EP2841614B2 - Method for manufacturing a znalmg coated sheet comprising the application of mechanical forces on the coating and corresponding sheet - Google Patents

Description

The present invention relates to a sheet comprising a steel substrate having two faces each coated with a metallic coating comprising zinc, magnesium and aluminum.

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

Metallic coatings comprising mainly zinc and aluminium in small proportions (typically of the order of 0.1% by weight) are traditionally used for their good protection against corrosion. These metallic coatings are now competing in particular with coatings comprising zinc, magnesium and aluminium.

Such metallic coatings will be generally referred to herein as zinc-aluminum-magnesium or ZnAlMeg coatings.

US 2011/0008644 discloses a method for preparing ZnAlMg coated sheets and joining them with a second sheet using an adhesive.

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

In the automotive industry in particular, sheet metal is frequently assembled using adhesives to produce certain parts of vehicles, such as, for example, door sills.

These adhesives are chosen from structural, reinforced structural (for example, “crash” type) or semi-structural adhesives, sealing mastics or even packing mastics which are of various chemical natures, such as epoxy, polyurethane or rubber.

In the automotive industry, the bond between a sheet metal and an adhesive is usually assessed by means of a tensile test on a specimen formed of two tabs of the sheet metal, these tabs being bonded on part of their surface by the adhesive.

On this occasion, the adhesion of the adhesive to the sheet metal is assessed on the one hand by measuring the tensile stress at break and on the other hand the compatibility of the adhesive and the sheet metal by visually determining the nature of the break.

• la rupture cohésive, lorsque la rupture a lieu dans l'épaisseur de l'adhésif,
• la rupture adhésive (voir figure 5), lorsque la rupture a lieu à une des interfaces entre les languettes et l'adhésif,
• la rupture cohésive superficielle (voir figure 6), lorsque la rupture a lieu dans l'adhésif au voisinage d'une interface entre les languettes et l'adhésif.

On this occasion, we can mainly observe three types, or facies, of rupture:

• cohesive failure, when the failure occurs within the thickness of the adhesive,
• adhesive rupture (see figure 5 ), when the breakage occurs at one of the interfaces between the tabs and the adhesive,
• superficial cohesive failure (see figure 6 ), when the rupture occurs in the adhesive in the vicinity of an interface between the tabs and the adhesive.

In the automotive industry, we seek to avoid adhesive failures which reflect poor compatibility of the adhesive with the sheet metal.

However, tensile tests show that the fractures are predominantly adhesive when using certain adhesives commonly used in the automotive industry on ZnAlMg coated sheets. Up to 100% adhesive fractures can be observed with certain ZnAlMg coatings and certain adhesives.

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

An aim of the invention is therefore to propose a method for producing a ZnAlMg coated sheet which has better compatibility with adhesives and therefore limits the risks of adhesive rupture.

For this purpose, the invention has as its first subject a method according to claim 1.

The method may also comprise the features of claims 2 to 11, taken alone or in combination.

The invention also relates to an assembly according to claim 12.

• la figure 1 est une vue schématique en coupe illustrant la structure d'une tôle obtenue par un procédé selon l'invention, 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 vue schématique illustrant une éprouvette utilisée pour un test de traction ;
• les figures 5 et 6 sont des clichés montrant respectivement une rupture cohésive superficielle et une rupture adhésive.

The invention will now be illustrated by examples given for information purposes only, and without limitation, and with reference to the appended figures in which:

• there figure 1 is a schematic sectional view illustrating the structure of a sheet obtained by a method according to the invention, and
• THE Figures 2 and 3 show results of XPS spectroscopy analysis of the exterior surfaces of metal coatings,
• there figure 4 is a schematic view illustrating a test specimen used for a tensile test;
• THE Figures 5 and 6 are images showing respectively a superficial cohesive rupture and an adhesive rupture.

Sheet 1 of the figure 1 comprises a steel substrate 3 covered 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 on the figure 1 in order to facilitate 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 µm and is conventionally intended to protect the substrate 3 against corrosion.

The coating 7 comprises zinc, aluminum and magnesium. According to the invention, each coating 7 comprises between 0.1 and 10% by weight of magnesium and between 0.7 and 6% 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 1 and 6% by weight of aluminum.

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

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

A substrate 3 is used, obtained for example by hot and then cold rolling. 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 can make it possible, among other things, to improve the ductility or adhesion of the coatings 7 on the substrate 3. The person skilled in the art who knows their effects on the characteristics of the coatings 7 will know how to use them depending on the additional purpose sought. The bath can finally contain residual elements originating 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 deposition of the coatings 7, the substrate 3 is for example wrung out using nozzles projecting a gas on either side of the substrate 3. The coatings 7 are then left to cool in a controlled manner.

