EP0769567B1 - Process for coating a sheet - Google Patents

Abstract

A metal coating still in the liquid state is treated by projecting powder onto it. The powder is produced by atomisation and the agglomerated particles produced resemble the elementary grains contained in the agglomerated particles. The average size of the particles is greater than the thickness of the liquid coating.

Description

The invention relates to a method for treating a metal layer. sheet coating in which a powder is sprayed onto the layer metallic when it is, at least partially, in the liquid state

This process is commonly used during galvanizing of sheets steel, for example to achieve an aspect with a large configuration number of small flowers, as described in document FR-A-2 236 013.

In one of the embodiments described in this document, we project on the coating a powdery substance capable of undergoing thermal transformation in contact with liquid coating zinc (page 1, lines 31-36), like zinc powder (creator of crystalline germs, page 1, lines 37-38) or as salt powders decomposing below the zinc melting temperature (crystallization primers - page 2, lines 14-19).

For this particular type of galvanization, the sheet is quenched in a bath of liquid metal, we extract the sheet from the bath to cause a layer of said liquid metal on the sheet, a powder is sprayed onto said liquid layer driven and then solidifies said layer.

At the end of the bath, you can adjust the thickness of the metal layer trained before spraying the powder.

By this galvanizing process, coatings are commonly obtained thickness between 10 and 50 µm.

A variant of this galvanizing process consists in spraying the powder well after soaking: we heat the sheet to melt, at least partially, the coating layer, we spray the powder on the layer melted, then solidified.

At the time of spraying, the powder is inserted into the layer metallic, i.e. generally dispersed or dissolved.

The sheets coated according to this process exhibit, for example, better surface appearance (disappearance of flowering) or are easier to apply shape (better stampability).

Thanks to this process, it is also possible to widen the range of coating compositions.

One can also obtain "composite" coatings, of the inclusions type "granular" in a metallic matrix.

Among the sprayed powders, oxide powders can be used, or mixtures of metallic powders and oxide powders.

The quality of such coatings depends, among other things, on the morphology of powder and spray conditions on the sheet; these two parameters indeed determine the homogeneity of the distribution of the material projected from a on the surface of the molten metal layer, on the other hand in the depth of this layer.

To best disperse and / or dissolve the powder in the layer metallic, generally very fine powders are used.

However, the use of very fine particle size powders presents serious disadvantages, especially when the elementary grains of the powder have a size less than 10 µm.

The handling of fine particle size powders requires expensive waterproof protections, or even explosion-proof protections even more expensive to guard against the risk of explosion.

To avoid any risk of explosion when handling powders, especially when it contains a highly oxidizable component like magnesium, patent JP 02 093053 proposes to use powders atomized.

However, in the case of relatively thick metallic layers, it is sometimes difficult to spray the powder so that it enters the layer depth close to the sheet metal substrate, preventing to treat the coating (or the metallic layer entrained) in all its thickness and to obtain a homogeneous coating in the thickness.

When the projected material comprises several basic components, it should adapt and control the powder spraying process to achieve an identical and homogeneous distribution of all the components of base of the powder in the treated metal layer.

In particular, it is then often advisable to prepare a mixture perfectly homogeneous of these basic components, which requires new constraints, especially when the physical characteristics (for example example: density, grain sizes) of these components are very different.

The invention aims to treat a metal layer by spraying homogeneously in the depth, and, when the powder of treatment includes several basic components, spread them out one very evenly throughout the metal layer.

The subject of the invention is a method of treating a metal layer sheet metal coating, especially in a sheet galvanizing line, in which a powder is sprayed onto said metal layer when it is, at least partially, in the liquid state, characterized in that the said preparation is prepared powder by atomization in a manner suitable for it to be present under form of atomization agglomerates gathering elementary grains to dispersing and / or dissolving in said metallic layer in the liquid state, to that the average size of said agglomerates is greater than the thickness of said metallic layer in the liquid state, and so that said agglomerates have a sufficiently weak cohesion to disappear under the impact of projection of the agglomerate on said metal layer, so as to ensure the dispersion of the elementary grains of the agglomerate in the part of the said metallic layer which is in the liquid state.

