METHOD FOR PRODUCING AN ENAMELLED STEEL SUBSTRATE

METHOD FOR PRODUCING AN ENAMELLED STEEL SUBSTRATE

Field of the Invention [0001] The present invention is related to a method for producing an enamel layer on a steel substrate, such as a steel sheet or formed product. The invention is equally related to the enamelled steel substrate as such, preferably obtained by the method of the invention.

General Background

[0002] The protection of metallic surfaces by application of a layer of vitrified enamel is well-known, and is widely used due to the enamel's resistance to high temperature and because it gives the surface a protection against chemical aggression. Vitrified enamelled products are thus widely used in different applications such as in washing machines, sanitary ware, cooking range, domestic appliances, as well as construction materials. [0003] A number of processes exist for producing enamelled steel products. The conventional process for producing a white enamelled steel sheet, involves the following steps: applying a first layer of enamel, containing adhesion promoting oxides, such as cobalt, nickel, copper, antimony or molybdenum oxides, a first firing operation,

- applying a second layer of white cover enamel, and

- a second firing operation. [0004] This is the so-called ^λ2-coat, 2 fire' approach (2C/2F) . The adhesion of the first enamel layer on the steel is obtained by firing around 800°C-850°C, via oxido-reduction chemical reactions between the elements in the steel, such as carbon and iron, and the adhesion promoting oxides in the enamel. These oxides however give the enamel a dark colour. Consequently, the application of a second layer of white enamel is required to obtain a white enamel steel sheet. [0005] In order to avoid the use of a large quantity of enamel and double firing which are expensive, it is known to apply a direct white enamelling (DWE) process (^λl coat/1 fire' ) , that allows to obtain a white enamelled steel sheet by applying a single layer of enamel on the steel sheet, and then subjecting the steel sheet to a single firing operation. That process comprises the steps of:

- an extended surface preparation procedure, consisting of

• degreasing, pickling and rinsing of a decarburized steel sheet to remove a given quantity of iron. The pickling is necessary to obtain the correct roughness. A decarburised substrate is necessary to obtain the correct surface of the enamelled product . • applying a layer of nickel by a chemical treatment, applying a layer of enamel, and

- firing the layer of enamel typically at a temperature range of 750 to 900⁰C.

[0006] In this case, the enamel does not comprise adhesion promoting oxides, so that it does not change colour. The adhesion in this type of enamelling is due to the prior pickling and nickling operation. However, this type of pre-treatment operation is environment-unfriendly and costly.

[0007] To avoid the pre-treatment steps associated with DWE, a method was developed wherein a ground enamel and cover enamel are applied, and subsequently fired together ( ^λ2 coat, 1 fire' ) . A disadvantage of this method however, is that it needs large quantities of enamel (2 enamel coats) .

[0008] Besides this, several pre-coat chemistries and techniques to deposit them are known in the art. All aim to deliver a precoated steel suitable for direct white enamelling without pickling and nickeling operations and requiring only one enamel coating and one firing treatment.

• The document EP-A-0964078 focuses on Zn and Zn-alloy precoatings, applied by hot dipping or by electrolytic plating, and includes all Zn or Zn-alloy coatings with a thickness of the Zn-coating between lμm and 30 μm, in particular 7μm to 25μm. The chemistry includes all Zn-contents above 50% with a content of the other alloy components of up to 15% (Al, Fe, Mg, Si, Cr, Ni, Co, Cu, Mn) . The patent applies for decarburised steel surfaces (C < 0.08%, in particular <0.004%) or IF steel surfaces (all carbon tied in precipitates) .

• The documents WO-A-0250326 and WO-A-0252055 describe a nickel-molybdenum alloy coating applied by electrolytic or electroless plating, which is then subjected to a heat treatment in a temperature range between 500⁰C and 900⁰C.

• The documents JP-A-04107752 and JP-A-04107753 describe an iron-molybdenum coated cold rolled steel sheet for direct adhesion of the enamel. The iron-molybdenum coating is obtained by electrolytic plating in a bath containing e.g. iron sulfates and molybdenum ammonium salts. After plating, the plated steel sheet is heat treated at temperatures between 500 ⁰C and 900 ⁰C.

