EP2954086B1 - Metal sheet with a znalmg coating having a particular microstructure, and corresponding production method - Google Patents

Description

The present invention relates to a sheet comprising a substrate of which at least one face is coated with a metal coating comprising Al and Mg, the remainder of the metal coating being Zn, unavoidable impurities and possibly one or more additional elements. chosen from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni or Bi, the content by weight of each additional element in the metal coating being less than 0.3%.

Galvanized metal coatings consisting essentially of zinc and 0.1 to 0.4% by weight of aluminum are traditionally used for their good protection against corrosion.

These metal coatings are now in competition with, in particular, coatings comprising zinc and magnesium and aluminum additions of up to 10% and up to 20% by weight, respectively.

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

The documents US 3,505,043 and EP 2 119 804 disclose such coatings.

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

These sheets are for example intended for the automotive field, household appliances or construction.

They can be put in paintings before or after their formatting by the users of these domains. When they are painted before shaping, they are called "coated" sheets, these being particularly intended for household appliances or construction.

In the case of coated steel sheets, the entire process of making sheet metal being provided by the steelmaker, the costs and constraints related to the painting of users are reduced.

However, it is observed that known metal coatings may be subject to problems of delamination of the paint layers, leading to local corrosion of the sheet.

An object of the invention is to provide a coated sheet whose corrosion resistance, when it is painted, is increased.

For this purpose, the first object of the invention is a sheet according to claim 1.

The sheet may also comprise the features of claims 2 to 12, taken alone or in combination.

The invention also relates to a method according to claim 13.

The method may also include the features of claims 14 and 15, taken alone or in combination.

• la figure 1 est une vue schématique en coupe illustrant la structure d'une tôle selon l'invention, après peinture,
• les figures 2 à 4 sont des schémas illustrant la microstructure de la surface à l'état brut des revêtements métalliques de la tôle de la figure 1,
• la figure 5 est un diagramme illustrant les résultats de tests de délamination menés sur un échantillon de tôle suivant l'invention et de tôles qui ne sont pas conformes à l'invention, et
• la figure 6 est un diagramme illustrant des courbes de densité de courant et de potentiel de corrosion représentatives de différentes phases.

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

• the figure 1 is a schematic sectional view illustrating the structure of a sheet according to the invention, after painting,
• the Figures 2 to 4 are diagrams illustrating the microstructure of the raw surface of the metal coatings of the sheet metal of the figure 1 ,
• the figure 5 is a diagram illustrating the results of delamination tests carried out on a sample of sheet according to the invention and sheets which do not conform to the invention, and
• the figure 6 is a diagram illustrating current density and corrosion potential curves representative of different phases.

Sheet 1 of the figure 1 comprises a substrate 3 of steel covered on each of its two faces 5 by a metal coating 7, itself covered by a film of paint 9, 11.

It will be observed that the relative thicknesses of the substrate 3 and of the various layers covering it have not been respected 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 later. As a variant (not shown), only one of the faces 5 has a coating 7.

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

The coating 7 comprises zinc, aluminum and magnesium. The weight content of aluminum t _Al of the metal coating 7 is between 3.6 and 3.8%. The weight content of magnesium t _Mg of the metal coating 7 is between 2.7 and 3.3%.

Preferably, the magnesium t _{Mg content} is between 2.9 and 3.1%.

Preferably, the mass ratio AI / (AI + Mg) is greater than or equal to 0.45, or even greater than or equal to 0.50, or even greater than or equal to 0.55.

As illustrated by Figures 2 to 4 the coating 7 has a particular microstructure with a lamellar matrix 13 of ternary eutectic Zn / Al / MgZn ₂ . As we see on the figure 3 the lamellar matrix 13 forms grains separated by seals 19.

In a preferred form of the invention, the ternary eutectic constitutes the entire microstructure of the coating.

The interlamellar distance of the lamellar matrix 13 can vary quite strongly within its grains, especially in the vicinity of the structures possibly encompassed by this matrix, structures which will now be described.

In addition to the aforementioned lamellar matrix 13, the surface and cross-sectional microstructure may comprise Zn dendrites and Zn / MgZn ₂ binary eutectic flowers in small amounts. which are not too detrimental to the improvement of the delamination resistance obtained according to the invention.

To do this, the cumulative surface levels of 15 Zn dendrites and flowers 17 binary eutectic Zn / MgZn ₂ to the outer surface 21 in the raw state are limited.

