FI2841614T4 - Method for producing a ZnAlMg coated sheet comprising applying mechanical forces to the coating and the corresponding sheet - Google Patents

Claims (12)

1. METHOD FOR MANUFACTURING A ZNALMG COATED SHEET COMPRISING THE APPLICATION OF MECHANICAL FORCES ON THE COATING AND CORRESPONDING SHEET

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

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

   Metal coatings comprising substantially zinc and aluminium in a small proportion (typically of the order of 0.1% by weight) are conventionally used for their good corrosion protection.

   Such metal coatings are now in competition especially with coatings comprising zinc, magnesium and aluminium.

   Such metal coatings will be referred to generally herein by the expression zinc- aluminium-magnesium or ZnAlMg coatings.

   US 2011/0008644 discloses a method of preparing metal sheets having ZnAlMg coatings and assembly with a second metal sheet by means of an adhesive.

   The addition of magnesium considerably increases the corrosion resistance of such coatings, which can allow the thickness thereof to be reduced or the guarantee of corrosion protection over time to be increased.

   In the automotive industry especially, metal sheets are frequently assembled by means of adhesives in order to produce some vehicle parts, such as, for example, door sills.

   The adhesives are chosen from structural, reinforced structural (for example "crash" type) or semi-structural adhesives, mastic sealants or alternatively bedding mastics, which have very varied chemical natures, such as epoxy, polyurethane or rubber.

   In the automotive industry, the association of a metal sheet with an adhesive is conventionally evaluated by means of a tensile test on a specimen formed of two strips of the metal sheet, the strips being bonded over a portion of their surface by the adhesive.

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

   Three types, or patterns, of failure can principally be observed on this occasion: - cohesive failure, when failure occurs within the thickness of the adhesive, - adhesive failure (see Figure 5), when failure occurs at one of the interfaces between the strips and the adhesive, - Superficial cohesive failure (see Figure 6), when failure occurs within the adhesive in the vicinity of an interface between the strips and the adhesive.

   In the automotive industry, it is desirable to avoid adhesive failures, which reflect poor compatibility of the adhesive with the metal sheet.

   However, tensile tests reveal predominantly adhesive failures when some adhesives conventional for the automotive industry are used on metal sheets having ZnAIMg coatings.

   Up to 100% adhesive failures can thus be observed with some ZnAIMg coatings and some adhesives.

   Such proportions of adhesive failure are not acceptable for automotive manufacturers, which might limit the use of these new ZnAlMg coatings for some applications.

   It is therefore an object of the invention to propose a method for producing a metal sheet having ZnAlMg coatings which has better compatibility with adhesives and therefore limits the risks of adhesive failure.

   To that end, the invention relates firstly to a method according to claim 1. The method may also have the features of claims 2 to 11, taken in isolation or in combination.

   The invention relates also to an assembly according to claim 12. The invention will now be illustrated by examples which are given by way of example and without implying any limitation, and with reference to the accompanying figures, in which: - Figure 1 is a schematic view in section showing the structure of a metal sheet obtained by a method according to the invention; and - Figures 2 and 3 show results of analysis by XPS spectroscopy of the outer surfaces of the metal coatings; - Figure 4 is a schematic view showing a specimen used for a tensile test; - Figures 5 and 6 are negative images showing a superficial cohesive failure and an adhesive failure, respectively.

   The metal sheet 1 of Figure 1 comprises a steel substrate 3 covered on each of its two faces 5 by a metal coating 7. It will be seen that the relative thicknesses of the substrate 3 and of the coatings 7 covering it have not been observed in Figure 1 in order to facilitate the illustration.

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

   The coating 7 generally has a thickness of less than or equal to 25 um and is conventionally provided in order to protect the substrate 3 against corrosion.

   The coating 7 comprises zinc, aluminium and magnesium.

   According to the invention, each coating 7 comprises between 0.1 and 10% by weight magnesium and between 0.7 and 6% by weight aluminium.

