Method for producing a coated metal strip having an improved appearance

Manufacturing process of a coated metal band that has an improved appearance.

The invention relates to a method of manufacturing a metal band that has an improved appearance, more particularly intended to be used for the manufacture of surface parts for motor land vehicles, but is not limited thereto.

Steel sheets intended for the manufacture of parts for motorized land vehicles are generally coated with a corrosion-resistant metal layer, based on zinc, deposited either by hot tempering in a liquid zinc-based bath, or either by electrodeposition in an electrolytic bath comprising zinc ions.

A hot tempering coating process is described by EP1 466 994.

Galvanized sheets intended for the manufacture of surface parts, are then shaped and assembled to form a blank box, which is then coated with at least one layer of paint, which ensures increased protection against corrosion as well as a good surface appearance .

In this regard, conventionally, car manufacturers first apply a single layer of cataphoresis to the box, then a sizing paint layer, a base paint layer, and possibly a varnish layer. To obtain a satisfactory painted surface appearance, a total paint thickness between 90 and 120 µm is generally applied, consisting of a cataphoresis layer 20 to 30 µm thick, a sizing paint layer of 40 to 50 µm, and a base paint layer of 30 to 40 µm, for example.

In order to limit the thickness of the paint systems to a value below 90 µm, some car manufacturers have proposed either to avoid the cataphoresis stage, or also to limit the number of paint layers to increase productivity. However, until now, this reduction in the thickness of the paint system is always done to the detriment of the final painted surface aspect of the piece and is not done industrially.

In fact, zinc-based coatings that serve as a base substrate have what is called a surface ripple, which can only be compensated by significant layers of paint today, under penalty of presenting an unacceptable "orange peel" appearance for body parts.

The waviness W (waviness in English) of the surface is a soft, pseudo-periodic, rather large wavelength (0.8 to 10 mm) geometric irregularity that is distinguished by the roughness R corresponding to the geometric irregularities of lengths of small wave (<0.8 mm).

In the present invention, the arithmetic mean Wa of the undulation profile, expressed in µm, has been retained to characterize the undulation of the sheet surface, and the undulation measurements were made with a cut-off threshold of 0.8 mm and designated by Wa0.8.

The purpose of the invention is therefore to make available a method of manufacturing a metal band coated with an anti-corrosion coating, whose ripple Wa0.8 is reduced relative to the prior art bands, thus allowing to manufacture painted metal parts that need a total thickness of paint reduced in relation to the prior art pieces.

In this regard, a first object of the invention is constituted by a method of manufacturing a metal strip having a metallic corrosion protection coating, comprising the steps consisting of:

<dl><dt>-</dt><dd>run the metal band through a molten metal bath comprising between 2% by weight and </dd></dl>

8% by weight of aluminum, from 0 to 5% by weight of magnesium and up to 0.3% by weight of elements

of addition, the zinc complement and unavoidable impurities, keeping the indicated bath at a

temperature between 350 and 700oC, then

<dl><dt>-</dt><dd>Dry the coated metal strip by means of nozzles projecting a gas on either side of </dd></dl>

the band, and then

<dl><dt>-</dt><dd>Cool the coating in a controlled manner until complete solidification, making the </dd></dl>

indicated cooling at a speed below 15oC / s between the temperature at the drying outlet and

the beginning of solidification, and then at a speed greater than or equal to 15oC / s between the beginning and the

end of solidification.