• The documents JP-A-04107754 and JP-A-04107755 describe an iron-cobalt-molybdenum coated steel sheet obtained by electrolytic plating, followed by a heat treatment at temperatures between 500⁰C and 900⁰C. The plating technology has some disadvantages related to environmental issues, resulting from the chemical compounds like salts and sulfates used in the plating bath.

[0009] Document - FR2805277 - is related to a method for direct enamelling of steel sheets, which are covered by a polymer based corrosion protection layer. The surface density of the layer is chosen sufficiently low such that a degreasing step is not required prior to the application of the enamel, while at the same time, the density is high enough to ensure adequate corrosion protection. This technique does however not allow to obtain optimum characteristics in terms of adhesion. The strict requirements in terms of surface density also complicate the process.

[0010] Document - US1962617 - is related to the manufacture of enamel ware, involving the application of a coating of adhesion promoting oxides such as Cobalt oxides to a steel surface. The oxides are mixed with a solvent and a suspending agent such as ammonium linoleate, clay or bentonite, before being applied to the surface and subsequently drying the surface.

Aims of the invention

[0011] The invention aims to provide a method for producing a steel substrate, in particular a steel sheet which is directly enamelled in white or colour by a cover coat enamel layer, which does not have the drawbacks of the state of the art. In particular, the present invention aims to provide a method for producing a steel substrate wherein a strong adhesion between steel sheet and enamel is observed, and which is produced with any kind of steels suited for enamelling, and in an environment-friendly and simple process.

Summary of the invention

[0012] The invention is related to a method and a product such as described in the appended claims.

[0013] The invention is firstly related to a method for producing an enamelled steel substrate, said method comprising the steps of :

- providing a steel substrate, - applying to a surface of said steel substrate a solution comprising a solvent, a polymer precursor, and at least one metal or metal oxide, said metal or metal oxide being suitable for promoting the adhesion of an enamel layer to the surface of the steel substrate,

- curing said steel sheet, thereby removing said solvent, and forming an organic layer comprising said at least one metal or metal oxide,

- applying to said organic layer, an enamel layer, followed by a firing step, to obtain the enamelled steel substrate.

[0014] According to the invention, when not in oxide form, the metal is either present in unbound form or in an alloy with one or more other metals suitable for promoting enamel adhesion, for example an alloy of one or more transition metals and/or Sb. The metal is not present in the form of a non-oxide ceramic, such as a carbide or suicide, nor as any other organometallic compound. [0015] Preferred embodiments are described in any combination of claim 1 with one or more of the subclaims 2 to 11.

[0016] Preferably, said metal is chosen from the group consisting of Sc, Ti, V, Co, Cu, Ni, Fe, Mn, Mo, W and Sb and wherein said metal oxide is the oxide of a metal chosen from the group consisting of V, Co, Cu, Ni, Fe, Mn,

Mo, W and Sb.

[0017] Preferably, said metal is chosen from the group consisting of Ni, Cu, Co, Mo and wherein said metal oxide is the oxide of a metal chosen from the group consisting of Ni, Cu, Co, Mo.

[0018] Preferably, said at least one metal or metal oxide is/are added to said organic layer in the form of a powder.

[0019] Preferably, said powder has a mean particle size smaller than 2 microns.

[0020] Advantageously, said organic layer has a thickness between 100 nm and 10 microns, preferably between 100 nm and 6 microns.

[0021] Preferably, said solution is applied to the substrate by coil coating, dipping or spraying.

[0022] Preferably, said curing step takes place at a temperature between 80⁰C and 250⁰C. [0023] Preferably, said firing step is performed at a temperature between 700°C and 900⁰C.

[0024] Preferably, the firing step is preceded by a step of drying the enamel layer.

[0025] Preferably, said steel substrate is subjected to a step of forming and/or cutting, after the step of applying said organic layer and before the step of applying said enamel layer.

[0026] The present invention as a second object is equally related to a steel substrate, comprising on the surface of the steel substrate an organic coating, consisting of a polymer layer comprising at least one metal or metal oxide, said metal or metal oxide being suitable for promoting the adhesion of an enamel layer to the surface of the steel substrate.

