Methods for producing a pre-lacquered metal sheet having Zn—Al—Mg coatings and corresponding metal sheet

Abstract

A method is provided. The method includes providing a steel substrate having two faces coated by dipping the substrate in a bath, altering layers of magnesium oxide or magnesium hydroxide formed on the outer surfaces of the metal coatings by applying mechanical forces, rinsing and drying the outer surfaces, applying a conversion solution on the outer surfaces and painting the outer surfaces of the metal coatings. A metal sheet is also provided.

Claims

1. A method for producing a metal sheet, the method comprising the following steps in this order: providing a skin-passed steel substrate having two faces each coated with a metal coating, each metal coating comprising zinc, between 0.1 and 20 wt % of aluminum, and between 0.1 and 10 wt % of magnesium; altering layers of magnesium oxide or magnesium hydroxide formed on the outer surfaces of the metal coatings, said altering step comprising applying mechanical forces on the outer surfaces of the metal coatings; non-oxidizing degreasing the outer surfaces of the metal coatings; rinsing and drying the outer surfaces of the metal coatings; applying a conversion solution on the outer surfaces of the metal coatings; drying the outer surfaces of the metal coatings; painting the outer surfaces of the metal coatings to cover each of them with a paint film comprising at least one polymer chosen from the group consisting of melamine-cross-linking polyesters, isocyanate-cross-linking polyesters, polyurethanes and halogenated derivatives of vinyl polymers, excluding cataphoretic paints wherein the mechanical forces crack the layers of magnesium oxide or magnesium hydroxide.

2. The method according to claim 1, wherein the altering step includes applying an acid solution on the outer surfaces of the metal coatings.

3. The method according to claim 2, further comprising after the step of application of the acid solution and before the non-oxidizing degreasing step a step of rinsing the outer surfaces of the metal coatings.

4. The method according to claim 3, further comprising after the rinsing step and before the non-oxidizing degreasing step a step of drying the outer surfaces of the metal coatings.

5. The method according to claim 2, wherein the mechanical forces are applied on the outer surfaces of the metal coatings before applying the acid solution or when the acid solution is present on the outer surfaces.

6. The method according to claim 5, wherein the mechanical forces are applied by passing through a roller leveler.

7. The method according to claim 1, wherein the metal coatings comprise between 0.3 and 10 wt % of magnesium.

8. The method according to claim 7, wherein the metal coatings comprise between 0.3 and 4 wt % of magnesium.

9. The method according to claim 1, wherein the metal coatings comprise between 0.5 and 11 wt % of aluminum.

10. The method according to claim 9, wherein the metal coatings comprise between 0.7 and 6 wt % of aluminum.

11. The method according to claim 10, wherein the metal coatings comprise between 1 and 6 wt % of aluminum.

12. The method according to claim 1, wherein a weight ratio between the magnesium and the aluminum in the metal coatings is less than or equal to 1.

13. A metal sheet being obtained by the method according to claim 1.

14. The method according to claim 1, wherein the step of providing the skin-passed steel substrate includes hot dipping a steel substrate in a bath to coat the two faces and cooling the coated substrate.

15. The method according to claim 1, wherein the metal coating consists of zinc, between 0.1 and 20 wt % of aluminum, and between 0.1 and 10 wt % of magnesium.

16. The method according to claim 1, wherein the metal coating consists of zinc, between 0.1 and 20 wt % of aluminum, between 0.1 and 10 wt % of magnesium, and up to 0.3 wt % of one or more additional element(s) selected from the group consisting of Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be illustrated through non-limiting examples provided for information, in reference to the appended figures, in which:

(2) FIG. 1 is a diagrammatic cross-sectional view illustrating the structure of a metal sheet obtained using a method according to the present invention, and

(3) FIGS. 2 and 3 show the results of XPS spectroscopy analysis of the outer surfaces of the metal sheets.

DETAILED DESCRIPTION

(4) The metal sheet 1 of FIG. 1 comprises a steel substrate 3 covered on each of its two faces 5 by a metal coating 7. The coatings 7 are respectively covered by an upper paint film 9 and a lower paint film 11.

(5) It will be noted that the relative thicknesses of the substrate 3 and of the various layers covering are not shown to scale in FIG. 1 in order to facilitate the illustration.

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

(7) The coating 7 generally has a thickness smaller than or equal to 25 m, for example, and traditionally aims to protect the substrate 3 from corrosion.

(8) The coating 7 comprises zinc, aluminum and magnesium. It is in particular preferred for the coating 7 to comprise, for example, between 0.1 and 10 wt % of magnesium and between 0.1 and 20 wt % of aluminum.

