Note: Descriptions are shown in the official language in which they were submitted.
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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.
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[0004] This is the so-called `2-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 (`1
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 900 C.
[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
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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 ('2 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 1pm and 30 }gym,
in particular 7pm to 25pm. 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 500 C and 900 C.
= 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
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salts. After plating, the plated steel sheet is heat
treated at temperatures between 500 C and 900 C.
= 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 500 C and 900 C. 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
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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
5 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
silicide, nor as any other organometallic compound.
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[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 80 C and 250 C.
[0023] Preferably, said firing step is performed at
a temperature between 700 C and 900 C.
[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
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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
microns and preferably between 100 nm and 6 microns.
[0028] Preferably, said substrate is a steel sheet.
10 [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.
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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 1000nm. 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
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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 250 C 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 100nm and 10 microns, more
preferably between 100nm 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.
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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
5 adherence promotors.
[0038] The cover coat enamel layer is finalized by a
firing step, according to a known technique, preferably at
a temperature between 700 C and 900 C, and possibly
preceded by drying the enamel layer (for wet applicative
10 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
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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.
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Cl C2 C3 C4 C5 C6 C7 C8
Beetafin 35 53 36 36.5 36 27.5 35 28
LS9010
NiO 15 11 11 14.5 8
Co304 18 15 18
Water 50 36 53 49 56 54.5 50 54
total 100 100 100 100 100 100 100 100
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 90 C 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 80 C 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 100pm. Good bond
was observed in all cases because the surface of the enamel
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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.
composition Thickness 830 C- 830 C- 840 C- 840 C- 860 C-
TOC }gym 3'30" 4' 3'30" 4' 4'
C2 2 1
C2 1.5 1
C3 1.6 1
Cl 3.9 1
C4 2.4 3
C5 2.2 2
Table 2 : bond according to EN10209 obtained for different
TOC containing NiO and fired at different temperatures and
times
composition Thickness 820 C- 840 C- 860 C- 840 C-
TOC }gym 4' 4' 4' 7'
C6 2.1 2 1
C6 2.8 2 1
C7 1.5 1
Table 3 : bond according to EN10209 obtained for different
TOC containing Co304 and fired at different temperatures
and times.
