EP1115914B1 - Zinc coated steel plates coated with a pre-lubricating hydroxysulphate layer and methods for obtaining same - Google Patents

Abstract

The invention concerns a steel plate coated with a metal layer based on zinc and a zinc hydroxysulphate layer, whereof the surface density of sulphur is more than 0.5 mg/m2. The invention also concerns a method for obtaining said plate by treating a zinc coated sheet metal: either in a highly alkaline sulphate solution under polarization; or in a sulphate solution containing Zn2+ ions without polarization. The hydroxysulphate deposit brings about a pre-lubricating effect applicable to operations for forming sheet metal.

Description

The invention relates to the prelubrication of the galvanized surface of metal sheets coated with zinc or zinc-based alloy and the treatment of these sheets in aqueous solutions containing sulphates.

The document titled "The influence of some sulphur-containing anions on the anodic behavior of zinc in an alkaline medium", written by SS Abd El Rehim et al., Published in "Journal of Electroanalytical Chemistry" 401 (1996) 113-118, described, in an alkaline medium (| NaOH | = 0.1 M, thus pH = 13), the oxidation of zinc to zinc hydroxide ion Zn (OH) ₄ ⁼ in a first step, then the oxidation of this hydroxide ion zinc oxide ZnO ₂ in a second step; this document also describes the incidence of sulfur species in solution, in particular SO4 ⁼ sulfate ions, on the second oxidation step.

According to this document, if the Zn (OH) ₄ ⁼ ion concentration reaches the solubility product, zinc hydroxide Zn (OH) ₂ precipitates and forms a passivating film on the surface.

Referring to Figures 2 and 4, this document (see page 115) teaches that the presence of SO4 anions stimulates the dissolution of zinc; this effect could from the adsorption of these anions on the galvanized surface which facilitates the oxidation of zinc from this surface by these adsorbed anions.

Referring to Figures 2 and 5, this document (see page 115) teaches also that too much concentration of anions in the alkaline solution causes the rupture of the passivation film formed after the first step oxidation.

The concentration area studied in this document covers the range 0.05 to 1.7 mole SO4 ⁼ per liter.

JP 61-60915, 63-46158, 63-46159 and EP 0 339 578 describe anodic treatments of galvanized steel sheets in solutions aqueous substances containing sulphates and the use of these treatments to color the galvanized surfaces of these sheets.

The concentration of electrolyte in the treatment solution is included between 70 and 200 g / l; the concentration of sodium sulphate is, for example, 150 g / l, ie about 1 mole / liter.

JP 63-274797 discloses the use of a treatment of the same type to improve the phosphatibility of coated steel sheets by electrodeposition of a nickel-based alloy containing nickel.

The treatment solution then contains, besides the sulphates (for example: magnesium, sodium or aluminum sulfate at 150 g / l), acids carboxylic acids (for example: citric, maleic, salicylic at 30 or 40 g / l).

The pH of the treatment solution is between 4 and 5.5.

The anodic treatment is conducted at a current density of between 30 and 200 A / dm ² until the quantity of electricity consumed is between 50 and 500 C / dm ² of surface to be treated.

According to this document, this treatment improves phosphatibility because it results in the elimination of surface traces of zinc hydroxide Zn (OH) ₂ - and improves the surface reactivity.

The plates resulting from these treatments under anodic polarization in aqueous solutions containing at least 0.05 moles of sulfate ions per liter, especially more than 10 g / l of sulphate ions, pose stamping problems and formatting.

• on applique sur cette surface une solution aqueuse d'un sel aquasoluble d'un métal alcalin, notamment du phosphate de potassium,
• on sèche ladite surface,
• puis on effectue au moins une opération de huilage de ladite surface.

EP 0 489 105 discloses a method for treating the metal surface of a sheet, in particular a steel sheet, intended to prepare said sheet metal for stamping and / or to protect it against corrosion, in which :

• an aqueous solution of an aquasoluble salt of an alkali metal, in particular potassium phosphate, is applied to this surface;
• said surface is dried,
• then at least one oiling operation of said surface.

The conditions of application and drying are adapted to obtain a phosphate deposition of density of between 5 and 40 mg / m ² .

The oil used can be a temporary protection oil against corrosion and / or lubricating oil for shaping, in particular stamping.

Thus, before storage and / or transport, a first oiling can be carried out protection ; after destocking, we can carry out a second oiling of lubrication to prepare the shaping, especially by stamping.

According to this document EP 0 489 105, the phosphating treatment, 5 prior to oiling, can significantly improve the lubrication at the moment of formatting: this is therefore a prelubrication treatment.

The galvanized sheet obtained is provided with a pre-lubricating depot based on phosphate.

The lubricating effect of this phosphating treatment is sometimes insufficient on galvanized sheets; Moreover, this treatment generates effluents containing phosphates, which one wishes to avoid.

The object of the invention is to provide a pre-lubricated galvanized sheet more performing better than those obtained by phosphating and offering treatment pre-lubrication of galvanized sheets more efficient than a treatment of phosphating and more environmentally friendly at the level of effluents that it generates.

• ladite couche métallique est elle-même revêtue d'une couche à base d'hydroxysulfate de zinc,
• la densité surfacique de soufre correspondant à ladite couche d'hydroxysulfate est supérieure à 0,5 mg/m².