   Yet more preferably, the coating 7 comprises more than 0.3% by weight magnesium, or even between 0.3% and 4% by weight magnesium, and/or between 1 and 6% by weight aluminium.

   Preferably, the ratio by mass Mg/Al between the magnesium and the aluminium in the coating 7 is less than or equal to 1, or even strictly less than 1, or even strictly less than 0.9. In order to produce the metal sheet 1, the following procedure, for example, may be used.

   A substrate 3 obtained, for example, by hot- and then cold-rolling is used.

   The substrate 3 is in the form of a strip which is unwound in a bath in order to deposit the coatings 7 by hot dipping.

   The bath is a bath of molten zinc comprising magnesium and aluminium.

   The bath may also comprise up to 0.3% by weight of each of the optional alloying elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.

   These different elements can make it possible, inter alia, to improve the ductility or the adhesion of the coatings 7 to the substrate 3. The person skilled in the art who knows the effects thereof on the properties of the coatings 7 will know how to use them in accordance with the desired additional aim.

   Finally, the bath may comprise residual elements coming from the supply ingots or resulting from the passage of the substrate 3 in the bath, such as iron in a content of up to 5% by weight and generally between 2 and 4% by weight.

   After the coatings 7 have been deposited, the substrate 3 is, for example, dried by means of nozzles which project a gas on either side of the substrate 3. The coatings 7 are then allowed to cool in a controlled manner.

   The strip so treated can then be subjected to a step known as a skin-pass step, which allows it to be work-hardened so as to eliminate the elasticity plateau, set the mechanical properties and impart thereto a roughness adapted to the subsequent operations to which the metal sheet is to be subjected.

   The means of regulating the skin-pass operation is the elongation rate, which must be sufficient to achieve the objectives and minimal in order to preserve the capacity for subsequent deformation.

   The elongation rate is conventionally between 0.3 and 3%, preferably between 0.3 and 2.2%.

   The metal sheet 1 so obtained can be coiled before being cut, optionally shaped.

   The metal sheet 1 is assembled with other metal sheets by users.

   It may be oiled, in the conventional manner, for the purpose of temporary protection.

   As is illustrated schematically in Figure 1, an adhesive 13 can be applied locally to an outer surface 15 of a coating 7 in order to allow the metal sheet 1 to be assembled with another metal sheet, for example, and thus form a motor vehicle part.

   The adhesive 13 may be any type of adhesive or mastic conventionally used in the automotive industry.

   Analyses by XPS spectroscopy (X-ray photoemission spectroscopy) of the outer surfaces 15 of the coatings 7 have revealed the presence predominantly of magnesium oxide or magnesium hydroxide, even when the coatings 7 have similar contents of aluminium and magnesium.

   However, in the conventional coatings comprising substantially zinc and aluminium in a small proportion, the outer surfaces of the metal coatings are covered with a layer of aluminium oxide, despite the very low aluminium content.

   For similar contents of magnesium and aluminium, it would therefore be expected to find predominantly aluminium oxide or at the very least a mixture of magnesium oxide and aluminium oxide.

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

   It will be noted that these analyses by XPS spectroscopy were carried out on samples of metal sheets 1 which had not been subjected to corrosive environments.

   The formation of the layers of magnesium oxide or magnesium hydroxide is therefore associated with the deposition of the coatings 7. Figures 2 and 3 show 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) in an analysis by XPS spectroscopy.

   The corresponding atomic percentages are shown on the y-axis and the analysis depth on the x-axis.

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

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

   It thus appears that the layers of magnesium oxide or magnesium hydroxide are not removed by the conventional skin-pass steps or, moreover, by conventional alkaline degreasing operations and the conventional surface treatments.

   According to the invention, the method for producing the metal sheet 1 comprises at least one step of disrupting, by the application of mechanical stress, layers of magnesium oxide or magnesium hydroxide present on the outer surfaces 15 of the coatings 7, prior to any subseguent application of an adhesive 13. Such mechanical stress is applied by a leveller.

   The mechanical stress can serve for disrupting the layers of magnesium oxide or magnesium hydroxide by their action alone.

