EP0765952B1 - Process for preparing the metallic surface of an object made of a steel sheet for 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 surface preparation process. metallic of a sheet steel object for enameling.

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

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

The cover layer is the top layer.

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

In direct enamelling, as in any technique you can also superimpose several layers of enamel covered.

To enamel a sheet in direct mode, by direct application of a cover layer, we prepare the metal surface to be glazed so to obtain, in particular, good adhesion of the enamel layer to the sheet.

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

To prepare a metal surface for direct enameling, we know a first process in which the metal surface is scoured then the surface with a sulfate solution containing nickel as a cation diaper trainer; this surface treatment is also called "nickel plating"; pickling and surface treatment are generally carried out by immersion or by sprinkling.

Document FR-A-1237493 thus describes a direct enameling process "where only the finishing layer is necessary" (page 1, column 2, line 12) where the surface to be enameled is first pickled in citric acid medium or glycolic at a temperature above 71 ° C; before application of the enamel layer, chemical nickel plating is carried out suitable for depositing between 0.3 and 4 g / m ² of nickel; as indicated on page 4, column 2, line 23, it may be advantageous to add a small proportion of phosphoric acid to the bath (pickling).

The document FR-A-2020356 also describes a method of surface preparation for enamelling in direct mode, in which one pickles at least 20 g / m2 (page 1, lines 11 to 15 - page 6, lines 20 and following) and then treating said surface in two stages: a first stage thin layer phosphating (page 6, lines 12 to 15) then a second nickel plating step; page 6, line 20 and following, we describe a electrolytic pickling process and, under the conditions described, also specifies that a removal of 20 g / m2 is achieved.

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

So, in this first process, the necessary material removal 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 inconvenience of processing large volumes of sludge, document FR 2 593 522 proposes to replace the pickling and sulphate treatment by phosphating with a solution essentially containing nickel as a layer-forming cation; but the adhesion of the enamel proved to be very insufficient in this case, and in any case less than that obtained in the case of the first process cited.

We finally know a third method of surface preparation in which we also treat the surface with a phosphate solution, but after a light pickling: it is important that the solution contains molybdenum, in addition to nickel as before.

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

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

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

But, according to this third process, the treatment effluents then contain heavy metals, in particular molybdenum, and can be expensive to deal with.

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

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

To this end, the invention relates to a process for preparing the metal surface of a sheet steel object, for enameling in direct mode, in which it is stripped and then the said surface is treated, characterized by stripping said surface under conditions suitable for removing not more than 15 grams of steel per square meter of said surface and then treats the pickled surface 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.

Strong acids are preferably used for pickling, which reduces the pickling time.

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

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

According to a preferred embodiment of the invention, the nickel-plating conditions 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, finds that a light pickling is enough to obtain a reproducible an enamel that is both strongly adherent and has an appearance of satisfactory surface.

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

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

From a steel sheet, for make a direct enamelled finished object, such as a saucepan or a make a direct enamelled finished object, such as a saucepan or a cooking pot, it is necessary to proceed with a shaping of the sheet, obviously before the mesh.

As shaping, for example by stamping, is deemed destroy the surface preparation for direct enameling, even form before surface preparation; because, a sheet which would be, in reverse order, pickled and then shaped (including oiling and degreasing) no longer has 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 downside to the order of these manufacturing steps is that it is more difficult to prepare the surface of already shaped objects than that a sheet metal strip; on a strip of sheet metal in fact, it is possible in particular strip and process easily continuously and at high speed.

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

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

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

To strip (and phosphatize) the sheet, we can then take advantage high-performance industrial strip stripping installations sheet metal.

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

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

The invention will be better understood on reading the description which will follow, given by way of example, and with reference, for example 3, to the figure 1 which represents the adhesion of layers of enamel for two different and depending on surface preparations (abscissa axis) of the quantity of material removed during preliminary pickling (g / m2 / side).

According to the main embodiment of the invention, an object is taken metallic to enamel.

This object is produced by shaping a sheet of sheet steel.

According to the invention, the sheet metal is scoured then a first operation is carried out amorphous phosphating surface treatment.

The composition of the pickling bath is known in itself; for shorten the pickling time, preferably using base baths 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.

We choose the phosphating conditions to deposit a layer at least 0.2 g / m2 per side on the sheet to be glazed.

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

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

The phosphating solution used does not contain metals heavy and phosphating effluents do not contain either.

