EP0765952A1 - Process for preparing the metallic surface of an object, especially a steel sheet, for a direct enamelling - Google Patents

Abstract

Preparation of the metal surface of an object, notably a steel sheet, for subsequent direct enamelling, comprises: (a) a preliminary pickling or degreasing operation; (b) amorphous phosphating to form a layer of at least 0.2 g/m<2> on the metal surface; and (c) nickel-plating. In a complete sequence for the fabrication of an object formed from an enamelled steel sheet it is possible to form the object after the pickling stage, notable after the phosphating stage and before the final nickel-plating stage.

Description

The invention relates to a process for preparing metal surfaces for enamelling, in particular sheet steel.

To enamel a sheet, one can successively deposit on the sheet a layer of enamel of "mass" then a layer of enamel called "covered".

The mass enamel layer serves as a bonding layer to the sheet.

The cover layer is the top layer.

But one can also enamel a sheet without using a bonding layer, therefore without mass enamel: this is commonly called enameling "in direct mode".

In direct enameling, as in any enameling technique, several layers of covered enamel can also be superimposed.

To enamel a sheet in direct mode, by direct application of a covering layer, the metal surface to be enameled is prepared in order to obtain, in particular, good adhesion of the enamel layer to the sheet.

Overall then, to make an object like a pot or an enameled pan in direct mode, we shape a sheet, we prepare its surface, we coat it with enamel covered frit and we cook the enamel.

To prepare a metal surface for direct enameling, a first process is known in which the metal surface is etched and then the surface is treated with a sulphate solution containing nickel as a layer-forming cation; this surface treatment is also called "nickel plating".

Pickling and surface treatment are generally carried out by immersion or by spraying.

The disadvantage of this first method is that it requires a deep pickling, corresponding to a removal of material greater than 15 g / m2 on the surface to be enameled, to obtain enamels having satisfactory qualities in terms of adhesion and surface appearance.

Thus, in this first process, the removal of necessary material is commonly of the order of 25 g / m2 of surface to be glazed.

This deep pickling is a drawback insofar as it generates very large volumes of sludge to be treated.

To avoid the disadvantage of treating large volumes of sludge, document FR 2 593 522 proposes to replace the pickling and sulphate treatment by phosphating using a solution containing essentially nickel as a layer-forming cation; but the adhesion of the enamel has proved to be very insufficient in this case, and in any case less than that obtained in the case of the first process mentioned.

Finally, a third surface preparation process is known in which the surface is also treated with a phosphate solution, but after a light pickling: it is important then that the solution contains molybdenum, in addition to nickel as previously.

According to this third method, it would therefore no longer be necessary to pickle the metal surface to be enamelled as strongly to obtain an almost satisfactory adhesion and surface appearance: a lighter pickling, less than 15 g / m2 (per side), would suffice.

We then manage to reduce by approximately 40% the amount of sludge generated during surface preparation.

Other treatments of this type can be envisaged, for example with solutions containing antimony.

However, according to this third method, the treatment effluents then contain heavy metals, in particular molybdenum, and can be expensive to treat.

Furthermore, in practice, it seems difficult to achieve the same levels of enamel adhesion as in the first method described above, in particular in a reproducible manner.

The object of the invention is to obtain, on the metallic surface of an object, an enamel of strong adhesion and of good surface appearance by a preparation of the sheet metal generating less bulky effluents and easier to treat.

To this end, the subject of the invention is a process for preparing the metal surface of an object, in particular made of sheet steel, for enamelling in direct mode, in which the surface is scoured and then treated, characterized in that the etched surface is treated in two stages, a first stage of so-called "amorphous" phosphating treatment suitable for forming a layer of at least 0.2 g / m2 on said surface, then a second stage of treatment known as " nickel plating ".

Strong acids are preferably used for pickling, which makes it possible to reduce the pickling time.

For the amorphous phosphating treatment, a phosphate solution forming a layer is therefore used; this treatment is known in itself, in particular for the preparation of a sheet for stamping.

