EP2841615B1 - Method for manufacturing an oiled znalmg coated sheet and corresponding sheet - Google Patents

Description

Below is described a sheet comprising a steel substrate having two faces each coated with a metal coating comprising zinc, magnesium and aluminum.

Such sheets are more particularly intended for the manufacture of parts for the automotive industry, without being limited thereto.

Metallic coatings essentially comprising zinc and aluminum in small proportion (typically of the order of 0.1% by weight) are traditionally used for their good protection against corrosion. These metallic coatings are now in competition with coatings including zinc, magnesium and aluminum.

Such metallic coatings will be generally designated here under the term of zinc-aluminum-magnesium coatings or ZnAIMg.

The addition of magnesium significantly increases the corrosion resistance of these coatings, which can make it possible to reduce their thickness or to increase the guarantee of protection against corrosion over time.

Requirement US 2010/055344 describes a ZnAIMg coated steel sheet subjected to skin-pass steps, application of an oil, alkaline cleaning then application of a primer and a lacquer. Requirement JP 2003/013192 describes a steel sheet coated with ZnAIMg subjected to a skin-pass and then to the application of an oil. Requirement JP 2007/131906 describes a process for the preparation of sheets coated with a ZnAIMg coating and varnished comprising the steps consisting in supplying steel sheets coated with quenching by a ZnAIMg coating, subjecting them to polishing, oxidizing the surface by immersion in an alkaline solution, optionally immerse in a sulfuric acid solution containing Co and / or Ni ions, possibly chromating, then applying a varnish.

Sheet metal coils with such surface coatings can sometimes remain in storage sheds for several months and must not see this surface deteriorate by the appearance of surface corrosion, before being shaped by the end user. In particular, no corrosion initiation should appear whatever the storage environment, even in the event of exposure to the sun and / or to a humid or even salty environment.

Standard galvanized products, that is to say whose coatings mainly consist of zinc and aluminum in low proportion, are also subject to these stresses and are coated with a protective oil which is generally sufficient to provide corrosion protection during storage.

However, the present inventors have noted, with the ZnAIMg coated sheets, phenomena of dewetting of the protective oil and of tarnishing, in particular of the entire surface which is no longer covered with oil.

An object of the invention is to improve the temporary protection of sheets with ZnAIMg coatings.

To this end, the invention firstly relates to a method according to claim 1.

The method may also include the features of claims 2 to 14, taken in isolation or in combination.

The application describes a sheet 1 having two faces 5 each coated with a metallic coating 7 comprising zinc, aluminum and magnesium and with a layer of oil, the metallic coatings 7 comprising between 0.1 and 20% by weight of aluminum and between 0.1 and 10% by weight of magnesium, the sheet being capable of being obtained by the process described below.

• la figure 1 est une vue schématique en coupe illustrant la structure d'une tôle obtenue par le procédé décrit ci-après, et
• les figures 2 et 3 montrent des résultats d'analyse par spectroscopie XPS des surfaces extérieures des revêtements métalliques,
• la figure 4 est un cliché illustrant le phénomène de démouillage; et
• la figure 5 présente des courbes illustrant les résultats de tests de vieillissement en exposition naturelle sous abri effectués sur différentes éprouvettes de tôles traitées selon l'invention ou non traitées.

The invention will now be illustrated by examples given for information, and not limitation, and with reference to the appended figures in which:

• the figure 1 is a schematic sectional view illustrating the structure of a sheet obtained by the process described below, and
• the figures 2 and 3 show results of analysis by XPS spectroscopy of the exterior surfaces of metallic coatings,
• the figure 4 is a photo illustrating the dewetting phenomenon; and
• the figure 5 presents curves illustrating the results of aging tests in natural exposure under shelter carried out on different test specimens of sheets treated according to the invention or not treated.

Sheet 1 of the figure 1 comprises a steel substrate 3 covered on each of its two faces 5 by a metallic coating 7.

It will be observed that the relative thicknesses of the substrate 3 and of the coatings 7 covering it have not been observed on the figure 1 to facilitate representation.

The coatings 7 present on the two faces 5 are similar and only one will be described in detail below.

The coating 7 generally has a thickness less than or equal to 25 μm and aims conventionally to protect the substrate 3 against corrosion.

The coating 7 comprises zinc, aluminum and magnesium. It is particularly preferred that the coating 7 comprises between 0.1 and 10% by weight of magnesium and between 0.1 and 20% by weight of aluminum.

