EP1678343A1

EP1678343A1 - A satainless steel strip coated with aluminium.

Info

Publication number: EP1678343A1
Application number: EP04775357A
Authority: EP
Inventors: Anna Hultin Stigenberg
Original assignee: Sandvik Intellectual Property AB
Current assignee: Sandvik Intellectual Property AB
Priority date: 2003-09-05
Filing date: 2004-08-31
Publication date: 2006-07-12
Also published as: SE527393C2; JP2007504364A; CN1846014A; KR20060090804A; SE0302395L; WO2005024093A8; US20070082214A1; WO2005024093A1; SE0302395D0

Abstract

The invention provides a coated stainless steel strip (1) product with a dense and evenly distributed aluminum layer (2) on one side or both sides of said strip. Said layer consists of essentially pure aluminum, the thickness of said layer is maximally 15 µm, the tolerance of said layer is maximally +/- 30 % of the layer thickness, the Cr content of the steel strip substrate is at least 10 %, and that the layer has such a good adhesion so that the coated steel strip can be bent 180° over a radius maximally equal to the thickness (3) of said strip without showing any tendency to flaking or the like. The Al-coated strip product is suitable for applications in environments with high humidity or in wet conditions, such as outdoor life applications, sports and sea-life, household and personal care.

Description

A stainless steel strip coated with aluminium

The present invention relates to a method of manufacturing aluminum-coated stainless steel in a continuous roll-to-roll process, which results in an excellent adhesion of a thin covering layer of aluminum. In particular, it relates to aluminum-coated stainless steel strips, which exhibit an excellent adhesion of a thin layer of aluminum on the steel surface and which are suitable for a cost-efficient and productive manufacturing of components for anti-corrosive applications.

Background of the Invention and prior Art It is known that aluminum coatings can be used in anti-corrosive applications. However, for components in smaller dimensions, which are to be produced in a cost- efficient and productive way, there are difficulties in finding a method that can attain the quality and productivity requirements. For productivity reasons, a roll-to-roll coating process is imperative, and for quality reasons, a thin layer with excellent adhesion is needed. The superior adhesion is required for the functional quality of the final product, but also to enable a cost- efficient and productive manufacturing of components. Thus, a coated strip material with inferior adhesion would cause problems with, e.g., flaking, and this would result in a low yield and also in a disturbance caused by the flakes themselves of the manufacturing process as such, especially if the manufacturing process is in a continuous line.

Moreover, more frequent stops would be needed for quality inspections and for cleaning the process line from flakes. All in all, poor adhesion of the coating would result in a non-acceptable high manufacturing cost and low quality. Known, conventional methods of coating steel with aluminum in a roll-to-roll process are the following: - Cladding the substrate steel material with aluminum foil. The cladding process "metallurgically" bonds metals together, producing a continuous strip. This is a relatively simple and straightforward technology, with a high yield to a low cost. However, the method has some major drawbacks. First of all, there is often a problem with poor adhesion. Further, it is technically difficult to achieve good, uniform thin coatings with cladding techniques .

- Dipping can be used to apply low melting point metals to a substrate material by performing the dipping in a melt bath. One obvious drawback with this method is that aluminum has a rather high melting point (658°C). This results in a high complexity for controlling the process parameters and in a difficulty to achieve an evenly distributed thin covering layer with a good adhesion. There are also some vapor deposition methods that can be used for depositing aluminum. Most methods are batchlike processes, but there are also some continuous processes . One example of a roll-to-roll method making use of electron-beam deposition is disclosed in WO 98/08986, which describes a method of manufacturing ferritic stainless