(9) Also preferably, the coating 7 comprises more than 0.3 wt % of magnesium, or even between 0.3 wt % and 4 wt % of magnesium and/or between 0.5 and 11 wt % or even between 0.7 and 6 wt % of aluminum.

(10) Preferably, the Mg/Al weight 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.

(11) The paint films 9 and 11 are for example polymer-based. Preferably, they comprise at least one polymer chosen from the group consisting of melamine-cross-linking polyesters, isocyanate-cross-linking polyesters, polyurethanes and halogenated derivatives of vinyl polymers, excluding cataphoretic paints.

(12) The films 9 and 11 typically have thicknesses comprised between 1 and 200 m.

(13) To produce the metal sheet 1, the following method may for example be used.

(14) The installation used may comprise a single line or, for example, two different lines to carry out the metal coatings and the painting, respectively. In the event two different lines are used, they may be situated on the same site or different sites. In the rest of the description, an alternative will be considered as an example where two separate lines are used.

(15) In a first line for producing metal coatings 7, a substrate 3 is used that is for example obtained by hot, then cold rolling. The substrate 3 is in the form of a band that is caused to pass through a bath to deposit the coatings 7 by hot dipping.

(16) The bath is a molten zinc bath containing magnesium and aluminum. The bath may also contain up to 0.3 wt % of each of the optional additional elements, such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.

(17) These different elements may make it possible, inter alia, to improve the ductility or adhesion of the coatings 7 on the substrate 3. One skilled in the art who knows their effects on the characteristics of the coatings 7 will know how to use them based on the complementary aim sought. The bath may lastly contain residual elements coming from supply ingots or resulting from the passage of the substrate 3 in the bath, such as iron with a content of up to 5 wt %, and generally comprised between 2 and 4 wt %, for example.

(18) After depositing the coatings 7, the substrate 3 is for example spun dry using nozzles projecting a gas on either side of the substrate 3.

(19) The coatings 7 are then left to cool in a controlled manner.

(20) The band thus treated may next undergo a so-called skin-pass step, which makes it possible to cold work it so as to erase the elasticity plateau, set the mechanical characteristics and give it a roughness suitable for the stamping operations and the painted surface quality one wishes to obtain. The means for adjusting the skin-pass operation is the elongation level, which must be sufficient to achieve the aims and small enough to preserve the subsequent deformation capacity. The elongation level is typically comprised between 0.3 and 3 wt %, and preferably between 0.3 and 2.2%.

(21) The band may optionally be wound before being sent to a pre-lacquering line.

(22) The outer surfaces 15 of the coatings 7 are subjected to the following steps therein: degreasing, for example by applying an alkaline solution, then rinsing and drying, then surface treatment to increase the adherence of the paint and the corrosion resistance, then rinsing and optionally drying, then painting.

(23) The purpose of the degreasing step is to clean the outer surfaces 15 and therefore remove the traces of organic dirtying, metal particles and dust.

(24) Preferably, this step does not alter the chemical nature of the outer surfaces 15, with the exception of altering any aluminum oxide/hydroxide surface layer. Thus, the solution used for this degreasing step is non-oxidizing. As a result, no magnesium oxide or magnesium hydroxide is formed on the outer surfaces 15 during the degreasing step, and more generally before the painting step.

(25) The surface treatment step comprises applying, on the outer surfaces 15, a conversion solution that reacts chemically with the outer surfaces 15 and thus makes it possible to form conversion layers on the outer surfaces 15. Preferably, the conversion solution does not contain chromium. It may thus be a hexafluorotitanic or hexafluorozirconic acid-based solution.

(26) The painting may for example be done by depositing two successive layers of paint, i.e., a layer of primer and a topcoat layer, which is generally the case to produce the upper film 9, or by depositing a single layer of paint, which is generally the case to produce the lower film 11. Other numbers of layers can be used in certain alternatives.

(27) The layers of paint are for example deposited using roller coaters.

(28) Each deposition of a layer of paint is generally followed by baking in a furnace.

(29) The metal sheet 1 thus obtained can once again be wound before being cut, optionally shaped and assembled with other metal sheets 1 or other elements by users.

(30) The inventors have shown that the use of a step for altering a magnesium oxide or magnesium hydroxide layer present on the outer surface 15 of each coating 7 makes it possible to improve the corrosion resistance of the metal sheet 1, and in particular to limit the bubbling phenomenon of the paint films 9 and 11 when the metal sheet 1 is subjected to a corrosive environment.

(31) A magnesium oxide or magnesium hydroxide layer here refers to a layer that may contain compounds of the Mg.sub.xO.sub.y type, or compounds of the Mgx(OH)y type, or a mixture of those two types of compounds.

(32) In fact, XPS (X-ray Photoemission Spectroscopy) spectroscopic analyses of the outer surfaces 15 of the coatings 7 have shown the preponderant presence of magnesium oxide or magnesium hydroxide before painting, even when the coatings 7 have similar aluminum and magnesium content levels.

