WO2013004916A2

WO2013004916A2 - Raw material, method and steel product

Info

Publication number: WO2013004916A2
Application number: PCT/FI2012/050715
Authority: WO
Inventors: Arimo Lankila
Original assignee: Rautaruukki Oyj
Priority date: 2011-07-06
Filing date: 2012-07-06
Publication date: 2013-01-10
Also published as: FI20115720L; WO2013004916A3; FI20115720A0

Abstract

The present invention relates to a raw material and method for producing a coating on a steel substrate and to a steel product. The method comprises producing the metal coating (6) by applying on the surface of the steel substrate (4) substantially liquid raw material comprising one or more inorganic coloring agent for providing a throughout colored metal coating (6)and subjecting the steel strip to a heat treatment. The inorganic coloring agent is preferably sintered to the metal particles for adhering the inorganic coloring agent to the metal particles in the raw material.

Description

Raw material, method and steel product Field of the invention

The invention relates to coating technology of different steel products, such as continuously coated steel strips having metal coating on their surfaces. Especially the present invention relates to raw material for producing a metal coating on a steel substrate and especially to raw material as defined in the preamble of independent claim 1. The present invention further relates to a method for producing a metal coating on a substrate of steel and more particularly to a method as defined in the preamble of independent claim 8. The present invention relate also to a steel product and particularly to a steel product as defined in the preamble of independent claim 19, and to an arrangement for producing a metal coating on a steel strip according to the preamble of independent claim 26.

Background of the invention It is generally well known that steel substrates and products are coated with metal coatings such as zinc and aluminium based coatings for example to enhance environmental resistance, such as protection against corrosion. It is also known to produce different kinds of metal coatings on steel substrates and products for other purposes. Known methods for providing a metal coating on a steel substrate comprise for example hot dipping, such as hot dip galvanizing, electroplating, such as zinc electroplating, or by applying in different ways a coating material containing liquid zinc on the surface of a steel substrate. In many applications the steel substrate is further provided with a separate surface coloring applied on the metal coating. In the prior art, the surface coloring is formed as a separate coloring layer on the metal coated steel substrate. In other words, methods of prior art for providing the surface coloring comprise different painting methods with or without metallic particles, which has, in addition to a additional process step, disadvantages related to temperature resistance and ability for cathodic protection. Thus the problem relating to the prior art coloring methods is that the coloring is provided only as a separate layer on the metal coating provided to the steel substrate. In addition, this separate coloring layer is typically organic compound. The separate surface layer may become detached from the inorganic metal coating due to environmental conditions and thus the coloring layer on the metal coating is exposed. Furthermore mechanical stress such as abrasion or impact may break the coloring surface layer and even detach part of the same. Accordingly the prior art does not provide a durable color in surface coating to be used with steel substrates. Brief description of the invention

The object of the present invention is to provide a raw material, a method, a steel product and an arrangement so as to overcome or at least alleviate the prior art disadvantages. The objects of the present invention are achieved with a raw material according to the characterizing portion of claim 1. The objects of the present invention are further achieved with a method according to the characterizing portion of claim 8. The objects of the present invention are also achieved with a steel product according to the characterizing portion of claim 19 and with an arrangement according to the characterizing portion of claim 26. The preferred embodiments of the invention are disclosed in the dependent claims.

Basically the present invention is based on the idea of providing the throughout colored metallic coating by applying the raw material including the metal particles or metal alloy particles in solid form on the steels substrate. This way it may be possible to avoid some limitations from the liquid metallurgy.

Solution is achieved by providing a substantially liquid raw material for producing a coating on at least part of a steel substrate by applying the substantially liquid raw material on steel substrate, the substantially liquid raw material comprising metal powder of at least one metal or metal alloy and preferably at least one binder agent and possibly also solvent or other fluid to provide the fluidity. The substantially liquid raw material further comprises at least one inorganic coloring agent for forming a throughout colored metal coating. The inorganic coloring agent may be sintered to the particles of the metal powder for adhering the coloring agent to the metal powder. As can be understood the colored metallic coating may be substantially inorganic coating. The present invention provides therefore a method for producing a metal coating on at least part of a steel substrate by applying on the surface of the steel substrate substantially liquid raw material comprising one or more inorganic coloring agent for providing a throughout colored metal coating. The method comprises producing the metal coating by applying on the surface of the steel substrate substantially liquid raw material comprising one or more inorganic coloring agents for providing a throughout colored metal coating. The coating is further subjected to heat treatment for providing a metallic bonding between the particles of the powder of at least one metal or metal alloy and the steel substrate or between the particles of the powder of at least one metal or metal alloy and the steel substrate and between the particles of the powder of at least one metal or metal alloy. Therefore the present invention also provides a steel product having a metal coating on at least part of a surface of the steel product, the metal coating consisting at least partly of metal powder applied on the surface of the steel product such that the metal coating further comprises inorganic coloring agent embedded into the metal coating. The metal coating consists at least partly of metal powder applied on the surface of the steel product, the metal coating comprising one or more inorganic coloring agents embedded into the metal coating. The particles of the metal powder may be further sintered to each other and to the steel substrate. The present invention further provides an arrangement for producing a metal coating on a steel strip, preferably as a continuous process. The arrangement comprises a coating unit for producing a metal coating on at least part of the steel strip. The coating unit is arranged to apply on at least part of the steel strip one or more coating layers of substantially liquid raw material together with inorganic coloring agent for producing a throughout colored metal coating on the steel strip. The arrangement further may comprise a post-heating unit for heat treating the coated steel strip for providing metal bonding between the particles of the particles of the powder of at least one metal or metal alloy and the steel substrate, or between the particles of the powder of at least one metal or metal alloy and the steel substrate and between the particles of the powder of at least one metal or metal alloy.