For this purpose, the subject of the invention is a steel sheet coated with a zinc-based metal layer, characterized in that:

• said metal layer is itself coated with a layer based on zinc hydroxysulfate,
• the surface density of sulfur corresponding to said hydroxysulphate layer is greater than 0.5 mg / m ² .

Other advantageous features of the sheet according to the invention are indicated in the dependent claims.

Several hydroxysulfation processes make it possible to obtain the sheet metal zinc-coated prelubricated according to the invention.

• le pH de ladite solution est supérieur ou égal à 12, et inférieur à 13.
• la quantité de charges électriques, circulant pendant le traitement au travers de ladite surface et engendrant sur ladite surface le dépôt d'une couche comprenant du soufre, est adaptée pour que la quantité de soufre obtenue dans ladite couche d'hydroxysulfate dépasse 0,5 mg/m².

The invention therefore also relates to a first method for obtaining a sheet according to the invention from a steel sheet coated with a zinc-based metal layer comprising the steps of applying to the surface galvanized from the starting sheet an aqueous treatment solution containing more than 0.07 moles of sulfate ions per liter, anodically polarizing said surface so as to circulate a polarization current, then rinsing said surface and then drying it characterized in that

• the pH of said solution is greater than or equal to 12, and less than 13.
• the quantity of electric charges circulating during the treatment through said surface and generating on said surface the deposition of a layer comprising sulfur, is adapted so that the amount of sulfur obtained in said hydroxysulfate layer exceeds 0.5 mg / m ² .

Other advantageous characteristics of the first method of obtaining of the sheet according to the invention are indicated in the dependent claims.

In practice, it is possible to determine, for predetermined conditions of use of the treatment solution, the minimum amount of charges that must be circulated to obtain a deposit having a sulfur content of 0.5 mg / m ² ; to implement this method, it is then necessary that the amount of charges used is greater than this minimum amount.

The sheet obtained by this process, then oiled, offers very good tribological properties well suited to shaping, especially by stamping: this hydroxysulfation treatment therefore has a Prelubrication.

By optimizing the thickness of the deposited hydroxysulfate layer, the effect The prelubricant obtained is greater than that provided by a treatment in a phosphate solution as described in EP 0 489 105.

It can be seen that this prelubrication effect comes from the nature of the deposit: the infra-red reflection spectrum of the deposit obtained is represented in FIG. 3 (% reflectance as a function of the wavenumber in cm ^-1 ); this is essentially a layer of zinc hydroxysulfate, also called basic zinc sulfate; this hydroxysulfate would have the general formula: [Zn _x (SO ₄ ) _y (OH) _z , t H2O], where 2 x = 2 y + z, with y and z different from zero; preferably, z is greater than or equal to 6; according to the spectrum of FIG. 3, x = 4, y = 1, z = 6 and t = 5.

To be effective at the level of pre-lubrication, it is appropriate that this layer of zinc hydroxysulfate adheres to the treated surface: the conditions relating to the pH of the treatment solution and the drying step at the end of treatment are crucial for this purpose.

If the pH of the solution is less than 12, we do not form adhering hydroxysulfates on the surface to be treated; if the pH of the solution is greater than or equal to 13, the hydroxysulfate is redissolved and / or decomposed into zinc hydroxides; we then find ourselves in conditions similar to those described in the document of S.S. Abd El Rehim previously cited.

After rinsing but before drying, the layer deposited on the sheet has the appearance of a gel still not adherent; drying is suitable for eliminate the residual liquid water from the deposit and makes it easier to adhere the layer on the sheet.

When using sodium sulphate in the solution, if the concentration sodium sulphate is less than 10 g / l in the solution, there is little formation of hydroxysulfate layer on the surface; more generally, he it is therefore important for the concentration of sulphate ions to be greater than 0.07 moles per liter.

Preferably, the concentration of sulfate ions is less than or equal to 1 mol / liter; in the case of the use of sodium sulphate, at concentrations greater than 142 g / l (equivalent to 1 mol SO4 ⁼ / liter), for example 180 g / l, a decrease in the formation efficiency of the layer is observed hydroxysulphate.

It has been found that the prelubricating effect of the treatment is obtained only if the thickness of the deposited layer corresponds to more than 0.5 mg / m ² in sulfur equivalent, preferably at least 3.5 mg / m ² in sulfur equivalent.

On the other hand, it was found that the prelubricant effect of the hydroxysulphate layer decreased if the amount of sulfur deposited was well above 30 mg / m ² , apparently due to the degradation of the adhesion of the hydroxysulfate layer. this layer; it was also found that if the amount of sulfur deposited exceeded 27 mg / m ² , the phosphatability of the sheet obtained was degraded, the deposited phosphate crystals then being too large.

Thus, to obtain a significant prelubricant effect, the amount of hydroxysulfates deposited should be greater than 0.5 mg / m ² and less than or equal to 30 mg / m ² in sulfur equivalent, preferably between 3.5 and 27 mg / m ² in sulfur equivalent.

It is therefore appropriate that the applied charge density is adapted to this amount of hydroxysulfates capable of providing this significant prelubricant effect.

