WO2021214802A1

WO2021214802A1 - Coated metallic product

Info

Publication number: WO2021214802A1
Application number: PCT/IT2020/050102
Authority: WO
Inventors: Alessandra Primavera; Selena TIBURZIO
Original assignee: Danieli & C. Officine Meccaniche S.P.A.
Priority date: 2020-04-22
Filing date: 2020-04-22
Publication date: 2021-10-28

Abstract

Coated metal product suitable to be subjected to temperatures above 900 °C and comprising an internal metal part (11), and a protective layer (15) suitable to prevent the hot oxidation due to oxidative processes which occur at a temperature above 900°C.

Description

COATED METALLIC PRODUCT

FIELD OF THE INVENTION

Embodiments described here concern a coated metal product suitable to be subjected to temperatures above 900 °C and on which, externally, there is a protective layer for protection from hot oxidation phenomena, in particular from oxidation phenomena that occur when the metal product is subjected to a temperature above 900 °C, in particular for example subjected to heating and/or more complex heat treatments.

BACKGROUND OF THE INVENTION

It is known that in the iron and steel processes for the production and working of metal products, in particular products with a large surface, such as for example slabs and blooms, or long products, such as for example billets, arises the phenomenon of oxidation and scale formation on their external surface, with consequent loss of material that can be sold.

Such scale is associated with the formation of oxides, in particular iron oxides, on the surface of the product, and therefore with surface oxidation reactions.

The formation of surface scale is a very significant problem, which has a considerable impact on the production yield of steel plants.

It has in fact been estimated that the weight losses of metal on the mass of the final product at the end of the process, with respect to the total weight of the mass initially cast and/or charged, can amount to about 2-3%.

It has also been found that, approximately, 0.2% of these losses occur in the casting area, 0.8% in the heating furnace area, 0.7-1% in the rolling step and 0.6- 0.8% in the heat treatment and storage area. Losses of this size, although obviously subject to variations according to the type of product and the specific working methods, translate into a high economic impact for the producers.

Possible causes that lead to the formation of scale can be, for example, the numerous working steps typically performed in contact with the air, or thermal cycles to increase and reduce the temperature to which the metal product is subjected.

Particularly critical working steps in this sense are the heat treatments, for example in the heating furnace, which have the function of bringing the metal products to an optimal thermal level for subsequent working, bringing or keeping the cast metal products at temperature, making their thermal profile uniform, or heating products coming from external storage areas, kept at room temperature or at temperatures lower than the desired one.

When the formation of scale occurs in the initial or intermediate working steps of the iron and steel processes, as mentioned above, it interferes with the working operations that take place downstream, and also reduces the mass and value of the final product compared to the one worked.

In fact, in certain processes, for example hot rolling, the presence of surface scale on the metal products can damage the surface of the product, since, because the scale is pressed by the rollers toward the inside of the metal product, it can remain incorporated in the surface of the metal product, leading to surface irregularities that compromise the quality of the final product.

The formation of scale therefore entails, not only an economic disadvantage due to the mass losses of the metal products, but also the deterioration of the quality of the product, due to fragments of scale that remain adhering to the product at the end of the process.

The presence of such scale, as well as the disadvantages described heretofore, also entails problems from a plant engineering point of view, as the fragments of scale can enter the interstices of the machines, for example into bearings or other rotating members, making maintenance difficult and contributing to decrease the usefu l life of the elements of the line.

Furthermore, when the fragments remain attached to the surface of the rolling rollers, they can leave impressions on the surfaces of many types of rolled metal products, compromising their quality.

A known method for removing, at least partly, the scale from the surface of the products is the so-called descaling operation, carried out for example by means of jets of water.

However, descaling entails a cleaning operation, both in the transit areas of the product and also in the descaling area, which also entails the need to separate the descaling water from the scale removed.

Moreover, often, the descaling systems currently in use are unable to completely eliminate the scale from the surface of the product. Ideally, if the scale is kept intact and adheres firmly to the metal product, it could possibly carry out a protective action on the product, during the heat treatments. However in reality such circumstance does not typically occur, due to the inevitable breakage of the scale that occurs during plant operations.

Since the scale in fact, mainly consists of oxides, it has mechanical characteristics that are significantly different from those of the metal product from which it originates, in particular being more fragile and less elastic.

The breakage of the scale, promotes the entry, into the metal product, of air, humidity and oxidizing agents, which react with the most exposed layer of metal and oxides, promoting the formation of further oxides, for example ferrous and/or ferric.

