EP2841614A1

EP2841614A1 - Method for producing a metal sheet having zn-al-mg coatings, comprising the application mechanical stress to the coatings and an adhesive, and corresponding metal sheet and assembly

Info

Publication number: EP2841614A1
Application number: EP13727381.9A
Authority: EP
Inventors: Joëlle RICHARD; Eric Jacqueson; Audrey LHERMEROULT; Pascale FELTIN; Jean-Michel LEMAIRE; Jean-Michel Mataigne
Original assignee: ArcelorMittal Investigacion y Desarrollo SL
Current assignee: ArcelorMittal SA
Priority date: 2012-04-25
Filing date: 2013-04-25
Publication date: 2015-03-04
Anticipated expiration: 2033-04-25
Also published as: TR201906423T4; EP2841614B1; ES2724539T3; WO2013160569A1; PL2841614T3; HUE044446T2; WO2013160868A1

Abstract

The method comprises at least the following steps: providing a steel substrate (3) having two faces (5) that are coated by quenching the substrate (3) in a bath; at least by applying mechanical stress to the outer surfaces (15) of the metal coatings (7), altering the magnesium hydroxide or magnesium oxide layers formed on said outer surfaces (15); and applying an adhesive (13) locally on at least one outer surface (15) of a metal coating (7).

Description

The present invention relates to a sheet comprising a steel substrate having two faces, each coated with a coating, and a sheet of metal having two faces each coated with a coating. 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.

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

Such metal coatings will generally be referred to herein as zinc-aluminum-magnesium or ZnAIMg coatings.

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

In the automotive industry in particular, the sheets are frequently assembled by means of adhesives for the production of certain parts of the vehicles, such as, for example, door sills.

These adhesives may be structural, structural (for example "crash" type) or semi-structural adhesives, sealing mastics or setting mastics that are of various chemical natures, such as epoxy, polyurethane or rubber.

In the automotive industry, the combination of a sheet with an adhesive is usually evaluated by means of a tensile test on a specimen formed of two tongues of the sheet, these tongues being glued on a part of their surface by 'adhesive.

On this occasion, the adhesion of the adhesive to the sheet is evaluated on the one hand by measuring the tensile stress at break and on the other hand the compatibility of the adhesive and the sheet by visual determination of the nature of the break.

We can observe on this occasion three main types, or facies, of rupture:

- the cohesive rupture, when the rupture takes place in the thickness of the adhesive, the adhesive rupture (see FIG. 5), when the rupture takes place at one of the interfaces between the tongues and the adhesive,

the superficial cohesive rupture (see FIG. 6), when the rupture takes place in the adhesive in the vicinity of an interface between the tongues and the adhesive.

In the automotive industry, it is sought to avoid adhesive fractures that translate poor compatibility of the adhesive with the sheet.

However, the tensile tests show breaks mostly adhesive when using some adhesives usual for the automotive industry on ZnAIMg coated sheets. As a result, up to 100% of adhesive breaks can be observed with some ZnAIMg coatings and some adhesives.

Such proportions of adhesive breakage are not acceptable for automobile manufacturers, which could limit the use of these new ZnAIMg coatings for certain applications.

An object of the invention is therefore to provide a method of producing a ZnAIMg coated sheet which has a better compatibility with adhesives and therefore limits the risk of adhesive failure.

For this purpose, the invention firstly relates to a method according to claim 1. The method may also include the features of claims 2 to 22, taken alone or in combination.

The invention also relates to a sheet according to claim 23 and to an assembly according to claim 24.

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

FIG. 1 is a diagrammatic sectional view illustrating the structure of a sheet obtained by a method according to the invention, and

FIGS. 2 and 3 show XPS spectroscopic analysis results of the outer surfaces of the metal coatings,

FIG. 4 is a schematic view illustrating a specimen used for a tensile test;

- Figures 5 and 6 are views respectively showing a superficial cohesive failure and adhesive failure.

Sheet 1 of FIG. 1 comprises a substrate 3 made of steel coated on each of its two faces 5 by a metal coating 7.

It will be observed that the relative thicknesses of the substrate 3 and the coatings 7 covering it have not been respected in FIG. 1 in order to facilitate the representation. The coatings 7 present on the two faces 5 are similar and only one will be described in detail later.

The coating 7 generally has a thickness less than or equal to 25 μηι and conventionally aims 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 between 0.3% and 4% by weight of magnesium and / or between 0.5 and 11% or even between 0.7 and 6% in weight of aluminum, or even between 1 and 6% by weight of aluminum.

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

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

Using a substrate 3 obtained for example by hot rolling and 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 molten zinc bath containing magnesium and aluminum. The bath may 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 may make it possible, inter alia, to improve the ductility or the adhesion of the coatings 7 to the substrate 3. The skilled person who knows their effects on the characteristics of the coatings 7 will be able to use them according to the complementary aim sought. . The bath may finally contain residual elements coming from the 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 spun by means of nozzles throwing 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 allows it to be hardened so as to erase the elastic bearing, to fix the mechanical characteristics and to give it a roughness adapted to the subsequent operations that the 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 ability to subsequent deformation. The elongation rate is usually between 0.3 to 3%, and preferably between 0.3 and 2.2%.