FeCrAl-steel strips, by bringing about an aluminum coating of a substrate material in a roll-to-roll process. However, the method described in this patent application is optimized for a product suitable for use in a high temperature corrosive environment, thus requiring a material with a good high-temperature strength and also a good high-temperature corrosion resistance, i.e., oxidation resistance. In this context aluminum plays also a role of being an oxide-forming element, which is beneficial for the high-temperature corrosion resistance. This implies that the substrate material be alloyed with rare earth metals, and also that the aluminum coating is made on both sides of the strip. Moreover, this patent application suggests that a homogenization annealing at a temperature of 950-1150 °C is made in connection to the coating, in order to have the aluminum evenly distributed in the ferrite . This means that the final product in this case is not a coated product with an aluminum layer on the surface. Hence, it is rather a FeCrAl strip product with a uniform distribution of the alloying elements, including also aluminum. Further, this means that there are no special requirements on an oxide free interface and as to good adhesion of the layer. There is, e.g., no other cleaning done before the PVD coating step than ordinary wet cleaning by de-ionized water, to take away residuals of oil. Since the role of aluminum is to diffuse into the ferrite, there is also no requirement on any special evenness of the layer. This method, as disclosed in WO 98/08986, can thus not be used for the present invention. One other example of an apparatus used in a continuous vapor deposition process is described in US 4655168, in which a uniform distribution of thickness is achieved by using special control panels inside the vacuum chamber. The example given is for Zn-coating of a mild steel, but mentioned is that also aluminum can be coated in accordance to said invention. The method is however quite different from the present invention. There is for instance a roll over which the strip is guided that is heated to a temperature above melting temperature for the substance to be coated, and in the case of aluminum this would mean above 658 °C. This is a temperature in which structural stability of some stainless steels can be affected negatively. The source of energy for the evaporation to take place is not mentioned and there is also no discussion about any ion etching. There are also no special requirements on an oxide free interface or a good adhesion of the layer. It is described that the layer is uniformly distributed, but no details are given, and no range of tolerances is defined. Also, the system of controlling the distribution of the deposited substance seems to be rather complicated. This method, as described in US 4655168, can thus not be used in the present invention. One further example of aluminum coating using vapor deposition plating is described in US 5429843, in which a substance is applied to the surface of a steel material, in a vacuum atmosphere. The steel material is held at a temperature between 100 and 400 °C to form active spots in the surface to enable required properties, e.g., good adhesion. Also ion beam irradiation is used in connection to the coating process, but is done in the same chamber as the coating. The formed layer is to be used as an adhesive layer in a subsequent painting process. In principle two different combinations of coatings are described in this invention, Al+Zn and Al+Ti. However, in both cases it is shown that coating of essentially pure aluminum can not be used for the intended application. For Al+Zn, a co- evaporation of Al and Zn is done, so as to produce an Al/Zn- coating with a Zn-content of between 3-30% as the optimum. For Al+Ti, a two-layer coating is used to achieve acceptable properties, and with the prerequisite that the layer adjacent to the steel must be the Ti-layer. It is shown that if essentially pure aluminum is coated, problems occur with pitting corrosion starting in pin holes in the coating, and thus also creating a galvanic cell that eventually accelerates corrosion of the steel sheet material. One major difference to the present invention is that the substrate material is a plain steel and not a stainless steel, and also that inhibitors, such as Zn- addition or a Ti-layer, are used to avoid a galvanic corrosion to occur. The method is also significantly different in that the used process is a batch-type coating of steel sheet, and not as in the present invention, a continuous coating in a roll-to-roll process of a stainless strip material. This method, as described in US 5429843, can thus not be used in the present invention. In view of the above, it is an object of the present invention to provide a new roll-to-roll process to accomplish a thin and continuous aluminum coating with excellent adhesion on a stainless steel surface. Moreover, it is an object of the present invention to make possible a cost-efficient and productive manufacturing of components in anti-corrosive applications of the coated material . A further objective of the present invention is to obtain a coating with a thickness as uniform as possible. These and further objects have been achieved in a surprising way by providing a coated steel product according to the features as defined in claim 1. Further preferred embodiments of the present invention are defined in the dependent claims.

