EP2954086A1

EP2954086A1 - Metal sheet with a znaimg coating having a particular microstructure, and corresponding production method

Info

Publication number: EP2954086A1
Application number: EP13762578.6A
Authority: EP
Inventors: Christian Allely; Luc Diez; Tiago MACHADO AMORIM; Jean-Michel Mataigne
Original assignee: ArcelorMittal SA
Current assignee: ArcelorMittal SA
Priority date: 2013-02-06
Filing date: 2013-07-08
Publication date: 2015-12-16
Anticipated expiration: 2033-07-08
Also published as: EP2954086B1; JP2016514202A; CN105247094B; PL2954086T3; LT2954086T; MX360981B; KR102070480B1; HRP20170460T1; DK2954086T3; RU2636215C2; JP6185084B2; RU2015137791A; ES2620112T3; BR112015018780A2; HUE032189T2; US20150368778A1; CN105247094A; MA38321B1; BR112015018780B1; SI2954086T1

Abstract

The invention concerns a metal sheet comprising a substrate (3) of which at least one face (5) is coated with a metal coating (7) having an aluminium weight content t_Al of between 3.6 and 3.8% and a magnesium weight content t_Mg of between 2.7 and 3.3%. The coating has a microstructure comprising a Zn/AI/MgZn₂ ternary eutectic lamellar matrix and optionally: - Zn dendrites with a cumulative surface content less than or equal to 5.0%; - Zn/MgZn₂ binary eutectic flowers with a cumulative surface content less than or equal to 15.0%; - Zn/AI binary eutectic dendrites with a cumulative surface content less than or equal to 1.0%; - and MgZn₂ islands with a cumulative surface content less than 1.0%.

Description

ZnAIMg coated sheet with special microstructure and

corresponding production method.

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 optionally 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 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.

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 according to the invention, after painting,

FIGS. 2 to 4 are diagrams illustrating the microstructure of the raw surface of the metal coatings of the sheet of FIG. 1,

FIG. 5 is a diagram illustrating the results of delamination tests carried out on a sheet sample according to the invention and of sheets which do not conform to the invention, and

FIG. 6 is a diagram illustrating current density and corrosion potential curves representative of different phases.

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, itself covered by a paint film 9, 1 1.

It will be observed that the relative thicknesses of the substrate 3 and the different layers 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. 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 μηι and aims to protect the substrate 3 against corrosion.

The coating 7 comprises zinc, aluminum and magnesium. The weight content of aluminum t _A i 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 FIGS. 2 to 4, the coating 7 has a particular microstructure with a lamellar matrix 13 of ternary eutectic Zn / Al / MgZn ₂ . As seen in FIG. 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 contents of Zn dendrites and flowers

Zn / MgZn ₂ binary eutectic on the outer surface 21 in the raw state are limited.

Preferably, the cumulative surface area of Zn dendrites at 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 Zn / Al binary eutectic dendrites or MgZn ₂ islands, 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 area 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 islets 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 between 1 and 200 μηι. 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 ^< and 460 ^< C.

At the inlet of the bath, the substrate 3 has an immersion temperature Ti such that:

(2.34 xt _AI + 0.655 xt _Mg -10.1) x 10 ^-6 <exp (-10584 / Ti) 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 of preferably greater than or equal to 15 Os between the onset of solidification (i.e., when the coating 7 falls just below the temperature of the liquidus) 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 to achieve the upper film 9, or by depositing a single layer of paint, which is generally the case to achieve the lower film 1 1. Other numbers of layers can be used in some 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 metal 1 according to the invention was prepared and samples of sheets not corresponding to the invention by varying the immersion temperature Ti, t _A i and t _Mg of the samples. The corresponding microstructures were analyzed to determine the existing structures and their cumulative surface contents.

^* 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

8μηι.