The present invention provides a throughout colored metal coating to be formed on a steel substrate and also a steel product having a throughout colored metal coating. The coating according to the present invention prevents environmental conditions from detaching the color from the coating. Furthermore, the coating of the present invention provides an abrasion resistant coloring as scratching or the like does not remove or detach coloring from the coating. Therefore the present invention also provides a durable coloring of a coating. Sintering the metal particles, such as zinc particles, of the coating to each other and/or to the surface of the steel product or providing a metallic bonding between the coating and the steel product provides a durable color in the coating with excellent cathodic corrosion protection which may be difficult or even impossible to obtain with known resin paint coating solutions including metal particles or the like.

Furthermore, the method according to the present invention obviates a separate coloring or painting process step with organic coatings, which is usually made in addition to zinc plating. This reduces environmental load and realizes economical savings in manufacturing of steel products having colored metal based coatings.

Brief description of the figures

In the following the invention will be described in greater detail, in connection with preferred embodiments, with reference to the attached figure 1 showing schematically an arrangement for providing a metal coating on steel strip according to the present invention. Detailed description of the invention

The present invention relates to a substantially liquid raw material for producing a throughout colored metal coating on steel substrate or steel product. The substantially liquid raw material is thus a fluid, such as pasta, suspension, slurry or liquid. The substantially liquid raw material comprises powder or particles of at least one metal . In this context the metal particles relate to both pure metal particles and metal alloy particles. In one embodiment the metal particles or powder comprise zinc particles or aluminium particles or both zinc and aluminium particles or particles comprising zinc and aluminium. Alternatively the raw material may comprise also particles or powder of other metals, such as chromium or magnesium. The metal is in the raw material as powder of as particles having the average diameter between 0,5 and 150 micrometers, preferably between 0,5 to 50 micrometers, most preferably the metal particle size is small, for instance 0.5- 10 micrometer because smaller particle size may lead to better surface quality and/or color stability and further it makes possible to achieve thin coatings. The liquid raw material comprises preferably at least 50 weight-% of metal powder or particles, more preferably at least 60 weight-%, and most preferably at least 75 weight-%. Preferably, the metal powder or particle consists of metallic zinc or metallic zinc with small amount of aluminium (Al) and magnesium (Mg) or the like. The zinc concentration must be high enough to promote electrical conductivity in the sintered dry film. Otherwise, cathodic protection will not be provided to the substrate steel . Additional metal particles such as magnesium (Mg) and/or aluminium may be alloyed in small amounts, such as 2-8 weight-% in total, to improve formability, for example.

The substantially liquid raw material may comprise also at least one binder agent which acts as a medium for binding the metal particles or the powder together. In one embodiment the substantially liquid raw material comprises one or more of the following binder agents such as water-based inorganic zinc or aluminium silicate or other binder agents. It should be understood that the present invention is not restricted to any binder agent, but the binder agent may be chosen according to one embodiment or application of the present invention. The binder may also be used for facilitating the binding the metal particles or powder to the steel substrate, prior to forming the metallic bonding, for instance.

The substantially liquid raw material of the present invention is used for producing a metal coating on at least part of a steel substrate by applying the substantially liquid raw material on steel substrate. According to the present invention the substantially liquid raw material further comprises at least one inorganic coloring agent for forming a throughout colored metal coating on the steel substrate or product. The inorganic coloring agent may be added to the substantially liquid raw material as particles mixed to the raw material . Alternatively the inorganic coloring agent may be at least partially dissolved to the substantially liquid raw material . In a yet alternative embodiment the inorganic coloring agent is adhered to particles of the metal powder. In another embodiment the inorganic coloring agent may also be adhered or comprised to the binder agent.

In one embodiment the inorganic coloring agent is a powder or particle material mixed to the substantially liquid raw material . The coloring agent particles or powder may have average particles diameter between 0, 1 and 100 micrometers, preferably between 0,5 to 5 micrometers, most preferably 0,5 to 1 micrometers. Thus in this embodiment the inorganic coloring agent is embedded to the substantially liquid raw material as separate particles.