The nickel-plating treatment is known in itself for the preparation of a surface for direct enameling after a deep pickling and corresponds for example to the treatment of the first method of the prior art described above.

According to a preferred embodiment of the invention, the nickel-plating conditions are fixed in order to obtain a deposit of between 0.5 and 2.5 g / m2 on the surface to be glazed.

Thus, thanks to the surface treatment according to the invention, which comprises two stages, one of amorphous phosphating, the other of nickel plating, it is found that a light pickling is sufficient to obtain a reproducible enamel at a time strongly adherent and having a satisfactory surface appearance.

Light stripping means a stripping corresponding to a material removal of at most 15 g / m2, when the surface to be stripped is made of sheet steel.

Thanks to the invention, the quantity of sludge to be treated is therefore limited, the presence of heavy metals in the surface treatment effluents is avoided, while obtaining in direct mode a layer of enamel as adherent as by the first process cited. the prior art, namely a deep pickling followed by a single nickel-plating treatment with a sulphate solution.

From a metal sheet, in particular a steel sheet, to manufacture a finished enamelled object in direct mode, like a saucepan or a pot, it is necessary to carry out a shaping of the sheet, obviously before enamelling.

As the shaping, for example by stamping, is deemed to destroy the surface preparation for direct enameling, it is even shaped before the preparation of the surface; for, a sheet which, in the reverse order, would be pickled and then shaped (including oiling and degreasing) no longer had sufficient surface reactivity to make the surface treatment effective.

• mise en forme des objets à partir d'une bande de tôle, à savoir successivement huilage, mise en forme à proprement parler et dégraissage.
• préparation de surface des objets, à savoir décapage puis traitement de surface ;
• émaillage à proprement parler.

Thus, in the prior art for manufacturing an object made of enamelled sheet metal, the succession of manufacturing steps then presents the following order:

• shaping of objects from a sheet metal strip, namely successively oiling, shaping proper and degreasing.
• surface preparation of objects, namely pickling and surface treatment;
• enamelling itself.

The drawback of the order of these manufacturing steps is that it is more difficult to prepare the surface of already shaped objects than that of a sheet metal strip; indeed, on a sheet metal strip, it is possible to pickle and treat easily continuously and at high speed.

The invention also aims to simplify the method of manufacturing an object in enamelled sheet.

To this end, the invention therefore also relates to a method of manufacturing an object made of enameled metal sheet in direct mode, which comprises a shaping operation, a preparation of the surface according to the invention, and an operation enameling, characterized in that the shaping operation is carried out after pickling and after the first stage of surface treatment called amorphous phosphating.

Thanks to the invention which relates to the two-stage surface treatment, the etched and then phosphated surface nevertheless retains sufficient reactivity after shaping to make the nickel-plating treatment effective and give the enamel good adhesion and a good appearance. of surface.

To strip (and phosphatize) the sheet, we can then take advantage of very efficient industrial installations for strip strip stripping.

This new order of manufacturing operations also brings an additional advantage in terms of shaping since, thanks to the prior phosphating, the tribological properties of the surface are significantly improved, and therefore the lubrication between the sheet and the shaping tools. form.

Finally, this new order of manufacturing operations makes it possible to supply already pickled and phosphated sheets, suitable for direct enameling after possible shaping and a simple nickel-plating treatment; for its part, the enameler no longer needs pickling facilities and is freed from the treatment of pickling sludge.

The invention will be better understood on reading the description which follows, given by way of example, and with reference, for example 3, to the FIG. 1 which represents the adhesion of layers of enamel for two different surface preparations and as a function (abscissa axis) of the quantity of material removed during the prior pickling (g / m2 / face).

According to the main embodiment of the invention, a metallic object to be enameled is taken.

This object is produced by shaping a sheet metal blank, here of steel. According to the invention, the sheet is etched then a first amorphous phosphating surface treatment is carried out.

The composition of the pickling bath is known in itself; to shorten the pickling time, preferably using baths based on strong acid, for example sulfuric acid.

