WO2011069663A2

WO2011069663A2 - Environmentally friendly protective coatings for substrates

Info

Publication number: WO2011069663A2
Application number: PCT/EP2010/007521
Authority: WO
Inventors: Douglas Jesus Figueroa Gordon; Tim Harding; Sivasambu BÖHM; Wu Li
Original assignee: Tata Steel Uk Limited
Priority date: 2009-12-11
Filing date: 2010-12-10
Publication date: 2011-06-16
Also published as: CN102652157A; WO2011069663A3; EP2510061A2

Abstract

A coated cold-rolled steel substrate comprising a water-reducible coating composition, which coating composition comprises water, an organic solvent, a silane as binder, a coating additive and a metal, wherein the metal is a metal particle or a metal alloy particle having an aluminium content greater than 50 weight % and a balance of less than 50 weight % of a non-aluminium metal, and wherein the coating has a wet film thickness in the range of 6 μm to 90 μm.

Description

ENVIRONMENTALLY FRIENDLY PROTECTIVE COATINGS FOR SUBSTRATES

The present invention relates to a coated cold-rolled steel substrate comprising a water reducible coating composition and a process for the manufacture of a cure coated steel article, which steel article is produced by hot-forming or cold-forming. water reducible coating composition and a coated metal substrate wherein use is made of the said water reducible coating composition. The invention further relates to processes for the manufacture of hot-formed and cold-formed metal articles, wherein use is made of the coated metal substrate, hot-formed and cold-formed articles produced by said processes and a cured coated substrate intermediate.

The permanent deformation of a metal may be accomplished by applying a thermo-mechanical treatment to the metal surface. The primary objective of applying such a treatment is to produce a metal article having a certain shape or size or to improve certain physical properties such as strength.

When a bare metal substrate such as steel is used, the heating must be performed in a non- oxidising atmosphere to suppress the formation of oxide scales and decarburisation of its surface layers. The steel substrate is typically heated up to austenisation temperatures ~950'C for approximately 5-10 minutes. The requirement to heat steel to temperatures of approximately 950^°C in an inert atmosphere further increases the costs of the hot-forming process. Following the heat treatment the steel surface is exposed to atmospheric oxygen prior to hot-forming, which inevitably leads to scale formation. The scale layer is rough, brittle, flakes off and does not constitute a basis for subsequent processing such as spot welding or painting.

An additional process step is therefore required to remove scales from bare steel surfaces. Scale removal can be achieved through shot blasting, pickling or the like, but this results in a considerable cost increase. Further, if shot blasting is employed, the shape of the formed article may deteriorate.

Anti-scale coatings can be used to reduce oxide scale formation during hot-forming. Such coatings are based on compositions containing binders, metallic pigments and high concentrations of organic and/or inorganic solvents. However, the use of such solvents in high concentrations can be problematic both in terms of handling and the environment. As a consequence it is often necessary to incorporate incinerators into coating lines.

Another problem of current anti-scale coatings is that after the hot-forming process many of the coatings show reduced welding performance and in certain cases are not spot weldable, meaning a further process step is required to remove the coating. This effect is typically observed when after hot-forming, the electrical resistance of the coating is too high (> 5 mOhms), which prevents sufficient flow of welding current and leads to an inconsistent welding range.

It is an object of the present invention to provide a protective coating for a metal substrate such as steel, which after a high temperature heat treatment or high temperature service conditions, reduces the occurrence of oxide scales at the steel surface.

It is an object of the present invention to provide a protective coating for a metal substrate such as steel, which after a high temperature heat treatment or high temperature service conditions is resistant to corrosion.

It is an object of the present invention to provide a protective coating for a metal substrate such as steel, which after a high temperature heat treatment or high temperature service conditions allows the steel substrate to be spot welded.

It is an object of the present invention to provide a protective coating for a metal substrate such as steel, which is paintable.

It is an object of the present invention to provide a protective coating, which coating is produced using a more environmentally friendly process.

