carried out via a dipping, atomizing or spraying process. However, all other conventional application methods are also encompassed according to the invention. Wet coating processes permit a uniform, effective and cost-efficient application of the adhesion promoter layer.

In particular, in the wet coating process the adhesion promoter is in the form of an aqueous solution, wherein the solvent comprises water. The type of solvent can be varied however. In particular, the solvent mixtures according to the invention are encompassed, such as for example mixtures of water and alcohols, such as ethanol, isopropanol or t-butyl alcohol. The mixing ratios are not limited and, for example, range from 90:10 to 10:90, preferably from 80:40 to 20:80, more preferably from 60:40 to 40:60 (Water:alcohol). Thus a particularly cost-efficient, effective and environmentally friendly application is possible. Similarly, the adhesion promoter can be in the form of an aqueous dispersion, suspension or emulsion.

In the case of an application of the adhesion promoter layer via a wet coating process, the method preferably also comprises a step of drying the coated component before the enameling. The drying can take place, for example, at temperatures above room temperature of 21° C., such as for example ranging from 30 to 200° C., preferably 50 to 100° C. The drying can take place, for example with IR radiation or with hot air. This permits a cost-efficient and rapid procedure, wherein in particular steel components with a complex three-dimensional geometry (for example with undercuts) can be thoroughly coated.

In a preferred embodiment of the wet coating, in particular, the coating takes place in a standard commercially available dipping or spray washing system. The method is carried out such that after washing and optionally pre-treating the steel substrate, the application of the adhesion promoter layer takes place in the rinsing bath of an existing washing system. This is followed by drying in the drying stage of the enameling washing system and the application of the enamel layer. This has the advantage that the drying stage of the washing system can be used for the drying of the adhesion promoter layer and no additional drying is required after the application of the adhesion promoter layer. This embodiment of the method is thus particularly advantageous from an energy perspective.

As an alternative to the wet coating process, in embodiments the coating can be implemented via a powder coating process. Suitable process conditions for powder coatings are generally known and not limited according to the invention. The application in the form of a powder coating has the advantage that the coating can be carried out very effectively, cost-efficiently and with little outlay in terms of equipment. In particular, the step of drying, which is required in a wet coating process, is dispensed with in powder coating.

In a preferred embodiment, the method further comprises a step of cleaning the steel component before coating. Suitable cleaning processes are not limited according to the invention. Preferably the step of cleaning, however, is selected from the group comprising washing, annealing, degreasing, pickling or combinations thereof. Thus a particularly effective and cost-efficient cleaning can be carried out.

The substrate surface can also be mechanically roughened before the application of the adhesion promoter, for example by a blasting process using glass granulate or corundum, or otherwise chemically or physically treated in order to achieve improved adhesion. Alternatives to the mechanical roughening, for example, are pickling, etching, vaporization, eroding or milling. In addition, the substrates to be coated are optionally cleaned in order to remove dirt and grease residues, by immersing the substrates in cleaning baths, for example filled with a conventional detergent solution. If required, this process can be assisted by the application of ultrasound.

The layer thickness of the resulting adhesion promoter layer is not limited according to the invention. In embodiments, the adhesion promoter layer is applied in a layer thickness ranging from 25 μm or less, preferably 0.01-20 μm, more preferably 0.05-15 μm, further preferably 0.1-10 μm. As a result, an excellent adhesion can be achieved in combination with an effective and cost-efficient procedure. The layer thickness of the adhesion promoter layer is measured by means of white light interference or X-ray fluorescence.

Enameling processes for the application of the enamel layer, such as process conditions and educt and enamel compositions are commonly known and are not explained in more detail here. “Enamel” within the meaning of the present invention is a mass with an inorganic composition, generally based on silicates and oxides, which is generally produced in a glassy solidified form by melting, sintering or fritting, which means a melting process interrupted shortly before melting together. This mass, sometimes comprising additives, is generally applied in one or more layers to a carrier material and melted at high temperatures and for a brief firing period, wherein generally it is desired to coat the carrier material.

The layer thickness of the resulting enamel layer is not limited according to the invention. In embodiments, the enamel layer is applied in a layer thickness ranging from 1 mm or less, preferably 1-500 μm, more preferably 5-250 μm, further preferably 10-180 μm. As a result, it is possible to achieve particularly good chemical and mechanical properties of the enamel, an excellent adhesion to the substrate, together with an effective and cost-efficient conduction of the method. The layer thickness of the enamel layer is measured microscopically on a cross section.

