EP1826289A1 - A steel sheet coated with an aluminium based coating, said sheet having high formability - Google Patents

A steel sheet coated with an aluminium based coating, said sheet having high formability Download PDF

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Publication number
EP1826289A1
EP1826289A1 EP06447030A EP06447030A EP1826289A1 EP 1826289 A1 EP1826289 A1 EP 1826289A1 EP 06447030 A EP06447030 A EP 06447030A EP 06447030 A EP06447030 A EP 06447030A EP 1826289 A1 EP1826289 A1 EP 1826289A1
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EP
European Patent Office
Prior art keywords
aluminium
steel sheet
metal oxide
oxide layer
layer
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP06447030A
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German (de)
French (fr)
Inventor
Marijke De Meyer
Serge Claessens
Franz Horzenberger
Zinedine Zermout
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OCAS NV
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OCAS NV
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Publication date
Application filed by OCAS NV filed Critical OCAS NV
Priority to EP06447030A priority Critical patent/EP1826289A1/en
Priority to EP07712332A priority patent/EP1989339A2/en
Priority to PCT/EP2007/051843 priority patent/WO2007099097A2/en
Publication of EP1826289A1 publication Critical patent/EP1826289A1/en
Withdrawn legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
    • C23C28/321—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer with at least one metal alloy layer
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/32—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer
    • C23C28/322—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one pure metallic layer only coatings of metal elements only
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/34—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates
    • C23C28/345—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates with at least one oxide layer
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D
    • C23C28/30—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer
    • C23C28/34—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates
    • C23C28/345—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates with at least one oxide layer
    • C23C28/3455—Coatings combining at least one metallic layer and at least one inorganic non-metallic layer including at least one inorganic non-metallic material layer, e.g. metal carbide, nitride, boride, silicide layer and their mixtures, enamels, phosphates and sulphates with at least one oxide layer with a refractory ceramic layer, e.g. refractory metal oxide, ZrO2, rare earth oxides or a thermal barrier system comprising at least one refractory oxide layer