Preferably, the cumulative surface area of Zn dendrites on the outer surface 21 in the raw state is less than 5.0%, even 3.0%, even 2.0%, or even 1.0%, and ideally zero and the cumulated surface content of Zn / MgZn ₂ binary eutectic flowers 17 on the outer surface 21 in the raw state is less than 15.0%, even 10.0% or even 5.0%, or even 3%, 0% and ideally zero.

The microstructure may also comprise binary eutectic dendrites Zn / Al or islands of MgZn ₂ , in very small quantities because these structures greatly deteriorate the delamination resistance of the coated sheets according to the invention.

In any case, the cumulative surface content of Zn / Al binary eutectic dendrites on the outer surface 21 in the raw state is less than 1.0% and the cumulative surface area of MgZn ₂ islands on the surface in the raw state is less than 1.0% and these accumulated contents are preferably zero.

Likewise, the respective cumulative cross-sectional contents of Zn / Al binary eutectic dendrites and MgZn ₂ islands are preferably zero.

Thus, in general, the microstructure will consist of a lamellar matrix 13 of ternary eutectic and possibly Zn dendrites, Zn / MgZn ₂ binary eutectic flowers 17, binary eutectic dendrites Zn / Al and of islands of MgZn2. However, depending on the presence of additional optional elements mentioned below, the microstructure may also comprise small amounts of other structures included in the lamellar matrix 13 of ternary eutectic.

The cumulative surface contents for each structure are for example measured by taking at least 30 views with an X1000 magnification of the outer surface 21 in the raw state (that is to say without polishing but optionally degreased by organic solvent) thanks to a scanning electron microscope.

For each of these views, the contours of the structure whose content is to be measured are extracted, and then, for example, using the AnalySIS Docu 5.0 software from Olympus Soft Imaging Solutions GmbH, the occupancy rate of the outer surface 21 by the structure in question. The occupancy rate thus calculated is the cumulative surface content of the structure in question.

The paint films 9 and 11 are for example based on polymers. These polymers may be polyesters or halogenated derivatives of vinyl polymers such as plastisols, PVDF, etc.

The films 9 and 11 typically have thicknesses of between 1 and 200 μm. To produce the sheet 1, one can for example proceed as follows.

The installation used may comprise a single line or, for example, two different lines for producing respectively the metal coatings and the painting. In the case where two different lines are used, they may be located on the same site or on separate sites. In the remainder of the description, for example, a variant in which two distinct lines are used is considered.

In a first embodiment of metal coatings 7, a substrate 3 is used, for example obtained by hot rolling and then cold rolling. The substrate 3 is in the form of a strip which is scrolled in a bath to deposit the coatings 7 by hot quenching.

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

These various additional 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 purpose research. The bath may finally contain residual elements from the ingots or resulting from the passage of the substrate 3 in the bath, such as iron at a content of up to 0.5% by weight and generally between 0.1 and 0.4% by weight.

The bath has a temperature Tb of between 360 ° C and 480 ° C, preferably between 420 ° C and 460 ° C.

où Ti est exprimée en degrés Kelvin.At the inlet of the bath, the substrate 3 has an immersion temperature Ti such that: $$\left(\mathrm{2},\mathrm{34}\times {\mathrm{t}}_{\mathrm{al}}+\mathrm{0},\mathrm{665}\times {\mathrm{t}}_{\mathrm{mg}}-\mathrm{10},\mathrm{1}\right)\times {\mathrm{10}}^{-\mathrm{6}}\le \exp \left(-\mathrm{10584}/\mathrm{Ti}\right)$$

where Ti is expressed in degrees Kelvin.

Such an immersion temperature Ti makes it possible to obtain the abovementioned microstructure with little or no structures included in the lamellar matrix 13.

Generally, this temperature Ti is determined on site from a measurement made a few meters upstream of the bath by a pyrometric technique then application of a thermal model to calculate the temperature Ti.

To vary Ti and satisfy the above equation, the cooling conditions of the substrate 3 are modified upstream of the bath. This cooling can be provided by blowing inert cooling gas on both sides 5 of the substrate 3 by means of cooling boxes, the pressure of the gas can be regulated. It is also possible to play on the speed of travel of the substrate 3 in the cooling zone or on the temperature of the substrate 3 at the entrance of this zone, for example.