The composition of the coatings 7 of the sheets 1 according to the invention had a content t _A i of 3.7% and a content t _Mg of 3.0%. As indicated under the abscissa axis in the figure 5, the other tested coating compositions had values of t _A i 0.3%, 1, 5%, 6.0% and 1 1, 0% and _Mg t 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 °, 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 sheets tested are plotted on the ordinate in FIG.

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.

In FIG. 6, the corrosion potential with respect to a KCI saturated calomel reference (ECS) electrode is plotted on the abscissa and the current density on the 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.

FIG. 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

1. Sheet (1) comprising a substrate (3) of which at least one face (5) is coated with a metal coating (7) comprising Al and Mg, the remainder of the metal coating (7) being Zn, impurities inevitable and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, the weight content of each additional element in the metal coating (7) being lower than at 0.3%, the metal coating (7) having a content by weight of aluminum t _A i of between 3.6 and 3.8% and a magnesium content by weight of _Mg of between 2.7 and 3, 3%

the metal coating (7) having a microstructure comprising a lamellar matrix (13) of ternary eutectic Zn / Al / MgZn ₂ and optionally:

- Zn dendrites (15) with a cumulative surface content on the outer surface (21) of the coating (7) in the raw state less than or equal to 5.0%,

Zn / MgZn ₂ binary eutectic flowers (17) with a cumulative surface content on the outer surface (21) of the coating (7) in the raw state less than or equal to 15.0%,

Zn / AI binary eutectic dendrites with a cumulative surface content on the outer surface (21) of the metal coating (7) in the raw state of less than 1.0%,

MgZn ₂ islands with a cumulative surface content on the outer surface (21) of the coating (7) in the raw state less than 1, 0%.

2. Sheet according to claim 1, wherein the magnesium content t _Mg is between 2.9 and 3.1%.

3. Sheet according to claim 1 or 2, wherein the mass ratio AI / (AI + Mg) is greater than or equal to 0.45.

4. Sheet according to one of the preceding claims, wherein the microstructure does not include binary eutectic dendrite Zn / Al.

5. Sheet according to one of the preceding claims, wherein the microstructure does not include a block of MgZn ₂ .

6. Sheet according to one of the preceding claims, wherein the cumulative surface area of Zn / MgZn ₂ binary eutectic flowers (17) on the outer surface (21) of the coating (7) in the raw state is less than 10.0%.

Sheet according to Claim 6, in which the cumulative surface area of Zn / MgZn ₂ binary eutectic flowers (17) on the outer surface (21) of the coating (7) in the raw state is less than 5.0. %.

8. Sheet according to one of the preceding claims, wherein the cumulative surface content of Zn / MgZn ₂ binary eutectic flowers (17) on the outer surface (21) of the coating (7) in the raw state is less than 3.0%.

9. Sheet according to claim 8, wherein the cumulative surface content of Zn dendrites (15) on the outer surface (21) of the coating (7) in the raw state is less than 2.0%.

Sheet according to claim 9, wherein the cumulative surface area content of Zn dendrites (15) on the outer surface (21) of the coating (7) in the raw state is less than

1, 0%.

1 1. Sheet according to claim 10, wherein the microstructure consists solely of ternary eutectic (13).

12. Sheet according to one of the preceding claims, wherein the metal coating (7) is covered with at least one layer of paint and / or an oil layer.

13. A method of producing a sheet (1) according to any one of the preceding claims, the method comprising at least steps of:

supplying a substrate (3) made of steel,

depositing a metal coating (7) on at least one face (5) by quenching the substrate (3) in a bath, the substrate having an immersion temperature Ti at the inlet into the bath such that

(2.34 xt _A i + 0.655 xt _Mg -10.1) x 10 ^-6 <exp (-10584 / Ti) where Ti is expressed in Kelvin degrees, and

- solidification of the metal coating (7).

14. The production method according to claim 13, wherein the cooling rate of the coating (7) between the onset of solidification and the end of solidification is greater than or equal to 1 Ω / s.

15. The production method according to claim 14, wherein the cooling rate of the coating (7) between the onset of solidification and the end of solidification is greater than or equal to 20 Os.