A STEEL SHEET COATED WITH AN ALUMINIUM BASED COATING.

Field of the Invention

[0001] The present invention is related to a steel sheet coated with an aluminium-based coating by hot dip coating.

State of the art [0002] 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. [0003] 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. [0004] 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.

[0005] 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. [0006] 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, an oxide of a metal (e.g. Cr, V, Mn, Nb, W) is formed on a steel sheet, wherein said oxide can be reduced by molten aluminium. By dipping the sheet in an Al or Al alloy- comprising bath, a layer of alumina (A1203) 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. This effect is obtained because the alumina layer slows down the diffusion of iron atoms into the bath. This document therefore provides a method of controlling and limiting the thickness of the intermetallic Fe-Al layer without requiring the

presence of silicon in the molten aluminium bath. However, allowing a metal oxide coating to react with the aluminium of the bath is inconvenient, because after a certain time of operation of the aluminium-based bath, there is a danger of the bath becoming 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.

[0007] Another drawback of the technique of JP2002- 226959 lies in the fact that the immersion time of the sheet in the Al-based bath is determined by the time required for the reduction of the metal oxide by the molten aluminium, in order to form the layer of alumina. Long immersion times may be necessary because of this, and the process may be slowed down. [0008] Document JP-09053187 is related to steel sheets coated by hot dip aluminizing. According to this document, an oxide film is formed on the sheet in a heating stage. Then, an oxide film of Fe and Al, or Fe, Al and one or ≥two kinds among Si, Mn and Mg is formed on the surface of the steel to be plated, after which an alloy plating layer of Al or Al and one or ≥two kinds among Si, Mn and Mg is formed on the surface.

Aims of the invention

[0009] The present invention aims to provide a steel sheet coated with an aluminium-based coating, which overcomes the drawbacks of the prior art. Preferably, said sheet has an excellent formability, and is free from surface aspect defects . The invention equally aims to provide 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

[0010] The present invention is related to products and processes such as described in the appended claims .

[0011] 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. [0012] The metal oxide can be thermodynamically inert with respect to aluminium at the temperature of the molten metal bath, normally between 500 and 800 ⁰ C. Examples of 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 .

[0013] 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 : 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 . [0014] 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.

[0015] Based on the previous paragraphs, it should be clearly understood that in the present description, a metal oxide which is ^λ stable with respect to molten aluminium' is defined as : ^x thermodynamically inert with respect to molten aluminium OR exhibiting slow kinetics in its reaction with molten aluminium, so that no substantial reaction occurs between the metal oxide and the aluminium during a brief immersion time of a few seconds, typically between 3s and 9s, in a bath comprising molten Al' , typically at a bath temperature between 500 ⁰ C and 800 ⁰ C. It is specifically pointed out that the scope of the invention includes metal oxides (e.g. Cr-oxides) who are not regarded as inert with respect to molten aluminium but who exhibit 'slow' kinetics. The term ^N slow' is defined by the explicit reference to the time of immersion. For a given immersion time (for example 3s) , any metal oxide which does not substantially react with molten aluminium,

even though it is not thermodynamically inert with respect to molten aluminium, falls within the scope of the present invention.

[0016] 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.

[0017] A preferred embodiment of the invention therefore involves a steel sheet, with the stable metal oxide layer described above, and in contact therewith an intermediate layer of iron-aluminium alloy and in contact with said intermediate layer, a layer of substantially pure aluminium and inevitable impurities (corresponds to the Alupur ^® -coating) , wherein the intermediate layer has 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.

[0018] 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.

[0019] 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 lμm.

[0020] It is pointed out here that the present invention in the first place provides a solution to the problem of intermediate Fe-Al layers of excessive thickness. In many prior art techniques, this problem was solved by adding silicon to the hot dip bath, as described above. The present invention offers a solution to this problem, which allows to control the thickness of the intermediate layer without requiring silicon in the bath. This is a beneficial result of the present invention, but it must be emphasized that the absence of silicon as such is not an essential feature of the invention : the invention can be applied even with a bath comprising silicon. In the latter case, the thickness of the intermediate layer will still be controlled and limited as a consequence of the stable oxide layer and iron oxide layer, while this control/limitation may be further strenghtened by the presence of silicon in the bath. At the same time, silicon will make the coating more brittle, and is thus not a desired element in the preferred embodiment. So the only essential features of the invention are the presence of the stable oxide layer and of the iron oxide layer applied on top of that . The presence

or absence of silicon is not an essential feature, but merely serves to define the preferred embodiment (no silicon) and a less-preferred second embodiment (with silicon) . [0021] 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 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.