After deposition of 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 to solidify.

Alternatively, a brushing may be performed to remove the coating 7 deposited on a face 5 so that only one of the faces 5 of the sheet 1 will be finally coated with a coating 7.

Controlled cooling of the or each coating 7 is provided at a rate preferably greater than or equal to 15 ° C / s between the onset of solidification (i.e., when the coating 7 falls just below the liquidus temperature). ) and the end of solidification (that is to say when the coating 7 reaches the temperature of the solidus). More preferably, the cooling rate of the or each coating 7 between the onset of solidification and the end of solidification is greater than or equal to 20 ° C / s.

The band thus treated can then be subjected to a so-called skin-pass step which allows the harden and give it a roughness facilitating its subsequent shaping.

The band may optionally be wound before being sent to a prelacing line.

The outer surfaces 21 of the coatings 7 are optionally subjected to a degreasing step and possibly to a surface treatment step to increase the adhesion of the paint and the corrosion resistance.

The possible steps of degreasing and surface treatment may include other sub-stages of rinsing, drying ....

The painting can then be performed for example by depositing two layers of successive paints, namely a primer layer and a layer of finish which is generally the case for making the upper film 9, or by deposition of a single layer of paint, which is generally the case for making the lower film 11. Other numbers of layers can be used in certain variants .

The deposition of the paint layers is provided for example by roller coaters.

Each deposit of a paint layer is generally followed by a step of cooking in an oven.

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

Trial 1

A sample of sheet 1 according to the invention was prepared and samples of sheets not corresponding to the invention by varying the immersion temperature Ti, t _Al and t _Mg samples. The corresponding microstructures were analyzed to determine the existing structures and their cumulative surface contents. Microstructure of the coating - cumulative surface contents Trial t _Al (%) t _Mg (%) Ti (K) Ternary Eutectic (%) Dendrites of Zn (%) Binary Eutectic Flowers Zn / MgZn2 (%) Zn / AI binary eutectic dendrites (%) Islets of MgZn2 (%) * 1 3.7 3.0 753 100 0 0 0 0 2 3.7 3.0 713 95 0 0 5 0 3 * 3.7 3.3 753 100 0 0 0 0 4 3.7 3.3 713 80 0 15 0 5 * according to the invention

Trial 2

A sample of sheet metal 1 according to the invention and sheets not corresponding to the invention were subjected to delamination tests to measure their resistance to paint corrosion.

More specifically, the coatings of the sheets tested had thicknesses of 8 μm.

The composition of the coatings 7 of the sheets 1 according to the invention had an _Al content of 3.7% and an _Mg content of 3.0%. As indicated under the abscissa axis on the figure 5 , Other coating compositions tested had t _Al values of 0.3%, 1.5%, 6.0% and 11.0% and t _Mg 1.0%, 1.5%, 3, 0 and 3.0%.

The microstructure of the sheet according to the invention consisted solely of ternary eutectic and was obtained by immersion in a coating bath at a temperature Tb = 460 ° C., the strip having a temperature Ti = 480 ° C.

Corrosion tests were in accordance with VDA 621-415 (10 cycles).

Specifically, the test plates were phosphated, covered with a layer of cataphoresis and scratched to the substrate with a blade 1 mm wide.

The maximum delamination widths Ud measured in mm at the end of the corrosion tests for the various plates tested are plotted on the ordinate on the figure 5 .

As can be seen, the delamination widths are optimal for the sheet according to the invention.

Surprisingly, it is found that increasing the cumulative levels of aluminum and magnesium beyond the values of the invention, deteriorates the resistance to delamination and therefore to corrosion.

The inventors estimate at the moment that this good resistance to paint corrosion is due to the particular microstructure of the coatings 7 which makes it possible to limit the risks of electrical coupling between their different structures and the lamellar matrix 13.

Because of the small presence of structures embedded in the lamellar matrix 13, on the outer surface 21 of each coating 7, the risks of selective dissolution of these phases are indeed reduced.

On the figure 6 , the corrosion potential with respect to a saturated calomel reference electrode KCI (ECS) is plotted on the abscissa and the current density as ordinate. Curve 23 corresponds to a composition comprising 3.7% by weight of Al and 3.0% by weight of Mg, the remainder being Zn. This curve is therefore representative of the lamellar matrix 13.