One or more of the above objects is achieved if metal substrates such as steel are coated with the water reducible coating compositions described hereunder. According to a first aspect of the invention there is provided a coated cold-rolled steel substrate comprising a water-reducible coating composition, which coating composition comprises water, an organic solvent, a silane as binder, a coating additive and a metal, wherein the metal is a metal particle or a metal alloy particle having an aluminium content greater than 50 weight % and a balance of less than 50 weight % of a non-aluminium metal, and wherein the coating has a wet film thickness in the range of 6 m to 90 μιτι.

The inventors have found that such a water reducible coating is more environmentally friendly and very suitable for coating bare steel substrates that are to be hot-formed. Advantageously, cold-rolled steel substrates which are first coated with the water reducible coating and then hot- formed exhibit a reduction in oxide scale formation at the steel surface, improved corrosion resistance, improved paintability and a conductivity that permits hot-formed coated steel substrates to be spot welded.

The coating should contain a metal particle and/or a metal alloy particle having an aluminium content greater than 50 weight% otherwise the coatings have reduced corrosion resistance, exhibit reduced spot weldability and are less effective at preventing oxidation of the steel substrate.

Preferably, the coating comprises a metal particle and/or a metal alloy particle comprising zinc and/or magnesium as the non-aluminium metal. The presence of a non-aluminium metal particle or a metal alloy particle such as zinc and/or magnesium further improves the corrosion resistance of the coating since zinc and magnesium prevent oxidation of iron in steel by forming a protective barrier and by acting as a sacrificial anode if this protective barrier is damaged.

The metal particle or metal alloy particle which comprises greater than 50 weight % aluminium and a balance of less than 50 weight % of a non-aluminium metal such as zinc and/or magnesium may be provided in the form of a flake, an alloyed flake, a pigment, a powder or mixtures thereof. Aluminium and zinc flakes may be produced by milling and preferably by ball milling. Flakes having a width of 80pm or less and a thickness of 1 pm can be manufactured in this way. Extremely thin flakes may be provided if a physical vapour deposition method is used. The preferred particle size is between 5 and 30pm.

The inventors found it advantageous to use metal particles or metal alloy particles that comprised a non-aluminium metal content of at least 10 weight % to improve the sacrificial corrosion protection of the coatings. The inventors also found that by increasing the non- aluminium metal content to 20 and 30 weight %, further improvements in sacrificial corrosion protection could be obtained and that the overall corrosion protection of the coatings also improved without having a detrimental effect on either the anti-scale properties or the conductivity of the coating. Another advantage of the water reducible coating is that the aluminum and non aluminum metals such as zinc and magnesium act to increase the conductivity of the coating to an extent where it is possible to spot weld hot-formed steel substrates. The inventors also found that bare cold-rolled steel substrates which were coated with the water reducible coating and which were heat treated under inert or reducing conditions and subsequently hot-formed, exhibited a further reduction in oxide scale formation at the steel surface, improved corrosion resistance properties and improved welding range.

The observed reduction in oxide scale formation at the steel surface may also be attributed, in part, to the presence of the silane binder, which during a heat treatment that precedes hot- forming and/or during hot-forming, converts the coating composition into a dense protective coating (coating consolidation). Advantageously, the thermal energy from the heat treatment also sinters the metal particle and/or the metal alloy particle to further reduce the occurrence of oxides at the steel surface. Preferably, the heat treatment is carried out under non-oxidising conditions to minimise oxidation of the coating. When coated onto a cold-rolled substrate, the water reducible coating has a wet film thickness in the range of 6 μιη to 90 μητ The wet film thickness should be 90 μιτι at most since thicker coatings are not compatible with high speed coil coating lines. The wet coating thickness should be no lower than 6 μητι otherwise the coating may not be robust enough to survive the heat treatment and/or hot-forming.

In a preferred embodiment of the invention the cold-rolled steel substrate is a hot-formable steel or a boron steel. The steel may be a steel strip, sheet or blank. Advantageously, hot- formable steels and boron steels which have been coated with the water reducible coating are able to withstand high temperature hot-forming conditions and also exhibit a reduction in the extent of oxidation at the steel surface.