In another embodiment the inorganic coloring agent is provided to the metal particles in the substantially liquid raw material . Thus the inorganic coloring agent may be mechanically adhered to the metal particles or sintered or fused or chemically adhered to the metal particles. The inorganic coloring agent may be added to the metal particles by sintering or agglomerating the at least one inorganic coloring agent to the metal particles. The sintering or agglomeration may be carried out in a sintering furnace or the like such that particles of inorganic coloring agent and the metal particles adhere to each other due to elevated temperature. In an alternative embodiment the metal particles are milled together with particles or powder of the inorganic coloring agent for mixing and adhering coloring agent to the metal particles. The milling of the metal particles together with at least one inorganic coloring agent is carried out in a ball mill, rod mill or in another grinding machine suitable for adhering particles of the inorganic coloring agent to the metal particles and mixing them together. During milling of the inorganic coloring agent together with metal particles 8 the average diameter of the metal particles 8 and the particles of the inorganic coloring agent, and the raw material, may decrease. Milling may provide a raw material in which the inorganic coloring agent is mechanically adhered to the metal particles. When the inorganic coloring agent is adhered to the metal particles, the most of the metal particles and the raw material become sintered or fused together. It should be mentioned that also chemical methods for adhering the inorganic coloring agent to the metal particles may be used. Furthermore, other known methods may be used for adhering solid inorganic coloring agent mechanically or chemically to the metal particles. Thus the inorganic coloring agent may also be in vapor or liquid phase for adhering to the metal particles. In one embodiment the metal particles are coated at least partly with an inorganic coloring agent using a vapor phase deposition method. According to the above mentioned the present invention provides use of substantially liquid raw material comprising metal particles and inorganic coloring agent adhered to at least part of the metal particles for producing a throughout dyed metal coating on a steel substrate. The inorganic coloring agent may be or comprise depending on the desired color, one or more of cobalt, iron, chromium or titanium, or compounds or oxides thereof, such as Co₂O₄, CoAI₂O₄, Co(AI,Cr)₂O₄, Co(AI,Cr)₂O₄, (Co,Ni,Zn)₂(Ti,AI)O₄, (Fe,Cr)O₃, Fe(Fe,Cr)₂O₄, Cu(Cr,Fe)₂O₄, CuCr₂O₄, Cu(Cr,Fe)₂O₄, Co(Cr,Fe)₂O₄, Cr₂O₃, (Ni,Sb,Ti)O₂, (Ni,Cr,Sb,Ti)O₂, (Cr,Sb,Ti)O₂. However, it should be understood that basically the present invention is not restricted to any specific type of inorganic coloring agent. Preferably metal oxides or compounds are used as inorganic coloring agent because of their high temperature resistant.

In a preferred embodiment the substantially liquid raw material comprises inorganic coloring agent between 5 to 40 weight-%, preferably about 20 weight-% depending on the desired color of the coating. The substantially liquid raw material may therefore be used for producing a throughout colored metal coating on a steel substrate or product.

The above described substantially liquid raw material may be used for producing a metal coating on at least part of a steel substrate or product. The substantially liquid raw material comprises powder or particles of at least one metal or metal alloy. The method for producing a metal coating comprises applying on the surface of the steel substrate substantially liquid raw material comprising one or more inorganic coloring agent for providing a throughout colored metal coating. In other words the method of the present invention comprises applying on the surface of the steel substrate substantially liquid raw material comprising metal powder or particles together with at least one inorganic coloring agent for providing a throughout colored metal coating. The substantially liquid raw material may be applied on the surface of the steel substrate or product by spreading, painting, spraying, rolling or pouring the substantially liquid raw material on the surface of the steel substrate or by dipping the steel substrate into the substantially liquid raw material. Thus the substantially liquid raw material may be applied on the surface of the steel substrate or product using a brush, spatula or sprayer. The method may be used for producing a throughout colored metal coating having thickness at least 10 micrometers, or at least 50 g/m². The colored metal coating may be provided by providing several layers on top of each other and one layer may be 20 to 100 micrometers thick. The overall thickness of the coating may be for example 150 micrometers or 137 g/m². The coating comprises preferably at least 50 weight-% of metal powder or particles, more preferably at least 60 weight-%, or most preferably at least 75 weight-%. The metal powder is preferably pure metal, such as pure metallic zinc or aluminium or pure metallic zinc with small amount of aluminium (Al) and magnesium (Mg) or the like, as mentioned earlier.