Thus, preferably, the charge density applied is preferably between 10 and 100 C / dm ² of surface to be treated.

If the charge density exceeds 100 C / dm ² , it is found that the amount of sulfur deposited on the surface no longer increases and even decreases.

This first method of obtaining a sheet according to the invention therefore allows forming on a galvanized surface a layer based on hydroxysulphate, at the once thick enough and adherent.

Thanks to the anodic polarization of the galvanized surface to be treated, we are witnessing rapid dissolution of the zinc in the immediate vicinity of the galvanized surface, which promotes the precipitation of zinc salts on this surface.

Thus, in order to carry out this treatment as productively as possible with satisfactory faradic efficiency, it is necessary to deposit the hydroxysulphate layer under a high polarization current density, especially greater than 20 A / dm ² .

For a current density less than or equal to 20 A / dm ² , the deposition efficiency is very low and the sulfur content of the deposited layer does not provide the optimum prelube effect.

Experimental tests have shown that, for a predetermined electrical charge density, for example 20 C / dm ² , the amount of sulfur deposited on the surface to be treated was an increasing homogeneous function of the current density in the range of values between 20 and 200 A / dm ² ; preferably, therefore, a current density as high as possible, for example 200 A / dm ² , is chosen.

As a counter-electrode, it is possible to use a titanium cathode.

The temperature of the treatment solution is generally understood between 20 ° C and 60 ° C; preferably, one proceeds to a higher temperature or equal to 40 ° C, so as to increase the conductivity of the solution and to reduce ohmic losses.

The speed of circulation of the solution on the surface of the sheet has not, here, of determining incidence on the treatment according to the invention.

After formation of the hydroxysulfate layer on the surface, rinse abundantly the surface treated with demineralised water; as part of this first method, the rinsing step is important to remove the reagents alkaline on the surface of the deposit, which would cause corrosion problems.

The sheet thus prelubricated by the treatment according to the invention has a homogeneous coloring, a little more sustained compared to that of a sheet metal untreated galvanized; this treatment, however, does not color the sheet, as in JP 61-60915, 63-46158, 63-46-159 and EP 0 339 578 already cited; observed under the microscope, the deposit resulting from the treatment according to the invention is in the form of sparse plates; it has been noticed that the density of plates increased with the amount of sulfur deposited per unit area.

In this first method for obtaining a prelubricated galvanized sheet according to the invention, the zinc necessary for the formation of the pre-lubricant deposit of zinc hydroxysulfate comes from the anodic dissolution of zinc under the effect polarization of the galvanized surface.

This first method of obtaining obviously has the disadvantage cost effective to require a polarization facility.

• une étape d'application, sur la surface zinguée de la tôle de départ, d'une solution aqueuse de traitement contenant plus de 0,01 mole d'ions sulfate SO₄= par litre,
• et une étape postérieure de séchage,

   caractérisé en ce que :

• ladite solution de traitement contient des ions Zn²⁺ à une concentration supérieure à 0,01 mole/litre,
• les conditions d'application, notamment la durée, la température de ladite solution, la concentration en ions SO₄ ⁼ et en ions Zn²⁺ dans ladite solution, sont adaptées pour que la quantité de soufre obtenue dans ladite couche d'hydroxysulfate dépasse 0,5 mg/m².

In order to overcome this drawback, the subject of the invention is also a second method for obtaining a sheet according to the invention from a steel sheet coated with a zinc-based metal layer comprising:

• a step of applying, on the galvanized surface of the starting sheet, an aqueous treatment solution containing more than 0.01 mole of SO ₄ sulfate ions per liter,
• and a subsequent drying step,

characterized in that

• said treatment solution contains Zn ²⁺ ions at a concentration greater than 0.01 mol / liter,
• the conditions of application, particularly the duration, temperature of said solution, the concentration of ions SO ₄ ⁼ and Zn ²⁺ ions in said solution, are adapted so that the quantity of sulfur obtained in said hydroxysulphate layer exceeds 0 , 5 mg / m ² .

Other advantageous characteristics of the second method of obtaining of the sheet according to the invention are indicated in the dependent claims.

This second method of obtaining a sheet according to the invention does not require no polarization installation.

For example, solutions of treatment by dissolution of zinc sulfate in pure water are prepared; for example, zinc sulfate heptahydrate (ZnSO ₄ , 7H ₂ O) is used; the concentration of ions Zn ²⁺ is then equal to that of the anions SO ₄ ⁼ .

The pH of the treatment solution used for this second process is generally much less basic than that of the treatment solution used for the first method; the pH of the treatment solution corresponds preferably at the natural pH of the solution, without addition of base or acid; the value of this pH is generally between 5 and 7.

The treatment solution is applied to a galvanized sheet metal surface in a conventional manner, for example by dipping, spraying or coating.

Application conditions such as the time for soaking and spraying or the amount for coating, such as the solution temperature, such as the SO ₄ ⁼ and Zn ²⁺ ion concentrations, are adapted in a manner known in the art. itself so that the amount of sulfur obtained in the final hydroxysulfate layer exceeds 0.5 mg / m ² .

It has been found that if the concentration of SO ₄ ions ⁼ and / or if the concentration of Zn ²⁺ ions was less than 0.01 mol / liter, it was impossible to form such a hydroxysulfate layer.