Definitions

  • the present invention is related to a steel sheet coated with an aluminium-based coating by hot dip coating, the coated steel sheet according to the invention having a high level of formability.
  • Aluminium-based coated steel sheets present excellent corrosion resistance, and can be used for manufacturing parts for power-driven ground vehicles or cladding for buildings depending on the composition of the aluminium-based coating.
  • the aluminium-based coating is substantially composed of pure aluminium, as in the coated steel product Alupur ® , a thick intermediate layer of iron and aluminium is formed at the steel/aluminium interface during the coating operation.
  • the thickness of this intermediate layer is typically 20 ⁇ m. Due to the low ductility of this thick intermediate layer of iron and aluminium, the aluminium coated steel sheet is characterized by a low formability.
  • the aluminium-based coating is composed of by weight, 90 % aluminium and 10 % silicon, such as in the Alusi ® -product, a thin intermediate layer of iron, aluminium and silicon is formed at the steel/aluminium-silicon interface, which is thinner (typically up to 5 ⁇ m) compared to the intermediate layer of iron-aluminium in the case of a pure aluminium coating. Silicon is added for reasons of reaction control between iron and aluminium, thereby allowing to control (and limit) the thickness of the intermediate layer.
  • the Alusi ® coating itself is on the other hand brittle and not ductile due to the presence of silicon.
  • the aluminium-based coating is composed of, by weight, 55 % aluminium, 43.4 % zinc and 1.6 % silicon (Aluzinc ® )
  • a thin intermediate layer of iron, aluminium, zinc and silicon is formed at the steel/aluminium-zinc-silicon coating interface, which is thinner (typically a few ⁇ m) compared to the intermediate layer of iron-aluminium interface in the case of a pure aluminium coating.
  • the silicon keeps the intermediate layer under control, but the Aluzinc ® coating itself is brittle and not ductile due to the presence of silicon. Consequently, the aluminium-silicon coated steel sheet and the aluminium-zinc-silicon coated steel sheet are also characterized by a low formability.
  • Japanese patent 2002-226959 discloses a process for manufacturing a steel with an aluminium-based coating, produced by hot dip coating the steel sheet in an aluminium-based bath, wherein before hot dip coating, the steel sheet is coated with a metal (e.g. Cr, V, Mn, Nb, W), whose oxide can be reduced by molten aluminium to form the oxide of the metal on the surface.
  • a metal e.g. Cr, V, Mn, Nb, W
  • a layer of alumina is thus formed on the surface of the steel sheet, and the growth of the interfacial layer composed of Fe-Al or Fe-Al-Si is controlled to a thickness of 2 ⁇ m or less.
  • dross inter-metallic compound particles
  • This dross has a density lower than that of the liquid aluminium or aluminium alloy. It rises to the surface of the bath and is entrained by the steel sheet, creating surface aspect defects of the coated steel sheet.
  • a part of the metal can also be incorporated in the aluminium-based coating, and modifies its characteristics. Consequently, the surface of the bath must be cleaned to remove this dross, and the bath must be often replaced in order to avoid the presence of the metal in the aluminium-based coating.
  • the present invention aims to provide a steel sheet coated with an aluminium-based coating, said sheet having an excellent formability, and which is free from surface aspect defects, and a process for producing such a coated steel sheet by hot dip coating the steel sheet in a molten bath of aluminium or aluminium alloy having a long life service.
  • the present invention is related to products and processes such as described in the appended claims.
  • a metal oxide layer is applied to at least one face of a steel sheet, and an iron oxide layer is applied on top of and in contact with that metal oxide layer.
  • the invention is firstly related to the steel sheet comprising the bi-layer consisting of metal oxide and iron oxide.
  • the iron oxide is reduced into metallic iron during annealing prior to dipping and dissolves gradually when being immersed in the molten Al bath, while the metal oxide is stable with respect to molten aluminium. This means that the metal oxide does not or not substantially react with molten aluminium, when the steel sheet, comprising the metal oxide layer and metallic iron layer, is immersed into a bath of molten aluminium or aluminium-alloy.
  • the metal oxide can be thermodynamically inert with respect to aluminium at the temperature of the molten metal bath, normally between 500 and 800°C.
  • metal oxides which are thermodynamically inert in said temperature range are : CaO, MgO, LiO 2 , La 2 O 3 , BeO, ThO 2 , SrO, ZrO 2 .
  • the metal oxide can be non-inert with respect to aluminium, but can exhibit slow kinetics in its reaction with molten aluminium, so that no substantial reaction occurs between the metal oxide and the aluminium during the time of immersion in the bath, typically a few seconds.
  • the latter type of metal oxides are: TiO 2 , SiO 2 , V 2 O 3 , MnO, B 2 O 3 , Ta 2 O 5 , Nb 2 O 5 , Cr 2 O 3 .
  • the invention is equally related to a steel sheet coated with an aluminium-based coating, said coating comprising a metal oxide layer on at least one of the faces of said steel sheet, said metal oxide layer being stable (in the sense described above) with respect to molten aluminium, and in contact with said metal oxide layer an intermediate layer comprising an iron-aluminium alloy, and in contact with said intermediate layer an aluminium or aluminium-alloy layer.