These oxides increase in volume causing the detachment of the scale and consequently increasing the oxidizing effect of the contact between the surface of the product and the oxidizing agents.

Another disadvantage is that the oxidizing agents can also react with the carbon possibly present in the metal product, producing phenomena of surface decarburization which can alter the composition and content of the surface layers of the metal product.

In the state of the art, methods are known to prevent, or limit, the formation of scale, by coating the surface of the product with layers of mixed oxides, in order to form a barrier between the metal product and the external environment.

Examples of this type are reported in the patent documents CN1935921A, JP5171261A, CN101462859A, JPl 1222564A.

However these technologies based on the use of oxides have some disadvantages.

A first disadvantage is that during a heating thermal cycle, for example in a heating furnace, the different layers of material present in the metal product, for example the metal layer, the layers of iron oxide and the layers of coating oxides, can have different coefficients of thermal expansion, which lead to an increase in the internal stress of the material, generating tensions in the structure on a molecular level.

Such tensions then give rise to cracks, in which contact between the product and the oxidizing agents can take place once more, thus triggering new oxidative processes.

In cases where the metal products are subjected to very high temperatures, typically above 900°C, these phenomena of thermal expansion and internal stresses of the material are further accentuated, therefore, typically, the protection provided by known solutions, is not adequate.

Furthermore, at these high temperatures, phenomena of diffusion of oxygen ions through the surface layers can occur, with counter-diffusion of iron ions toward the outside.

The diffusion effects produce oxidation reactions, which lead to the formation of scale and subtract mass from the product.

Documents FR-A-2,830,856, WO-A-03/033435 and WO-A-03/033436 describe a coating precursor comprising a silicone resin, a mineral filler and an organic solvent and a method for coating a substrate.

Document EP-A-1,197,585 describes a method for repairing a thermal barrier coating of a component designed for use in a gas turbine engine.

Document EP -A- 1,088, 908 describes a method for smoothing the surface of a protective ceramic coating.

Document WO-A-2017/156312 describes a silicon-based high temperature coating. There is therefore the need to provide metal products than can overcome at least one of the disadvantages of the state of the art.

In particular, one purpose of the present invention is to provide a metal product which allows to reduce the wastes and the costs of the corresponding steel manufacturing processes, in particular associated with the phenomena of scale formation.

It is also a purpose of the present invention to provide a metal product which is resistant to the phenomena of hot oxidation, that is which occur at high temperatures, for example during heating or other heat treatments, limiting, or totally preventing, the loss of mass associated with surface oxidation. In particular, it is a purpose of the present invention to reduce oxidation in the heating area by at least 30%, preferably even over 60%.

Another purpose of the present invention is to provide a high quality metal product, in particular eliminating or at least reducing the surface defects associated with the presence of scale during the working steps subsequent to the heat treatments.

Another purpose of the present invention is to provide a metal product resistant to the phenomena of hot oxidation and decarburization, even in the presence of thermal cycles that provide significant temperature variations, such as for example the cycles that occur in the heating furnace.

Another purpose of the present invention is to provide a coated metal product which can be simple to obtain and the costs of which are contained.

The Applicant has studied, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

SUMMARY OF THE INVENTION

The present invention is set forth and characterized in the independent claim. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea.

In accordance with the above purposes, the present invention concerns a coated metal product suitable to be subjected to temperatures above 900 °C.

The coated metal product comprises an internal metal part, which has an external surface, and a protective layer, disposed on the external surface and suitable to limit the hot oxidation of the external surface due to oxidative processes which occur at a temperature above 900°C.

The protective layer comprises a polymer-derived ceramic coating, said polymer being silicon based. In some embodiments, the polymer-derived ceramic coating comprises phases of amorphous and/or crystalline silica, and/or of silicon oxide carbide and/or of graphitic carbon, or combinations thereof.

In some embodiments, the polymer-derived ceramic coating is obtainable starting from one or more ceramic precursor polymers, or from a mixture of ceramic precursor polymers, selected from: silicone resins, silicone oils, silicone pastes, other siloxane polymers, carbosilanic polymers, silazanic polymers, or combinations thereof.

In some embodiments, the polymer-derived ceramic coating, also, comprises first fillers with reducing characteristics chosen in a group comprising: elemental Iron powder, elemental Silicon powder, Iron-Silicon powder, Silicon Carbide powder, ferro-alloy powder or combinations thereof.