The pickling conditions are chosen so as to remove at most 15 g / m2 of steel (per side).

The amorphous phosphating bath is known in itself. The phosphating conditions are chosen to deposit a layer of at least 0.2 g / m2 per side on the sheet to be glazed.

Preferably, a phosphating solution and suitable application conditions are chosen so that the weight of the layer does not exceed 2 g / m2 per side.

Preferably, a suitable phosphating solution is chosen to obtain a layer based on amorphous phosphate of sodium, calcium and iron.

The phosphating solution used does not contain heavy metals, nor does the phosphating effluent contain it.

The rinsing and drying operations which accompany pickling and phosphating are known in themselves and are not described here in detail.

Still according to the invention, the second surface treatment of the preparation process according to the invention is then carried out.

This treatment is of a conventional type of nickel plating with a solution containing nickel cations, known in itself for preparing a metal surface for direct enameling.

The solution contains, for example, sulphate or phosphite anions.

Preferably, the nickel-plating conditions are adapted in a manner known per se to obtain a deposit of 0.5 to 2.5 g / m2 on the surface of the object to be enamelled.

The nickel-plating effluents are those which are usually found upstream of the direct enameling installations of the prior art and do not pose any particular problem for their treatment; an advantage of the invention is that they do not contain heavy metals.

The rinsing and drying operations which accompany nickel plating are known in themselves and are not described here in detail.

The object thus prepared according to the invention is now ready for direct enameling.

The object is coated with a single layer of covered enamel which is cooked in a manner known per se.

According to the invention, and even when only a light pickling has been carried out to prepare the surface, an enamel layer is obtained having a strong adhesion and a good surface appearance.

Light stripping means a stripping corresponding to a material removal of at most 15 g / m2 if the surface of the object to be glazed is made of steel.

By strong adhesion is meant an adhesion at least comparable to that which would be obtained by preparing the surface of the object according to the first process cited in the prior art; this process differs essentially from that of the invention by a greater stripping and by the absence of the phosphating treatment.

Thanks to the invention, a quantity of pickling sludge is also generated which is much lower than that of the first process cited in the prior art; the quantity of sludge can in particular be reduced by 40%.

Finally, still thanks to the invention, the surface treatment effluents do not contain heavy metals, unlike the third process cited in the prior art, using treatment solutions containing molybdenum.

The invention can also be implemented in the context of a method for manufacturing an object in enamelled sheet in direct mode.

For example, we start from a strip of steel sheet leaving a cold rolling train, more precisely from the work hardening cage.

According to the invention, the sheet metal strip is stripped and then a first amorphous phosphating surface treatment is carried out.

Advantageously, the pickling and the phosphating treatment are carried out online after work hardening, at a high rate, for example at a speed of 30 m / min. ; pickling time can be significantly reduced compared to the pickling times of the preparation processes of the prior art: for example 2 to 25 seconds compared to commonly more than five minutes in the prior art.

As before, pickling conditions are chosen in a manner known per se to obtain strong adhesion of the enamel while removing at most 15 g / m2 per side.

An amorphous phosphating bath known in itself is used, such as the bath commercially known as Bonderite 901 from the company PARKER, at a temperature of approximately 60 ° C.

As before, adequate phosphating conditions are chosen, in order to obtain a layer or a deposit of at least 0.2 g / m 2 per face.

The pickling sludge and the phosphating effluents can be treated in effluent treatment installations provided elsewhere for treating the rolling and work hardening effluents.

The pickled and phosphated sheet can be wound and sent to the enamellers, who carry out the shaping and enameling directly after simple nickel plating.

The pickled and phosphated sheet can then be considered as an intermediate commercial product, prepared for direct enameling.

Advantageously, the phosphating treatment provides temporary protection against corrosion and prepares for shaping.

According to the order of the manufacturing method according to the invention, the shaping is therefore then carried out, before the second nickel-type surface treatment.

Thus, the sheet is oiled, it is shaped, in particular by stamping, and the part formed is degreased, for example in an alkaline solution.