  • the aluminium-based coating coated on a steel sheet according to the invention is essentially silicon-free (content of silicon less than 0.5% by weight), while at the same time, the thickness of the intermediate layer is preferably not more than 5 ⁇ m.
  • the stable oxide layer makes it possible to control the intermediate layer to maximum 5 ⁇ m, even though no substantial Si is present in the molten Al-bath.
  • a preferred embodiment of the invention therefore involves a steel sheet, with the stable metal oxide layer described above, and in contact therewith a layer of substantially pure aluminium and inevitable impurities (corresponds to the Alupur ® -coating), and with a thickness of preferably not more than 5 ⁇ m.
  • this aluminium-based coated steel sheet therefore has the advantage of having a high formability, due to a controlled thickness of the intermediate layer and due to the non-brittle behaviour of the coating itself, as a consequence of the absence of Si.
  • Another embodiment involves a steel sheet with an aluminium-based coating comprising 42 to 46% of zinc, the complement being aluminium and inevitable impurities (composition close to an Aluzinc ® -coating but without the Si), while the intermediate layer is preferably not more than 5 ⁇ m in thickness.
  • the aluminium-alloy layer according to the invention comprises:
  • the invention is equally related to the preferred process of producing a coated steel sheet according to the invention, said process comprising the following steps :
  • the metal oxide layer and the iron oxide layer can be formed by gas phase deposition.
  • the gas phase deposition steps are preferably applied in a pre-heating zone before the annealing furnace in a hot dip processing line.
  • the annealing step is performed in said annealing furnace.
  • the process can thus be performed in a standard hot dip processing line, supplemented by suitable gas phase deposition, e.g. Chemical Vapour Deposition facilities before the annealing section.
  • the iron oxide is reduced and forms a metallic iron layer.
  • the reduction of the iron oxide is depending on the annealing temperature and the oxidation potential of the atmosphere, making a lower dew point necessary at lower temperature to completely reduce the iron oxide.
  • the iron oxide will be reduced from 350 °C onwards, and at a dew point of +10 °C, from 455 °C onwards.
  • the typical dew point range in an annealing furnace is between -40 °C and -30 °C.
  • the typical annealing temperature is between 750 °C and 850 °C and the typical overageing temperature is between 400 °C and 500 °C.
  • Iron oxide will thus always be reduced, already during the last part of the heating up to annealing temperature.
  • the iron reacts with aluminium to form an intermediate layer made of iron-aluminium alloy, which is essential for a good wetting of the surface.
  • the thickness of this intermediate layer is controlled by the presence of the metal oxide layer which is stable with respect to molten aluminium, and which prohibits diffusion of iron from the steel sheet into the bath.
  • the thickness of the iron-oxide layer defines the thickness of the final intermediate layer of iron-aluminium. Due to the fact that the metal oxide does not substantially react with aluminium, no metal will enter the bath and form a dross after a given operation time.
  • the gas phase deposition of the metal oxide layer preferably takes place at a temperature between 350°C and 550°C.
  • the thickness of the metal oxide layer in the oxide coated steel sheet (intermediate product) and in the aluminium-based coated steel sheet (final product) is preferably 5 to 1000 nm.
  • the iron oxide layer in the intermediate product is preferably between 5 and 1000nm thick.
  • the molten aluminium bath comprises no or no substantial amount of silicon (preferably max. 0.5% in weight).
  • the thickness of the intermediate layer is nevertheless controlled to a low value, preferably max. 5 ⁇ m, by the above-described influence of the stable oxide layer. Therefore, when the method of the invention is applied with a molten aluminium bath consisting of pure aluminium and inevitable impurities, the result is a steel sheet with an aluminium-based coating, comprising said metal oxide layer, an intermediate layer comprising essentially aluminium and iron (e.g. a FeAl 3 layer + a Fe 2 Al 5 layer), and having a thin thickness, preferably less than 5 ⁇ m.
  • the resulting aluminized steel sheet has excellent formability, due to the thin intermediate layer, while the coating has a high resistance to cracking, due to the absence of silicon.
  • the composition of the bath may be other than pure aluminium, e.g. comprising aluminium and zinc, but it is preferably without a deliberate addition of silicon. Silicon is therefore preferably present at impurity level, which is preferably not more than 0.5 % by weight.
  • the bath comprises by weight, 42 to 46 % zinc, the complement being aluminium and inevitable impurities. This could be defined as the composition for obtaining an Aluzinc ® -coating, but without the 1-2% Si.
  • the intermediate layer of iron-aluminium alloy comprises aluminium, zinc and iron, and its thickness is preferably less than 5 ⁇ m.
  • the composition of the molten aluminium-alloy bath can still comprise silicon, e.g., the composition may be (by weight) :
  • the invention is equally related to a power-driven ground vehicle and to a cladding for buildings made of a steel sheet coated with an aluminium-based coating, according to the invention.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Ceramic Engineering (AREA)
  • Coating With Molten Metal (AREA)