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

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

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

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

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

The nickel-plating effluents are those usually found in upstream of the prior art direct enameling installations and do not pose no 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 the 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 enamel and covered in a manner known in itself.

According to the invention, and even though only stripping was carried out light to prepare the surface, we obtain a layer of enamel with strong adhesion and 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 adhesion at least comparable to that which we would obtain by preparing the surface of the object according to the first cited process of the prior art; this process essentially differs of that of the invention by a greater stripping and by the absence of the phosphating treatment.

Thanks to the invention, a quantity of sludge is also generated. pickling 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 effluents for treating surface does not contain heavy metals, unlike the third cited process of the prior art, using treatment solutions containing molybdenum.

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

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

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

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

As before, we choose pickling conditions for a in itself known manner to obtain a strong adhesion of the enamel while removing at most 15 g / m2 per side.

We use an amorphous phosphating bath known in itself, as the bath commercially known as Bonderite 901 from the Company PARKER, at a temperature of around 60 ° C.

We choose, as before, adequate conditions of phosphating, to obtain a layer or a deposit of at least 0.2 g / m2 per side.

Pickling sludge and phosphating effluents can be treated in effluent treatment facilities provided elsewhere for treating effluents from rolling and work hardening.

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 a intermediate commercial product, primed for direct enameling.

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

According to the order of the manufacturing process according to the invention, the procedure is so after shaping, before the second surface treatment nickel-plating type.

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

Thanks to the phosphating treatment, and while using an oil conventional stamping, the coefficient of friction of the treated sheet against the shaping tools is weaker than with a sheet not treated, which facilitates the shaping operation.

We then proceed as before to the second processing of surface of the preparation process according to the invention, that is to say at nickel plating.

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

We cover the sheet with a single layer of covered enamel that we bake in a manner known in itself: we then obtain the sheet metal object enameled.

According to the invention, and even when the shaping is carried out after pickling, we obtain a layer of enamel with adhesion quite comparable to that obtained in the first mode of production ; this result indicates that, even after shaping, the surface pickled and phosphated remained reactive enough to make effective nickel treatment.

Thanks to the manufacturing process according to the invention, we move advantageously part of the surface preparation treatment, especially pickling, 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 processing facilities effluents and sludge.

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

The following examples illustrate the invention.

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

Comparative example 1:

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

The sheet to be enameled is a decarbonated steel sheet with a thickness of 1 mm, called SOLFER from SOLLAC.

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

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 acid sulfuric.

The stripping 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 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 stripped surface is then treated by immersion for 3 to 6 minutes in a nickel-plating solution at around 70 ° C containing 11 g / l approximately 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 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 FERRO company and a layer of approximately 300 g / m2 is applied per side.

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

An enamelled steel sheet is then obtained.

We then assess the qualities of adhesion and surface appearance of enamel.

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

The skilled person assesses the surface quality in a known manner in itself, in particular by visually checking the absence of defects such as bites, broths, nail blemishes.

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

Comparative example 2:

The purpose of this example is to illustrate the third process cited in the art previous in which one carries out a light stripping followed by a single treatment of the metal surface to be glazed with a solution of phosphating containing nickel and molybdenum.

We proceed on the same substrate as in Comparative Example 1, we form and degrease as before.

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

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

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

The stripped surface is then treated by immersion for 6 to 12 minutes in a phosphate solution at around 60 ° C.

The phosphating solution is commercially available under the name VP 10091 of 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 as 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 deal with.

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 we can see, the molybdenum / nickel proportion is much higher higher in the deposit than in the treatment solution, which indicates that molybdenum is preferentially deposited with nickel; so the proportion of nickel deposited remains lower than that of Comparative Example 1.

Then coat with enamel frit, bake as in the example comparison 1 and we obtain a sheet of enamelled steel.

As previously assessed, the qualities of adhesion and appearance enamel surface; the following results are obtained: adhesion: 3 - surface appearance: "good".

Thus, as already indicated in the preamble, this process does not allow achieve the same level of adhesion as the first process cited in the art previous (cf. comparative example 1).

Example 1:

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

We always proceed on the same substrate as in the example comparison 1 that is shaped and degreased in a solution alkaline.

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

The stripping 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 amount of sludge generated as in Comparative Example 2.

The stripped surface is then treated by immersion for 0.5 to 6 minutes in a phosphate solution at around 60 ° C.

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

This solution mainly contains the following elements: P205: 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 / side; 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 the comparative example # 1.

Then coated with enamel and baked as in the comparative example 1 to obtain an enamelled steel sheet.