The figure 6 shows that the risk of corrosive coupling of the lamellar matrix 13 is greater with structures containing Al (curve 25), Mg (curve 27) and Zn (curve 29).

In general, the sheets 1 according to the invention are not necessarily marketed in painted form ("prepainted" sheets) and / or they may be coated with at least one layer of oil.

Claims (15)

1. Metal sheet (1) comprising a substrate (3) having at least one face (5) coated by a metal coating (7) comprising Al and Mg, the remainder of the metallic coating (7) being Zn, unavoidable impurities and possibly one or more additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating (7) is less than 0.3%, the metal coating (7) having an aluminium content by weight t_Al of between 3.6 and 3.8% and a magnesium content by weight t_Mg of between 2.7 and 3.3 %,
   the metal coating (7) having a microstructure comprising a lamellar matrix (13) of ternary eutectic of Zn/Al/MgZn₂ and:

   - dendrites of Zn (15) with an accumulated surface content at the outer surface (21) of the coating (7) in the raw state null or of less than or equal to 5.0%,

   - flowers of binary eutectic of Zn/MgZn₂ (17) with an accumulated surface content at the outer surface (21) of the coating (7) in the raw state null or of less than or equal to 15.0%,

   - dendrites of binary eutectic of Zn/Al with an accumulated surface content at the outer surface (21) of the metal coating (7) in the raw state null or of less than or equal to 1.0%,

   - islets of MgZn₂ with an accumulated surface content at the outer surface (21) of the coating (7) in the raw state null or of less than or equal to 1.0%.
2. Metal sheet according to claim 1, wherein the t_Mg magnesium content is between 2.9 and 3.1%.
3. Metal sheet according to Claim 1 or 2, wherein the weight ratio Al/(Al+Mg) is greater than or equal to 0.45.
4. Metal sheet according to any preceding claim, wherein the microstructure contains no dendrite of binary eutectic Zn/Al.
5. Metal sheet according to any preceding claim, wherein the microstructure comprises no islet of MgZn₂.
6. Metal sheet according to any preceding claim, wherein the accumulated surface content of the flowers of binary eutectic Zn/MgZn₂ (17) at the outer surface (21) of the coating (7) in a raw state is less than 10. 0%.
7. Metal sheet according to claim 6, wherein the accumulated surface content of the flowers of binary eutectic Zn/MgZn₂ (17) at the outer surface (21) of the coating (7) in a raw state is less than 5.0%.
8. Metal sheet according to one of the preceding claims, wherein the accumulated surface content of the flowers of binary eutectic Zn/MgZn₂ (17) at the outer surface (21) of the coating (7) in a raw state is less than 3.0% .
9. Metal sheet according to Claim 8, wherein the accumulated surface content of dendrites of Zn (15) at the outer surface (21) of the coating (7) in a raw state is less than 2.0%.
10. Metal sheet according to claim 9, wherein the accumulated surface content of dendrites of Zn (15) at the outer surface (21) of the coating (7) in a raw state is less than 1.0%.
11. Metal sheet according to claim 10, wherein the microstructure consists solely of ternary eutectic (13).
12. Metal sheet according to any preceding claim, wherein the metal coating (7) is covered with at least a paint layer and/or an oil layer.
13. Method of manufacturing a metal sheet (1) according to any preceding claim, wherein the method comprises at least the steps of:

    - providing a substrate (3) made of steel,

    - deposition of a metallic coating (7) on at least one face (5) by dipping the substrate (3) in a bath, wherein the substrate has an immersion inlet temperature Ti at the inlet in the bath such that $$\left(\mathrm{2},\mathrm{34}\times {\mathrm{t}}_{\mathrm{Al}}+\mathrm{0},\mathrm{655}\times {\mathrm{t}}_{\mathrm{Mg}}-\mathrm{10},\mathrm{1}\right)\times {\mathrm{10}}^{-\mathrm{6}}\le \exp \left(-\mathrm{10584}/\mathrm{Ti}\right)$$

    

    where T is in degrees Kelvin, and

    - solidification of the metal coating (7).
14. Manufacturing method according to claim 13, wherein the rate of cooling the coating (7) between the start of solidification and the end of solidification is higher than or equal to 15°C/s.
15. Manufacturing method according to claim 14, wherein the rate of cooling the coating (7) between the start of solidification and the end of solidification is higher than or equal to 20°C/s.

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