Thanks to the phosphating treatment, and while using a conventional stamping oil, the coefficient of friction of the treated sheet against the forming tools is lower than with an untreated sheet, which facilitates the operation of shaping.

The second surface treatment of the preparation process according to the invention is then carried out as before, that is to say nickel plating.

The shaped and nickel-plated sheet is now ready for direct enameling.

The sheet is coated with a single layer of covered enamel which is fired in a manner known per se: the object is then obtained in enameled sheet.

According to the invention, and even when the shaping is carried out after pickling, an enamel layer is obtained having an adhesion entirely comparable to that obtained in the first embodiment; this result indicates that, even after shaping, the etched and phosphated surface remained sufficiently reactive to make the nickel-plating treatment effective.

Thanks to the manufacturing method according to the invention, part of the surface preparation treatment, in particular pickling, is advantageously displaced before shaping: pickling can then be carried out continuously on the sheet metal strip directly downstream of the rolling, on large capacity industrial installations which benefit from widely dimensioned effluent and sludge treatment installations.

The amorphous phosphating treatment, which is an integral part of the preparation for enameling, facilitates shaping.

The following examples illustrate the invention.

In the examples which follow, the composition of treatment solutions and deposits made on the surface of steel sheets is indicated; the analysis methods used to obtain these compositions are: chromatonionic analysis, atomic absorption and analysis by inductively coupled plasma for solution analyzes, as for deposit analyzes which are carried out after re-solution.

Comparative example 1 :

The purpose of this example is to illustrate the first method cited in the prior art in which a deep pickling is carried out followed by a single nickel-plating treatment of the metal surface to be glazed.

The sheet to be enamelled is a sheet of decarburized steel 1 mm thick, called SOLFER from the company SOLLAC.

The sheet is shaped, then degreased in an alkaline solution.

The sheet is then pickled by immersion for 6 to 11 minutes in an acid solution at approximately 70 ° C. containing approximately 70 g / l of sulfuric acid.

The pickling time is adjusted to obtain a weight loss of the sheet from 20 to 40 g / m2 per side.

This weight loss is necessary to obtain the qualities of adhesion and surface appearance of the enamel to be deposited.

This stripping generates sludge to be treated; the amount of sludge generated is proportional to the target weight loss.

The etched surface is then treated by immersion for 3 to 6 minutes in a nickel-plating solution at approximately 70 ° C. containing approximately 11 g / l of nickel sulphate, the pH of which has been adjusted to approximately 2.8 by addition of sulfuric acid. .

The nickel-plating time is adjusted to obtain a nickel-plating deposit of between 0.5 and 2.5 g / m2 per side.

The coated surface is then coated with enamel.

Take for example a liquid white enamel called L138 from the company FERRO and deposit a layer of approximately 300 g / m2 per side.

The enamel is then baked under conditions specific to its composition, here about 3 minutes at around 820 ° c.

An enamelled steel sheet is then obtained.

The qualities of adhesion and surface appearance of the enamel are then evaluated.

The adhesion of the enamel layer to the sheet steel substrate is evaluated according to standard EN 10209, which defines a scale of five ratings, from the value 1 for excellent adhesion to the value 5 for poor adhesion.

A person skilled in the art evaluates the surface quality in a manner known per se, in particular by visually verifying the absence of defects such as bites, broths, nail blows.

Here, the enamel obtained has a level of adhesion of 1 and a surface appearance qualified as good.

Comparative example 2 :

The purpose of this example is to illustrate the third method cited in the prior art in which a light pickling is carried out followed by a single treatment of the metal surface to be enameled with a phosphating solution containing nickel and molybdenum.

The procedure is carried out on the same substrate as in Comparative Example 1, it is shaped and degreased as before.

The sheet is then pickled by immersion for 4 to 12 minutes in an acid solution at approximately 65 ° C. containing approximately 25 g / l of sulfuric acid.

The pickling time is adjusted to obtain a weight loss of the sheet from 5 to 15 g / m2 per side.