The strip thus treated can then be subjected to a stage called skin-pass which allows it to be work-hardened so as to erase the elasticity level, to fix the mechanical characteristics and to give it a roughness adapted to the subsequent operations which the sheet must undergo.

The means of adjusting the skin-pass operation is the elongation rate 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 sheet 1 thus obtained can be coiled before being cut, possibly shaped. Sheet 1 is assembled with other sheets by users.

It can, in a classic way, be oiled for temporary protection purposes.

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

X-ray Photoemission Spectroscopy (XPS) analyses of the exterior surfaces of the coatings 7 revealed the predominant presence of magnesium oxide or magnesium hydroxide, even when the coatings 7 have similar aluminum and magnesium contents.

However, in conventional coatings comprising mainly zinc and aluminium in small proportions, the outer surfaces of the metal coatings are covered with a layer of aluminium oxide, despite the very low aluminium content. For similar magnesium and aluminium contents, one would therefore have expected to find predominantly aluminium oxide or at least a mixture of magnesium and aluminium oxides.

XPS spectroscopy was also used to measure the thickness of the magnesium oxide or magnesium hydroxide layers present on the exterior surfaces 15. These layers appear to be a few nm thick.

It should be noted that these XPS spectroscopy analyses were carried out on sheet metal samples 1 that 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.

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 XPS spectroscopy. The corresponding atomic percentages are plotted on the ordinate and the analysis depth 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 classic 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 XPS spectroscopy analyses 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 classic skin-pass steps, nor by classic alkaline degreasing and classic surface treatments.

According to the invention, the method for producing the sheet metal 1 comprises at least one step of alteration, by application of mechanical forces, of layers of magnesium oxide or magnesium hydroxide present on the external surfaces 15 of the coatings 7, before possible subsequent application of an adhesive 13.

Such mechanical forces are applied by a planer.

These mechanical efforts can have the function of altering, through their sole action, layers of magnesium oxide or magnesium hydroxide.

The planer, which is characterized by the application of plastic deformation by bending between rollers, is adjusted to deform the sheet metal passing through it sufficiently to create cracks in the layers of magnesium oxide or magnesium hydroxide.

The application of mechanical forces to the outer surfaces 15 of the metal coatings 7 may be combined with the application of an acid solution or the application of a degreasing agent, for example based on an alkaline solution, to the outer surfaces 15.

The acidic 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 may be between 0.2 s and 15 s, more preferably between 0.5 s and 15 s, depending on the pH of the solution, the time and the manner in which it is applied.

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

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

The possible degreasing step aims to clean the exterior surfaces 15 and therefore to remove traces of organic dirt, metal 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 surface layer of aluminum oxide/hydroxide. Thus, the solution used for this degreasing step is non-oxidizing. Therefore, no magnesium oxide or magnesium hydroxide is formed on the exterior surfaces 15 during the degreasing step and more generally before the step of applying the adhesive 13.

If a degreasing step is used, it occurs before or after the acid solution application step.

The possible degreasing step and the step of applying the acid solution occur before a possible surface treatment step, that is to say a step making it possible to form on the external surfaces 15 layers (not shown) 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 applying to the exterior surfaces 15 a surface treatment solution which reacts with the exterior surfaces 15 to form said layers.

In some embodiments, the surface treatment solution is a conversion solution and the layers formed are conversion layers. Preferably, the conversion solution does not contain chromium. It may 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 while it is present on the exterior surfaces 15 to promote the action of the acid solution.

In this case, the mechanical efforts can be less intense.

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

In the latter case, it is the surface treatment solution used 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 above 30°C.

In order to illustrate the invention, tensile tests were carried out and will be described by way of non-limiting example.

As illustrated by the figure 4 , each test piece 17 is prepared as follows. Tabs 29 are cut from the sheet metal 1 to be evaluated. These tabs 29 have dimensions of 25 mm by 100 mm. The tabs 29 are glued by a joint 31 of the adhesive BM1496V, which is a so-called "crash" glue based on epoxy and marketed by the company Dow Automotive.

This adhesive was selected because it is one of the adhesives that most commonly leads to adhesive failures.

The test piece 27 thus formed is then brought to 180°C and maintained at this temperature for 30 minutes.

The tensile test is then carried out at an ambient temperature of 23°C by imposing a tensile speed of 10 mm/min on a tab 29, parallel to it, while the other tab 29 is fixed. The test is continued until the test piece 27 breaks.