In a preferred embodiment of the invention the non-aluminium metal comprises zinc, magnesium, nickel, copper, tin, molybdenum or mixtures thereof. The presence of zinc and/or magnesium improves the sacrificial corrosion protection properties of the water reducible coating when it is applied on steel. The addition of nickel, copper, tin or molybdenum can further increase the conductivity of the coating to an extent whereby it is possible to spot weld the formed steel substrate. In a preferred embodiment of the invention the aluminium and/or non-aluminium metal particle or alloy particle is encapsulated with silica, titania, acrylates or derivatives thereof. Preferably, the non-aluminium is magnesium and/or zinc. The encapsulation of aluminium, magnesium or zinc may be necessary to prevent said metals reacting with water, which is present in the water reducible coating composition of the invention.

In a preferred embodiment of the invention the water reducible coating composition contains in weight %: 15 to 80 %, preferably 25 to 70 % water; 0 to 35 %, preferably 0 to 25 % , more preferably 0 to 10 % and even more preferably 0 to 2 % organic solvent; 10 to 35 %, preferably 15 to 30 % silane binder; 0.5 to 3 %, preferably 1 to 2 % of a coating additive and 10 to 35 %, preferably 15 to 30 % metal, wherein the water reducible coating composition amounts to 100 weight %.

It was found that coatings having a water content between 15 and 80 weight % were particularly suitable for coil coating and that the viscosity and the solids content could be controlled by varying the amount of water in the coating or by adding a coating additive such as a viscosity modifier. It is preferable to keep the organic solvent content as low as possible and preferably the organic solvent content is 0%. Increasing the silane binder content to above 30 weight % may reduce the conductivity of the coating whereas coatings having a binder content below 10% may not provide sufficient mechanical and barrier properties during the heat treatment and/or hot forming. The use of a metal content above 30 weight % can lead to coatings which reduce physical properties, whereas a metal content below 10 weight % could lead to coatings that do not have sufficient, anti-corrosion and conductive properties. It is preferable that the water reducible coating composition comprises a coating additive content of 0.5 to 3% to improve the processability of the coating. Coatings comprising a coating additive content below 0.5 weight % do not provide enough functional benefits, for example, the dispersability and/or wettability of the coating is reduced, whereas coatings comprising a coating additive content above 3% will be detrimental to the processability of the coating.

Preferably the organic solvent of the coating composition is a water-compatible high boiling point solvent which has a boiling point at atmospheric pressure above 100^°C. High boiling point solvents in accordance with the invention comprise tri-ethyleneglycol, tetra-ethyleneglycol, di- propyleneglycol, tri-propyleneglycol and the monomethyl, dimethyl and ethyl ethers of these glycols. Alternatively, the organic solvent of the coating composition is a by-product of the hydrolysis reaction between the silane binder and water and may be present only in small amounts. The presence and use of such solvents leads to improvements in coating processability and can reduce coating defects such as blistering during curing.

Preferably the coating additive is provided to enhance coating processability and/or coating performance. Coating additives such as DisperpBYK 192 or other copolymers having pigment affinic groups are able deflocculate pigments through steric stabilisation. Coatings comprising such additives show improved wetting and reduced viscosity, which enables higher pigment loads to be used. Other coating additives include polysiloxanes and/or hydrophobic solids in glycol, which are suitable defoamers for water based systems. An example of a particularly suitable defoamer is BYK019. Moreover, coatings comprising such additives are easier to apply on substrates through rolling, brushing and spraying applications. Polyether modified siloxanes may also be used to further improve substrate wetting. Alternative additives such as an ion exchange bentonite clay and more preferably a cerium doped bentonite clay can also be used.

The water reducible coating composition may be applied on steel using a coil coating process. The use of a coil coating process requires the coating to have a certain solids content and to be of a certain viscosity. Preferably the coating has a viscosity of 150 to 1000 mPas and a solids content in the range of 20-50%; the viscosity of the coating depends on the application method.