Advantageously according to this method, the loss of weight on pickling can be approximately 50% less than that of Comparative Example 1 and the quantity of sludge generated by pickling is reduced in the same proportions.

The etched surface is then treated by immersion for 6 to 12 minutes in a phosphating solution at approximately 60 ° C.

The phosphating solution is commercially available under the name VP 10091 from the company CHEMETALL.

This solution mainly contains the following elements: P205: 15 to 20 g / l - sodium: 4 to 6 g / l - nitrates (expressed as NO3-): 3 to 4 g / l - nickel: 1 to 2 g / l - sulfates (expressed in SO4 =): 0.5 to 1.5 g / l - fluorine (expressed in F-): 0.5 to 1 g / l - silicon: 0.1 to 0.3 g / l - iron : 0.01 to 0.1 g / l - ammonium (expressed as NH4 +): 0.08 to 0.12 g / l - molybdenum: 0.03 to 0.1 g / l - calcium: 0.05 to 0 , 5 g / l.

The treatment time is adjusted to obtain a deposit of between 1 and 1.5 g / m2 per side.

The phosphating solution here contains heavy metals, in particular molybdenum, and generates effluents which can be expensive to treat.

The deposit obtained typically contains the following elements: P2O5: 0.1 to 0.2 g / m2 / side - Na: 0.05 to 0.1 g / m2 / side - Ni: 0.05 to 0.1 g / m2 / side - Mo: 0.05 to 0.1 g / m2 / side - Ca: 0.05 to 0.1 g / m2 / side.

As can be seen, the molybdenum / nickel proportion is much higher in the deposit than in the treatment solution, which indicates that the molybdenum deposits preferentially over nickel; thus, the proportion of nickel deposited remains lower than that of Comparative Example 1.

Then coated with enamel frit, baked as in Comparative Example 1 and obtained an enameled steel sheet.

The qualities of adhesion and surface appearance of the enamel are evaluated as before; the following results are obtained: adhesion: 3 - surface appearance: "good".

Thus, as already indicated in the preamble, this method does not make it possible to achieve the same level of adhesion as the first method cited in the prior art (cf. comparative example 1).

Example 1 :

The purpose of this example is to illustrate the surface preparation process according to the invention.

The procedure is always carried out on the same substrate as in Comparative Example 1 which is formed and which is degreased in an alkaline solution.

The sheet is then pickled by immersion for 1.5 to 4.5 minutes in an acid solution at approximately 70 ° C. containing approximately 70 g / l of sulfuric acid.

The pickling time is adjusted to obtain a weight loss of the sheet from 5 to 15 g / m2 per side, with the same advantage in terms of the amount of sludge generated as in Comparative Example 2.

The etched surface is then treated by immersion for 0.5 to 6 minutes in a phosphating solution at approximately 60 ° C.

The phosphating solution is commercially available under the name Bonderite 901 from the company PARKER.

This solution mainly contains the following elements: P2O5: 5 to 15 g / l - sodium: 10 to 20 g / l - nitrates (expressed as NO3-): 0 to 4 g / l - calcium: 5 to 20 g / l - and, for the elements Ni, Mo, Si, Fe, SO4, F: <0.05 g / l.

The treatment time is adjusted to obtain a deposit of between 0.2 and 2 g / m2 per side.

The deposit obtained typically has the following elements: P2O5: 0.02 to 0.5 g / m2 / side - Na: 0.02 to 0.1 g / m2 / side - Ca: 0.2 to 0.5 g / m2 / face; the nickel and molybdenum contents are not measurable and are less than 0.005 g / m2 / side.

The phosphated surface is then treated by immersion for 3 to 6 minutes in a nickel-plating solution as in Comparative Example No. 1.

Then coated with enamel and baked as in Comparative Example 1 to obtain an enameled steel sheet.

The qualities of adhesion and surface appearance of the enamel are evaluated as before; the following results are obtained: adhesion: 1 - surface appearance: "good".

The level of adhesion is comparable to that of Comparative Example 1, that is to say that obtained with the first process cited in the prior art.