More preferably, the coating 7 comprises more than 0.3% by weight of magnesium or even between 0.3% and 4% by weight of magnesium and / or between 0.5 and 11% or even between 0.7 and 6% by weight of aluminum, or even between 1 and 6% by weight of aluminum.

Preferably, the Mg / Al mass ratio between the magnesium and the aluminum in the coating 7 is strictly less than or equal to 1, preferably strictly less than 1, or even strictly less than 0.9.

To make the sheet 1, one can for example proceed as follows.

Using a substrate 3 obtained for example by hot rolling then cold. The substrate 3 is in the form of a strip which is passed through a bath to deposit the coatings 7 by hot dipping.

The bath is a bath of molten zinc containing magnesium and aluminum. The bath can also contain up to 0.3% by weight of each of the optional addition elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.

These various elements can make it possible, among other things, to improve the ductility or the adhesion of the coatings 7 on the substrate 3. Those skilled in the art who know their effects on the characteristics of the coatings 7 will know how to use them as a function of the complementary aim sought . The bath may finally contain residual elements from the feed ingots or resulting from the passage of the substrate 3 in the bath, such as iron at a content of up to 5% by weight and generally between 2 and 4% by weight .

After depositing the coatings 7, the substrate 3 is for example drained by means of nozzles projecting a gas on either side of the substrate 3. The coatings 7 are then allowed to cool in a controlled manner.

The strip thus treated can then be subjected to a so-called skin-pass step which makes it possible to work harden it so as to erase the bearing of elasticity, to fix the mechanical characteristics and to confer on it a roughness suitable for subsequent operations such as sheet metal. must undergo.

The means of adjusting the skin-pass operation is the rate of elongation which must be sufficient to achieve the objectives and minimum to maintain the capacity for subsequent deformation. The elongation rate is usually between 0.3 to 3%, and preferably between 0.3 and 2.2%.

The outer surfaces 15 of the coverings 7 are then oiled to provide temporary protection. The oils used can conventionally be Quaker or Fuchs oils and the grammage of the layers of oils deposited on each external surface 15 is for example less than or equal to 5 g / m ² . The layers of oil deposited were not shown on the figure 1 .

The sheet 1 thus obtained can be wound before being cut, possibly shaped and assembled with other sheets 1 or other elements by users.

Analyzes by XPS spectroscopy (X ray Photoemission Spectroscopy) of the outer surfaces 15 of the coatings 7 revealed the preponderant presence of magnesium oxide or magnesium hydroxide, even when the coatings 7 have aluminum and similar magnesium contents .

However, in the usual coatings essentially comprising zinc and aluminum in small proportion, the external surfaces of the metallic coatings are covered with a layer of aluminum oxide, despite the very low aluminum content. For similar magnesium and aluminum contents, one would therefore have expected to find predominantly aluminum oxide.

XPS spectroscopy has also been used to measure the thickness of the magnesium oxide or magnesium hydroxide layers present on the exterior surfaces 15. It appears that these layers have a thickness of a few nm.

It will be noted that these analyzes by XPS spectroscopy were carried out on sheet metal samples 1 which had not been subjected to corrosive environments. The formation of magnesium oxide or magnesium hydroxide layers is therefore linked to the deposition of coatings 7.

The figures 2 and 3 illustrate respectively the spectra of the elements for the energy levels C1s (curve 17), O1s (curve 19), Mg1s (curve 21), Al2p (curve 23) and Zn2p3 (curve 25) during an analysis by spectroscopy XPS. The corresponding atomic percentages are plotted on the ordinate and the depth of analysis on the abscissa.

The sample analyzed on the figure 2 corresponds to coatings 7 comprising 3.7% by weight of aluminum and 3% by weight of magnesium and subjected to a conventional skin-pass step with an elongation rate of 0.5% while the sample of the figure 3 has not been subjected to such a step.

On these two samples, it can be estimated from the analyzes by XPS spectroscopy that the thickness of the magnesium oxide or magnesium hydroxide layers is approximately 5 nm.

It thus appears that these layers of magnesium oxide or magnesium hydroxide are not removed by the conventional skin-pass stages, nor, moreover, by conventional alkaline degreasing and conventional surface treatments.

The inventors have also found that ZnAIMg coated sheets have a low ability to be wetted with oil. This visually translates into a deposition of protective oil in the form of droplets while it is continuous or film-forming on conventional galvanized coatings.

The inventors have also observed phenomena of dewetting of the deposited oil, so that certain zones are no longer covered with oils. Such an area is identified by the reference 41 on the figure 4 . Temporary protection is therefore heterogeneous.