In some embodiments, the polymer-derived ceramic coating comprises, as an alternative or in addition to the first fillers, second fillers, which include one or more minerals.

The second fillers are advantageously able to contrast and reduce the formation of a molten layer of Fayalite and therefore contrast its harmful effects on the oxidation of the substrate the formation of compounds with a low melting point, such as Fayalite in particular, in the temperature range comprised between 1100-1300°C, typical of the heat treatments to which the metal product is subjected, is deleterious for the oxidation of the substrate, as explained in detail below.

In some embodiments, the second fillers contain a mineral source of Forsterite.

In some embodiments, the above-mentioned mineral source of Forsterite comprises Olivine. In possible implementations, the Olivine has a fraction of Forsterite higher than 50%, in particular higher than 60%, more particularly higher than 75%, even more particularly higher than 85%.

In some embodiments, the above-mentioned mineral source of Forsterite comprises Magnesium Oxide.

In some embodiments, the above-mentioned mineral source of Forsterite comprises Olivine and Magnesium Oxide and the weight ratio between Olivine and Magnesium Oxide is between 2 and 8, in particular between 3 and 7, more particularly between 3.5 and 6, even more particularly between 4 and 5.5.

The second fillers, thanks to their reactivity, reduce the deleterious effect of Fayalite which is typically generated at a temperature above 1150°C: in fact, from this temperature upward the Fayalite would melt, generating a liquid layer that promotes the mobility of the ions and therefore causes oxidation.

Favorably, in the embodiments which provide to use second fillers comprising a source of Forsterite, the latter is able to form a solid solution with Fayalite, able to significantly raise the melting temperature.

Advantageously, in the embodiments where the source of Forsterite comprises Magnesium Oxide, the latter is able to form forsterite in situ, with the above advantages.

In some embodiments, the weight ratio between the first fillers and the second fillers is comprised between 0.1 and 0.6, in particular between 0.15 and 0.5, more particularly between 0.15 and 0.4.

In some embodiments, the protective layer consists of only the polymer- derived ceramic coating.

In some embodiments, the protective layer consists of the polymer-derived ceramic coating and of the second fillers.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, characteristics and advantages of the present invention will become apparent from the following description of some embodiments, given as a non-restrictive example with reference to the attached drawings wherein:

- fig. 1 is an example schematic representation of a coated metal product in accordance with some embodiments;

- fig. 2 is an example schematic representation of a coated metal product in accordance with some embodiments;

- fig. 3 is an example schematic representation of a coated metal product in accordance with some embodiments;

- fig. 4 is an example schematic representation of a coated metal product in accordance with some embodiments.

To facilitate comprehension, the same reference numbers have been used, where possible, to identify identical common elements in the drawings. It is understood that elements and characteristics of one embodiment can conveniently be incorporated into other embodiments without further clarifications.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

We will now refer in detail to the various embodiments of the invention, of which one or more examples are shown in the attached drawings. Each example is supplied by way of illustration of the invention and shall not be understood as a limitation thereof. For example, the characteristics shown or described insomuch as they are part of one embodiment can be adopted on, or in association with, other embodiments to produce another embodiment. It is understood that the present invention shall include all such modifications and variants.

Before describing these embodiments, we must, also, clarify that the present description is not limited in its application to details of the components as described in the following description using the attached drawings. The present description can provide other embodiments and can be obtained or executed in various other ways. We must, also, clarify that the phraseology and terminology used here is for the purposes of description only and cannot be considered as limitative.

Hereafter we will use the term “bulk”, referred to a material, referring to the part of the material far enough from the regions of the material where exchanges of matter, momentum and heat occur, so as not to perceive their effects.

In the present description, we will refer to the region of separation, or interphase, between two phases or two different materials, which have different chemical-physical, composition or crystallographic properties, in which, by way of example, the transition from one phase to another or from one material to another occurs, such as a separation surface.

However, such simplification, adopted for simplicity of language, must not be understood in a limiting sense with respect to the real morphology of the region of separation.

Here and in this description, with the expression “metal product” we mean a product composed of metal material.

Such metal material essentially comprises metallic Iron, possibly with the presence of other elements suitable to give the metal product the desired characteristics.

For example the metal material can comprise steels with different carbon contents, special steels, high alloy steels, cast iron or other types of metal alloys.

Embodiments described here, using figs. 1, 2, 3 and 4, concern a coated metal product 10 suitable to be subjected to a temperature above 900 °C.