According to the invention, it was therefore possible to obtain an enamel layer having satisfactory qualities of adhesion and surface appearance while limiting the amount of sludge generated by pickling and without producing effluents containing heavy metals.

Example 2 :

The purpose of this example is to illustrate the process for manufacturing an object made of enamelled sheet metal according to the invention, in which it is shaped after pickling, in particular after the phosphating treatment and before the nickel-plating treatment.

The procedure is always carried out on the same substrate as in Comparative Example 1 which is degreased in an alkaline solution.

The sheet is then etched by immersion for 5 to 15 seconds in an acid solution at approximately 100 ° C. containing 750 g / l of sulfuric acid approximately.

The pickling time is adjusted to obtain a weight loss of the sheet from 5 to 15 g / m2 per side, with the same advantage in terms of the amount of sludge generated as in Comparative Example 2.

The pickled surface is then treated by immersion for 5 to 25 seconds in the same phosphating solution as in Example 1, at approximately 60 ° C.

Also as in Example 1, the phosphating time is adjusted to obtain a deposit of between 0.2 and 2 g / m2 / face having the same composition approximately.

According to this variant of the invention, the phosphated sheet is then shaped to form an object, according to a conventional sequence comprising an oiling of the sheet, the shaping proper and an alkaline degreasing.

Advantageously, while using a conventional oil, the shaping is facilitated by the prior phosphating treatment: there is indeed a decrease in the coefficient of friction compared to that which would be observed with the same oil on the same sheet. , raw or coming directly from pickling.

After shaping, the metal surface of the object is then treated by immersion for 3 to 6 minutes in a nickel-plating solution as in Comparative Example No. 1.

Then coated with enamel and baked as in Comparative Example 1.

We then obtain an object in enamelled sheet.

The qualities of adhesion and surface appearance of the enamel are evaluated as before; the following results are obtained: adhesion: 1 - surface appearance: "good".

The level of adhesion and the surface appearance are comparable to those of Example 1, which indicates that the shaping did not deteriorate the surface reactivity obtained during pickling, although carried out after pickling.

According to the invention, the advantages already mentioned in Example 1 are added to the possibility of carrying out the first stages of surface preparation, namely pickling and phosphating, on high-speed industrial lines, in particular directly at the exit from rolling. , more precisely of work hardening.

According to this variant of the invention, the shaping is also facilitated.

Example 3 :

The purpose of this example is to illustrate, in the surface preparation process according to the invention, the importance of the amorphous phosphating step for obtaining good adhesion, when only a light pickling is carried out beforehand. not more than about 15 g / m2 / side.

On the one hand, a series of comparative samples is prepared according to the procedure of Comparative Example 1, with the difference that the conditions of prior stripping are varied to obtain different weight losses: 0, 5, 10, 15, 20, 25 g / m2 / side respectively for samples Ec1, Ec2, Ec3, Ec4, Ec5 and Ec6.

• on fait varier les conditions de décapage préalable pour obtenir les mêmes différentes pertes de poids : 0, 5, 10, 15, 20, 25 g/m2/face respectivement pour les échantillons E1, E2, E3, E4, E5 et E6.
• on ajuste les conditions de phosphatation amorphe (à l'aide de la même solution dénommée Bonderite 901) pour obtenir un poids de couche de phosphatation de environ 1,4 g/m2/face.

On the other hand, a series of samples is prepared according to the procedure of Example 1, with the difference that:

• the conditions of prior pickling are varied to obtain the same different weight losses: 0, 5, 10, 15, 20, 25 g / m2 / face respectively for the samples E1, E2, E3, E4, E5 and E6.
• the amorphous phosphating conditions are adjusted (using the same solution called Bonderite 901) to obtain a phosphate layer weight of approximately 1.4 g / m2 / face.

The curve in FIG. 1 represents the adhesion results obtained for the two series of samples (on the ordinate: 1 for very good adhesion, 5 for poor adhesion) as a function of the amount of material removed during pickling (on the abscissa). : 0 to 25 g / m2 / side).