The steel substrate or product may be pre-treated before the coating step for enhancing the adhesion of the coating on the surface of the steel substrate or product. The pre-treatment of the steel substrate may comprise washing the surface of the steel substrate and/or roughening the surface of the steel substrate and/or forming a primer coating on the surface of the steel substrate, the metal coating being formed on the primer coating and/or treating the surface of the steel substrate with activation agent for enhancing the adhesion of the metal coating to the steel substrate. The roughening, such as mechanical roughening, may be carried out for example by shot blasting, abrasion or by some other known roughening method. The preheating and the roughening of the surface of the steel substrate enhance the adhesion between the coating 6 and the steel substrate 4. The surface of the steel product or substrate may be washed with washing agent. The primer coating may be for example a zinc-phosphate-layer provided on the steel substrate or also a separate metallic coating as described later in this description. The steel substrate 4 may further be pre-heated to an elevated temperature, more detailed to a temperature below the melting temperature of the metal or metal alloy concerned. However, it should be understood that during pre-heating the temperature of the steel substrate may be raised, momentarily, above the melting point of the metal or metal alloy, for example to temperature of 400 to 700 °C, but the temperature is lowered such that the temperature is below the melting temperature during the coating. Therefore, the elevated temperature of the pre-heating in the context of the present application refers to temperature of the steel substrate during coating. The pre-heating may be carried out by a pre-heating furnace or by burner or some other suitable heating means. In pre-heating the temperature of the steel substrate is raised to an elevated temperature for subject the coating 6 to be produced, preferably to a temperature at least 0,7 times the melting temperature of the metal or metal alloy and below the melting temperature of the metal or metal alloy, most preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal or metal alloy in the coating. The mentioned temperature range provides metallic bonding between the particles of the metal powder in the coating 6, or between the metal particles of the metal powder and between the metal particles and the steel substrate. The metallic bonding may be formed by diffusion bonding between the particles of the metal powder and between the metal particles and the steel substrate. The phases at the interface of steel substrate 4 and coating 6, or the boundary layer between the steel substrate 4 and the coating 6, which are formed during or after plating and sintering can be Fe-Zn or Fe-AI-Zn or Fe-AI-Mg-Zn intermetallic compounds similar to compounds after immersion galvanizing, i.e. hot-dip galvanizing and post-dip annealing, ie. annealing after hot-dip galvanizing.

Thus heat treatment provides adhesion between the produced coating 6 and the steel substrate and also hardens or solidifies the coating 6.

According to one embodiment the steel substrate is coated by multi-stage metal coating process. According to this embodiment, the coating process includes at least two stages: at least the first stage and the second stage. In said first stage the steel strip is galvanized as usual, such as hot-dip galvanized as usual, and subsequently, in said second stage, subjected to the coating process, such as by using said coating unit 2, according to the independent claim 1. It can be understood that the metal coating obtained thereby includes different metal layers: first metal layer which includes pure metal without coloring agent and second metal layer which includes coloring agents. The first metal layer is applied in the first stage before the second metal layer applied in the second stage. In case the second stage is accomplished when the heat of metal coating gained in the first stage is still available, i .e. the first metal layer is still warm, the second metal layer, i .e. the throughout colored metal coating deposited in the second stage will form metal bonding with first metal layer, ie. with pure metal layer obtained in the first stage. In other words, the throughout colored metal coating may be applied to the steel strip having warm or even melt first metal layer, preferably zinc layer, on the steel strip after first stage. Solution provides for layers which are tightly attached to each other due the metal bonding. This may be accomplished in commonly used hot dip galvanizing - lines in the industry. Therefore, providing a coating on a steel substrate may be carried out after galvanizing at least part of the steel substrate 4 such that the throughout colored metal coating 6 is formed on a zinc layer for providing a metal coating having a zinc layer and a throughout colored metal layer 6 on the zinc layer. The arrangement according to the present invention, as discussed above, may be provided to a continuous galvanizing line downstream of a galvanizing unit, preferably hot-dip galvanizing unit, such that the throughout colored metal coating 6 is formed on a zinc layer for providing a metal coating having a zinc layer and a throughout colored metal layer 6 on the zinc layer.

So it must be perceived that throughout colored metal coating can be preferably, but not necessarily, understood as a metallic layer on the multilayer metallic coating including also non-colored metallic layers beneath the throughout colored metallic layer. Similarly, it can be understood, that in this embodiment, the apparatus arrangement comprises at least two coating units, first coating unit (not drawn) and the second coating unit 2 for first and the second coating stages, respectively. The coated steel substrate or product may be further post-treated with one or more post-treating methods. In the post-heating the coating 6 is subjected to a heat treatment for providing a metallic bonding between the particles of the powder of at least one metal or metal alloy and the steel substrate 4, or by subjecting the coating 6 to a heat treatment for providing a metallic bonding between the particles of the powder of at least one metal or metal alloy in substantially liquid raw material and the steel substrate 4 and between the particles of the powder of at least one metal or metal alloy in substantially liquid raw material. The post heating may be provided with a furnace, such as galvanneal furnace, burner or some other known heating method. The post-heating may also be carrying out such that the temperature of the particles of the powder of at least one metal or metal alloy is raised to a elevated temperature below the melting temperature of the metal or metal alloy concerned, preferably to a temperature at least 0,7 times the melting temperature of the metal or metal alloy and below the melting temperature of the metal or metal alloy, most preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal or metal alloy. Also the post heating may be provided such that it provides metallic bonding between the particles of the metal powder in the coating 6, or between the metal particles of the metal powder and between the metal particles and the steel substrate. The metallic bonding may be formed by diffusion bonding between the particles of the metal powder and between the metal particles and the steel substrate. Thus heat treatment provides adhesion between the produced coating 6 and the steel substrate and also hardens or solidifies the coating 6. Also liquid substances such as solvents may evaporate during the heat treatment. Temperatures above the melting point of the coating are not preferably used because melted metals or metal alloys may repel the coloring agent.