In some embodiments, the coated metal product 10 of the present invention can have variable shapes and sizes, and for example be configured as a slab, a billet, a bloom or any other type of product whatsoever used in the steel industry.

In embodiments described by way of example in fig. 1, the coated metal product 10 of the present invention has an internal metal part 11 and a protective layer 15.

The internal metal part 11 comprises the bulk 12 of the coated metal product 10, substantially consisting of metal material, as defined above, and corresponding to the innermost portion of the internal metal part 11.

The internal metal part 11 also comprises a layer of oxides 13, mainly consisting of iron oxides and disposed outside the bulk 12, which develops when the metal material comes into contact with an oxidizing agent.

The oxides can have variable iron contents and oxidation states, and can be present in the form of different crystalline phases, for example hematite, magnetite, wiistite.

The internal metal part 11 therefore has an external surface 14, on which the protective layer 15 is disposed, suitable to prevent the hot oxidation of the external surface 14, due to oxidative processes that occur at temperatures above 900°C.

Here and in the present description by “high temperatures” we mean temperatures above 900°C.

The oxidative processes can comprise for example surface oxidation and/or decarburization reactions, due to oxidizing agents present in the surrounding oxidizing environment.

The oxidizing environment can be any liquid or aeriform environment whatsoever, for example air, comprising one or more oxidizing agents, or oxidizing chemical species, for example oxygen, carbon dioxide, water, also in the form of water vapor. However such definition does not exclude the presence of other chemical species, such as for example nitrogen, nitrogen oxides, sulfur oxides, carbon monoxide, methane.

The oxidizing environment can also comprise chemical species typical of the environments associated with the heating furnaces used in the steel industry, such as for example heating furnaces that use fuel.

In these cases, due to the combustion reactions, the oxidizing environment can have low oxygen fractions and, in addition to the chemical species already mentioned, also volatile chemical species associated with the fuel, partly or totally combusted, or even residues of unburnt fuel, such as hydrocarbons.

As will be described below, the protective function of the protective layer 15 is carried out, at least, with one or more of the following advantageous effects:

- a barrier effect, that is prevent, or at least limit, the contact between the oxidizing agents and the metal material contained in the internal metal part 11 of the coated metal product 10;

- a kinetic effect, that is prevent, or at least slow down, the diffusion of oxidizing agents from the oxidizing environment toward the internal metal part 11 of the coated metal product 10 and/or of metal material from the internal metal part 11 toward the surface of the coated metal product 10; these diffusion phenomena being the more significant the higher the temperature.

- a reactive or sacrificial effect, that is chemically neutralize, completely or only partly, the oxidizing agents before they come into contact with the metal material contained in the internal metal part 11 of the coated metal product 10. In embodiments described with figs. 2 and 3, the protective layer 15 comprises a polymer-derived ceramic coating 16, said polymer being silicon-based.

In some embodiments, the ceramic coating 16 can comprise a material, or a mixture of materials, possibly in homogeneous phase, suitable to guarantee the cohesion of the protective layer 15. In some embodiments, Si-O, Si-O-C, O-Si-O, Si-Si, Si-C, Si-N bonds can be present in such ceramic coating 16.

Advantageously, the Si-0 bonds have high bonding energies (about 452 KJ/mol) compared to other analogous chemical bonds, such as for example C-0 (about 358 KJ/mol), promoting the resistance of the ceramic coating 16 to high temperatures.

Furthermore, the chemical structure of the ceramic coating 16 can be very compact and cross-linked, limiting, or completely preventing, the diffusion of oxidizing agents toward the internal metal part 11 of the coated metal product 10 and thus contributing to providing an effective barrier effect and kinetic effect of the protective layer 15.

In some embodiments, the ceramic coating 16 can comprise phases of amorphous and/or crystalline silica, and/or of silicon oxide carbide, and/or of graphitic carbon, or combinations thereof.

The phases of crystalline silica can for example comprise quartz and/or cristobal ite.

In general, the ceramic coating 16 can include silicates in amorphous or crystalline phases.

In some embodiments, the polymer-derived ceramic coating 16 can be obtainable starting from one or more silicon-based ceramic precursor polymers, or from a mixture of silicon-based ceramic precursor polymers.

In some embodiments, the ceramic precursor polymers can be selected from: silicone resins, silicone oils, silicone pastes, or other siloxane polymers, carbosilane polymers, silazanic polymers, or combinations thereof.