Brief Description of the Invention

The present invention relates to a method of manufacturing aluminum-coated stainless steel in a continuous roll-to-roll process, which results in an excellent adhesion of a thin covering aluminum layer. The aluminum-coated stainless steel strips must have such a good adhesion of the thin layer that they are suitable for a cost-efficient and productive manufacturing of components in anti-corrosive applications. The final product, in the form of aluminum-coated strip material, is suitable for uses as an anti-corrosive component in consumer-related applications that are occasionally used in environments with high humidity or in wet conditions. This component of aluminum-coated stainless steel can then protect another metallic part from corrosion by galvanic currents, thus acting as a sacrificial anode. The aluminum layer is deposited by means of electron beam evaporation (EB) in a roll-to-roll process, to an evenly distributed layer with a thickness of preferably less than 15 μm. The substrate material should be a stainless steel with a Cr content above 10% (by weight) and with a strip thickness of usually less than 3 mm. As a first step, the roll-to-roll process may also include an etch chamber, in order to remove the oxide layer that otherwise normally is present on a stainless steel.

Brief Description of the Drawings

Figure 1 shows an illustration of a test specimen in accordance with the present invention, i.e. a coated stainless steel' strip with a thin dense aluminum layer with good adhesion before a test of said adhesion in a 180° bend test over a radius maximally equal to the thickness of said strip. Figure 2 shows an illustration of a test specimen in accordance with the present invention, i.e., a coated stainless steel strip with a thin dense Al layer with good adhesion, and after a bending in a bend test as described in Fig 1. Figure 3 shows a photo of a cross-section of a coated stainless steel strip specimen in thickness 0.3 mm and with a thin coating of 2 μm of aluminum, which has been bent in a 180° bending over a radius of 0.3 mm. There is no tendency at all of any flaking. Figure 4 shows a schematic picture of the roll-to-roll production line according to the invention. Detailed Description of the Invention

Description of the Coating and the Use of the Invention The final product, in the form of an aluminum-coated strip material, is suitable to be used as an anti-corrosive component in consumer-related applications such as outdoor life applications, sports and sea-life applications, household applications and applications for personal care. In principle, these are all applications that occasionally are used in environments with high humidity or in wet conditions. At the same time, these types of applications are often expected to be nice-looking throughout its product lifetime, with a shiny appearance, or just a "high quality" appearance. Dull surfaces, with spots or even rust, are normally not acceptable. To prevent the final product from corroding, it is suitable to have at least one component made of aluminum- coated stainless steel. This component can then protect another metallic part from corrosion by galvanic currents, thus acting as a sacrificial anode. Both one-sided or two- sided coating can be applied, but the advantage of using stainless steel as the substrate material is that a onesided coating is enough from an anti-corrosion point of view, since a stainless material has a good basic corrosion resistance in itself. Also, if the substrate material is made of a steel more noble than the part that is to be protected, the aluminum content that is needed for protecting during the life-time of the critical parts can be reduced to a minimum, which has a positive effect on the cost. One-sided coating is also preferred from a cost perspective . The method described in the present invention is suitable for thin coatings of essentially pure aluminum at a thickness of up to 15 μm, but preferably thinner. An aluminum layer of normally 0.1 to 15, usually 0.1 -12 more normally 0.1 - 10 and preferably 0.1 - 7 or even 0.1-5 μm in total. If thicker layers are to be coated, an optimum in cost versus properties may be achieved by using multi- layers with up to 10 layers, and where each layer is between 0.1 to 8 μm thick, suitably between 0.1 to 6 μm, or more suitably 0.1 to 5 μm, preferably 0.1 to 3 μm and even more preferably 0.1 to 2 μm. The tolerances obtained by EB technique are usually very good. Thus, the tolerances of each layer may be maximally +/- 30% of the layer thickness in strip widths up to 400 mm, normally +/- 20%, and preferably +/- 10%. This means that very tight tolerances can be achieved, which is of benefit for the precision during usage and the quality of the product. The thin layer must also have a good adhesion with regard to the applications and their uses. During usage it is not acceptable that the aluminum starts to flake off. Furthermore, the layer/layers according to the present invention should be able to use without any bonding layer, i.e. should be applied directly on to the substrate. The coating layer should have superior adhesion to the substrate without any bonding layer or bond-coat. An illustration of the good adhesion is that the coated stainless steel strip according to the present invention should be able to be bent 180° over a radius maximally equal to the thickness of said strip without showing any tendency to flaking or the like. (See Fig 1-2) The coating layer should be sufficiently resistant in order to withstand the wear and shear exerted by the treated material, on the other hand it should not be too thick, due to primarily economical reasons. For anti- corrosive applications, the ratio between the thickness of the coating and the substrate material should be between 0.1% to 12%, normally 0.1 to 10% and usually 0.1 to 7.5% but most preferably between 0.1-5%. In variation to the above-described coating of a thin covering aluminum layer, also a combination of aluminum coating with coatings of other metallic elements such as Ti, Ni and/or Mo, may be done. By using the multiple layer system of up to 10 multiple layers, a coating consisting of a combination of several layers of different metallic coatings, and with aluminum in at least one of the layers, can even further enhance the possibility to tailor-made the corrosion properties, and is preferable to use in applications intended for use in very severe environments. The final product in the form of a coated strip material in accordance to the present invention should also be capable of being readily manufactured to components suitable for applications as described above, in a cost- efficient and productive manufacturing process, including forming steps such as deep-drawing, punching, stamping, or the like. [cf. Figures 1 and 2]