The curve marked with squares - "phosphated" - corresponds to the samples prepared according to the invention, the curve marked with diamonds - "not phosphated" - corresponds to the samples prepared according to the prior art with simple nickel plating after pickling.

This example illustrated by FIG. 1 clearly shows that there are significant differences in adhesion between the two surface preparation processes, as soon as the pickling is less than or equal to 20 g / m2 / face, in particular when it is at more than 15 g / m2 / side.

FIG. 1 also shows that in order to obtain sufficient adhesion of the enamel layer when proceeding according to the invention, it is advisable for the preliminary stripping to be preferably greater than or equal to 5 g / m2 / face: the level of adhesion is then 3; to obtain the same level of adhesion by proceeding without amorphous phosphating, the surface should be stripped at least at the level of 15 g / m2 / face.

Example 4 :

The purpose of this example is to show that the etched and phosphated surface retains, after shaping, sufficient reactivity to make effective a direct nickel-plating treatment and to obtain good adhesion of the enamel layer.

The operation of shaping a sheet consists of oiling it, deforming it, in particular by stamping, and finally degreasing it.

Such a shaping operation would therefore be likely to degrade the phosphating layer and the underlying pickling surface, in particular by friction.

We will first show that the presence of an amorphous phosphating layer on the sheet facilitates shaping, insofar as it improves the tribological properties of the surface.

• M1 : acier non phosphaté huilé par une huile de protection,
• M2 : acier phosphaté puis huilé par une huile de protection,
• M3 : acier non phosphaté huilé par une huile de protection puis une huile d'emboutissage,
• M4 : acier phosphaté puis huilé par une huile de protection puis une huile d'emboutissage.

Four samples are prepared:

• M1: non-phosphated steel oiled with a protective oil,
• M2: phosphated steel then oiled with a protective oil,
• M3: non-phosphated steel oiled with a protective oil and then a stamping oil,
• M4: phosphated steel then oiled with a protective oil then a stamping oil.

Unprocessed steel is called untreated raw steel.

• dégraissage de l'échantillon une solution à 90°C, puis rinçage à 90°C, enfin rinçage à froid.
• décapage par une solution d'acide sulfurique (H2SO4 à 600 g/l) à 90°C dans des conditions adaptées pour enlever sur chaque face 10 g/m2.
• rinçage acide de la surface décapée, puis traitement par une solution de phosphatation dénommée Bonderite (cf. exemple 1) à 70°C pendant 10 secondes, puis rinçage et séchage.

A sample of pickled steel covered with an amorphous phosphate layer of at least 0.2 g / m2 obtained under the following conditions is called phosphated steel:

• degreasing of the sample a solution at 90 ° C, then rinsing at 90 ° C, finally cold rinsing.
• pickling with a sulfuric acid solution (H2SO4 at 600 g / l) at 90 ° C under suitable conditions to remove on each side 10 g / m2.
• acid rinsing of the pickled surface, then treatment with a phosphating solution called Bonderite (cf. example 1) at 70 ° C for 10 seconds, then rinsing and drying.

Protective oil is an oil usually used for temporary protection against corrosion, in particular for the storage of sheets.

Stamping oil is an oil usually used for stamping operations, suitable for improving the tribological properties of a sheet surface.

The tribological properties of the surfaces of the different samples are measured in the following manner, all the samples having the same dimensions.

The sample to be measured is clamped in a blank greenhouse with a predetermined clamping force F _s .

The tribological properties are characterized by then measuring the maximum drawing force F _d of the sheet in the blank holder.

This maximum drawing force F _d is obviously proportional to the clamping force F _s .

The lower the maximum drawing force F _d for a given clamping force F _s , the better the tribological properties of the surface.

• pour l'échantillon M3 , F_d est 3% à 4% inférieur aux forces de serrage mesurées pour l'échantillon M1 dans les mêmes conditions.
• pour les échantillons M2 et M4 , F_d est 8% à 11% inférieur aux forces de serrage mesurées pour l'échantillon M1 dans les mêmes conditions.