The invention may also have the characteristic object of the claim 2 and also relates to the use, for implementing the method according to the invention, of a powder having one or more of Characteristics which are the subject of claims 3 to 5.

The invention will be better understood on reading the description which will follow, given as an example:

Installation of sheet metal coating by dipping with powder spraying is known in itself and will not be described here in detail; she understands , in particular a sheet dipping bath, wringing means for regulating the thickness of the metal layer on the sheet at the end of the bath, and a device of powder projection on the metallic layer still in the molten state after spinning.

To prepare the powder to be sprayed, an atomization installation is used of a known type and we proceed in a manner known in itself, from the or basic components of the material to be sprayed onto the sheet.

By way of nonlimiting example, one starts with one or more basic powders ; the nature and proportions of these basic powders are adapted from a known manner to the type of metal layer treatment to be performed.

The atomization installation is supplied from a composition of these basic powders, for example suspensions and / or liquid solutions of these basic powders.

According to another variant of the invention, it is possible to start directly from suspensions, in particular colloidal, and / or solutions containing the basic components of the material to be sprayed.

The feed composition is atomized to form a powder atomized, composed of agglomerates gathering elementary grains of basic powder, possibly of different natures and different morphologies.

When the material to be sprayed contains several components in predetermined proportions, atomization can be used to preformulate a mixture, in the sense that one adapts in a manner known per se the atomization conditions to obtain, within each agglomerate of spray powder, a mixture of grains of the different components in said proportions.

The conditions are determined in a manner known per se atomization to obtain, according to the invention, agglomerates having a average size greater than the thickness of the molten metal layer after spinning.

The atomized powder, because it consists of agglomerates of large size, can be handled with even less explosive or toxic risk.

Choose sufficiently fine base powders; so the agglomerates are made up of sufficiently fine elementary grains to ensure a homogeneous dispersion of the material to be sprayed in the layer metallic.

Preferably, these elementary grains have an average size less than about half the thickness of the molten metal layer.

Thus, preferably, in a manner known per se, the atomization conditions so that the ratio (average size of agglomerates) / (average size of elementary grains) is greater than approximately 4.

To the supply composition of the atomization installation, we add as necessary a binder intended to reinforce if necessary the cohesion of agglomerates of the atomized powder.

As binder, polyvinyl alcohol can in particular be used.

We determine, also in a manner known in itself, the atomization conditions as well as the nature and proportion of binder in the feed composition, to obtain cohesion of the agglomerates, certainly weak but sufficient.

By sufficient cohesion is meant a cohesion which is preserved in all powder handling operations located between atomization itself and the projection onto the sheet at the outlet of the coating bath.

By weak cohesion is meant cohesion which disappears under the impact projection of the agglomerate on the sheet, to ensure the dispersion of the grains elements of the agglomerate in the metallic layer in the molten state.

The porosity measurement, i.e. the ratio (total pore volume) / (volume total of an agglomerate), is a known means of assessing the cohesion of agglomerates.

Preferably, we adapt the proportion of binder and the conditions atomization to obtain agglomerates of porosity greater than approximately 30%, the porosity measured taking into account only the size pores greater than 0.01 µm.

We will now describe globally the process for processing a metallic coating layer according to the invention, in the case of the galvanizing of sheet metal.

We proceed to dip coating a sheet with projection of powder in a manner known per se, except that feeds the powder spraying device with the atomized powder previously described.

The sheet to be coated is here a steel sheet.

The metallic coating bath is here a zinc bath.

According to other variants of the invention, other types of metallic dip coating bath, such as zinc alloy baths or aluminum.

So we dip the sheet to be coated, wring it out to obtain a known in itself a predetermined thickness of metal layer driven on the sheet, then, while the metal layer is still in the state melted, the atomized powder is sprayed onto said layer.

Preferably, the temperature of the metal layer is higher at least about 10 ° C to that of its melting temperature.