At the end of the test, the maximum tensile stress is noted and the nature of the rupture is visually assessed.

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 to the process. These coatings have thicknesses of approximately 10 µm. Sheet 1 was also previously oiled with a Quaker 6130 oil and a weight of 2.5 g/m ² .

As illustrated by Table 1 below, the sheets 1 which have undergone mechanical treatment to alter layers of magnesium oxide or magnesium hydroxide promote the appearance of superficial cohesive ruptures, unlike the reference sheets for which only adhesive ruptures are observed.

Reference sheet 1 had not undergone any mechanical treatment to alter layers of magnesium oxide or magnesium hydroxide. Test reference sheet 1 had been subjected to a plane tensile deformation of 10%. Table 1 Reference Test 1 Tensile stress at break (in MPa) 15.5±0.1 24.2±0.3 Types of breakup 100% RA 25% RCS 75% RA

This effect is enhanced by the application of an acid solution, possibly as a surface treatment, or a degreasing step.

Claims (12)

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

   - providing a steel substrate (3) having two faces (5) each coated with a metal coating (7) obtained by dipping the substrate (3) in a bath and cooling, each metal coating (7) comprising zinc, between 0.7 and 6% by weight aluminium and between 0.1 and 10% by weight magnesium, and then

   - disrupting, at least by applying mechanical stress applied by a leveller to the outer surfaces (15) of the metal coatings (7), layers of magnesium oxide or magnesium hydroxide formed on the outer surfaces (15), wherein the mechanical stress is adapted to crack the layers of magnesium oxide or magnesium hydroxide which have formed on the outer surfaces (15) of the metal coatings (7), then

   - applying an adhesive (13) chosen from structural, reinforced structural or semi-structural adhesives, mastic sealants and bedding mastics, locally to at least one outer surface (15) of a metal coating (7),

   - assembling with a second metal sheet by way of the adhesive (13).
2. Method according to claim 1, wherein the metal coatings (7) comprise between 0.3 and 10% by weight magnesium, preferably between 0.3 and 4% by weight magnesium.
3. Method according to claim 1 or 2, wherein the metal coatings (7) comprise between 1 and 6% by weight aluminium.
4. Method according to any one of the preceding claims, wherein the ratio by mass between the magnesium and the aluminium in the metal coatings (7) is strictly less than or equal to 1, preferably strictly less than 1, and preferably strictly less than 0.9.
5. Method according to any one of the preceding claims, the method further comprising, prior to the step of applying the adhesive (13), a step of degreasing by applying an alkaline solution to the outer surfaces (15) of the metal coatings (7).
6. Method according to any one of the preceding claims, the method further comprising, prior to the step of applying the adhesive (13), a step of applying a surface treatment solution to the outer surfaces (15) of the metal coatings (7) in order to form on the outer surfaces (15) layers which improve the corrosion resistance and/or the adherence.
7. Method according to any one of the preceding claims, the method further comprising, prior to the step of applying the adhesive (13), applying an acidic solution to the outer surfaces (15) of the metal coatings (7).
8. Method according to claim 7, wherein the acidic solution is applied to the outer surfaces (15) of the metal coatings (7) for a period of between 0.2 s and 15 s, preferably between 0.5 s and 15 s.
9. Method according to any one of claims 7 to 8, wherein the acidic solution has a pH of between 1 and 4, especially between 1 and 3.5, typically between 1 and 3, preferably between 1 and 2.
10. Method according to any one of claims 7 to 9, wherein the acidic solution is an acidic surface treatment solution, preferably an acidic conversion solution.
11. Method according to any one of claims 7 to 10, wherein mechanical stress is applied to the outer surfaces (15) of the metal coatings (7) prior to the application of the acidic solution or while the acidic solution is present on the outer surfaces (15).
12. Assembly of a metal sheet (1) with a second metal sheet obtained by a method according to any one of the preceding claims, said assembly comprising:

    - a first metal sheet (1) having two faces (5) each coated with a metal coating (7) comprising zinc, aluminium and magnesium, the metal coatings (7) comprising between 0.7 and 6% by weight aluminium and between 0.1 and 10% by weight magnesium,

    - a second metal sheet,

    the first metal sheet (1) and the second metal sheet (1) being assembled by way of an adhesive (13) chosen from structural, reinforced structural or semi-structural adhesives, mastic sealants and bedding mastics, the adhesive (13) being applied locally to at least one outer surface (15) of a coating (7) of the first metal sheet (1).

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