In a preferred embodiment of the invention the silane binder is an epoxy functional silane, an amino functional silane or a mixture thereof. Preferably the silane binders are fully miscible with water. The inventors found that silanes have good compactability with organic compounds that may also be present in the water reducible coating composition. Moreover, silanes provide robustness to the water reducible coating composition at ambient conditions before the coated steel substrate is subjected to the heat treatment and hot-forming. Suitable epoxy functional silane binders include glycidyloxypropylalkoxysilane, 3-glycidyloxypropyltrimethoxysilane, 3- glycidyloxypropyltriethoxysilane, 3-glycidyloxypropylmethyldimethoxysilane and 3- glycidyloxypropylmethyldiethoxysilane or a mixture thereof. Suitable amino functional silane binders include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3- aminopropylmethyldimethoxysilane and 3-aminopropylmethyldiethoxysilane or a mixture thereof. Other functional silanes such as alkyl, acrylate and vinyl functional silanes can also be added to improve coating performance. The use of such binders permits the manufacture of water reducible coatings that possess improved anti-scale and anti-corrosion properties.

In a preferred embodiment of the invention the weight ratio of the silane binder and the metal is between 4: 1 and 1 :4 and preferably between 2: 1 and 1 :2. If the binder: metal ratio is above 4: 1 then the spot weldability of the coated steel substrate decreases. Moreover, if the metal: binder ratio is above 4:1 then the mechanical and barrier properties of the coating are diminished.

According to a second aspect of the invention there is provided a process for the manufacture of a cure coated steel article, which comprises the steps of:

(i) providing the coated steel substrate of the first aspect of the invention

(H) curing the coating of the coated steel substrate wherein the substrate reaches a peak metal temperature in the range of 80^°C to 400^°C. Curing the water reducible coating of the coated steel substrate, at least partially removes water and the organic solvent from the coating. Preferably water and the organic solvent are fully removed. The curing step also partially sinters the particulate metal which should increase coating adherence, robustness and the conductivity of the water reducible coating prior to the heat treatment and hot-forming. Advantageously the curing may be performed using electromagnetic radiation such as infrared and preferably near infrared radiation. The use of infrared radiation means that fully cured coatings can be obtained within a period of 90 seconds or less, preferably 60 seconds or less. Curing the coating also improves coating integrity and coating stability during handling and blanking operations. In a preferred embodiment of the invention there is provided a process for the manufacture of a cured coated steel substrate according to the second aspect of the invention, which after being subjected to a heat treatment of at least 700°C has an electrical resistance of 5 mOhms or less. Electrical resistances of 5 mOhms or less are obtainable when heat treatments are carried out up to 1000°C and preferably between 700 and 900°C. Cure coated steel substrates were deemed to have bad spot weldability if the measured electrical resistance was greater than 5 mOhms.

In a preferred embodiment of the invention the cured coating of the cure coated metal article contains in weight % 25 to 75%, preferably 40 to 60% silane binder, 1 to 6% , preferably 2 to 4% of a coating additive and 25 to 75%, preferably 40 to 60% metal. Cured coatings having a silane binder content above 75 weight % exhibited a significant reduction in electrical conductivity whereas cured coatings having a silane content below 25 weight % have reduced physical properties. Cured coatings which comprise a metal content above 75 weight % exhibit a significant reduction in the physical properties of the coating. On the other hand if the metal content is too low, i.e., below 25 weight % then the coating does not exhibit sufficient electrical conductivity and consequently a reduction in the spot weldability of the cure coated substrate is observed. With respect to the coating additives, such additives are used to improve the dispersability, wetting and processability of the coating before it is cured. These additives will remain in the coating until the coating is heated prior to hot-forming the coated substrate.

In a preferred embodiment of the invention there is provided a process for the manufacture of a cured coated steel substrate wherein the coating has a dry film thickness in the range of 2μιη to 30μπτ The inventors found that coatings having a dry film thickness below 2 yim were not robust enough to withstand subsequent processing steps and coatings having a dry film thickness above 30 μηι were not suitable for coil coating and had a tendency to crack due to a reduction in coating flexibility. In a preferred embodiment of the invention there is provided a process for the manufacture of a cure coated steel article according to the second aspect of the invention, which further comprises the steps of:

(i) subjecting the cured coated steel substrate to a heat treatment to reach the austenisation temperature of the substrate, and

(ii) hot-forming the cured coated steel substrate at a temperature above the austenisation temperature of the substrate, thus obtaining a hot-formed coated steel article.