We adapt the spraying conditions of the atomized powder so that the agglomerates which compose it break under the effect of projection impact against the sheet, which releases and disperses the elementary grains in the layer molten metal.

This ensures a uniform distribution of the material projected into the metallic layer, especially in the depth, and by this, we obtain better quality coated sheets.

The average size of the agglomerates of the atomized powder being greater than the thickness of the metal layer, it is easier than in art prior to spraying said powder with sufficient energy for the elementary grains are dispersed in the depth of the layer, at vicinity of the sheet metal substrate, which represents an essential advantage of the invention.

We no longer directly handle fine-grained powder, which avoids delicate security problems (environment, explosion).

We also find that it is much easier to implement the powder spraying device from these atomized powders and which even saves energy (gas flow and pressure in said device): it It’s much easier to spray a powder large particle size, like an atomized powder with large agglomerates, only a powder of fine particle size.

When the material to be sprayed contains several components and uses the atomization operation to preformulate a mixture of these components, the piloting constraints of the projection installation; indeed, the mixture being preformulated, one obtains much more easily than in the processes of the prior art a uniform and homogeneous distribution of the elementary grains of the material to project into the metallic layer.

So we can easily use a multi-component atomized powder, where the agglomerates comprise coated "heart" elementary grains by another matter.

By way of nonlimiting example, the elementary grains of heart are silica particles and the coating material consists of particles of metal of the same nature as that of the sheet dipping bath, here by example, zinc particles.

When the atomized powder is projected, the periphery of the agglomerates melts first, releasing the oxide particles it contains at heart to disperse them in the molten metal layer.

Thus, the powder to be sprayed, thanks to its method of preparation (atomization) and the size of its agglomerates, makes it possible to treat layers metallic (in the molten state) easily and very homogeneously.

Example :

The purpose of this example is to illustrate the invention in the case of a treatment of a metallic layer of coating with a metallic zinc powder.

More specifically, this processing takes place in an operation of galvanization, where, after quenching, the metal layer to be treated has a thickness of about 10 µm.

As a base powder, we take a zinc powder whose grains have an average size of around 4.5 µm.

A suspension of this base powder is prepared and diluted with polyvinyl alcohol at about 3% by weight (relative to the weight of powder of based).

This suspension (or feed composition) is atomized so to obtain batches of powder to be sprayed with large agglomerates average between 24 µm and 56 µm.

The porosity of the powders obtained is approximately 33.8% and the porosity internal has an average diameter of 0.06 µm.

At the end of galvanizing hardening, the metal layer is treated still in the liquid state by spraying a batch of powder and then solidifying.

For all batches of powder (average size of agglomerates included between 24 µm and 56 µm), a perfectly homogeneous treated coating is obtained in all depth.

Claims (5)

1. Process for treating a metal coating layer on a sheet, especially in a sheet galvanizing line, in which a powder is sprayed onto the said metal layer when it is at least partially in the liquid state, characterized in that the said powder is prepared by atomization in a suitable manner:

   in order for it to be in the form of atomized agglomerates which are comprised of elementary particles to be dispersed and/or dissolved in the said metal layer in the liquid state,

   in order for the average size of the said agglomerates to be greater than the thickness of the said metal layer in the liquid state and

   in order for the cohesion of the said agglomerates to be sufficiently weak for them to disappear under the impact of spraying the agglomerate onto the said metal layer so as to ensure that the elementary particles of the agglomerate are dispersed in that part of the said metal layer which is in the liquid state.
2. Process according to Claim 1, characterized in that the said powder is prepared in a suitable manner in order for the average size of the said particles to be less than approximately half the thickness of the said layer.
3. Use of an atomized powder for implementing the process according to either of Claims 1 and 2, characterized in that its open porosity, corresponding to pores having a size of greater than 0.01 µm, is greater than 30%.
4. Use of an atomized powder for implementing the process according to either of Claims 1 and 2, characterized in that the (average agglomerate size)/(average particle size) ratio is greater than approximately 4.
5. Use of an atomized powder for implementing the process according to either of Claims 1 and 2, characterized in that it contains components of various kinds and/or various morphologies.