By ceramic precursor polymers we mean polymeric materials which at room temperature appear in the liquid state, with more or less high viscosity, or in the solid state, obtainable in the form of powders, and which, following heating at temperatures above 200°C, can undergo cross-linking chemical reactions, which modify their chemical structure.

Depending on the type and composition of the ceramic precursor polymers and the surrounding environment, further increases in temperature, for example reaching temperatures comprised between 400°C and 1400°C, can accentuate the cross-linking reactions and/or trigger further reactions, for example decomposition, thermal degradation, pyrolysis processes or elimination reactions, leading to the formation of the ceramic coating 16.

Advantageously, by controlling the composition of the ceramic precursor polymers with respect to the thermodynamic conditions in which the cross- linking reactions occur, it is possible to increase the compactness of the ceramic coating 16, contributing to improve the barrier effect and the kinetic effect.

The siloxane polymers, or polysiloxanes, have Si-0 bonds with variable cross- linking degree, to which organic functional groups (-R1, -R2) of variable type can be linked.

Such siloxane polymers can have a molecular structure which comprises units of the type -Si(Rl)(R2)-0-.

The carbosilanic polymers, or polycarbosilanes, have Si-C bonds with variable cross-linking degree, to which organic functional groups (-R1, -R2, -R3, R4) of variable type can be linked.

Such carbosilanic polymers can have a molecular structure which comprises units of the type -Si(Rl)(R2)-C(R3)(R4)-.

The silazanic polymers, such as polysilazanes or perhydridosilazanes, have Si- N bonds with variable cross-linking degree, to which organic functional groups (- Rl, -R2, -R3) of variable type can be linked. Such silazanic polymers can have a molecular structure which comprises units of the type -Si(Rl)(R2)-N(R3)-.

Silicone resins, silicone oils and silicone pastes, can have both cross-linked and also linear molecular structures, with organic functional groups (-R1, -R2, - R3, -R4).

In some embodiments, the organic functional groups (-R1, -R2, -R3, -R4) can comprise functional groups selected from: hydrogen (-H), alkyl, aryl, alkoxyl groups, possibly in turn substituted with other substituents.

Possible alkyl groups can be methyl groups, possible aryl groups can be phenyl groups and possible alkoxyl groups can be methoxy groups.

Advantageously, organic functional groups can be chosen that allow to maximize the cross-linking of the polymer chains, helping to improve the barrier effect and the kinetic effect of the protective layer 15.

In some embodiments, polymethylhydrosiloxane (PMHS) and/or polydimethylsiloxane (PDMS) and/or polyperhydrosilazane and/or polyphenylsiloxane, or combinations thereof can be used as ceramic precursor polymers.

Advantageously, ceramic precursor polymers in which at least one of the organic functional groups linked to a silicon atom (-R1, -R2) is a hydrogen with a hydride character, such as for example polyalkylhydridosiloxanes, in particular polymethylhydrosiloxane (PMHS), or polyperhydrosilazane, can have reducing characteristics.

Such characteristic contributes to improving the reactive effect of the protective layer 15 of the coated metal product 10, since the hydruric hydrogens can reduce the oxidizing agents, deactivating them, before they react with the metal material of the internal metal part 11.

In some embodiments, the ceramic coating 16 can comprise an organic- inorganic hybrid material.

In embodiments described by way of example with figs. 2 and 3 and 4, the ceramic coating 16 can also comprise inorganic fillers 17, 18.

Advantageously, when the coated metal product 10 is subjected to high temperatures, the presence of the fillers 17, 18 allows to compensate for possible mechanisms that lead to a loss of mass and volume contraction of the ceramic coating 16. Such characteristic confers mechanical stability upon the protective layer 15 of the coated metal product 10.

In embodiments described by way of example with fig. 1, the protective layer

15 consists only of the polymer-derived ceramic coating 16.

In embodiments described by way of example with figs. 2 and 3, the ceramic coating 16 comprises the inorganic fillers 17, 18, in particular first inorganic fillers 17, hereafter first fillers 17, and/or second inorganic fillers 18, hereafter second fillers 18.

In embodiments described by way of example with fig. 4, the ceramic coating

16 comprises only the second fillers 17.

In some embodiments, the ceramic coating 16 comprises first fillers 17 with reducing characteristics, selected from a group comprising: elemental Iron powder, also called metallic Iron, and/or elemental Silicon powder, also called in some cases metallic Silicon, Iron-Silicon powder, and/or Silicon Carbide powder, and/or ferro-alloy powders or combinations thereof.