Abstract

The present invention is related to a steel sheet comprising a metal oxide layer in contact with said steel face, and on top of that oxide layer, an iron oxide layer, wherein said iron oxide layer is reduced during annealing prior to dipping and gradually dissolves when being immersed in the molten Al bath, while said metal oxide layer is stable with respect to molten Al, and to an aluminium-based steel sheet produced by immersing said sheet into a hot dip molten metal bath comprising Al. According to the preferred embodiment, the metal oxide layer and iron oxide layers are applied via gas phase deposition.

Description

    Field of the Invention
  • The present invention is related to a steel sheet coated with an aluminium-based coating by hot dip coating, the coated steel sheet according to the invention having a high level of formability.
    • State of the art
  • Aluminium-based coated steel sheets present excellent corrosion resistance, and can be used for manufacturing parts for power-driven ground vehicles or cladding for buildings depending on the composition of the aluminium-based coating.
  • When the aluminium-based coating is substantially composed of pure aluminium, as in the coated steel product Alupur®, a thick intermediate layer of iron and aluminium is formed at the steel/aluminium interface during the coating operation. The thickness of this intermediate layer is typically 20 µm. Due to the low ductility of this thick intermediate layer of iron and aluminium, the aluminium coated steel sheet is characterized by a low formability.
  • When the aluminium-based coating is composed of by weight, 90 % aluminium and 10 % silicon, such as in the Alusi®-product, a thin intermediate layer of iron, aluminium and silicon is formed at the steel/aluminium-silicon interface, which is thinner (typically up to 5 µm) compared to the intermediate layer of iron-aluminium in the case of a pure aluminium coating. Silicon is added for reasons of reaction control between iron and aluminium, thereby allowing to control (and limit) the thickness of the intermediate layer. However, the Alusi® coating itself is on the other hand brittle and not ductile due to the presence of silicon.
  • In the same way, when the aluminium-based coating is composed of, by weight, 55 % aluminium, 43.4 % zinc and 1.6 % silicon (Aluzinc®), a thin intermediate layer of iron, aluminium, zinc and silicon is formed at the steel/aluminium-zinc-silicon coating interface, which is thinner (typically a few µm) compared to the intermediate layer of iron-aluminium interface in the case of a pure aluminium coating. Again, the silicon keeps the intermediate layer under control, but the Aluzinc® coating itself is brittle and not ductile due to the presence of silicon. Consequently, the aluminium-silicon coated steel sheet and the aluminium-zinc-silicon coated steel sheet are also characterized by a low formability.
  • Japanese patent 2002-226959 discloses a process for manufacturing a steel with an aluminium-based coating, produced by hot dip coating the steel sheet in an aluminium-based bath, wherein before hot dip coating, the steel sheet is coated with a metal (e.g. Cr, V, Mn, Nb, W), whose oxide can be reduced by molten aluminium to form the oxide of the metal on the surface. A layer of alumina is thus formed on the surface of the steel sheet, and the growth of the interfacial layer composed of Fe-Al or Fe-Al-Si is controlled to a thickness of 2 µm or less. However, the use of a metal oxide coating able to react with the aluminium of the bath is inconvenient, because after a certain time of operation of the aluminium-based bath, the bath will be saturated with the metal. The metal will react with the other components of the bath in order to form inter-metallic compound particles, called dross. This dross has a density lower than that of the liquid aluminium or aluminium alloy. It rises to the surface of the bath and is entrained by the steel sheet, creating surface aspect defects of the coated steel sheet. Furthermore a part of the metal can also be incorporated in the aluminium-based coating, and modifies its characteristics. Consequently, the surface of the bath must be cleaned to remove this dross, and the bath must be often replaced in order to avoid the presence of the metal in the aluminium-based coating.
    • Aims of the invention
  • The present invention aims to provide a steel sheet coated with an aluminium-based coating, said sheet having an excellent formability, and which is free from surface aspect defects, and a process for producing such a coated steel sheet by hot dip coating the steel sheet in a molten bath of aluminium or aluminium alloy having a long life service.
    • Description of the invention
  • The present invention is related to products and processes such as described in the appended claims.