According to the above mentioned the produced coating may be subjected to a heat treatment before or after applying the coating 6 on the steel substrate 4. Thus the heat treatment step may be carried out by heating the steel substrate 4 before applying the substantially liquid raw material on the surface of the steel substrate 4 or after applying the substantially liquid raw material on the surface of the steel substrate 4. Alternatively the heat treatment may be carried out by heating the steel substrate 4 before applying the substantially liquid raw material on the surface of the steel substrate 4 and after applying the substantially liquid raw material on the surface of the steel substrate 4. When the heat treatment is carried out by elevating the temperature to a temperature below the melting temperature of the metal or metal alloy concerned, most preferably when the heat treatment is carried out in temperature 0,7 - 0,9 times the melting temperature of the metal or metal alloy, such as zinc or zinc alloy, the mentioned metal bonding is achieved without melting the metal or metal alloy. If the temperature is below 0,7 times the melting temperature of the metal or metal alloy metal bonding may be difficult to achieve with reasonable heating times. In the heat treatment the particles of the metal powder may be sintered to each other or that the particles of the metal powder mat be sintered to each other and to the steel substrate 4 during the heat treatment. In one embodiment the post treating is carried out by heat treating the coated steel substrate at a temperature of at least 400 °C but below the melting temperature of the metal or metal alloy concerned. Heat treatment may also be carried out in a galvannealing furnace. The heat treatment further enhances the adhesion between the colored coating and the steel substrate or product. Furthermore, the post- treatment may comprise sintering the produced coating such that the particles of the metal powder in the raw material are sintered together. Alternative the sintering may be carried out such that the particles of the metal powder are sintered together and to the steel substrate. The heat treatment may be carried out also by infrared furnace or convection furnace. The heat treatment time may vary, depending on the heating temperature, between 5 to 600 seconds, preferably between 30 to 150 seconds. The above mentioned heat treatments are preferably carried out in protective gas atmosphere. The protective gas atmosphere may be argon atmosphere or atmosphere comprising a mixture of hydrogen and nitrogen. The protective atmosphere prevents or diminishes oxidation of the metal coating and/or the steel substrate. The arrangement is provided with means for providing the protective gas atmosphere to the preheating unit 10 for carrying out the pre-heating protective atmosphere. The means for producing the protective gas atmosphere may for example comprise a pre-heating chamber substantially isolated from the ambient atmosphere for example with gas curtains or the like. The means for providing the protective gas atmosphere may also comprise gas supplies for supplying protective gas into the heat treatment chamber. This provides for non-oxidized surface prior coating enhancing the adhesion. The protective atmosphere prevents or diminishes oxidation of the metal coating and/or the steel substrate and provides for a surface with good adhesion potential . Thus the protective atmosphere is preferable in preheating step before the coating of the steel substrate, and the steel substrate 4 is transported to the coating unit 2 in protective gas atmosphere.

The sintering phenomenon may be strengthened, in addition to the above mentioned the heat treatment, by performing the above mentioned post- heat treatments under pressure or subjecting the coated substrate to pressure after or during the heat treatment. The pressure may be provided by using for example a pair hot rollers between which a steel substrate or steel strip is conducted for providing a pressure to the coated steel substrate.

In another embodiment the heat treatment, the pre-heat treatment and/or post-heat treatment may carried at a temperature which is above the melting temperature of the metal of the metal particles or metal alloy particles coating, such as zinc particles or zinc alloy particles. The heat treatment temperature may for example be approximately 500 °C, and the metal particles or the metal alloy particles may partly melt during heat treatment such that metal bond are formed between the particles and the between the particles and the steel substrate. The coated steel substrate or product may further be coated with a substantially transparent surface coating. The surface coating may be provided for example by providing a varnish coating on the colored metal coating or by providing a surface coating on the metal coating by sol-gel process or by some vapor phase coating method such as chemical vapor deposition. The coating method of the present invention may thus use the above described raw material for producing a throughout colored metal coating. The method may be carried out separate steel products or continuously on a steel strip.

The above described substantially liquid raw material and the coating method may be utilized for providing a steel product having a metal coating on at least part of a surface of the steel product, the metal coating consisting at least partly of metal powder applied on the surface of the steel product. According to the present invention the metal coating further comprises inorganic coloring agent embedded into the metal coating. The inorganic coloring agent may be adhered to the particles of the metal powder in the metal coating . Alternatively the inorganic coloring agent may be embedded between particles of the metal powder in the metal coating. In the latter embodiment the inorganic coloring agent may be as separate particles in the metal coating. In a yet alternative embodiment the particles of the metal powder are sintered to each other and to the inorganic coloring agent by heat treating the coated steel product.