In possible implementations, the ferroalloy powders can be chosen from Ferro- Chromium, Ferro-Molybdenum, Ferro-Manganese, Ferro-Silicon-Manganese powders.

In some embodiments, the first fillers 17 can typically be associated with low oxidation states, thus presenting reducing characteristics that can improve the reactive effect of the protective layer 15.

In this case, such reactive effect can also be indicated as sacrificial oxidation, with reference to the fact that the first fillers 17, in contact with an oxidizing agent, can oxidize instead of the metal material of the internal metal part 11 of the coated metal product 10, which is therefore protected.

The Iron and the Silicon used according to possible embodiments are supplied metallic and/or in low states of oxidation, or compounds thereof are supplied in low states of oxidation, with reducing characteristics.

In some embodiments, the first fillers 17 can comprise an Iron-Silicon powder, for example with a fraction of Silicon greater than 50% in weight with respect to the weight of the first fillers 17, in particular greater than 75%, even more particularly greater than 90%.

In other embodiments, the first fillers 17 can comprise a Silicon Carbide powder. In possible implementations, the first fillers 17 can consist only of Silicon Carbide powder.

In some embodiments, the first fillers 17 are uniformly mixed in the ceramic coating 16, presenting a homogeneous distribution.

Such characteristic allows, during use, to obtain uniform and effective protection of the protective layer 15 over the entire surface of the coated metal product 10.

In some embodiments, the ceramic coating 16 comprises, alternatively or in addition to the first fillers 17, second fillers 18.

Advantageously, such second fillers 18 are able to contrast and reduce the formation of a molten layer of Fayalite, or in general of compounds that have low melting temperatures, and therefore contrast their harmful effects on the oxidation of the substrate.

In embodiments in which the ceramic coating 16 comprises first fillers 17 and second fillers 18, the weight ratio between the first fillers 17 and the second fillers 18 is comprised between 0.1 and 0.6, in particular between 0.15 and 0.5, more particularly between 0.15 and 0.4.

The second fillers 18 can include one or more minerals.

The one or more minerals can have a melting temperature higher than a temperature comprised between 1100°C and 1300°C.Advantageously, such characteristic allows to obtain a protective layer 15 which maintains a stable and compact chemical structure even at high temperatures, preventing a lack of homogeneity which can generate scale or cracks.

Such characteristic also improves the barrier effect of the protective layer 15, since it prevents the formation of possible liquid, or viscous, phases in correspondence with the separation surfaces between the different layers of the coated metal product 10, which would promote the diffusion of Iron ions toward the surface, and therefore the oxidation processes.

In some embodiments, the one or more minerals can be mineral sources of silicates.

In some embodiments, the one or more minerals can comprise silicates, for example nesosilicates, or orthosilicates, possibly of the group of olivines.

In some embodiments, the one or more minerals present in the second fillers can comprise a Forsterite mineral source.

The high melting temperature of Forsterite further limits the formation of liquid or viscous phases in correspondence with the separation surfaces between the different layers of the coated metal product 10, improving the kinetic effect of the protective layer 15.

In some embodiments, the Forsterite mineral source can be Olivine, with a fraction of Forsterite higher than 50%, in particular higher than 60%, more particularly higher than 75%, even more particularly higher than 85%.

Advantageously, such high fraction of Forsterite limits, or completely prevents, possible chemical reactions between silicates and Iron compounds, which lead to the formation of compounds with a lower melting point, such as for example Fayalite, further limiting the spread of ionic species from and toward the protective layer 15 and further improving its kinetic effect. Such advantageous aspect is further enhanced in the event the second fillers 18 include Magnesium Oxide as described above with reference to some embodiments.

In some embodiments, for example in which first fillers 17 based on Ferro- Silicon powder and second Olivine-based fillers 18 are present, the weight ratio between Ferro-Silicon and Olivine can be for example lower than 1, in particular comprised between 0.1 and 0.9, more particularly between 0.15 and 0.8, even more particularly between 0.2 and 0.7.

In some embodiments, in which the first fillers 17 based on Silicon Carbide powder and second fillers 18 comprising Olivine are present, the weight ratio between Silicon Carbide and Olivine can be for example between 0.1 and 0.6, in particular between 0.15 and 0.5, more particularly between 0.2 and 0.4, even more particularly between 0.2 and 0.3.