  • According to the invention, a metal oxide layer is applied to at least one face of a steel sheet, and an iron oxide layer is applied on top of and in contact with that metal oxide layer. The invention is firstly related to the steel sheet comprising the bi-layer consisting of metal oxide and iron oxide. According to the invention, the iron oxide is reduced into metallic iron during annealing prior to dipping and dissolves gradually when being immersed in the molten Al bath, while the metal oxide is stable with respect to molten aluminium. This means that the metal oxide does not or not substantially react with molten aluminium, when the steel sheet, comprising the metal oxide layer and metallic iron layer, is immersed into a bath of molten aluminium or aluminium-alloy.
  • The metal oxide can be thermodynamically inert with respect to aluminium at the temperature of the molten metal bath, normally between 500 and 800°C. Examples of metal oxides which are thermodynamically inert in said temperature range are : CaO, MgO, LiO2, La2O3, BeO, ThO2, SrO, ZrO2.
  • Alternatively, the metal oxide can be non-inert with respect to aluminium, but can exhibit slow kinetics in its reaction with molten aluminium, so that no substantial reaction occurs between the metal oxide and the aluminium during the time of immersion in the bath, typically a few seconds. Examples of the latter type of metal oxides are: TiO2, SiO2, V2O3, MnO, B2O3, Ta2O5, Nb2O5, Cr2O3.
  • The invention is equally related to a steel sheet coated with an aluminium-based coating, said coating comprising a metal oxide layer on at least one of the faces of said steel sheet, said metal oxide layer being stable (in the sense described above) with respect to molten aluminium, and in contact with said metal oxide layer an intermediate layer comprising an iron-aluminium alloy, and in contact with said intermediate layer an aluminium or aluminium-alloy layer.
  • According to the preferred embodiment of the invention, the aluminium-based coating coated on a steel sheet according to the invention is essentially silicon-free (content of silicon less than 0.5% by weight), while at the same time, the thickness of the intermediate layer is preferably not more than 5 µm. The stable oxide layer makes it possible to control the intermediate layer to maximum 5µm, even though no substantial Si is present in the molten Al-bath.
  • A preferred embodiment of the invention therefore involves a steel sheet, with the stable metal oxide layer described above, and in contact therewith a layer of substantially pure aluminium and inevitable impurities (corresponds to the Alupur®-coating), and with a thickness of preferably not more than 5µm. Contrary to the prior art Alupur®-coated steel sheets, which have intermediate Fe-Al layers of about 20 µm thick, this aluminium-based coated steel sheet therefore has the advantage of having a high formability, due to a controlled thickness of the intermediate layer and due to the non-brittle behaviour of the coating itself, as a consequence of the absence of Si.
  • Another embodiment involves a steel sheet with an aluminium-based coating comprising 42 to 46% of zinc, the complement being aluminium and inevitable impurities (composition close to an Aluzinc®-coating but without the Si), while the intermediate layer is preferably not more than 5µm in thickness.
  • According to another embodiment, the aluminium-alloy layer according to the invention comprises:
    • 42 to 44% of zinc, 1 to 2% of silicon, the complement being aluminium and inevitable impurities (corresponds to Aluzinc®-coating, including the Si), or
    • 9 to 11% of silicon, the complement being aluminium and inevitable impurities (corresponds to Alusi®-coating).
    In the last two cases, the presence of Si, together with the application of the stable oxide layer, leads to an intermediate iron-aluminium layer with a thickness of preferably not more than 1µm.
  • The invention is equally related to the preferred process of producing a coated steel sheet according to the invention, said process comprising the following steps :
    • Providing a steel sheet,
    • Applying to at least one face of said sheet, a metal oxide layer,
    • Applying a layer of iron oxide in contact with said metal oxide layer,
    • Annealing said steel sheet comprising said metal oxide and said iron oxide layers, in a reducing atmosphere so that the iron oxide is reduced into a metallic iron layer, and
    • Immersing said annealed steel sheet into a bath of molten aluminium or aluminium-alloy so that said metallic iron is gradually dissolved in said bath with the formation of an intermediate layer of iron-aluminium and a layer of aluminium or aluminium-alloy is formed on said intermediate layer.