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

   The application of mechanical stress to the outer surfaces 15 of the metal coatings 7 can be combined with the application of an acidic solution or the application of degreasing, 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 yet 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 acidic solution can be between 0.2 s and 15 s, and yet more preferably between 0.5 s and 15 s, depending on the pH of the solution and the time at which and the manner in which it is applied. The solution can be applied by dipping, spraying or any other system. The temperature of the solution can be, for example, ambient temperature or any other temperature, and subsequent steps of rinsing and drying can be employed. More generally, the layers of magnesium oxide or magnesium hydroxide can be disrupted by applying an acidic solution and without applying mechanical stress. The purpose of any degreasing step is to clean the outer surfaces 15 and therefore remove traces of organic soiling, metal particles and dust. Preferably, this step does not modify the chemical nature of the outer surfaces 15, apart from disrupting any superficial layer of aluminium oxide/hydroxide. The solution used for the degreasing step is thus non-oxidising. Magnesium oxide or magnesium hydroxide is therefore not formed on the outer surfaces 15 in the degreasing step and, more generally, prior to the step of application of the adhesive

   13. If a degreasing step is employed, it takes place before or after the step of application of the acidic solution. The optional degreasing step and the step of applying the acidic solution take place before any surface treatment step, that is to say a step permitting the formation on the outer surfaces 15 of the layers (not shown) which improve the corrosion resistance and/or the adherence of other layers subsequently deposited on the outer surfaces 15.

   Such a surface treatment step comprises applying to the outer surfaces 15 a surface treatment solution which reacts with the outer surfaces 15 to form said layers.

   In some variants, 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 acid or hexafluorozirconic acid.

   In the case where the application of mechanical stress is combined with the application of an acidic solution, the mechanical stress will be applied preferably before the acidic solution or while the acidic solution is present on the outer surfaces in order to further the action of the acidic solution.

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

   In a variant, the step of applying the acidic solution and the surface treatment step are carried out together.

   In the latter case, it is the surface treatment solution employed that is acidic.

   In this case especially, the pH can 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 examples.

   As shown in Figure 4, each specimen 17 is prepared as follows.

   Strips 29 are cut from the metal sheet 1 to be evaluated.

   The strips 29 have dimensions of 25 mm by 100 mm.

   The strips 29 are bonded by a joint 31 of the adhesive BM1496V, which is an epoxy-based adhesive known as a "crash" adhesive marketed by Dow Automotive.

   This adhesive was chosen because it is one of the adhesives that cause the most adhesive failures.

   The specimen 27 so formed is then heated to 180*C and maintained at that temperature for 30 minutes. The tensile test is then performed at an ambient temperature of 23*C by imposing a tensile speed of 10 mm/min on one strip 29, parallel thereto, while the other strip 29 is fixed. The test is continued until the specimen 27 fails. At the end of the test, the maximum tensile stress is noted and the nature of the failure is evaluated visually. The tests were carried out using a metal sheet 1 the substrate 3 of which is an IFHR 340 steel of thickness 0.8 mm, covered with coatings 7 comprising 3.7% aluminium and 3% magnesium, the remainder being composed of zinc and impurities inherent in the process. The coatings have thicknesses of approximately 10 um. The metal sheet 1 has likewise been oiled beforehand with Quaker 6130 oil and a spread of
2. 2.5 g/m”. As shown by Table 1 below, the metal sheets 1 which have undergone a mechanical treatment of disruption of layers of magnesium oxide or magnesium hydroxide favour the appearance of superficial cohesive failures, contrary to the reference metal sheets for which only adhesive failures are found. The reference metal sheet 1 had not undergone any mechanical treatment of disruption of the layers of magnesium oxide or magnesium hydroxide. The metal sheet 1 referenced test had undergone 10% planar deformation by traction. — jees mer Tensile stress at failure 15.5 + 0.1 24.2+03 (in MPa) Types of failure 100% AF 25% SCF 75% AF Table 1 This effect is increased by application of an acidic solution, optionally as a surface treatment, or a degreasing step.