[0027] Advantageously, said organic coating is a Thin Organic Coating, having a thickness between 100 nm and 10 microns and preferably between 100 nm and 6 microns. [0028] Preferably, said substrate is a steel sheet. [0029] Finally, the present invention is also related to the use of a steel substrate as defined here above for producing an enamelled steel sheet or part.

Detailed description of the preferred embodiments of the present invention

[0030] According to the invention, a steel substrate

(e.g. a sheet) is coated with an organic coating, comprising metal or metal oxides that are suitable for promoting adhesion of an enamel layer. The organic coating consists of a polymer layer comprising in said layer one or more adhesion promoting metals and/or metal oxides, said adhesion promoting materials being present in the form of particles embedded in said matrix. Preferably, the coating is a so-called Thin Organic Coating, having a thickness between 100 nm and 10 microns. Subsequently, an enamel layer is applied to the substrate and subjected to a firing step. The organic coating is prepared from a solution comprising a solvent, e.g. water, and polymers dispersed, dissolved or emulsified into this solvent. The polymers are the precursors of the organic coating.

[0031] According to a preferred embodiment of the present invention, these precursors are loaded, i.e. mixed with a filler which is suitable for promoting the adhesion of an enamel layer to the surface of the steel substrate. In other words, the filler material is able to react at high temperature with the steel surface and elements present in the enamel composition, in order to form an interface in between. The filler is preferably administered to the solution in the form of a powder, the mean particle size being lower than 2 microns, more preferably between 1 and lOOOnm. Said powder is added to the solution by dispersion. The solvent comprising polymer and filler is applied to the steel sheet by a known technique, e.g. coil coating, dipping or spraying.

[0032] The filler material is metal or metal oxide, or a mixture of one or more metals, or a mixture of one or more metal oxides, or a mixture of metals and metal oxides. The filler can thus be a mixture of particles of different metals or of different metal oxides or of different metals and metal oxides, and/or the filler can comprise particles which are themselves consisting of a mixture of metals and/or metal oxides, e.g. an alloy of two or more adhesion promoting metals. The filler particles can be pre-coated with a polymer or other organic coating, to modify the chemistry of the surface of the filler particles, in order to facilitate dispersion of these particles. [0033] Metals/metal oxides which are suitable for adhesion promotion of enamel are known in the art, e.g. Cobalt or Cobalt oxide. Any of such known adhesion promotors can be used in the present invention. According to the preferred embodiment, one or more of the metals chosen from the group consisting of V, Co, Cu, Ni, Fe, Mn, Mo, W and Sb are used - pure or in oxide form - in the filler . All these metallic oxides of metals are suitable adhesion promotors as all can be reduced at low temperatures, and all are chemically and physically compatible with iron. For example, they can form as well titanates reacting with titanium dioxide from the glass composition .

[0034] A more preferred embodiment uses one or more of the metals Ni, Cu, Co, Mo and/or their oxides in the filler.

[0035] After the solution has been applied to the steel surface, the steel sheet is subjected to a curing step in order to remove the solvent and to form the organic coating onto the steel surface. This curing step may be performed according to known techniques for applying TOC, such as hot air (convection) curing at temperatures between 80° and 250⁰C or Infra Red curing. The result is an organic coating, preferably a Thin Organic Coating as defined above, consisting of a polymer layer and adhesion promoting materials embedded therein. The final thickness of the TOC is preferably between lOOnm and 10 microns, more preferably between lOOnm and 6 microns, more preferably between 1 and 3microns. [0036] According to a preferred embodiment, the following compositions of the TOC after curing are obtained by the method of the invention :

Polymer between 20wt% and 95wt%, more preferably between 33wt% and 80wt%. Adherence promotor (i.e. the filler, e.g. metal or metal oxide) : between 5 and 80wt%, more preferably between 20wt% and 66wt%. Expressed in surface density, the filler is preferably present in the TOC in a density of between 100 and 6000 mg/m2. [0037] Then the cover coat enamel layer is applied by a known technique, such as wet or dry electrostatic spraying, pneumatic spraying, dipping, or flow coating technologies. Possibly, the enamelling can be preceded by cutting or forming steps. The application of the enamel layer is not preceded by degreasing, pickling or nickling. A cover coat enamel is defined as an enamel applied as an outside surface, which is contrary to a ground coat enamel, used as a base layer for subsequent further treatment and coating. A cover coat enamel generally does not contain adherence promotors .