  • According to the invention, the metal oxide layer and the iron oxide layer can be formed by gas phase deposition. The gas phase deposition steps are preferably applied in a pre-heating zone before the annealing furnace in a hot dip processing line. The annealing step is performed in said annealing furnace. The process can thus be performed in a standard hot dip processing line, supplemented by suitable gas phase deposition, e.g. Chemical Vapour Deposition facilities before the annealing section.
  • During the annealing step, the iron oxide is reduced and forms a metallic iron layer.
    The reduction of the iron oxide is depending on the annealing temperature and the oxidation potential of the atmosphere, making a lower dew point necessary at lower temperature to completely reduce the iron oxide. According to equilibrium data, at a dew point of -30 °C, the iron oxide will be reduced from 350 °C onwards, and at a dew point of +10 °C, from 455 °C onwards. The typical dew point range in an annealing furnace is between -40 °C and -30 °C. The typical annealing temperature is between 750 °C and 850 °C and the typical overageing temperature is between 400 °C and 500 °C. Iron oxide will thus always be reduced, already during the last part of the heating up to annealing temperature.
    Once in contact with the molten aluminium or aluminium-alloy bath, the iron reacts with aluminium to form an intermediate layer made of iron-aluminium alloy, which is essential for a good wetting of the surface. The thickness of this intermediate layer is controlled by the presence of the metal oxide layer which is stable with respect to molten aluminium, and which prohibits diffusion of iron from the steel sheet into the bath. The thickness of the iron-oxide layer defines the thickness of the final intermediate layer of iron-aluminium. Due to the fact that the metal oxide does not substantially react with aluminium, no metal will enter the bath and form a dross after a given operation time.
  • The gas phase deposition of the metal oxide layer preferably takes place at a temperature between 350°C and 550°C. The thickness of the metal oxide layer in the oxide coated steel sheet (intermediate product) and in the aluminium-based coated steel sheet (final product) is preferably 5 to 1000 nm. The iron oxide layer in the intermediate product is preferably between 5 and 1000nm thick.
  • According to the preferred embodiment, the molten aluminium bath comprises no or no substantial amount of silicon (preferably max. 0.5% in weight). Despite the absence of silicon, the thickness of the intermediate layer is nevertheless controlled to a low value, preferably max. 5 µm, by the above-described influence of the stable oxide layer. Therefore, when the method of the invention is applied with a molten aluminium bath consisting of pure aluminium and inevitable impurities, the result is a steel sheet with an aluminium-based coating, comprising said metal oxide layer, an intermediate layer comprising essentially aluminium and iron (e.g. a FeAl3 layer + a Fe2Al5 layer), and having a thin thickness, preferably less than 5 µm. The resulting aluminized steel sheet has excellent formability, due to the thin intermediate layer, while the coating has a high resistance to cracking, due to the absence of silicon.
  • The composition of the bath may be other than pure aluminium, e.g. comprising aluminium and zinc, but it is preferably without a deliberate addition of silicon. Silicon is therefore preferably present at impurity level, which is preferably not more than 0.5 % by weight.
    According to a specific embodiment, the bath comprises by weight, 42 to 46 % zinc, the complement being aluminium and inevitable impurities. This could be defined as the composition for obtaining an Aluzinc®-coating, but without the 1-2% Si. In this case, the intermediate layer of iron-aluminium alloy comprises aluminium, zinc and iron, and its thickness is preferably less than 5 µm.
  • Even though the preferred embodiment uses a bath composition without Si-addition, the composition of the molten aluminium-alloy bath, according to other embodiments, can still comprise silicon, e.g., the composition may be (by weight) :
    • 9 to 11% silicon, the complement being aluminium and inevitable impurities. In that case, the intermediate layer of iron-aluminium alloy comprises : aluminium, iron, silicon (FeAl3 layer + Fe2Al5 layer + Al-Fe-Si τ5),.
    • 42 to 44 % zinc, 1 to 2 % silicon, the complement being aluminium and inevitable impurities. In that case, the intermediate layer of iron-aluminium alloy comprises : aluminium, zinc, iron, silicon.
    In the latter cases, the advantage of a controlled thickness of the intermediate layer is still present and may even be more pronounced due to the presence of Si, which makes it possible to obtain an intermediate layer with a thickness of max. 1µm. On the other hand, the presence of Si may make the aluminium-based coating more brittle.
  • The invention is equally related to a power-driven ground vehicle and to a cladding for buildings made of a steel sheet coated with an aluminium-based coating, according to the invention.