Preferably the metal is a hot-formable steel or a boron steel since such steels are able to withstand the high temperature conditions of the heat treatment and hot-forming. During the heat treatment the cured water reducible coating is consolidated to form a protective coating to reduce oxidation at the steel surface. The consolidated coating may also provide anti-corrosion properties if non-aluminium metals are provided that are corroded in preference to iron. Advantageously, the coating of the invention is robust and flexible enough to withstand hot- forming.

In a preferred embodiment of the invention there is provided a hot-formed coated steel article wherein the coating of the hot-formed coated steel article contains in weight %: greater than 50% aluminium; 0 to 35%, preferably 0 to 25% zinc; less than 50%, magnesium, and 0 to 10%, preferably 0 to 1 % of nickel, copper, tin or molybdenum. Coating corrosion performance is reduced if the coating of the hot-formed coated steel article comprises aluminium and non-aluminium metals in a weight % that is outside the ranges specified above. For instance, hot-formed coated steel articles which comprise a coating having an aluminium content greater than 50 weight % are characterised by improved barrier properties whereas coatings having a nickel, copper, tin or molybdenum content above 10% reduce the interfacial bonding between the steel substrate and the coating. Coatings which comprise 0 weight % of magnesium and/or zinc will exhibit anti-scale and barrier protection properties only, i.e. the coatings will not have any sacrificial corrosion protection. Magnesium improves the corrosion performance by acting as a sacrificial anode to zinc, thereby extending the lifetime of zinc and consequently the lifetime of the coated hot-formed steel article. Magnesium also limits the amount of zinc that is evaporated during the heat treatment and/or hot-forming at temperatures above 850^°C. If zinc and/or magnesium are provided to afford sacrificial corrosion protection to the coating then the zinc content should be no greater than 35 weight % otherwise after hot-forming the barrier properties of the coating will be compromised. The magnesium content should not exceed 50 weight % otherwise unfavourable surface oxides will accumulate at the coated surface; which oxides will be detrimental to the adhesion of subsequent surface coatings, i.e. when phosphonating or painting the coated hot-formed steel article.

In a preferred embodiment of the invention zinc and magnesium are provided to afford sacrificial corrosion protection, the lower limits of which are 10 % and 20 % respectively.

In a preferred embodiment of the invention there is provided a process for the manufacture of a cure coated cold-rolled steel substrate, which further comprises the steps of cold-forming the cure coated cold-rolled steel substrate to produce a cold-formed steel article having a temperature resistance of 550°C or less. By temperature resistance it is meant that the coating does not thermally degrade. Advantageously the coating of the invention is robust and flexible enough to withstand cold forming. Moreover the coating is resistant to temperatures of 550^°C or less and is suitable for protecting cold-formed coated steel articles which may experience temperatures of 550^°C or less during high temperature service conditions. In a preferred embodiment of the invention the coating of the cold-formed coated steel article contains in weight %: greater than 50%, preferably greater than 75% aluminium; less than 50%, preferably less than 20% magnesium and 0 to 10%, preferably 0 to 1 % of nickel, copper, tin or molybdenum. Coating performance is reduced if the coating of the cold-formed coated steel article comprises aluminium and non-aluminium metals in a weight % that is outside the ranges specified above. For example, coatings which contain less than 50% aluminium exhibited a reduction in corrosion durability and coatings having a nickel, copper, tin or molybdenum content above 1% reduced the interracial bonding between the coating and the cold-formed steel article.

The invention will now be further explained by the following non-limitative examples:

Figure 1 shows the results of a spot weldability test (BS1140) for a hot-formed coated boron steel article comprising the coating of Example 9. The spot welding test was carried out to determine the welding range. Splash occurs between the sheets in region C whereas a minimum spot weld diameter(dp) of min≥4Vt (t is the steel substrate thickness) has not been approached in region A. Region B shows the useful spot welding range for example 9. The dotted lines (i), (ii) and (iii) correspond to a weld size of 3.5, 4 and 5 Vt respectively.