Preferably, the processing solutions used contain between 20 and 160 g / l of zinc sulfate heptahydrate, corresponding to a molar concentration of Zn ²⁺ ions, or SO ₄ ⁼ between 0.07 and 0.55 mol / liter; in this concentration range, it was found that the deposition rate was little influenced by the value of the concentration.

After application and before drying, the layer deposited on the sheet is adherent; the drying is adapted to remove residual liquid water from the deposit.

Between the application step and the drying step, the sheet so as to eliminate the soluble part of the deposit obtained; the absence of rinsing and obtaining a partially solubilisable deposit with water which result are not very detrimental to the pre-lubricating effect, as long as the obtained deposit includes the prelubricating layer of insoluble hydroxysulfate with water in contact with the sheet.

The sheet obtained by this second process has intrinsic and extrinsic characteristics comparable to those of the sheet obtained by the first method; the reflection spectrum of the hydroxysulphate deposition by infra-red under grazing incidence is given in FIG. 4 (percentage reflectance as a function of the wavenumber in cm ^-1 ); it is also essentially a layer of zinc hydroxysulfate, which would have the same formula: [Zn _x (SO ₄ ) _y (OH) _z , tH 2 O], where 2 x = 2 y + z , with y and z different from zero; preferably, z is greater than or equal to 6; according to the spectrum of FIG. 4, x = 4, y = 1, z = 6 and t = 3; the deposition of hydroxysulfate obtained is finely crystallized and very covering.

• la polarisation anodique de la surface zinguée n'est pas nécessaire pour obtenir l'effet prélubrifiant recherché,
• le dépôt d'hydroxysulfate obtenu est plus homogène.

This second method has the following advantages over the first method:

• the anodic polarization of the galvanized surface is not necessary to obtain the desired prelubricant effect,
• the hydroxysulfate deposit obtained is more homogeneous.

• les conditions d'applications de la solution :
• 
  la durée d'application : le dépôt obtenu au bout de 300 secondes peut présenter une densité surfacique double de celui obtenu au bout de 100 secondes;
• le renouvellement de la solution au voisinage de la surface zinguée : en cas d'application au trempé, l'agitation appropriée du bain permet de doubler la vitesse de dépôt ;
• la température de la solution de traitement : on détermine le domaine de températures optimales, par exemple entre 40°C et 60°C.

Many parameters can have a significant impact on the speed and / or thickness of the hydroxysulfate deposit obtained:

• the application conditions of the solution:
• 
  the duration of application: the deposit obtained after 300 seconds may have a surface density double that obtained after 100 seconds;
• the renewal of the solution in the vicinity of the galvanized surface: in the case of dipping application, the appropriate agitation of the bath makes it possible to double the deposition rate;
• the temperature of the treatment solution: the optimum temperature range is determined, for example between 40 ° C. and 60 ° C.

With regard to the incidence of SO ₄ ⁼ and Zn ²⁺ ion concentrations in the treatment solution, it is found that there are concentration thresholds below which no pre-lubricating deposit is obtained, but also finds that too high concentrations do not significantly improve the rate of deposition and may even slightly decrease it.

To implement this second method, these parameters are optimized in a manner known per se to obtain a deposition of hydroxysulfate according to the invention, ie containing a quantity of sulfur greater than 0.5 mg / m ² .

According to a variant of this second method, the treatment solution contains an oxidizing agent for zinc, such as hydrogen peroxide; this oxidizing agent can have a very marked hydroxysulfation accelerating effect at low concentration; it was found that the addition of only 0.03%, ie 8 10 ^-3 mole / liter of hydrogen peroxide, or 2 ^× 10 ^-4 mole / liter of potassium permanganate in the solution allowed to double (approximately) the deposit rate; it has been found, on the contrary, that concentrations 100 times higher no longer made it possible to obtain this improvement in the rate of deposition.

In view of the prelubricant effect of the hydroxysulphate deposit, the invention has Finally, a method of forming a steel sheet coated with a zinc-based metal layer comprising the steps of treating the surface of said coated sheet according to the first or second method described above, applying a film of lubricating oil to said treated treated surface and to shape strictly said sheet.

These methods apply, for example, to electrogalvanized sheets; for dip galvanized sheets, the second treatment process; for zinc alloy coated sheets, use shall be made of example the second process of treatment without rinsing.

• la figure 1, en référence à l'exemple 1, illustre les résultats des tests d'emboutissabilité effectués sur différents échantillons de tôle traitée selon l'invention ou non traitée ; la zone hachurée correspond à la zone de rupture.
• la figure 2, en référence à l'exemple 4, illustre les variations de la quantité de soufre obtenu par le premier procédé selon l'invention, en fonction de la densité de charge de polarisation appliquée.
• les figures 3 et 4 représentent les spectres de réflexion infra-rouge de tôles revêtues d'une couche d'hydroxysulfate selon l'invention, respectivement selon le premier et selon le deuxième procédés d'obtention de cette tôle précédemment décrits.