The preferred immersion time is between 3s and 9s. The process of the invention is preferably a continuous process, wherein the line speed during the immersing step is situated between 80 and 120m/min.

[0022] 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-he,ating 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.

[0023] 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 ⁰ C, the iron oxide will be reduced from 350 ⁰ C onwards, and at a dew point of +10 ⁰ C, from 455 ⁰ C onwards. The typical dew point range in an annealing furnace is between -40 ⁰ C and -30 ⁰ C. The typical annealing temperature is between 750 ⁰ C and 850 ⁰ C and the typical overageing temperature is between 400 ⁰ C and 500 ⁰ C. Iron oxide will thus always be reduced, already during the last part of the heating up to annealing temperature . [0024] 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.

[0025] The gas phase deposition of the metal oxide layer preferably takes place at a temperature between 35O ⁰ C and 550 ⁰ 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 lOOOnm thick.

[0026] 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 FeAl ₃ layer + a Fe ₂ Al ₅ 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.

[0027] 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.

[0028] 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. [0029] 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 (FeAl ₃ layer + Fe ₂ Al ₅ 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.

[0030] 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. lμm. On the other hand, the presence of Si may make the aluminium-based coating more brittle . We refer to the remarks made above with respect to the essential features of the present invention. [0031] The invention is equally related to the use of steel sheet according to the invention for the production of 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. [0032] Both the steel sheets and the process according to the invention provide an advantage over the classic Al-based coated sheets (Alupur, Aluzinc, Alusi) and their production methods, in that sheets according to the

invention allow to control the intermediate Fe-Al layer to lower thicknesses, without requiring the presence of silicon in the Al-based bath. With respect to document JP2002-226959, the method of the present invention involves the application of a 'stable' (see definition above) metal oxide layer and of an Fe-oxide layer on said stable oxide layer. The intermediate product and the final product of JP2002-226959 differ from the corresponding products of the present invention. The intermediate product of JP-2002- 226959 comprises a metal-oxide layer, e.g. a Cr-oxide layer, without an additional Fe-oxide layer on top of that Cr-oxide layer. The final product of JP2002-226959 does not comprise a metal oxide layer anymore (this layer has been reduced to form alumina) , underneath the intermediate layer and the Al-based coating. Furthermore, the metal oxide layer produced according to JP2002-226959 is said to be reduced by molten aluminium, whereas according to the present invention, the metal oxide layer is either thermodynamically inert or exhibiting sufficiently slow kinetics in its reaction with molten aluminium, so that the metal oxide layer stays intact. It is clear that the mechanism by which JP2002-226959 controls the intermediate layer thickness is fundamentally different from the present invention. According to JP2002-226959 , an alumina layer is formed, to slow down diffusion of Fe-atoms into the Al- bath. The present invention prohibits said diffusion by providing a metal oxide layer which does not substantially react with the Al-bath. The Fe-Al intermediate layer is obtained by providing an additional Fe-oxide layer on top of the stable oxide layer. As stated above, the advantage of the present technique lies in the use of stable metal oxides which do not substantially react with the molten aluminium, and thus will not contaminate the bath. Because

of the different mechanism, immersion times in the bath can be limited to a few seconds .

[0033] With respect to JP09053187, the final product according to that document comprises an oxide film formed by heating a steel sheet (so presumably and Fe-oxide film) , an oxide film of Fe and Al or Fe, Al and another element, and an Al alloy plating layer. The Fe-oxide film formed by heating is not a ^λ stable' oxide layer in the sense of the present invention, nor is an oxide film of Fe and Al equal to an intermediate Fe-Al layer.