In some embodiments, the source of Forsterite present in the second fillers 18 can be the mineral Magnesium Oxide. Magnesium Oxide is able to form Forsterite in situ according to the reaction:

2MgO + Si0₂ = Mg₂Si0₄

In some embodiments, where the first fillers 17 based on Silicon Carbide powder and second fillers 18 comprising Magnesium Oxide are present, the weight ratio between Silicon Carbide and Magnesium Oxide can be for example between 0.1 and 0.6, in particular between 0.15 and 0.5, more particularly between 0.15 and 0.4.

In some embodiments, the second fillers 18 can consist exclusively of Magnesium Oxide.

In other embodiments, the second fillers 18 can comprise both Olivine, and also Magnesium Oxide, which advantageously act as a source of Forsterite. In some embodiments, the second fillers 18 can consist of, that is comprise exclusively, Olivine and Magnesium Oxide.

In some embodiments in which the second fillers comprise both Olivine, and also Magnesium Oxide, Olivine is present in a quantity in weight greater than Magnesium Oxide. For example, the weight ratio between Olivine and Magnesium Oxide can be between 2 and 8, in particular between 3 and 7, more particularly between 3.5 and 6, even more particularly between 4 and 5.5.

The Applicant has also verified that to improve the protective action of the protective layer 15, it is possible to provide a fraction of first 17 and second fillers 18, with respect to the protective layer 15 comprised in a range between 62% and 85% in weight, preferably between 68% and 80% in weight.

It is also an advantage of the present invention that the protective layer 15 has thermal expansion coefficients close to those of the bulk 12 of the coated metal product 10. Such characteristic allows to limit one of the disadvantages of the state of the art whereby the protective layers, following expansion effects due to high temperatures, can create internal stresses in the coated metal product 10, generating tensions in the structure at the molecular level and possibly leading to the detachment or cracking of the protective layer 15. Such characteristic therefore allows to obtain a coated metal product 10 which can be subjected and worked at high temperatures.

In some embodiments, the protective layer 15 of the coated metal product 10 of the present invention can also comprise additives, known per se, with thickening, dispersing, wetting, antifoaming, rheological modifying and other effects, according to requirements.

In some embodiments, these additives are added in percentages not higher than 5% in weight of the total mass of the coating composition.

In some embodiments described by way of example with fig. 3, the first fillers 17 can have a micrometric diameter, possibly with a granulometry of less than 20 pm, while the second fillers 18 can have a granulometry of less than 100 pm, in particular less than 60 pm.

In such embodiments, due to the small sizes, the first fillers 17 can easily disperse in the ceramic coating 16, guaranteeing an effective and uniform protection, from oxidation phenomena, over the entire surface of the coated metal product 10.

Furthermore, in these embodiments, the second fillers 18 keep can reduce the diffusion of iron Ions toward the surface and the effects related to the thermal expansion of the protective layer 15, when the coated metal product 10 is subjected to heat treatments at high temperature.

This advantage is also associated with the embodiments, described by means of fig. 4, in which the first fillers 17 are not present.

In alternative embodiments described by means of fig. 2, both the first fillers 17, and also the second fillers 18 can be supplied with granulometry in a range comprised between 5 and 60 pm, advantageously between 20 and 30 pm.

In these embodiments, the small sizes of the fillers 17, 18 can make the dispersion of the first 17, and of the second 18 fillers in the ceramic coating 16 more homogeneous.

Such characteristic allows to promote the sacrificial oxidation of the first fillers 17, by oxidizing agents that could possibly spread in the protective layer 15.

Furthermore, by reducing the sizes of the second fillers 18, their contact surface with the ceramic coating 16 and with the first fillers 17 is increased, promoting the formation of Magnesium Oxide phases with passivating properties.

It is obvious that second fillers 18 with smaller sizes, with the advantages they entail, can also be used in embodiments in which the first fillers 17 are not present, described by means of fig. 4.

The Applicant has also verified that an effective protective effect is obtained when the average thickness 19 (figs. 2 and 3) of the protective layer 15 is comprised between 5 and 100 pm, in particular comprised between 20pm and 60pm, preferably between 30pm and 50pm. The average thickness 19 can be substantially defined as the average distance between the external surface 14 of the internal metal part 11 and the external surface of the protective layer 15, unless there are possible protrusions due to the granulometry of the fillers 17, 18, as shown by way of example in fig. 3.

The Applicant has also verified that the protective layer 15 of the coated metal product 10 is removable by the action of jets of water.

Such characteristic is promoted by the different chemical structure of the bulk 12, of the layer of oxides 13 and of the protective layer 15.