Claims (28)

  1. A steel sheet coated on at least one of its faces with an oxide coating, wherein said oxide coating comprises :
    - a metal oxide layer in contact with said at least one face, and
    - an iron oxide layer in contact with said metal oxide layer,
    and wherein said metal oxide layer is stable with respect to molten aluminium.
  2. The steel sheet according to claim 1, wherein the thickness of said iron oxide layer is between 5nm and 1000nm.
  3. A steel sheet coated on at least one of its faces with an aluminium-based coating, wherein said aluminium-based coating comprises :
    - a metal oxide layer in contact with said at least one face,
    - in contact with said metal oxide layer : an intermediate layer of iron-aluminium alloy, and
    - in contact with said intermediate layer : an aluminium or aluminium-alloy layer, and wherein said metal oxide layer is stable with respect to molten aluminium.
  4. The steel sheet according to any one of claims 1 to 3, wherein said metal oxide is thermodynamically inert with respect to molten aluminium, in a temperature range of 500°C to 800°C.
  5. The steel sheet according to claim 4, wherein said metal oxide is chosen from the group consisting of : CaO, MgO, LiO2, La2O3, BeO, ThO2, SrO, ZrO2.
  6. The steel sheet according to any one of claims 1 to 3, wherein said metal oxide exhibits slow kinetics in its reaction with molten aluminium, in a temperature range of 500°C to 800°C.
  7. The steel sheet according to claim 6, wherein said metal oxide is chosen from the group consisting of : TiO2, SiO2, V2O3, MnO, B2O3, Ta2O5, Nb2O5, Cr2O3.
  8. The steel sheet according to any one of claims 3 to 7, wherein the thickness of said stable metal oxide layer is between 5nm and 1000nm.
  9. The steel sheet according to any one of claims 3 to 8, wherein said aluminium or aluminium-alloy coating layer is essentially silicon-free.
  10. The steel sheet according to claim 9, wherein said aluminium-alloy comprises, by weight, 42 to 46 % of zinc, the complement being aluminium and inevitable impurities.
  11. The steel sheet according to claim 9 or 10, wherein the thickness of said intermediate layer is less than 5µm.
  12. The steel sheet according to any one of claims 3 to 8, wherein said aluminium-alloy comprises, by weight, 9 to 11 % of silicon, the complement being aluminium and inevitable impurities.
  13. The steel sheet according to any one of claims 3 to 8, wherein said aluminium-alloy comprises, by weight, 42 to 44 % of zinc, 1 to 2 % of silicon, the complement being aluminium and inevitable impurities.
  14. The steel sheet according to claim 12 or 13, wherein the thickness of said intermediate layer is less than 1 µm.
  15. Power-driven ground vehicles made of the steel sheet according to any one of claims 3 to 14.
  16. Cladding for buildings made of the steel sheet according to any one of claims 3 to 14.
  17. A process for producing a coated steel sheet according to any one of claims 3 to 14, wherein said process comprises the steps of :
    - providing a steel sheet,
    - applying to at least one face of said sheet, a metal oxide layer,
    - applying a layer of iron oxide in contact with said metal oxide layer,
    - annealing said steel sheet comprising said metal oxide and said iron oxide layers, in a reducing atmosphere so that the iron oxide is reduced into a metallic iron layer, and
    - immersing said annealed steel sheet into a bath of molten aluminium or aluminium alloy wherein said metallic iron gradually dissolves into said bath to form said intermediate layer of iron-aluminium alloy and wherein said metal oxide layer is stable during immersion of the annealed steel sheet into said bath.
  18. The process according to claim 17, wherein said metal oxide is thermodynamically inert with respect to aluminium, at the temperature of the immersion, said temperature being between 500°C and 800°C.
  19. The process according to claim 18, wherein said metal oxide is chosen from the group consisting of : CaO, MgO, LiO2, La2O3, BeO, ThO2, SrO, ZrO2.
  20. The process according to claim 17, wherein said metal oxide is chosen from the group consisting of : TiO2, SiO2, V2O3, MnO, B2O3, Ta2O5, Nb2O5, Cr2O3.
  21. The process according to any one of claims 17 to 20, wherein said metal oxide layer is applied by gas phase deposition.
  22. The process according to any one of claims 17 to 21, wherein said iron oxide layer is applied by gas phase deposition.
  23. The process according to any one of claims 17 to 22, wherein the thickness of said metal oxide layer is between 5 nm and 1000 nm.
  24. The process according to any one of claims 17 to 23, wherein the thickness of said iron oxide layer is between 5 nm and 1000 nm.
  25. The process according to any one of claims 17 to 24, wherein said bath is essentially silicon-free.
  26. The process according to claim 25, wherein the aluminium-alloy molten bath comprises, by weight, 42 to 46 % of zinc, the complement being aluminium and inevitable impurities.
  27. The process according to any one of claims 17 to 24, wherein the aluminium-alloy molten bath comprises, by weight, 9 to 11 % of silicon, the complement being aluminium and inevitable impurities.
  28. The process according to any one of claims 17 to 24, wherein the aluminium-alloy molten bath comprises, by weight, 42 to 44 % of zinc, 1 to 2 % of silicon, the complement being aluminium and inevitable impurities.
EP06447030A 2006-02-28 2006-02-28 A steel sheet coated with an aluminium based coating, said sheet having high formability Withdrawn EP1826289A1 (en)