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

It can, conventionally, be oiled for temporary protection purposes.

As illustrated schematically in Figure 1, an adhesive 13 may be applied locally to an outer surface 15 of a coating 7 to allow for example to assemble the sheet 1 to another sheet and thus constitute a motor vehicle part. The adhesive 13 may be any type of glue or putty conventionally used in the automotive industry.

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

However, in the usual coatings essentially comprising zinc and aluminum in a small proportion, the outer surfaces of the metal coatings are covered with a layer of aluminum oxide, despite the very low aluminum content. For similar levels of magnesium and aluminum, one would have expected to find predominantly aluminum oxide or at least a mixture of magnesium oxides and aluminum.

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

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

FIGS. 2 and 3 respectively show the spectra of the elements for the energy levels C1 s (curve 17), 01 s (curve 19), Mg1 s (curve 21), Al2p (curve 23) and Zn2p3 (curve 25) during of an XPS spectroscopy analysis. The corresponding atomic percentages are plotted on the ordinate and the analysis depth on the abscissa.

The sample analyzed in FIG. 2 corresponds to coatings 7 comprising 3.7% by weight of aluminum and 3% by weight of magnesium and subjected to a stage conventional skin-pass with an elongation rate of 0.5% while the sample of Figure 3 was not subjected to such a step.

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

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

According to the invention, the method of producing sheet metal 1 comprises at least one step of alteration, by application of mechanical forces, of magnesium oxide or magnesium hydroxide layers present on the outer surfaces of the coatings. 7, before possible subsequent application of an adhesive 13.

Such mechanical forces can be applied by a planer, brushing devices, shot blasting ...

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

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

The application of mechanical forces on the outer surfaces 15 of the metal 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 outside 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 may 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 15 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 may for example be room temperature or any other temperature and subsequent rinsing and drying steps may be used. More generally, it is possible to alter the magnesium oxide or magnesium hydroxide layers by applying an acidic solution and without applying mechanical stresses.

The purpose of the degreasing step is to clean the outer surfaces and thus to remove traces of organic dirt, metal particles and dust.

Preferably, this step does not alter the chemical nature of the outer surfaces with the exception of the weathering of any surface aluminum oxide / hydroxide layer. Thus, the solution used for this degreasing step is non-oxidizing. Therefore, magnesium oxide or magnesium hydroxide are not formed on the outer surfaces during the degreasing step and more generally before the step of applying the adhesive 13.

If a degreasing step is used, it intervenes before or after the step of applying the acid solution.

The optional degreasing step and the step of applying the acid solution take place before a possible surface treatment step, that is to say a step for forming on the outer surfaces 15 layers (not shown). ) improving the corrosion resistance and / or adhesion of further layers subsequently deposited on the outer surfaces 15.

Such a surface treatment step comprises applying to the outer surfaces a surface treatment solution which reacts with the outer surfaces to form said layers.

In some embodiments, the surface treatment solution is a conversion solution and the formed layers are conversion layers. Preferably, the conversion solution does not contain chromium. It can thus be a solution based on hexafluorotitanic acid or hexafluorozirconic acid.

In the case where the application of mechanical forces is combined with the application of an acidic solution, the mechanical forces will preferably be applied before the acidic solution or while it is present on the outer surfaces to favor the action of the acid solution.

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

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

In the latter case, it is the surface treatment solution employed that is acidic. In this case in particular, the pH may be strictly greater than 3, especially if the surface treatment solution is applied at a temperature greater than 30 'Ό. In order to illustrate the invention, tensile tests have been carried out and will be described by way of non-limiting example.

As illustrated in FIG. 4, each specimen 17 is prepared in the following manner. Tabs 29 are cut into sheet 1 to be evaluated. These tongues 29 have dimensions of 25 mm per 100 mm. The tabs 29 are glued by a seal 31 of the BM1496V adhesive, which is an epoxy-based "crash" adhesive marketed by Dow Automotive.

This adhesive has been selected because it is one of the adhesives leading most to adhesive breaks.

The test piece 27 thus formed is then brought to the end and held there for 30 minutes.

The tensile test is then performed at an ambient temperature of 23 ° C by imposing a tensile speed of 10mm / min at one tab 29, parallel thereto, while the other tab 29 is attached. The test is continued until the test piece 27 is broken.

At the end of the test, the maximum tensile stress is recorded and the nature of the rupture is visually evaluated.

The tests were carried out with a sheet 1 whose substrate 3 is a 0.8 mm thick IFHR 340 steel covered with coatings 7 comprising 3.7% aluminum and 3% magnesium, the remainder being zinc and impurities inherent in the process. These coatings have thicknesses of approximately 10 μηι. Sheet 1 was also pre-oiled with Quaker 6130 oil and a basis weight of 2.5g / m ² .