Other advantages of the method and galvanized sheet of the invention will appear on reading the examples presented below without limitation of the present invention and with reference to the following figures:

• FIG. 1, with reference to example 1, illustrates the results of drawability tests carried out on different samples of treated sheet according to the invention or untreated; the shaded area corresponds to the rupture zone.
• FIG. 2, with reference to example 4, illustrates the variations of the amount of sulfur obtained by the first method according to the invention, as a function of the applied polarization charge density.
• FIGS. 3 and 4 show the infra-red reflection spectra of sheets coated with a hydroxysulfate layer according to the invention, respectively according to the first and second methods of obtaining this sheet previously described.

MATERIALS:

(1) The sheet used for the treatment tests is a steel sheet, grade ES-quality "aluminum-killed steel", thickness 0.7 mm, electroplated in a chloride bath on both sides. a metal layer of zinc approximately 7.5 μm thick.

2) The sulphate used to prepare the treatment solution of the first process is sodium sulphate; any other soluble sulfate can be used.

The sulphate used for the second process according to the invention is zinc sulphate heptahydrate ZnSO ₄ , 7 H ₂ O.

METHODS : 1) Extraction tests:

We have a stamping press adapted to realize buckets 50 mm inner diameter from sheet blanks diameter 90 mm; a punch having a diameter of 50 mm and having at the end a radius of curvature of 3 mm, a matrix of diameter 52.6 mm having a inner rim of radius of curvature 3.5 mm.

The stamping speed is set at 12 cm / min. ; the maximum force of blank is 150 kN.

The press is equipped with means of continuous control of the parameters stamping, in particular the blanking pressure, the stamping force and the punch race.

During a stamping operation of a given blank sheet under a sheet blank clamp pressure, one traces the curve of evolution of the stamping force as a function of the punch stroke; this curve goes through a maximum that sets the maximum stamping force in progress operation.

For a series of stamping operations under different pressures of blank, this gives a series of maximum force values stamping; we can then draw the evolution curve of the force maximum stamping according to the blanking pressure; these curves often correspond to straight lines, the slope of which friction of the punch and the matrix on both sides of the sheet.

A weak slope corresponds to low friction, that is to say to well-lubricated sheets on both sides.

This stamping test protocol therefore makes it possible to evaluate the level of lubrication of the surface of a sheet for stamping; to evaluate this level on one side, we affix on the other side a film of Teflon ® (punch side) so as to obtain on this other face a friction always constant with regard to the one exercising on the surface to be evaluated.

To evaluate the prelubricant effect of a surface treatment of the type of that of the invention, this protocol is applied to prelubricated sheets by pretreatment and oiled in a standard manner (with Teflon ® film on the untreated side ); the standard oiling consists here of applying, on the treated surface, the oil referenced 6130 of the QUAKER Company so as to obtain a layer of 1 g / m ² approximately.

2) Phosphatability tests:

To evaluate the phosphatability, especially after treatment hydroxysulfation according to the invention, the sheet samples are phosphated according to a predetermined protocol corresponding to conventional methods practiced in the automotive industry, using several conventional baths of surface treatment adapted to form a layer of phosphates of zinc, manganese and nickel; we can use the baths marketed for this purpose by the PARKER Companies or CFPI; so we generally use and successively: one or two alkaline degreasing baths, a refining bath, then a phosphating bath; each step is followed by rinsing with water.

After this treatment according to the predetermined protocol, the quality is evaluated. deposited phosphates layer, in particular in terms of morphology and chemical composition; for this purpose, electron microscopy is used for scanning and atomic absorption spectroscopy.

Example 1: Effect of a deposition of hydroxysulphates on the drawability of a galvanized sheet:

We start from samples of steel sheet as defined in paragraph MATERIALS.

In order to use the first method of obtaining sheet metal according to the invention, a solution of treatment by dissolution of sulfate of sodium in water (concentration: 60 g / l) and addition of sodium hydroxide to obtain pH = 12.7.

Sheet metal sample processing is carried out in a "cell to electrolyte circulation "where they are immersed in this solution brought to 40 ° C, and where they are polarized anodically with respect to a titanium cathode ; the "electrolyte circulation cell" is set so that the speed of the electrolyte in the vicinity and along the surface of the sheet to be treated is 100 m / min.

Four trials (# 1 to # 4) of treatments under the conditions reported in Table I; the reference sample ("Ref.") mentioned in this table did not undergo any particular surface treatment.

After immersion and anodic polarization, the treated samples are rinsed with deionized water. Definition of treatments for stamping tests Test no. Current density Density of charge Electrolysis time Amount deposited mg / m ² Sulfur Sodium Ref. 0 C / dm ² 0 s. 0.0 mg / m ² 5.0 mg / m ² 1 200 A / dm ² 5 C / dm ² 0.025 s 0.5 mg / m ² 5.5 mg / m ² 2 200 A / dm ² 10 C / dm ² 0.050 s 7.5 mg / m ² 4.8 mg / m ² 3 150 A / dm ² 20 C / dm ² 0.133 s 26.9 mg / m ² 4.8 mg / m ² 4 200 A / dm ² 50 C / dm ² 0.250 s 59.6 mg / m ² 5.8 mg / m ²

The surface of the samples is then analyzed in order to measure the amount of sulfur deposited on the surface; we also measure the amount of sodium on the surface; the results are reported in Table I.