The metal coating on the steel product may be a zinc-based coating, aluminium-based coating, or zinc-aluminium-based or zinc-aluminium- magnesium-based coating . Alternatively the metal coating comprises zinc particles, aluminium particles, magnesium particles or all aluminium and zinc and magnesium particles or particles comprises zinc or aluminium or magnesium or all of those. The inorganic coloring agent in the metal coating may comprise cobalt, iron, chromium or titanium or compounds or oxides thereof. The colored metal coating may be provided such that the thickness of the metal coating is at least 10 micrometers, or at least 50 g/m². The colored metal coating may be provided by providing several layers on top of each other and one layer may be 20 to 100 micrometers thick. The overall thickness of the coating may be for example 150 micrometers or 137 g/m². The steel product may further comprise a surface coating provided on the throughout colored metal coating. Preferably the surface coating is a substantially transparent surface coating provided on the metal coating for example with varnishing, vapor phase deposition method or sol-gel process. The present invention also relates to an arrangement for producing a metal coating 6 on a steel strip 4, as very schematically shown in figure 1, according to the above described method and the substantially liquid raw material. The arrangement may be provided for example to a continuous galvanizing line in which the steel strip is provided under protective gas atmosphere to the coating unit 2 while the steel strip still includes the heat from recrystallization annealing. The arrangement comprises a coating unit 2 for producing a metal coating 6 on at least part of the steel strip 4. The steel strip 4 is transported along a process line with rollers 20. The coating unit 2 is arranged to apply on at least part of the steel strip one or more coating layers of substantially liquid raw material together with inorganic coloring agent for producing a throughout colored metal coating 6 on the steel strip 4. The coating unit 2 may be arranged to spread, paint, spray or pour the substantially liquid raw material comprising inorganic coloring agent on the surface of the steel substrate 4 or by dipping the steel substrate 4 into the substantially liquid raw material comprising inorganic coloring agent. The substantially liquid raw material may be a raw material as described above.

In one preferred embodiment coating unit 2 comprises a bath of the substantially liquid raw material and the coating unit is arranged to dip the steel strip 4 into the substantially liquid raw material comprising inorganic coloring agent. The bath may be heated for maintaining the temperature of the steel strip or diminishing the temperature drop of the steel strip in the bath. The heating temperature must be selected so that there does not occur any possible excessive evaporation of substantial liquid raw material . As shown in figure 1 the arrangement may comprise a roughening unit 8 for roughening the surface of the steel strip 6. The roughening unit may be comprise means for abrasion or shot blasting at least one surface of the steel strip 4. The arrangement further comprises a pre-treating unit 10, such as recrystallization furnace at continuous galvanizing line. The pre-treating unit may comprise one or more of the following : a washer for washing the surface of the steel strip 4, a coating equipment for forming a primer coating on the surface of the steel substrate 4, the metal coating 6 being formed on the primer coating, or means for treating the surface of the steel substrate 4 with activation agent for enhancing the adhesion of the metal coating 6 to the steel substrate 4.

The arrangement of figure 1 further comprises a post-heating unit 12, such as galvannealing furnace, for heat treating the coated steel strip 4 for providing metal bonding between particles of the metal powder, or between the particles of the metal powder and the steel strip 4. Thus the post-heating unit may be arranged to subject the coating 6 to a temperature at least 0,7 times the melting temperature of the metal or metal alloy and below the melting temperature of the metal or metal alloy, most preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal or metal alloy. In one embodiment the post-heating unit 12 may be arranged to heat treat the coated substrate 4 at a temperature of at least 400 °C, such as between 400 and 550 °C. The post-heating unit 12 may be a galvannealing furnace, infrared furnace, or convection furnace. It should be understood, that the arrangement may be arranged to produce a coating only on one surface of the steel strip or to both sides of the steel strip. The arrangement may further comprise a pair rollers, preferably hot rollers, between which a steel substrate or steel strip is conducted for providing a pressure to the coated steel substrate. The pressure enhances the formation of metallic bonds, as mentioned above. The arrangement may further comprise a pre-heating unit arranged to heat the steel strip 4 to an elevated temperature below the melting temperature of the metal or metal alloy concerned, preferably for subject the coating 6 to a temperature at least 0,7 times the melting temperature of the metal or metal alloy in the coating and below the melting temperature of the metal or metal alloy in the coating, preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal or metal alloy in the coating. The pre-heating unit may be for instance in continuous hot-dip galvanizing line a recrystallization furnace.

It is apparent to a person skilled in the art that as technology advanced, the basic idea of the invention can be implemented in various ways. The invention and its embodiments are therefore not restricted to the above examples, but they may vary within the scope of the claims.

Claims

1. A substantially liquid raw material for producing a metal coating (6) on at least part of a steel substrate (4) by applying the substantially liquid raw material on steel substrate, the substantially liquid raw material comprising metal powder having particles of at least one metal and at least one inorganic coloring agent, characterized in that the inorganic coloring agent is sintered to the metal particles such that the inorganic coloring agent is adhered to the metal particles for forming a throughout colored metal coating (6) .