Example 1

100g of aluminium flake paste (Eckart Stapa ® Hydrolan 1515nl) containing 35 weight% of isoproanol and aluminium particles having a particle thickness of less than one micron and a particle size of 13 micron was added to a mixing vessel. To the mixing vessel 100g of isoproanol was also added under moderate agitation (500-800rpm) to form a mixture. To this mixture 100g of water and 100g of gamma-glycidyloxypropyltrimethoxysilane were added and the mixture was mixed for 10 minutes using an ultrasonic mixer. Finally, 0.4g of BYK348 and 1.6g of BYK24 were added. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 2

50g of gamma-glycidyloxypropyltrimethoxysilane was added to a mixing vessel. To the mixing vessel was added 50g of water to form a mixture, which was then mixed under moderate agitation (500-800rpm). To this mixture was added 100g aluminium flake paste (Eckart Stapa ® Hydrolan 1515nl) before the mixture was subsequently ground. A further 100g of water, 0.3g of BYK348 and 1.5g of BYK24 were then added to this ground mixture before the water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 3

100g of isoproanol and 3g of a low molecular weight unsaturated anionic polycarboxylic acid polymer (BYK® P 192) were added to a mixing vessel and were mixed under moderate agitation (500-800rpm) to form a mixture. To this mixture and under mechanical stirring (500rpm) was added 70g of aluminium flake paste (Eckart Stapa ® Hydrolan 1515nl) and 30g of zinc flake (Standart® GTT, Eckart). 100g of water and 100g of gamma- glycidyloxypropyltrimethoxysilane were then added to the mixture, which was subsequently mixed. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 4

10g of 3-aminopropyltriethoxysilane and 10g water were added to mixing vessel and subsequently mixed under moderate agitation (500-800rpm). To this mixture was added 5g of aluminium flake paste and the mixture was subsequently ground. Following grinding a further 5.6g of water, 0.02g of BYK348 and 0.1g of BYK25 were added before the water reducible coating composition thus obtained was applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 5

The water reducible coating was formed under the same conditions as in Example 1. To 100g of this solution a further 7g of tetraisopropyltitinate (20%) in isopropanol was subsequently added to the mixing vessel and mixed. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 6

100g of aluminium flake paste (Eckart Stapa ® Hydrolxal w1515nl) was added to a mixing vessel. 5g of BYK P192 and 100g of water were then added to the mixing vessel under moderate agitation (500-800rpm) to form a mixture. 60g of the mixture was then mixed with 30g of Hydrosil™ 2926 water-based epoxy functional silane (Evonik), 0.09g of BYK348 and 0.45g of BYK24. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 7

100g of aluminium flake paste (Eckart Stapa ® Hydroxal w1515nl) containing 35 weight% of water and aluminium particles having a particle thickness of less than one micron and a particle size of 13 micron was added to a mixing vessel. To the mixing vessel 100g of water was also added under moderate agitation (500-800rpm) to form a mixture. To this mixture 100g of water and 100g of gamma-glycidyloxypropyltrimethoxysilane were added and the mixture was mixed for 10 minutes using an ultrasonic mixer. Finally, 0.4g of BYK348 and 1.6g of BYK024 were added. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip.

Example 8

100g of water and 3g of a low molecular weight unsaturated anionic polycarboxylic acid polymer (BYK® P 192) were added to a mixing vessel and were mixed under moderate agitation (500-800rpm) to form a mixture. To this mixture and under mechanical stirring (500rpm) was added 70g of aluminium flake paste (Eckart Stapa ® Hydrolan 1515nl) and 30g of zinc flake (Standart® GTT, Eckart). 200g Hydrosil 2926 water-based epoxy functional silane (Evonik) were then added to the mixture, which was subsequently mixed. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip. Example 9

100g of aluminium flake paste (Eckart Stapa ® Hydrolxal w1515nl) was added to a mixing vessel. 5g of BYK P192 and 100g of water were then added to the mixing vessel under moderate agitation (500-800rpm) to form a mixture. 60g of the mixture was then mixed with 24g of Hydrosil™ 2926 water-based epoxy functional silane and 4g Hydrosil 2909 functional silane (Evonik), 0.09g of BYK348 and 0.45g of BYK24. The water reducible coating composition thus obtained was then applied on a boron steel strip by spraying to provide a uniform coating thereon. The coated steel strip was then cured using infrared heating to provide a cure coated steel strip. Although the water reducible coatings have been applied on boron steel strips by spraying, such coatings can also be applied by draw bar coating, roller coating, dipping or printing.