Sulfur is measured by X-ray fluorescence ("SFX"); the depth taken into account by this method of analysis is several microns; as the galvanized steel substrate does not contain sulfur (except quantities corresponding to unavoidable impurities), the signal given by this method of dosing then corresponds effectively to deposited sulfur during treatment; the amount of sulfur deposited is calculated from the signal measured, according to a pre-established law.

Sodium is measured, after leaching the surface with boiling water, by Atomic Absorption Spectroscopy ("AAS").

The amount of sodium observed at the surface of the reference sample is quite conventional for an electrogalvanized steel; quantities Approximately identical readings from treated samples indicate that the sodium from the treatment solution does not incorporate the deposit-based hydroxysulphate.

The drawability tests are then carried out as described in paragraph METHODS; the results are reported on a diagram giving on the ordinate the maximum drawing force in kN and the abscissa the force blank holder in kN; this diagram is shown in Figure 1, with the following correspondences between symbols and samples: sample reference: empty squares (□) or solid squares - test n ° 1: empty diamonds or full; test n ° 2: triangles (Δ) - tests n ° 3: crosses straight (+) - test n ° 4: inclined crosses (x); the hatched upper area corresponds to the area a break.

There is a very marked improvement in the stamping and a very pronounced prelubricant effect as soon as the amount of sulfur deposited reaches 7.5 mg / m ² ; such pretreatment corresponds to a quantity of charges of at least 10 C / dm ² (see Table I); the tests of Example 6 show that the prelubricant effect is obtained for lower amounts of sulfur, of the order of 3.5 mg / m ² .

Comparative Example 1 Highlighting the role of sulfur on the prelubricant effect:

On the same sheet samples as in Example 1, a comparative treatment under anodic polarization (charge density: 12 C / dm ² - current density: 9.8 A / dm ² ) was carried out in a solution at room temperature. same temperature and at the same pH as in Example 1, but not containing sulphates; this comparative treatment therefore consists in depositing a layer of zinc hydroxides in the place of the hydroxysulfates of Example 1.

The low level of current density used comes from the low level of conductivity of the electrolyte used which contains only sodium hydroxide.

The same drawability tests are carried out as in Example 1.

On the diagram (maximum stamping force - blanking force), we then obtain comparable results to those of the reference sample and Test No. 1 of Example 1.

Having no pre-lubricating effect with a deposit without sulfur, it is therefore confirmed that an amount of sulfur greater than 0.5 mg / m ² in the deposit is necessary to obtain the desired prelubricant effect.

Example 2 First polarization process: incidence of current density on the amount of sulfur deposited:

A series of treatments are performed on the same samples as in Example 1 and using the same solution as in Example 1.

Unlike in Example 1, the treatments are carried out in a "Rotating electrode cell"; the circular-shaped sample is plunged in this cell and is animated by a rotation movement; the speed of electrolyte flow in the vicinity of the sample therefore depends on the rotation speed.

All treatments are performed under the same charge density: 20 C / dm ² .

Each treatment is performed under a different current density; after treatment, the amount of sulfur deposited by a method is measured different from that described in Example 1, which involves the use of a Silicon-Lithium diode ("Si-Li") adapted for this purpose on an installation of scanning microscopy.

The results obtained are reported in Table II. Influence of the current density on the deposit. Current density (A / dm ² ) 25 50 75 100 125 150 175 200 Quantity deposited sulfur (mg / m ² ) 4.1 8.2 11.1 11.9 13.3 13.2 14.5 15.9

Thus, we observe that the deposition efficiency, expressed as the density of deposited sulfur, increases with current density.

Example 3 First method under polarization: incidence of the pH of the solution of treatment on the amount of sulfur deposited:

A series of treatments are performed on the same samples as in Example 1 and using a solution analogous to that of Example 1, at pH value close; the treatment facility is the same as in the example 2 ("rotating electrode cell").

All the treatments are carried out under the same charge density 20 C / dm ² and under the same current density 200 A / dm ² .

Each treatment is performed using a different pH solution; after treatment, the amount of sulfur deposited by the same method as in Example 2.

The results obtained are reported in Table III. Influence of the pH of the solution on the deposit. pH of the solution 11 12 12.7 13.4 Quantity deposited sulfur (mg / m ² ) 1 2 13 0

It can be deduced that the amount of sulfur deposited does not become significant, ie greater than 0.5 mg / m ² only if, approximately, 12 ≤ pH <13.

Example 4 First method under polarization: incidence of charge density (C / dm ² ) applied during the treatment on the amount of sulfur deposited:

A series of treatments are performed on the same samples as in Example 1 and using the same solution as in Example 1.

All treatments are performed under the same current density 200 A / dm ² , but with different charge densities.

After treatment, the amount of sulfur deposited by the same method than in Example 1.

The results are reported in FIG. 2, where the sulfur content in mg / m ^{2 was} plotted on the ordinate and the charge density in C / dm ² on the abscissa.

This figure shows that, beyond a charge density of the order of 100 C / dm ² , the amount of sulfur deposited decreases; this decrease could be explained by the bursting of the first hydroxysulphate deposits formed under the effect of the continued dissolution of the galvanized substrate.

Example 5 Effect of the sheet treatment according to the invention on the phosphatability:

Phosphatability tests as defined in paragraph METHODS above, on samples processed according to the invention in the conditions of Example 1, and having different surface contents in sulfur (Table I).