As illustrated in Table 1 below, the sheets 1 which have undergone a mechanical treatment of alteration of magnesium oxide or magnesium hydroxide layers promote the appearance of superficial cohesive fractures, unlike the reference sheets for which only adhesive breaks are noted.

The reference sheet 1 had not undergone any mechanical treatment of alteration of magnesium oxide or magnesium hydroxide layers. The sheet 1 referenced test had been subjected to a plane deformation by traction of 10%. Reference Test 1

Tensile stress at 15.5 ± 0.1 24.2 ± 0.3

rupture (in MPa)

Types of rupture 100% RA 25% RCS 75% RA

Table 1 This effect is increased by the application of an acid solution, optionally as a surface treatment, or a degreasing step.

Claims

1. Process for producing a sheet (1), the method comprising at least steps of:

- providing a substrate (3) made of steel having two faces (5) each coated with a metal coating (7) obtained by quenching the substrate (3) in a bath and cooling, each metal coating (7) comprising zinc, between 0.1 and 20% by weight of aluminum and between 0.1 and 10% by weight of magnesium, then

- alteration, at least by mechanical stresses on the outer surfaces (15) of the metal coatings (7), of magnesium oxide or magnesium hydroxide layers formed on the outer surfaces (15), and

- Application of an adhesive (13) locally on at least one outer surface (15) of a metal coating (7).

2. The method of claim 1, wherein the metal coatings (7) comprise between 0.3 and 10% by weight of magnesium.

3. Method according to claim 2, wherein the metal coatings (7) comprise between 0.3 and 4% by weight of magnesium.

4. Method according to one of the preceding claims, wherein the metal coatings (7) comprise between 0.5 and 1 1% by weight of aluminum.

5. The method of claim 4, wherein the metal coatings (7) comprise between 0.7 and 6% by weight of aluminum.

6. The method of claim 5, wherein the metal coatings (7) comprise between 1 and 6% by weight of aluminum.

7. Method according to one of the preceding claims, wherein the mass ratio between magnesium and aluminum in the metal coatings (7) is strictly less than or equal to 1, preferably strictly less than 1, and preferably less than 1, at 0.9.

8. Method according to one of the preceding claims, the method further comprising, before the step of applying the adhesive (13), a degreasing step by applying an alkaline solution on the outer surfaces (15) of the coatings. metal (7).

9. Method according to one of the preceding claims, the method further comprising, before the step of applying the adhesive (13), a step of applying a surface treatment solution on the outer surfaces (15). metal coatings (7) for forming on the outer surfaces (15) layers improving corrosion resistance and / or adhesion.

The method according to one of the preceding claims, the method further comprising, prior to the step of applying the adhesive (13), the application of an acidic solution to the outer surfaces (15) of the metal coatings (7). ).

1 1. A process according to claim 10, wherein the acidic solution is applied for a period of between 0.2 s and 15 s to the outer surfaces (15) of the metal coatings (7).

12. The method of claim 1 1, wherein the acid solution is applied for a period of between 0.2 s and 15 s on the outer surfaces (15) of the metal coatings (7).

13. The method of claim 12, wherein the acid solution is applied for a period between 0.5 s and 15 s on the outer surfaces (15) of the metal coatings (7).

14. Method according to one of claims 10 to 13, wherein the acidic solution has a pH between 1 and 4.

15. The method of claim 14, wherein the acidic solution has a pH of between 1 and 3.5.

The process of claim 15, wherein the acidic solution has a pH of from 1 to 3.

17. The method of claim 16, wherein the acidic solution has a pH of between 1 and 2.

18. The method according to one of claims 10 to 17, wherein the acidic solution is an acidic surface treatment solution.

The method of claim 18, wherein the acidic surface treatment solution is an acidic conversion solution.

20. Method according to one of claims 10 to 19, wherein mechanical forces are applied to the outer surfaces (15) of the metal coatings (7) before application of the acid solution or while the acid solution is present on the surfaces. outside (15).

21. Method according to one of the preceding claims, wherein the mechanical forces are applied by passing the sheet (1) in a planer.

22. Method according to one of the preceding claims, wherein the mechanical forces are adapted to crack the magnesium oxide or magnesium hydroxide layers formed on the outer surfaces (15) of the metal coatings (7).

23. Sheet (1) having two faces (5) each coated with a metal coating (7) comprising zinc, aluminum and magnesium, the metal coatings (7) comprising between 0.1 and 20% by weight of aluminum and between 0.1 and 10% by weight of magnesium, an adhesive (13) being applied locally on at least one outer surface (15) of a coating (7) , the sheet can be made by a method according to one of the preceding claims.

24. Assembly comprising:

a first sheet (1) according to claim 23,

- a second sheet,

the first sheet (1) and the second sheet (1) being assembled via the adhesive (13) of the first sheet (1).