For clamping forces F _s between 1 and 6 kN, it can be seen that:

• for sample M3, F _d is 3% to 4% lower than the clamping forces measured for sample M1 under the same conditions.
• for samples M2 and M4, F _d is 8% to 11% lower than the clamping forces measured for sample M1 under the same conditions.

We deduce that the amorphous phosphating treatment (M2 and M4) improves the tribological properties much more significantly than the application of a stamping oil (M3), which is not only an advantage for the shaping itself. even, but also limits a priori the risks of degradation of the surface by friction and therefore contributes to maintaining the reactivity of the surface.

Maintaining the surface reactivity in the event of friction is illustrated below in the example.

Then an M2 or M4 type sample (see above) is shaped by stamping.

The so-called "rubbed" zones and the so-called "non-rubbed" zones are identified on the shaped sample.

The "rubbed" areas are areas where the surface has been substantially modified by friction on the stamping tool, where therefore the surface reactivity may have been degraded.

In contrast, the non-rubbed areas are areas which do not seem to have undergone particular friction and which have retained the appearance they had before shaping.

• dégraissage électrolytique par traitement anodique dans une solution à 60°C, à pH # 12 , sous # 10 A/dm2 pendant 2 fois 30 secondes.
• nickelage électrolytique dans une solution à 60°C, sous 18 A/dm2 pendant 6,5 secondes.
• émaillage blanc par une composition référencée L138 de la Société FERRO, la cuisson de l'émail étant effectuée au four tunnel à environ 820°C.

After shaping, the sample is subjected to the following operations:

• electrolytic degreasing by anodic treatment in a solution at 60 ° C, pH # 12, under # 10 A / dm2 for 2 times 30 seconds.
• electrolytic nickel plating in a solution at 60 ° C, under 18 A / dm2 for 6.5 seconds.
• white enameling with a composition referenced L138 from the company FERRO, the enamel being fired in a tunnel oven at around 820 ° C.

• zones non frottées : adhérence = 1 (comme exemple 1)
• zones frottées : adhérence = 2 .

The adhesion of the enamel is then measured as before on rubbed areas and on non-rubbed areas; the results are as follows:

• areas not rubbed: adhesion = 1 (as example 1)
• rubbed areas: adhesion = 2.

The result obtained on rubbed areas must be compared with that obtained on a non-phosphate sample, directly nickel-plated after a pickling of the order of 10 g / m2 / face.

Referring to FIG. 1, it can be seen that for this type of non-phosphate sample, the adhesion is only 4.

It is deduced therefrom that, even on the rubbed areas, the enamel layer adheres to a completely acceptable level and that the amorphous phosphating treatment remains effective even if the surface is substantially modified by friction.

The shaping therefore does not degrade the surface reactivity, which makes it possible, according to the invention, to nickel directly after the shaping, without repeating stripping.

Still according to the invention, it is not necessary to protect the phosphating layer in particular before carrying out the shaping.

Claims (4)

Process for the preparation of the metallic surface of an object, in particular made of sheet steel, for enamelling in direct mode, in which the etched surface is then treated, characterized in that the etched surface is treated in two stages, a first so-called "amorphous" phosphating treatment step suitable for forming a layer of at least 0.2 g / m2 on said surface, then a second so-called "nickel-plating" treatment step. A method according to claim 1 characterized in that, in the case of a steel sheet, said surface is etched under conditions suitable for removing at most 15 grams of steel per square meter from said surface. Method according to any one of the preceding claims, characterized in that the nickel-plating conditions are adapted to obtain a deposit of between 0.5 and 2.5 g / m2 on the surface of the object to be enamelled. Method for manufacturing an enameled sheet metal object in direct mode, which comprises a shaping operation, a preparation of the surface by the method according to any one of the preceding claims, and an enameling operation, characterized in that the shaping operation is carried out after pickling and after the first stage of surface treatment called amorphous phosphating.

Images