After phosphatation tri-cation (including Mn, Ni), the microscopic observation of the surface, treated and phosphated, reveals that the size of phosphate crystals deposited increases with the initial surface sulfur content; this magnification seems acceptable in the case of the plates resulting from the tests n ° 2 and n ° 3, but seems unacceptable in the case of the test n ° 4; to allow good phosphating, it would be important that the amount of sulfur deposited from the treatment of the invention does not exceed 27 mg / m ^2.

• les quantités de manganèse et de nickel déposées sont bien corrélées à la quantité de phosphore, ce qui confirme la bonne phosphatation ;
• la quantité de soufre est inférieure à 1 mg/m², ce qui montre que le dépôt d'hydroxysulfate est éliminé lors de la phosphatation.

The analysis of the phosphate surfaces then reveals that:

• the quantities of manganese and nickel deposited are well correlated with the amount of phosphorus, which confirms the good phosphatation;
• the amount of sulfur is less than 1 mg / m ² , which shows that the deposition of hydroxysulfate is removed during phosphating.

After phosphating and painting treated sheets according to the tests No. 2 and No. 3 of Example 1, we obtain painted sheets with such a beautiful surface appearance that the reference sheet phosphated and painted in same conditions.

Example 6 Implementation of the second method without dipolar biasing:

With the aim of using the second method of obtaining sheet metal according to the invention, a solution of treatment by dissolution of sulfate of zinc heptahydrate in water; the solution is used at its natural pH, without addition of acid or base; the natural pH obtained is close to 7.

To apply the treatment solution, the samples of sheet metal in this solution, without electrical polarization.

After immersion, the treated samples are rinsed twice with water demineralized: first at 20 ° C for about 8 seconds, then at 50 ° C for about 5 seconds.

The rinsed samples are then dried.

A hydroxysulphate deposit is observed on the samples obtained; of rinsing, it is therefore considered that the deposits obtained are insoluble in the water.

• durée de trempé et vitesse d'agitation :
  • sans agitation : 5 mg/m² à 60 s., 10 mg/m² à 300 s.
  • avec agitation : 10 mg/m² à 60 s., 15 mg/m² à 120 s., 25 mg/m² à 300 s.
• température de la solution, utilisée sous agitation : densité de dépôt optimale entre 42 et 62°C, densité de dépôt réduite de 30% environ à 20°C ou à 70°C.

Using a solution of 90 g / l of zinc sulphate heptahydrate, the influence of the application conditions on the amount of sulfur contained in the deposit is summarized below:

• quenching time and stirring speed:
  • without agitation: 5 mg / m ² to 60 sec., 10 mg / m ² to 300 sec.
  • with agitation: 10 mg / m ² to 60 sec., 15 mg / m ² to 120 sec., 25 mg / m ² to 300 sec.
• temperature of the solution, used with stirring: optimal deposition density between 42 and 62 ° C, deposition density reduced by about 30% at 20 ° C or at 70 ° C.

Using a treatment solution at 50 ° C with stirring, it is found that the concentration of zinc sulfate heptahydrate little influence the deposition rate in the range of between 20 and 160 g / l; the amount of sulfur obtained in the deposit decreases even when this concentration increases: 5 mg / m ² for 20 g / l, and 3.5 mg / m ² only for 160 g / l after 10 seconds of soaking.

Using a treatment solution at 50 ° C. with stirring, containing 40 g / l of zinc sulfate heptahydrate and various zinc oxidizing agents, the amount of sulfur obtained in the deposit is measured after 10 seconds of treatment; the results obtained are reported in Table IV and illustrate the interest in using a low-concentration zinc oxidizing agent as an activating agent for the hydroxysulfation treatment according to the invention. influence of the oxidizing agent of zinc on the deposition rate. Activator of hydroxysulfatation in the treatment solution: Deposition hydroxysulfate obtained: amount of sulfur Without (reference) 4 mg / m ² CuSO ₄ , 5H ₂ O at 0.24 g / l 5 mg / m ² H ₂ O ₂ at 0.03% 12 mg / m ² H ₂ O ₂ at 3% 8 mg / m ² KMnO ₄ at 0.001 N = 2 10 ^-4 mole / liter 7.5 mg / m ² KMnO ₄ at 0.1 N, ie 2 10 ^-2 mol / liter 2 mg / m ²

Extraction tests carried out according to the protocol defined in the "Methods" paragraph on samples prepared using this second method have shown that a prelubricant effect comparable to that obtained on samples prepared with the aid of the second method was obtained. first process; samples with hydroxysulfate deposition containing respectively 3.5 mg / m ² , 4.3 mg / m ² and 6 mg / m ² of sulfur yielded lower maximum drawing forces than non treated and at most comparable to those of samples pre-lubricated by phosphating; these results confirm that the hydoxysulfated sheets according to the invention have drawing properties at least comparable and generally higher than those of the phosphated sheets of the prior art.

Example 7 Implementation of the second process without polarization by coating; Influence of the soluble part of the deposit on the prelubricant effect.

A solution of dissolution treatment of 25.7 g / l of zinc sulphate heptahydrate in water; the solution is used at its pH natural, without addition of acid or base; the natural pH obtained is close to 7.