In fact, the bulk 12 of the coated metal product 10 has a chemical structure mainly based on metal bonds, while the ceramic coating 16 of the protective layer 15 has a cross-linked chemical structure, in which there are directional chemical bonds with a covalent character.

Furthermore, the intermediate layer of oxides 13, can comprise several crystalline phases which are not cohesive with each other, thus presenting flaking, crumbling and brittle characteristics.

On the one hand, such characteristics make the protective layer 15 in itself very resistant, thus allowing to provide a stable and long-lasting protection to the coated metal product 10, even in conditions of movement, or of impacts, for example when moved along conveyor belts or in a transport vehicle.

On the other hand, due to the chemical characteristics of the lower layers, in particular of the layer of oxides 13, the protective layer 15, if necessary, can be easily removed, using, for example jets of air or water, also at high pressure.

The coated metal product 10 of the present invention can therefore be advantageously used in the steel industry, in particular in manufacturing processes of metal products that provide at least a heating operation at temperatures above 900°C.

In particular, the coated metal product 10, thanks to the reduced presence of scale that forms as a result of the heating, can subsequently be worked more efficiently than metal products of the state of the art.

It is clear that modifications and/or additions of parts may be made to the invention as described heretofore, without departing from the field of the present invention as defined by the claims.

It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art shall certainly be able to achieve many other equivalent forms of coated metal product 10, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby. In the following claims, the sole purpose of the references in brackets is to facilitate reading and they must not be considered as restrictive factors with regard to the field of protection claimed in the specific claims.

Claims

1. Coated metal product suitable to be subjected to temperatures above 900°C and comprising: an internal metal part (11) with an external surface (14); a protective layer (15) disposed on said external surface (14) and suitable to limit the oxidation of said external surface (14) due to oxidative processes which occur at a temperature above 900° C, wherein said protective layer (15) comprises a polymer-derived ceramic coating

(16), said polymer being silicon based, wherein said polymer-derived ceramic coating (16), also, comprises first fillers

(17) with reducing characteristics chosen in a group comprising: elemental Iron powder, elemental Silicon powder, Iron-Silicon powder, Silicon Carbide powder, ferro-alloy powder or combinations thereof and, as an alternative or in addition to said first fillers (17), second fillers (18) which include one or more minerals comprising a Forsterite mineral source.

2. Coated metal product as in claim 1, characterized in that said Forsterite mineral source comprises Olivine, with a fraction of Forsterite higher than 50%, in particular higher than 60%, more particularly higher than 75%, even more particularly higher than 85%.

3. Coated metal product as in claim 1 or 2, characterized in that said Forsterite mineral source comprises Magnesium Oxide.

4. Coated metal product as in claim 2 or 3, characterized in that said Forsterite mineral source comprises Olivine and Magnesium Oxide, wherein the weight ratio between Olivine and Magnesium Oxide is between 2 and 8, in particular between 3 and 7, more particularly between 3.5 and 6, even more particularly between 4 and 5.5.

5. Coated metal product as in any claim hereinbefore, characterized in that the weight ratio between said first fillers (17) and said second fillers (18) is comprised between 0.1 and 0.6, in particular between 0.15 and 0.5, more particularly between 0.15 and 0.4.

6. Coated metal product as in any claim hereinbefore, wherein said polymer- derived ceramic coating (16) comprises phases of amorphous and/or crystalline silica, and/or of silicon oxide carbide and/or of graphitic carbon, or combinations thereof.

7. Coated metal product as in any claim hereinbefore, wherein said polymer- derived ceramic coating (16) is obtainable starting from one or more ceramic precursor polymers, or from a mixture of ceramic precursor polymers, selected from: silicone resins, silicone oils, silicone pastes, other siloxane polymers, carbosilanic polymers, silazanic polymers, or combinations thereof.

8. Coated metal product as in claim 7, wherein said ceramic precursor polymers are: polymethylhydrosiloxane (PMHS) and/or polydimethylsiloxane (PDMS), polyperhydrosilazane, polyphenylsiloxane, or combinations thereof. 9. Coated metal product as in any claim hereinbefore, wherein the fraction of said first and second fillers (17, 18) with respect to said protective layer (15) is in a range between 62% and 85% in weight, in particular between 68% and 80% in weight.

10. Coated metal product as in any claim hereinbefore, wherein said protective layer (15) consists of said polymer-derived ceramic coating (16).

11. Coated metal product as in any claim hereinbefore, wherein said protective layer (15) consists of said ceramic coating (16) and of said second fillers (18).