2. A raw material according to claim 1, characterized in that the average diameter of the metal particles is between 0,5 to 150 micrometers, preferably between 0,5-50 micrometers and more preferably between 0,5- 10 micrometers.

3. A raw material according to claim 1 or 2, characterized in that the inorganic coloring agent comprises cobalt, iron, chromium or titanium, or compounds or oxides thereof.

4. A raw material according to any one of claims 1 to 3, characterized in that the substantially liquid raw material comprises inorganic coloring agent particles having average particles diameter of the inorganic coloring agent is between 0, 1 to 100 micrometer, preferably between 0,1-5 micrometer, and more preferably between 0,1- 1 micrometers.

5. A raw material according to any one of claims 1 to 4, characterized in that the substantially liquid raw material comprises a binder agent, or that the substantially liquid raw material comprises one or more of the following binder agents: water-based inorganic zinc or aluminium silicate.

6. A raw material according to any one of claims 1 to 5, characterized in that the substantially liquid raw material comprises inorganic coloring agent between 5 to 40 weight-%.

7. Use of substantially liquid raw material according to any one of claims 1 to 6 for producing a throughout colored metal coating (6) on a steel substrate or product (4) .

8. A method for producing a metal coating (6) on at least part of a steel substrate (4) using substantially liquid raw material comprising powder having particles of at least one metal or metal alloy, the method comprising producing the metal coating (6) by applying on the surface of the steel substrate (4) substantially liquid raw material comprising one or more inorganic coloring agents, characterized in that the method comprises a heat treatment step for subjecting the coating (6) to a heat treatment for providing a metallic bonding between the particles of the powder of the at least one metal or metal alloy and the steel substrate (4) or between the particles of the powder of the at least one metal or metal alloy and the steel substrate (4) and between the particles of the powder of the at least one metal or metal alloy such that a throughout colored metal coating (6) is formed.

9. A method according to claim 8, characterized by carrying out the heat treatment step such that the temperature of the particles of the powder of at least one metal or metal alloy is raised to elevated temperature below the melting temperature of the metal or metal alloy concerned, preferably the temperature is raised to a temperature at least 0,7 times the melting temperature of the metal or metal alloy and below the melting temperature of the metal or metal alloy, most preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal or metal alloy.

10. A method according to claim 8 or 9, characterized by

- carrying out the heat treatment step by heating the steel substrate (4) before applying the substantially liquid raw material on the surface of the steel substrate (4), or - by carrying out the heat treatment by heating the coated steel substrate (4) after applying the substantially liquid raw material on the surface of the steel substrate (4), or

- by carrying out the heat treatment by heating the steel substrate (4) before applying the substantially liquid raw material on the surface of the steel substrate (4) and by heating the coated steel substrate (4) after applying the substantially liquid raw material on the surface of the steel substrate (4) .

11. A method according to any one of claims 8 to 10, characterized by - heat treating the coated steel substrate (4) at a temperature of at least 400 °C, or

- heat treating the coated steel substrate (4) in a galvannealing furnace; or

- sintering the produced coating (6) such that the particles of the metal powder are sintered together or such that the particles of the metal powder are sintered together and to the steel substrate (4) .

12. A method according to any one of claims 8 to 11, characterized by applying the substantially liquid raw material on the surface of the steel product (4) by spreading, painting, spraying, pouring the substantially liquid raw material on the surface of the steel substrate (4) or by dipping the steel substrate (4) into the substantially liquid raw material .

13. A method according to any one of claims 8 to 12, characterized by producing a coating having thickness at least 10 micrometers, or at least 50 g/m2 or by producing a coating having overall thickness at least 150 micrometers or 137 g/m2.

14. A method according to any one of claims 8 to 13, characterized by pre-treating the steel substrate (4) with one or more of the following : - washing the surface of the steel substrate (4) ;

- roughening the surface of the steel substrate (4);

- forming a primer coating on the surface of the steel substrate (4), the metal coating (6) being formed on the primer coating; or - treating the surface of the steel substrate (4) with activation agent for enhancing the adhesion of the metal coating (6) to the steel substrate (4) .

15. A method according to any one of claims 8 to 14, characterized by using the substantially liquid raw material according to claim 1 to 9 for producing the metal coating (6) .

16. A method according to any one of claims 8 to 15, characterized by carrying the method for producing a coating continuously on a steel strip (4) .

17. A method according to any one of claims 8 to 16, characterized by carrying the method for producing a coating continuously on a steel strip

(4) by dipping the steel strip (4) into the substantially liquid raw material .

18. A method according to any one of claims 8 to 17, characterized in that the method is carried out after galvanizing at least part of the steel substrate (4) such that the throughout colored metal coating (6) is formed on a galvanizing layer for providing a metal coating having a galvanizing layer and a throughout colored metal layer (6) on the galvanizing layer.