The invention further comprises an enameled steel component that is obtainable by the above-described method. The steel component according to the invention differs structurally from enameled steel components which have been produced by the above-described conventional methods. This is why the enameled steel components according to the invention have an improved adhesion of the enamel to the steel substrate, even with a small overall quantity of bonding metal. This is why a plurality of types of steel can be used as the substrate. Furthermore, the enameled steel component has excellent chemical and mechanical stability, such as resistance to chemicals, water or steam, corrosion resistance, scratch resistance or cleanability. The enameled steel component is additionally suitable for a plurality of uses, in particular for uses in the food contact sector.

Enameled steel components according to the present invention are suitable for a plurality of applications. In particular, the enameled steel components can be used in a kitchen appliance, preferably an oven, in particular as an oven cavity, baking sheet or hob, and as accessory in or on the cooking compartment, such as for example hot air guide plates, grill shelves, rotisserie spit rods, support racks for baking sheets, telescoping extension rails, vapor slots and/or air outlet apertures.

Claims

1-15. (canceled)

16. A method for producing an enameled steel component, the method comprising:

coating a steel component with an adhesion promoter layer which comprises a bonding metal or a bonding metal oxide or a salt thereof, and a binder; and

enameling the coated steel component.

17. The method of claim 16, wherein the adhesion promoter layer is a sol-gel based system.

18. The method of claim 16, wherein the bonding metal is selected from a group comprising nickel, cobalt, copper, tin, iron, molybdenum or arsenic.

19. The method of claim 16, wherein the bonding metal is selected from a group comprising nickel, copper, iron or molybdenum.

20. The method of claim 16, wherein the bonding metal is present in elemental form, as an oxide or as a salt thereof.

21. The method of claim 16, wherein the binder is an inorganic binder.

22. The method of claim 19, wherein the inorganic binder is an oxide-based binder.

23. The method of claim 19, wherein the oxide-based binder is an aluminum oxide-based binder, titanium oxide-based binder or silicate-based binder.

24. The method of claim 16, wherein the binder is an organic binder which is thermally decomposed when the coated steel component is enameled.

25. The method of claim 24, wherein the binder is a polymer-based binder.

26. The method of claim 25, wherein the polymer-based binder is an epoxy-based binder, acrylate-based binder such as poly(meth)acrylate binder, polyester-based binder, polyurethane-based binder, polycarbonate-based binder, gum-based binder such as gum arabic binders, or any combination thereof.

27. The method of claim 16, wherein the steel component is coated via a wet coating process, with the adhesion promoter layer being applied in a form of an aqueous solution.

28. The method of claim 26, wherein the wet coating process is a process selected from a group comprising a dipping process, atomizing process or spraying process.

29. The method of claim 27, wherein the aqueous solution comprises a solvent which comprises water.

30. The method of claim 27, further comprising drying the coated steel component before the coated steel component is enameled.

31. The method of claim 16, wherein the steel component is coated via a powder coating process.

32. The method of claim 16, further comprising cleaning the steel component before the steel component is coated.

33. The method of claim 16, wherein the steel component is cleaned by washing, annealing, degreasing, pickling or any combination thereof.

34. The method of claim 16, wherein the adhesion promoter layer is applied in a layer thickness ranging from 25 μm or less, preferably 0.01-20 μm, more preferably 0.05-15 μm, further preferably 0.1-10 μm.

35. The method of claim 16, wherein the coated steel component is enameled with an enamel layer in a layer thickness ranging from 1 mm or less, preferably 1-500 μm, more preferably 5-250 μm, further preferably 10-180 μm, wherein the layer thickness is measured microscopically on a cross section.

36. An enameled steel component obtained by a method as set forth in claim 16.

37. The enameled steel component of claim 36 for installation in a kitchen appliance, preferably an oven, in particular as an oven cavity, baking sheet or hob, and as an accessory in or on the cooking compartment, such as for example hot air guide plates, grill shelves, rotisserie spit rods, support racks for baking sheets, telescoping extension rails, vapor slots and/or air outlet apertures.