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EP06447030A EP1826289A1 (en) 2006-02-28 2006-02-28 A steel sheet coated with an aluminium based coating, said sheet having high formability
EP07712332A EP1989339A2 (en) 2006-02-28 2007-02-27 A steel sheet coated with an aluminium based coating.
PCT/EP2007/051843 WO2007099097A2 (en) 2006-02-28 2007-02-27 A steel sheet coated with an aluminium based coating.

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WO2011101158A1 (en) * 2010-02-19 2011-08-25 Tata Steel Nederland Technology Bv Strip, sheet or blank suitable for hot forming and process for the production thereof
CN102560313A (en) * 2012-02-15 2012-07-11 北京海泰锐森环境能源技术开发有限公司 Hot dip aluminum plating method for steel structure member

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DE102020120580A1 (en) 2020-08-04 2022-02-10 Muhr Und Bender Kg METHOD OF MAKING COATED STEEL STRIP, AND METHOD OF MAKING A HARDENED STEEL PRODUCT

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WO2009080292A1 (en) * 2007-12-20 2009-07-02 Voestalpine Stahl Gmbh Method for the production of coated and hardened components made of steel, and coated and hardenable steel strip therefor
US9090951B2 (en) 2007-12-20 2015-07-28 Voestalpine Stahl Gmbh Method for producing coated and hardened components of steel and coated and hardened steel strip therefor
CN101918599B (en) * 2007-12-20 2016-06-01 沃斯特阿尔派因钢铁有限责任公司 Coating and the method for stiffener members and coating thereof and hardenable strip steel is made for preparing steel
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CN102762750A (en) * 2010-02-19 2012-10-31 塔塔钢铁荷兰科技有限责任公司 Strip, sheet or blank suitable for hot forming and process for the production thereof
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CN102762750B (en) * 2010-02-19 2014-06-04 塔塔钢铁荷兰科技有限责任公司 Strip, sheet or blank suitable for hot forming and process for the production thereof
US9593391B2 (en) 2010-02-19 2017-03-14 Tata Steel Nederland Technology Bv Strip, sheet or blank suitable for hot forming and process for the production thereof
CN102560313A (en) * 2012-02-15 2012-07-11 北京海泰锐森环境能源技术开发有限公司 Hot dip aluminum plating method for steel structure member

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