As previously assessed, the qualities of adhesion and appearance enamel surface; the following results are obtained: adhesion: 1 - surface appearance: "good".

The level of grip is comparable to that of the example Comparative 1, that is to say that obtained with the first process mentioned of the prior art.

According to the invention, it was therefore possible to obtain a layer of enamel 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 of manufacturing an object enamelled sheet according to the invention, in which it is shaped after pickling, in particular after the phosphating treatment and before nickel plating treatment.

We always proceed on the same substrate as in the example Comparative 1 which is degreased in an alkaline solution.

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

The stripping 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 amount of sludge generated as in Comparative Example 2.

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

As in example 1 also, the time of phosphating to obtain a deposit of between 0.2 and 2 g / m2 / side having the same composition approximately.

According to this variant of the invention, the next step is to shape of the phosphated sheet to form an object, according to a sequence classic including oiling the sheet, shaping properly talk and alkaline degreasing.

Advantageously, while using a conventional oil, the implementation shape is facilitated by the preliminary phosphating treatment: indeed observes a decrease in the coefficient of friction compared to the one we would observe with the same oil on the same sheet, raw or straight out of 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 1.

Then coat with enamel and bake as in the example comparative 1.

We then obtain an object in enamelled sheet.

As previously assessed, the qualities of adhesion and appearance enamel surface; 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 of Example 1 are added to the possibility of carrying out the first stages of surface preparation, at know pickling and phosphating, on large industrial lines flow, in particular directly at the rolling outlet, more precisely 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 process for preparing surface according to the invention, the importance of the phosphating step amorphous to obtain good adhesion, when not prior to a light stripping of not more than 15 g / m2 / side approximately.

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

• 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 Figure 1 represents the adhesion results obtained for the two series of samples (on the ordinate: 1 for a very good adhesion, 5 for poor adhesion) depending on the amount of material removed during stripping (on the abscissa: 0 to 25 g / m2 / face).

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

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

Figure 1 also shows that to obtain adhesion sufficient enamel layer when proceeding according to the invention, it it is advisable that the pre-stripping is preferably greater than or equal to 5 g / m2 / face: the level of adhesion is then 3; to get the same level of adhesion by proceeding without amorphous phosphating, it would be necessary strip the surface at least at the level of 15 g / m2 / face.

Example 4:

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

The operation of shaping a sheet consists of oiling it, deform, in particular by stamping, finally to degrease it.

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

We will first show that the presence of a layer of amorphous phosphating on the sheet facilitates shaping, in the measure where 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 the temporary corrosion protection, especially for the storage of sheets.

Stamping oil is an oil usually used for stamping operations, adapted to improve properties tribological of a sheet surface.

We measure the tribological properties of the surfaces of the different samples as follows, all samples showing 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 1 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 1 under the same conditions.

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

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

Then a sample of type M2 or M4 is formed (see above), by stamping.

We identify, on the shaped sample, so-called "rubbed" areas and so-called "non-rubbed" areas.

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

In contrast, the non-rubbed areas are areas that do not appear not have undergone any particular friction and which have retained the appearance that 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:

• non-rubbed areas: adhesion = 1 (as in example 1)
• rubbed areas: adhesion = 2.

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

Referring to Figure 1, we see that for this type non-phosphate sample, adhesion is only 4.

We deduce that, even on the rubbed areas, the enamel layer adheres to an entirely acceptable level and that treatment of amorphous phosphating remains effective even if the surface is substantially modified by friction.

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

Still according to the invention, it is not necessary to protect particularly the phosphating layer before carrying out the form.

Claims (3)

1. Process for preparing the metal surface of an object made of steel sheet for direct enamelling, in which the said surface is pickled and then treated, characterized in that:

   the said surface is pickled under suitable conditions for removing at most 15 grams of steel per square meter of the said surface;

   the pickled surface is treated in two steps, a first treatment step called "amorphous" phosphatizing suitable for forming a layer of at least 0.2 g/m² on the said surface, and then a second treatment step called "nickel plating".
2. Process according to Claim 1, characterized in that the nickel plating conditions are modified in order to obtain a coating of between 0.5 and 2.5 g/m² on the surface of the object to be enamelled.
3. Process for manufacturing an object made of direct-enamelled steel sheet, which comprises a forming operation, a surface preparation using the process according to either of the preceding claims and an enamelling operation, characterized in that the forming operation is carried out after pickling and after the first surface treatment step known as amorphous phosphatizing.