Table 1 below shows in weight % the contribution of each component to the water reducible coating composition of Examples 1-9. Examples of the curing cycles used are shown in Table 2. The peak metal temperature was measured by attaching a K-type thermocouple to the back of the steel strip sample. The cure coated steel strips were then subjected to a heat treatment (700-900"C) and assessed in terms of their anti-scale properties, corrosion resistance and electrical resistance. The conditions employed during the heat treatment and the results of the assessment are summarised in Table 3. Coating Examples

composition (wt%)

1 2 3 4 5 6 7 8 9

Water 24.9 49.8 24.8 50 23.2 64.2 48.4 54.4 64.6

Organic solvent 33.5 1 1.4 31 5.5 36.8 0* 10.0 6.1 0^*

Silane binder 24.9 16.6 24.8 33.1 23.2 13.3 24.9 20.0 13.3

Particulate metal 16.2 21.6 18.7 10.8 15.1 21.7 16.2 18.9 21.5

Coating additive 0.50 0.6 0.7 0.6 1.7 0.8 0.50 0.6 0.6

Total 100 100 100 100 100 100 100 100 100

Table 1 : Contribution in weight % of each component to the water reducible coating composition of Examples 1 -9. * Trace organic solvent.

Table 2: Curing cycles for coated boron steel substrates coated with the coating compositions of Example 1 and Example 5.

*** excellent, ** good , ^* bad

Table 3: Assessment of anti-scale properties, electrical resistance and corrosion resistance for boron steel substrates coated with the coating compositions of Examples 1 to 6 following a heat treatment and hot-forming.

Examples 1 , 2, 3 and 5 have an organic solvent content higher than 10% weight, but the other examples represent the preferred compositional range for the organic solvent form 0 to a maximum of 10% weight. In the coating compositions of examples 6 and 9, organic solvent will be present as a by-product of the hydrolysis reaction between water and the silane binder, but only in trace amounts. The coated steel strips were assessed to determine the presence and thickness of oxide scales after hot-forming. Coated steel strips were deemed to have excellent anti-scale properties if no oxide scales were observed or if oxide scales having a thickness of 5 μηι or less were formed at the steel surface. Coated steel strips were deemed to have good anti-scale properties if oxide scales having a thickness between 6 and 30 μπι were formed at the steel surface. Coated steel strips were deemed to have bad anti-scale properties if oxide scales having a thickness greater than 30 μηι were formed at the steel surface and/or if the coating cracked or delaminated from the steel surface. Example C1 relates to a bare boron steel substrate which was subjected to the heat treatment and hot-forming under oxidising conditions. The uncoated boron steel strip showed severe oxide scale formation with oxide scales possessing an average thickness of 80μηη. In comparison boron steels coated with the coating composition of Example 1 had excellent anti- scale properties following the heat treatment and hot-forming, which were carried out under the same conditions. Oxide scales having a thickness of 5 μηι were observed but the coating was free of cracks and free of flakes.

Corrosion resistance was assessed by visual inspection after subjecting the coated steel strip to a cyclic humidity test (relative humidity (100%). Coated steel strips were deemed to have excellent corrosion resistance if there was no formation of red rust. Coated steel substrates were deemed to have good corrosion resistance if 5 or less red rust spots were observed at the steel surface. Coated steel substrates were deemed to have bad corrosion resistance if severe red rust formation was observed. The bare boron steel substrate after heat treatment was subjected to a cyclic humidity test (relative humidity 100%) for 96 hours, after which severe red rust formation was observed. In contrast boron steel strips coated with the coating composition of Example 1 had excellent corrosion resistance after 96 hours and excellent corrosion resistance after 120 hours, with little to no red rust being observed.

Severe rust formation was also observed when bare boron steel substrates were exposed to a four hour salt spray test (ASTM B1 17). In contrast boron steel substrates coated with the coating composition of Example 3 showed no red rust formation even after a 120 hour salt spray test.