A homogeneous film of coating treatment solution is applied sheet samples; the solution is applied at room temperature.

5 to 60 seconds after application, the sample is dried.

The deposits obtained on the dried samples then all present the same thickness and the same amount of sulfur.

But the solubilizable part of the deposit depends on the delay (5 to 60 seconds) between application and drying, during which the hydroxysulfation reaction could develop.

For each sample, the amount of sulfur corresponding to the solubilizable part of the deposit and that corresponding to the insolubilizable part, that is, hydroxysulfate.

The results obtained are reported in Table V according to the drying time. soluble / insoluble part of the deposit obtained by coating: Drying time: 5 s. 20 s. 40 s. 60 s. Quantity of S in deposit (mg / m ² ) insoluble: 5 6 7 7 soluble: 9 8 7 7

It can therefore be seen that at room temperature and in the absence of an agent hydroxysulfation accelerator, the hydroxysulfation reaction continues up to 40 seconds at the expense of the soluble part of the deposit.

Comparing the results of stamping tests carried out on samples whose deposition has a significant soluble share to those samples where the soluble part has been removed by rinsing, it can be seen that total amount of sulfur deposited identical, the prelubricant effect of the deposit not rinsed is slightly inferior to that of the rinsed, insoluble and mainly consisting of hydroxysulfate.

This observation confirms that the prelube effect is mainly provided by the hydroxysulfate layer insoluble in water.

Claims (17)

1. Steel plate coated with a metal layer based on zinc, characterized in that:

   said metal layer is itself coated with a layer based on zinc hydroxysulphate,

   the surface density of sulphur corresponding to said hydroxysulphate layer is more than 0.5 mg/m².
2. Plate according to Claim 1, characterized in that said layer is water-insoluble.
3. Plate according to either one of Claims 1 to 2, characterized in that said surface density of sulphur is included between 3.5 and 27 mg/m².
4. Plate according to any one of Claims 1 to 3, characterized in that said hydroxysulphate corresponds to the general formula : [Zn_x (SO₄)_y(OH)_z, t H₂O] where 2 x = 2 y + z, with y and z different from zero.
5. Plate according to Claim 4, characterized in that z is greater than or equal to 6.
6. Plate according to Claim 5, characterized in that z = 6 and 3 ≤ t ≤ 5.
7. Method for obtaining a plate according to any one of Claims 1 to 6 from a steel plate coated with a metal layer based on zinc comprising the steps consisting in :

   applying on the zinc-coated surface of said plate an aqueous treatment solution containing more that 0.07 moles of sulphate ions per litre,

   anodically polarising said surface so as to cause a polarisation current to circulate,

   rinsing said surface, then drying it,

   characterized in that

   the pH of said solution is higher than or equal to 12, and lower than 13,

   the quantity of electric charges circulating during the treatment through said surface and generating on said surface the deposit of a layer comprising sulphur, is adapted so that the quantity of sulphur obtained in said hydroxysulphate layer exceeds 0.5 mg/m².
8. Method according to Claim 7, characterized in that the concentration of sulphate ions in said solution is less than or equal to 1 mole/litre.
9. Method according to either one of Claims 7 to 8, characterized in that said quantity of electric charges circulating during the treatment is adapted so that the quantity of sulphur in said layer is included between 3.5 and 27 mg/m² in sulphur equivalent.
10. Method according to any one of Claims 7 to 9, characterized in that the polarisation current density applied during the treatment is greater than 20 A/dm².
11. Method for obtaining a plate according to any one of Claims 1 to 6 from a steel plate coated with a metal layer based on zinc comprising :

    a step of application, on the zinc-coated surface of said plate, of an aqueous treatment solution containing more than 0.01 mole of sulphate SO₄ ⁼ ions per litre,

    and a subsequent drying step,

    characterized in that

    said treatment solution contains ZN²⁺ ions at a concentration higher than 0.01 mole/litre,

    the conditions of application, particularly the duration, temperature of said solution, the concentration of SO₄ ⁼ ions and of Zn ²⁺ ions in said solution, are adapted so that the quantity of sulphur obtained in said hydroxysulphate layer exceeds 0.5 mg/m².
12. Method according to Claim 11, characterized in that it comprises a step of rinsing before the drying and after the application.
13. Method according to either one of Claims 11 to 12, characterized in that it does not comprise a step of polarisation of said zinc-coated surface.
14. Method according to any one of Claims 11 to 13, characterized in that, in said treatment solution, the concentration of Zn²⁺ ions is approximately equal to that of the SO₄ ⁼ ions.
15. Method according to any one of Claims 11 to 14, characterized in that the concentration of Zn²⁺ ions and that of the SO₄ ⁼ ions are included between 0.07 and 0.55 mole/litre.
16. Method according to any one of Claims 11 to 15, characterized in that the pH of the treatment solution is included between 5 and 7.
17. Method for forming a steel plate coated with a metal layer based on zinc comprising the steps consisting in treating the surface of said coated plate, in applying a film of lubricating oil on said treated surface and in forming said plate proper, characterized in that said treatment is effected by the method according to any one of Claims 6 to 10 or by the method according to any one of Claims 11 to 16.

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