19. A steel product (4) having a metal coating (6) on at least part of a surface of the steel product (4), the metal coating (6) consisting at least partly of metal powder having particles applied on the surface of the steel product (4), the metal coating (6) further comprising one or more inorganic coloring agents embedded into the metal coating (6), characterized in that the particles of the metal powder are adhered to each other with a metal bond.

20. A steel product according to claim 19, characterized in that metal coating (6) is zinc-based coating, aluminium-based coating, or zinc- aluminium-based coating, or that the coating comprises zinc particles, aluminium particles or both aluminium and zinc particles or particles comprises zinc or aluminium or both.

21. A steel product according to claim 19 or 20, characterized in that the inorganic coloring agent comprises cobalt, iron, chromium or titanium or compounds or oxides thereof.

22. A steel product according to any one of claims 19 to 21, characterized in that the metal coating (6) is bonded to the steel product (4) by metallic bonding between the steel product (4) and the metal coating (6) .

23. A steel product according to any one of claims 19 to 22, characterized in that the coating comprises at least 50 weight-%, preferably at least 60 weight-%, or more preferably at least 75 weight-% of metal, such pure metallic zinc.

24. A steel product according to any one of claims 19 to 23, characterized in that the thickness at least 10 micrometers, or at least

50 g/m2 or by producing a coating having overall thickness at least 150 micrometers or 137 g/m2.

25. A steel product according to any one of claims 19 to 23, characterized in that the steel product (4) further comprises a zinc coating layer provided between the throughout colored metal coating (6) and the surface of the steel product (4) .

26. An arrangement for producing a metal coating (6) on a steel strip (4), the arrangement comprising: - a coating unit (2) for producing a metal coating (6) on at least part of the steel strip (4), the coating unit (2) is arranged to apply on at least part of the steel strip one or more coating layers of substantially liquid raw material together with inorganic coloring agent, characterized in that the arrangement further comprises a heating unit (10, 12) for heat treating the steel strip (4) for providing metal bonding between the particles of the particles of the powder of the at least one metal or metal alloy and the steel substrate (4) or between the particles of the powder of the at least one metal or metal alloy and the steel substrate (4) and between the particles of the powder of the at least one metal or metal alloy for producing a throughout colored metal coating (6) on the steel strip (4).

27. An arrangement according to claim 26, characterized in that the arrangement is arranged to produce a metal coating (6) on a steel strip (4) in continuous process.

28. An arrangement according to claim 27, characterized in that the arrangement is provided to a continuous galvanizing line.

29. An arrangement according to claim 28, characterized in that the arrangement is provided to a continuous galvanizing line downstream of a galvanizing unit such that the throughout colored metal coating (6) is formed on a galvanizing layer for providing a metal coating having a galvanizing layer and a throughout colored metal layer (6) on the galvanizing layer.

30. An arrangement according to any one of claims 26 to 29, characterized in that the arrangement comprises:

- a preheating unit (10) carrying out the heat treatment by heating the steel substrate (4) before applying the substantially liquid raw material on the surface of the steel substrate (4) in the coating unit (2), or - a post-heating unit ( 12) carrying out the heat treatment by heating the coated steel substrate (4) after applying the substantially liquid raw material on the surface of the steel substrate (4) in the coating unit (2), or - a preheating unit ( 10) carrying out the heat treatment by heating the steel substrate (4) before applying the substantially liquid raw material on the surface of the steel substrate (4) in the coating unit (2), and a post-heating unit (12) by heating the coated steel substrate (4) after applying the substantially liquid raw material on the surface of the steel substrate (4) in the coating unit (2) .

31. An arrangement according to any one of claims 26 to 30, characterized in that the arrangement comprises means for providing the protective gas atmosphere to the a pre-heating unit ( 12) for carrying out the pre-heating in the protective atmosphere.

32. An arrangement according to any one of claims 26 to 31, characterized in that the coating unit (2) is arranged to spread, paint, roll, spray or pour the substantially liquid raw material comprising inorganic coloring agent on the surface of the steel strip (4) or by dipping the steel strip (4) into the substantially liquid raw material comprising inorganic coloring agent.

33. An arrangement according to any one of claims 26 to 32, characterized in that the coating unit (2) comprises a bath of the substantially liquid raw material and the coating unit is arranged to dip the steel strip (4) into the substantially liquid raw material comprising inorganic coloring agent.

34. An arrangement according to any one of claims 26 to 33, characterized in that the coating unit (2) is arranged to apply on the surface of the steel strip substantially liquid raw material according to any one of claim 1 to 9.

35. An arrangement according to any one of claims 26 to 34, characterized in that the post-heating unit (12) is arranged to subject the coating (6) to a temperature at least 0,7 times the melting temperature of the metal and below the melting temperature of the metal, preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal.

36. An arrangement according to any one of claims 26 to 35, characterized in that the arrangement further comprises a pre-heating unit arranged to heat the steel strip (4) to an elevated temperature for subject the coating (6) to a temperature at least 0,7 times the melting temperature of the metal and below the melting temperature of the metal, preferably to a temperature between 0,7 - 0,9 times the melting temperature of the metal.