In addition, tarnishing phenomena, linked or not to dewetting, may appear after a few weeks under certain storage conditions.

The inventors have finally found that these drawbacks could be reduced or even eliminated, and the temporary protection improved by including in the process for producing a sheet 1 a step of altering the layers of magnesium oxide or magnesium hydroxide present. on the outer surfaces 15 of the coatings 7, before applying the oil.

This alteration step can be carried out by the means described in claim 1, such as, for example, the application of mechanical forces.

Such mechanical forces are applied by a leveler, a brushing device or a blasting device.

These mechanical forces may have the function of altering, due to their sole action, the layers of magnesium oxide or of magnesium hydroxide. Thus, the brushing and shot blasting devices can remove all or part of these layers.

Similarly, a leveler, which is characterized by the application of plastic deformation by bending between rollers, can be adjusted to deform the sheet which passes through it enough to create cracks in the layers of magnesium oxide or hydroxide magnesium.

The application of mechanical forces to the external surfaces 15 of the metallic coatings 7 can be combined with the application of an acid solution or the application of a degreasing, for example based on an alkaline solution, on the surfaces exterior 15.

The acid solution has for example a pH of between 1 and 4, preferably between 1 and 3.5, preferably between 1 and 3, and more preferably between 1 and 2. This solution can comprise, for example hydrochloric acid , sulfuric acid or phosphoric acid.

The duration of application of the acid solution can be between 0.2 s and 30 s, preferably between 0.2 s and 15 s, and more preferably between 0.5 s and 15 s, depending on the pH of the solution, when and how it is applied.

This solution can be applied by immersion, sprinkling or any other system. The temperature of the solution can for example be room temperature or a whole other temperature and subsequent rinsing and drying steps can be used.

More generally, the layers of magnesium oxide or magnesium hydroxide can be altered by applying an acid solution and without applying mechanical forces.

The purpose of the possible degreasing step is to clean the external surfaces 15 and therefore to remove the traces of organic dirt, metallic particles and dust.

Preferably, this step does not modify the chemical nature of the exterior surfaces 15, with the exception of the alteration of a possible layer of surface aluminum oxide / hydroxide. Thus, the solution used for this degreasing step is non-oxidizing. No magnesium oxide or magnesium hydroxide is therefore formed on the outer surfaces during the degreasing step and more generally before the oil application step.

If a degreasing step is used, it occurs before or after the step of applying the acid solution. The possible degreasing step and the step of applying the acid solution take place before a possible surface treatment step, that is to say a step making it possible to form layers (not shown) on the outer surfaces. ) improving the resistance to corrosion and / or the adhesion of other layers subsequently deposited on the external surfaces 15.

Such a surface treatment step comprises the application to the exterior surfaces 15 of a surface treatment solution which reacts chemically with the exterior surfaces 15. In certain variants, this solution is a conversion solution and the layers formed are conversion layers.

Preferably, the conversion solution does not contain chromium. It can thus be a solution based on hexafluorotitanic or hexafluorozirconic acid.

In the case where the application of mechanical forces is combined with the application of an acid solution, the mechanical forces will preferably be applied before the acid solution or when it is present on the external surfaces 15 to promote the action of the acid solution.

In this case, the mechanical forces may be less intense.

In a variant, the step of applying the acid solution and the step of surface treatment are combined.

In the latter case, the surface treatment solution is acidic. In this case in particular, the pH can be strictly greater than 3, in particular if the surface treatment solution is applied at a temperature above 30 ° C.

In order to illustrate the invention, various tests have been carried out and will be described by way of nonlimiting example.

The tests were carried out with a sheet 1 of which the substrate 3 is a steel covered with coatings 7 comprising 3.7% aluminum and 3% magnesium, the remainder consisting of zinc and impurities inherent in the process. These coatings have thicknesses of approximately 10 μm. Sheet 1 samples were oiled with Fuchs 4107S oil and a grammage of 1g / m ² .

As summarized in Table 1 below, certain samples had previously been subjected to an alkaline degreasing and / or to the application of an acid solution. In the latter case, the nature of the acid, the pH of the solution and the duration of application are indicated. The acid solutions were at room temperature. The samples, once oiled, were all first observed with the naked eye so as to assess the continuous or discontinuous nature of the layer of oil deposited. Table 1 Sample Alkaline degreasing Type of acid pH Acid exposure time in s Oil distribution observed with the naked eye 1 / / / / Discontinued 2 Gardoclean S5117 at 25g / l at a temperature of 55 ° C, applied for 15s, HCl 2 5 keep on going 3 / HCl 2 5 keep on going 4 / HCl 1 5 keep on going 5 / HCl 2 10 keep on going 6 / H2SO4 2 5 keep on going

The application of an acid solution, possibly combined with an alkaline degreasing, therefore makes it possible to improve the distribution of the oil and therefore the temporary protection. These visual observations were also confirmed by Raman spectroscopy analysis of the external surfaces of the samples.