The spot weldability of the coated steel strip was examined by measuring electrical resistance (BS1 140). A lower electrical resistance generally results in better spot weldability. After a review of the criteria for assessing spot weldability, coated boron steel strips were deemed to have excellent spot weldability if the measured electrical resistance was equal to 2 mOhm or less. Coated steel strips were deemed to have good spot weldability if the measured electrical resistance was between 2 and 5 mOhm. Coated steel strips were deemed to have bad spot weldability if the measured electrical resistance was greater than 5 mOhm.

Table 4 shows the electrical resistance of steel strips coated with the water reducible coating compositions of the invention after the heat treatment and before hot-forming. Example E relates to a cold rolled substrate (DP600CR), which has been coated with the coating composition of Example 9 and cured.

Table 4: Electrical resistance of coated boron steel strips.

Claims

1. A coated cold-rolled steel substrate comprising a water-reducible coating composition, which coating composition comprises water, an organic solvent, a silane as binder, a coating additive and a metal, wherein the metal is a metal particle or a metal alloy particle having an aluminium content greater than 50 weight % and a balance of less than 50 weight % of a non-aluminium metal, and wherein the coating has a wet film thickness in the range of 6 pm to 90 μιτι.

2. A coated steel substrate according to claim 1 wherein the cold-rolled steel substrate is a hot-formable steel, preferably a boron steel.

3. A coated steel substrate according to claim 1 or claim 2 wherein the non-aluminium metal comprises zinc, magnesium, nickel, copper, tin, molybdenum or mixtures thereof.

4. A coated steel substrate according to any one of the preceding claims wherein the aluminium and/or non-aluminium metal particle or alloy particle is encapsulated with silica, titania, acrylates or derivatives thereof. 5. A coated steel substrate according to any one of the preceding claims wherein the coating composition contains in weight %:

(i) water 15 to 80 %

(ii) organic solvent 0 to 35 %

(iii) silane binder 10 to 35 %

(iv) coating additive 0.

5 to 3 %

(v) metal 10 to 35 %

6. A coated steel substrate according to any one of the preceding claims wherein the silane binder is an epoxy functional silane, an amino functional silane or a mixture thereof.

7. A coated steel substrate according to any one of the preceding claims wherein the weight ratio of the silane binder and the particulate metal is between 4:1 and 1 :4 and preferably between 2: 1 and 1 :2.

8. A process for the manufacture of a cure coated steel article, which comprises the steps of:

(i) providing the coated steel according to any one of claims 1-7, and

(ii) curing the coating of the coated steel substrate, wherein the substrate reaches a peak metal temperature in the range of 80 to 400°C.

9. A process for the manufacture of a cure coated steel article according to claim 8, which cured coating has a dry film thickness in the range of 2 to 30 μητ

10. A process for the manufacture of a cure coated steel article according to claim 8 or claim 9, which cured coating contains in weight %:

(i) silane binder 25 to 75

(ii) coating additive 1 to 6 %

(iii) metal 25 to 75 %

1 1. A process for the manufacture of a cure coated article according to claims 8-10, which further comprises the steps of:

(ϋ) hot-forming the cured coated steel substrate at a temperature above the austenisation temperature of the substrate, thus obtaining a hot-formed coated steel article.

12. A process for the manufacture of a cure coated article according to claim 11 , which after being subjected to a heat treatment of at least 700^°C, preferably between 700 and 900°C has an electrical resistance of 5 mOhms or less according to standard BS1140.

13. A process for the manufacture of a cure coated article according to claim 11 or claim 12 wherein the coating of the hot-formed coated steel article contains in weight % :

(i) aluminium greater than 50%

(ii) zinc 0 to 35%

(iii) magnesium 0 to 50%

(iv) nickel 0 to 10%

(v) copper 0 to 10%

(vi) tin 0 to 10%

(vii) molybdenum 0 to 10%

14. A process for the manufacture of a cure coated article according to claims 8-10 which further comprises the steps of cold-forming to produce a cold-formed steel article having a temperature resistance of 550°C or less.

15. A process for the manufacture of a cure coated article according to claim 14 wherein the coating of the cold-formed coated steel article contains in weight % :

(i) aluminium greater than 50%

(ii) magnesium 0 to 50 %

(iii) nickel 0 to 10% (iv) copper 0 to 10%

(v) tin 0to 10%

(vi) molybdenum 0to 10%