(33) However, in the typical coatings essentially comprising zinc and aluminum in small proportions, the outer surfaces of the metal coatings are covered with a layer of aluminum oxide, despite the very low aluminum content level. For similar content levels of magnesium and aluminum, it would therefore have been expected to find a preponderant quantity of aluminum oxide.

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

(35) It will be noted that these XPS spectroscopic analyses were done on specimens of metal sheets 1 that had not been subjected to corrosive environments. The formation of layers of magnesium oxide or magnesium hydroxide is therefore related to the deposition of the coatings 7.

(36) FIGS. 2 and 3 respectively illustrate the spectrums of the elements for energy levels C1s (curve 17), O1s (curve 19), Mg1s (curve 21), Al2p (curve 23) and Zn2p3 (curve 25) during an XPS spectroscopic analysis. The corresponding atomic percentages are shown on the y-axis and the analysis depth on the x-axis.

(37) The sample analyzed in FIG. 2 corresponds to coatings 7 comprising 3.7 wt % of aluminum and 3 wt % of magnesium and subjected to a traditional skin-pass step with an elongation of 0.5%, while the specimen of FIG. 3 has not been subjected to such a step.

(38) On these two specimens, according to the XPS spectroscopic analyses, it may be estimated that the thickness of the layers of magnesium oxide or magnesium hydroxide is approximately 5 nm.

(39) It thus appears that these layers of magnesium oxide or magnesium hydroxide are not removed by the traditional skin-pass steps, or by the traditional alkaline degreasing and traditional surface treatments.

(40) According to the invention, the method for producing the metal sheet 1 comprises, before painting, a step for altering layers of magnesium oxide or magnesium hydroxide present on the outer surfaces 15 of the coatings 7.

(41) Such an alteration step may take place before or during the surface treatment step. It may for example take place on the production line for the coatings 7 or the pre-lacquering line.

(42) In a first embodiment, the alteration step comprises applying an acid solution, for example with a pH comprised between 1 and 4, preferably between 1 and 3.5, and still more preferably between 1 and 3, on the outer surfaces 15. The solution may for example comprise hydrochloric acid, sulfuric acid or phosphoric acid.

(43) The application duration of the acid solution may be comprised between 0.2 s and 30 s, preferably between 0.2 s and 15 s, and still more preferably between 0.5 s and 15 s, as a function of the pH of the solution, and the moment and manner in which it is applied.

(44) The solution may be applied by immersion, aspersion or any other system. The temperature of the solution may for example be the ambient temperature or any other temperature.

(45) In the first embodiment, the step for applying the acid solution takes place after the rinsing and drying step following the degreasing step. The application of the acid solution is followed by a rinsing and optionally drying step of the outer surfaces 15 before the application step for the conversion solution.

(46) In a second embodiment, the step for applying the conversion solution constitutes the alteration step for magnesium oxide or magnesium hydroxide layers present on the outer surfaces 15 of the coatings 7.

(47) In that case, the conversion solution used has a pH comprised between 1 and 2.

(48) The application times are similar to those of the first embodiment.

(49) In a third embodiment, the alteration step comprises the application of mechanical forces, and optionally the application of an acid solution, on the outer surfaces 15 of the metal coatings 7.

(50) Such mechanical forces may be applied by a roller leveler, brushing devices, shot-blasting devices, etc.

(51) These mechanical forces may serve, due to their action alone, to alter the layers of magnesium oxide or magnesium hydroxide. Thus, the brushing and shot-blasting devices may remove all or part of those layers.

(52) Likewise, a roller leveler, which is characterized by the application of a plastic deformation by bending between rollers, may be adjusted to deform the metal sheet that passes through it enough to create cracks in the layers of magnesium oxide or magnesium hydroxide.

(53) In the event 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 outer surfaces 15 to favor the action of the acid solution.

(54) In that case, the mechanical forces may be less intense.

(55) The acid solution may then be applied in the roller leveler.

(56) When an acid solution is used in combination with the application of mechanical forces, the pH of the acid solution may be higher, and in particular greater than 3.

(57) In this third embodiment, the alteration step takes place before the degreasing step.

(58) If the alteration step of the third embodiment comprises applying an acid solution, it is followed by a rinsing and optionally drying step for the outer surfaces 15 of the metal coatings 7.

(59) Specimens of the metal sheets 1 obtained using a method according to the invention, i.e., with a step for altering the layers of magnesium oxide or magnesium hydroxide present on the metal coatings, and metal sheets obtained traditionally, were subjected to corrosion resistance tests. It appears that the metal sheets 1 obtained using a method according to the invention have a better corrosion resistance.