Samples 1 to 6 were also exposed to the ambient atmosphere for 12 weeks under the conditions described in standard VDA230-213 in order to assess their temporary protection.

Monitoring of the evolution of the tarnishing during the test was carried out via a colorimeter measuring the difference in luminance (measurement of ΔL *). Any difference in luminance greater than 2 during the 12 week period is considered to be detectable with the naked eye and should therefore be avoided.

The results obtained for samples 1 to 6 are shown respectively on the figure 5 where time, in weeks, is plotted on the abscissa and the evolution of | ΔL * | is plotted on the ordinate.

Sample 1 (curve 51 on the figure 5 ), which constitutes the reference, has a ΔL greater than 2, which is consistent with the distribution of the discontinuous oil observed visually.

Samples 2 to 6 (curves 52 to 56 respectively on the figure 5 ) have a variation in luminescence of less than 2, therefore imperceptible to the naked eye.

Claims (14)

1. A method for improving temporary protection of a metal sheet (1) having two faces (5) each coated with a metal coating (7) comprising zinc, between 0.1 and 20 wt% of aluminum, and between 0.1 and 10 wt% of magnesium, the metal coatings (7) having been obtained by hot dipping a steel substrate (3) having two faces (5) in a bath then cooling,
   the process comprising a step of altering layers of magnesium oxide or magnesium hydroxide formed on the outer surfaces (15) of the metal coatings (7) by applying an acid solution on the outer surfaces (15) of the metal coatings (7) and/or by applying mechanical forces applied by a roller leveler, a brushing device or a shot-blasting device on the outer surfaces (15) of the metal coatings (7),
   before depositing a layer of oil on the outer surfaces (15) of the metal coatings (7).
2. The method according to claim 1, wherein the metal coatings (7) comprise between 0.3 and 10 wt%, preferably between 0.3 and 4 wt%, of magnesium.
3. The method according to one of the preceding claims, wherein the metal coatings (7) comprise between 0.5 and 11 wt%, notably between 0.7 and 6 wt%, preferably between 1 and 6 wt%, of aluminum.
4. The method according to one of the preceding claims, wherein the weight ratio between the magnesium and the aluminum in the metal coatings (7) is strictly less than or equal to 1, preferably strictly less than 1, and still more preferably strictly less than 0.9.
5. The method according to one of the preceding claims, the method further comprising a degreasing step by applying an alkaline solution on the outer surfaces (15) of the metal coatings (7).
6. The method according to one of the preceding claims, the method further comprising a surface treatment step by applying a surface treatment solution on the outer surfaces (15) of the metal coatings (7).
7. The method according to one of the preceding claims, wherein the alteration step comprises applying an acid solution on the outer surfaces (15) of the metal coatings (7).
8. The method according to claim 7, wherein the acid solution is applied during a duration comprised between 0.2 s and 30 s, notably between 0.2 s and 15 s, preferably between 0.5 s and 15 s, on the outer surfaces (15) of the metal coatings (7).
9. The method according to one of claims 7 to 8, wherein the acid solution has a pH comprised between 1 and 4, notably between 1 and 3.5, typically between 1 and 3, preferably between 1 and 2.
10. The method according to one of claims 7 to 9, wherein the acid solution is an acid surface treatment solution, preferably an acid conversion solution.
11. The method according to one of claims 7 to 10, wherein mechanical forces are applied on the outer surfaces (15) of the metal coatings (7) before applying the acid solution or when the acid solution is present on the outer surfaces (15).
12. The method according to claim 11, wherein the mechanical forces are applied by passing the metal sheet (1) through a roller leveler.
13. The method according to one of the preceding claims, wherein the alteration step comprises applying mechanical forces by a roller leveler, a brushing device or a shot-blasting device on the outer surfaces (15) of the metal coatings (7).
14. The method according to claim 13, wherein the alteration step comprises applying mechanical forces on the outer surfaces (15) of the metal coatings (7) to crack the layers of magnesium oxide or magnesium hydroxide.

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