[0038] The cover coat enamel layer is finalized by a firing step, according to a known technique, preferably at a temperature between 700⁰C and 900⁰C, and possibly preceded by drying the enamel layer (for wet applicative technologies) . The firing step causes the burning out of the organic layer. In other words, the polymer of the layer is burned and thereby removed.

[0039] The steel sheet can be decarburized or not, and can be any sheet suitable for enamelling, e.g. Al- killed, high-oxygen, Ti-added, Nb-added, Ti-Nb added, B- added steel.

[0040] According to the invention, the pre-coated steel sheet is coated with a single cover coat enamel layer, without any substantial adherence promoting metal oxides in the enamel, and subjected to a firing step. The adhesion-promoting metal oxides present in the pre-coat provide for a good adhesion of the enamel layer, without necessitating pre-treatments of the sheet, such as nickling. The enamel doesn't darken, due to the absence of adhesion promoting elements in the enamel layer itself.

[0041] Additional advantages of an organic coating according to the invention are related to specific capabilities of this particular type of coating, i.e. consisting of a polymer matrix as described above. It has been found that such coatings have low friction- characteristics, allowing the product on which the coating is present to be deformed, e.g. in a deep-drawing or other deformation process, without damaging the coating. This would not be possible when an organic coating based on clay or bentonite as described in the prior art.

[0042] Also contrary to the latter prior art coatings, the coatings of the invention provide corrosion protection comparable to the corrosion protection offered by oiling of cold rolled steel sheets. This is important because pre-treated products may be subject to longer periods of transport or storage before the enamelling step is performed. [0043] Finally, the coatings according to the invention are resistant to water, which cannot be said of clays or bentonite such as documented in the prior art. This allows the pre-treated products to be easily cleaned with water, e.g. after a period of storage, before performing the enamelling step.

[0044] These advantages provide the possibility to perform cutting and forming steps directly on the product provided with an organic coating according to the invention, said forming/cutting taking place before the enamelling step. Because of the low friction characteristics, no oil is needed during the forming process, so no degreasing step is required before enamelling. As stated, no pickling or nickling is required either, leading to a simplified process for obtaining enamelled products.

Examples

[0045] The formulations Cl to C8 listed in table 1 below were prepared (all numerical data provided in weight%) . After weighting the ingredients, the products were mixed together using first a high speed centrifugal mixer containing ceramic balls and then an ultrasonic cell in order to break the final aggregates.

Table 1

[0046] Products: Beetafin LS9010 is a current polyurethane dispersion manufactured by the company BIP Limited, UK. NiO and Co304 powders are current nano powders manufactured by Inframat Advanced Materials LLC, USA.

[0047] All the dilutions obtained were applied by spraying onto the previously degreased surface of steel grade suitable for enamelling (DC03ED, as defined in the norm EN10209) and cured at 90⁰C during 1 minute after spraying. The thickness of the thin organic coating was measured after curing (see tables 2 and 3) . [0048] Organic coated sheet steel as described before was covered directly after curing the thin organic coating without any additional surface treatment such as degreasing, with a conventional white cover coat enamel powder dispersed in water. Enamelled samples were first dried at about 80⁰C during 4 minutes and then fired. After firing at different temperature and time, the thickness of the enamel layer was measured and the bond of enamelled sheet steel was tested afterwards according to the norm EN10209 (tables 2 and 3) . For all samples, the thickness of the enamel after firing was found over lOOμm. Good bond was observed in all cases because the surface of the enamel layer is smooth and glossy, without any surface defect such as pin holes, craters or blisters.

[0049] A bond quoted 1 according to the norm EN10209 is the best result to obtain. A dense interface issued from the reaction between steel, enamel and TOC is fully covering the steel surface. According to the norm and general practice in this technical domain, bonds quoted 1 & 2 are very high quality, 3 is acceptable, 4 critical and 5 fully out of range.

Table 2 : bond according to EN10209 obtained for different TOC containing NiO and fired at different temperatures and times