Description of the substrate material to be coated The material to be coated should have a good basic corrosion resistance, preferably with a chromium content of more than 12%, or at least 11% or minimum 10%, depending on the composition of the other alloying elements. Materials that are suitable to use are alloys such as ferritic chromium steels of the type AISI 400-series, austenitic stainless steels of the type 300-series or precipitation hardenable stainless steels, such as the alloy disclosed in WO 93/07303. Also other stainless grades such as e g the AISI 200-series, may be used. The coating method may be applied on any kind of product made of said types of stainless steel alloys and in the form of strip, bar, wire, tube, foil, fiber etc., preferably in the form of strip or foil, that have good hot workability and also can be cold-rolled to thin dimensions. The alloy should also readily be manufactured to components in a productive manufacturing process including steps such as forming, deep drawing, punching, stamping, or the like. The thickness of the strip substrate material is usually between 0.015 to 3 mm, normally between 0.03-2.0 mm and preferably between 0.05 to 1.5 mm, and even more preferably between 0.05 mm to 1.0 mm. The width of the substrate material depends on if the coating is made before or after any foreseen slitting operation. Further, said width should preferably be selected to be a width suitable for further manufacturing to the final width of the component intended to be used in an anti-corrosive application. In principle, the width of the substrate material is therefore between 1 to 1500 mm, suitably 1 to 1000 mm, or preferably 1 to 500 mm, or even more preferably between 5 and 500 mm. The length of the substrate material is suitably between 10 and 20 000 m, preferably between 100 and 20 000 m. The substrate material should have a compos"ition suitable for use in environments with high humidity or wet conditions. This means usually a stainless steel of the type; Ferritic stainless steel, or an Austenitic stainless steel, or a Duplex stainless steel, or a Hardenable chromium steel, and with a composition of essentially:

0.001 - 1 % C, 10-26 % Cr, 0,01-8% Mn, 0.01-2% Si, 0.001- 25 % Ni, up to 6% Mo, 0.001-0.5%N, up to 1.5% Al , up to 2% Cu and rest essentially Fe; or Precipitation hardenable stainless steels of: 0.001-0.3 % C, 10-16% Cr, 4-12 % Ni , 0.1-1.5 % Ti , 0.01-1.0% Al , 0.1-6 % Mo, 0.001-4% Cu, 0.001-0,3 % M, 0.01-1.5% Mn, 0.01-1.5% Si, rest essentially Fe . Description of the Coating Method A variety of evaporation methods for the application of the coating media and the coating process may be used as long as they provide a continuous uniform and adherent layer. As exemplary methods can be mentioned chemical vapor deposition (CVD) , metal organic chemical vapor deposition (MOCVD) , physical vapor deposition (PVD) such as sputtering and evaporation by resistive heating, by electron beam, by induction, by arc resistance or by laser deposition methods, but for the present invention especially electron beam evaporation (EB) is preferred for the deposition. Optionally, the EB evaporation can be plasma activated to even further ensure good quality coatings of dense layers. For the present invention, it is a pre-requisite that the coating method is integrated in a roll-to-roll strip production line. The aluminum layer is then deposited by means of electron beam evaporation (EB) in a roll-to-roll process. If multi-layers are needed, the formation of them can be achieved by integrating several EB deposition chambers in-line. The deposition of aluminum should be done under reduced atmosphere at a maximum pressure of 0,01 mbar with no addition of any reactive gas to ensure pure aluminum films. The coating process according to the invention is performed at a rate of at least 5 meters per minute, preferably at least 8 m/min, or more preferably, at a rate of at least 10 m/min. To enable a good adhesion, different types of cleaning steps are used. First of all, the surface of the substrate material should be cleaned in a proper way to remove oil residues, which otherwise may negatively affect the efficiency of the coating process and the adhesion and quality of the coating layer. Moreover, the very thin native oxide layer that normally always is present on a stainless steel surface must be removed. This can preferably be done by including a pre-treatment of the surface before the deposition of aluminum. Therefore, in this roll-to-roll production line, the first production step is preferably an ion-assisted etching of the metallic strip surface to achieve good adhesion of the first covering aluminum layer [see Fig. 4] . As an alternative also pickling in e g HF may be used to remove oxides.

Description of a Preferred Embodiment of the Invention Two examples of embodiments of the invention will now be described in more detail . One example is based on a substrate material of type AISI 430, and the other is based on a substrate material of type AISI 301. The nominal chemical compositions of the substrate materials are:

AISI 430: max 0.12% C, max 1% Si, max 1% Mn, 16.0-18.0% Cr and rest is essentially Fe . AISI 301: max 0.15% C, max 1% Si, max 1% Mn, 16.0-18.0% Cr, 6.0-8.0 % Ni and rest essentially Fe Firstly, the substrate materials are produced by ordinary metallurgical steel making to a chemical composition as described above. They are afterwards hot- rolled down to an intermediate size, and thereafter cold- rolled in several steps with a number of recrystallization steps between said rolling steps, to a final thickness of 0.3 mm and a width of maximum 400 mm. The surface of the substrate material is then cleaned in a proper way to remove oil residuals from the rolling. Thereafter, the coating process takes place in a continuous process line, starting with decoiling equipment. The first step in the roll-to-roll process line can be a vacuum chamber or an entrance vacuum lock followed by an etch chamber, in which ion-assisted etching takes place in order to remove the thin oxide layer on the surface of the stainless substrate material. The strip then enters into the E-beam evaporation chamber (s) in which aluminum deposition takes place. An aluminum layer of normally 0.1 up to 15 μm is deposited, the preferred thickness depending on the application. In the two examples described here, a thickness of 2 μm is deposited by using one E-beam evaporation chamber. After the EB evaporation, the coated strip material passes through the exit vacuum chamber or exit vacuum lock before it is being coiled on to a coiler. The coated strip material can now, if needed, be further processed by, for example, rolling or slitting, to obtain the preferred final dimension for the manufacturing of components. The final product as described in the two examples, i.e., a coated 301 and 430 strip material, respectively, in a strip thickness of 0.3 mm and with a thin covering aluminum layer of 2 μm, has a very good adhesion of the coated layer and is thus suitable to be used in a cost- efficient and productive manufacturing of components in anti-corrosive applications. The good adhesion of the layers is further described in Figures 1-3. A substrate material of a stainless steel strip 1 that has been coated with a thin covering layer 2 so as to produce a coated strip product in accordance with the present invention, is put on to a support 4 with a shaped top that has a radius 5 that is maximally equal to the thickness 3 of said strip. A bend test is then performed in a way that bends said strip 180° over the radius 5 maximally equal to the thickness of said strip and the bending continues until the strip ends meet 6. When the bending has been completed in such a bend test, the test specimen is investigated and especially the quality of the layer after bending 7 and the quality of the substrate after bending 8 and the adhesion between said layer and substrate. The test specimens in accordance with the examples described here, do not show any tendency to any flaking, or the like. This is also shown in the picture in Fig 3, which is a photo taken of a cross-section of a test-specimen tested in a bend test as described in Fig 1- 2. The cross section of the sample in the photo is taken where the bending has been most severe, i.e. in the middle of the bend, 9. The roll-to-roll electron beam evaporation process referred to above is illustrated in Figure 4. The first part of such a production line is the uncoiler 13 within a vacuum chamber 14, then the in-line ion assisted etching chamber 15, followed by a series of EB evaporation chambers 16, the number of EB evaporation chambers needed can vary from 1 up to 10 chambers, this to achieve a multi-layered structure, if so desired. All the EB evaporation chambers 16 are equipped with EB guns 17 and water-cooled copper crucibles 18 for the evaporation. After these chambers comes the exit vacuum chamber 19 and the recoiler 20 for the coated strip material, the recoiler being located within vacuum chamber 19. The vacuum chambers 14 and 19 may also be replaced by an entrance vacuum lock system and an exit vacuum lock system, respectively. In the latter case, the uncoiler 13 and the coiler 20 are placed in the open air.

Claims

1. A coated stainless steel strip product with a dense and evenly distributed layer on one side or both sides of said strip characterized in that said layer consists of essentially pure aluminum and is applied directly on to the steel strip, the thickness of said layer is maximally 15 μm, the tolerance of said layer is maximally +/- 30% of the layer thickness, the Cr content of the steel strip substrate is at least 10%, and that the layer has such a good adhesion so that the coated steel strip can be bent 180° over a radius maximally equal to the thickness of said strip without showing any tendency to flaking or the like.

2. Product according to claim 1 characterized in that the thickness of the strip substrate is between 0,015 mm and 3 , 0 mm.

3. Product according to claim 1 or 2 , characterized in that it is made of a substrate of ferritic stainless steel, hardenable chromium steel, austenitic stainless steel, duplex stainless steel, or precipitation hardenable stainless steel .

4. Product according to any of preceding claims, characterized in that the layer has a multi-layer constitution of up to 10 layers.

5. Product according to claim 4 characterized in that each individual layer has a thickness of between 0.1 to 8 μm.

6. Product according to claim 5, characterized in that the layer has a multi-layer constitution of individual layers of different metallic coatings, such as Al, Ni, Ti,

Mo, and where at least one layer consists of essentially pure aluminum.

7. A product according to any of the preceding claims 1-6, characterized in that it is suitable for use in anti- corrosive applications acting as a sacrificial anode.

8. A product according to any of the preceding claims 1-7, characterized in that it is suitable for cost efficient and productive manufacturing of anti-corrosive components in applications occasionally used in environments with high humidity or in wet conditions, such as outdoor life applications, sports and sea-life applications, household applications and applications for personal care .

9. Method of manufacturing a coated stainless steel strip product according to any of the preceding claims 1-8, characterized in that said product is produced in a continuous roll-to-roll process included in a strip production line, with a minimum strip speed of at least 5 meters per minute, using electron beam evaporation comprising an etch chamber in-line.