In preferred embodiments, the process according to the invention may further comprise the following characteristics, taken alone or in combination:

<dl><dt>-</dt><dd>the cooling is carried out at a speed below 10oC / s between the temperature at the exit of the </dd></dl>

drying and the beginning of solidification, and then at a speed greater than or equal to 15oC / s between the

beginning and end of its solidification,

<dl><dt>-</dt><dd>the cooling is carried out at a speed below 10oC / s between the temperature at the exit of the </dd></dl>

drying and the beginning of solidification, and then at a speed greater than or equal to 20oC / s between the

beginning and end of its solidification,

<dl><dt>-</dt><dd>the coated metal band is dried such that the part of the band located between the line of </dd></dl>

dried and that ends at least 10 cm higher is in contact with an atmosphere whose

oxidizing power is less than that of an atmosphere consisting of 4% by volume of oxygen and 96%

by volume of nitrogen,

<dl><dt>-</dt><dd>The metal band is a steel band. </dd></dl>

The subject of the invention is also a cold rolled metal strip and hot temper coated but not subjected to the treatment of finishing laminators (skin-passée), which can be obtained by the process according to the invention, whose metallic coating comprises a 2 to 8% by weight of aluminum, from 0 to 5% by weight of magnesium, up to 0.3% by weight of additional elements, the complement consisting of zinc and unavoidable impurities, the said coating having an undulation Wa0.8 less than or equal to 0.5 µm, preferably less than or equal to 0.45 µm.

In preferred embodiments, the metal band according to the invention may further comprise the following characteristics, taken alone or in combination:

-metal coating does not contain magnesium, -metal coating contains from 1 to 4% by weight of magnesium, -the metal band consists of steel.

A subject of the invention is also a metal part obtained by deformation of a metal strip without being subjected to the treatment of finishing mills, whose coating has a Wa0.8 ripple of 0.48 µm or less, preferably less than or equal to 0 , 43.

The object of the invention is also a metal part obtained by deformation of a metal band that has undergone a pass operation through finishing mills before deformation, whose coating has a Wa0.8 ripple of 0.35 µm or less, of preference less than or equal to 0.32, even 0.31 µm.

The characteristics and advantages of the present invention will appear better in the course of the description that follows, given by way of non-limiting example.

The first stage of the process according to the invention consists in continuously passing a metal band, such as a steel band, through a coating bath containing molten metal, contained in a crucible.

The speed of the band in the industrial lines is generally between 40 m / min and 200 m / min, for example, and is preferably greater than 120 m / min, even greater than 150 m / min.

The composition of the coating bath to be used in the process according to the invention is zinc based and contains from 2% to 8% by weight of aluminum, which is therefore an indispensable element of the coating. The inventors have observed that a coating comprising less than 2% by weight of aluminum did not allow for improvement of the corrugation by the process according to the invention. Similarly, a coating comprising more than 8% by weight of aluminum does not allow this effect sought by the invention to be obtained. This element also improves corrosion resistance.

The bath can also contain up to 5% by weight of magnesium in order to improve the corrosion resistance of the galvanized coating and in particular its resistance against red oxide. To obtain a remarkable effect on corrosion resistance, it is preferred to add at least 0.1% by weight, even 0.2% by weight and preferably at least 1% by weight. Its content in the bath is limited to 5%, even 4% by weight as the coating obtained could present problems of fragility and adhesion in the subsequent shaping, for higher contents. In addition, the inventors have observed that the contribution of this element does not once again question the results obtained on the undulation by the contribution of aluminum.

The bath composition may also contain up to 0.3% by weight of optional addition elements such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. These different elements may allow, among others, to improve the corrosion resistance of the coating or its fragility or adhesion, for example. The person skilled in the art who knows its effects on the characteristics of the coating will know how to use them according to the complementary purpose sought. It has also been found that these elements did not interfere with the ripple domain obtained by the process according to the invention. In some circumstances, however, it will be preferred to limit the titanium content by less than 0.01%, even less than 0.005%, as this element can cause contamination problems of degreasing and phosphating baths of car manufacturers.

Finally, the bath may contain unavoidable impurities that come from the feeding bars of the crucible or also from the passage of the band through the bath. It will be possible to cite in particular, iron, etc ...

The bath is maintained at a temperature between the initial solidification temperature of a liquid mixture + 10 ° C and 700 ° C, the initial solidification temperature of a liquid mixture varying according to its composition. For the range of coatings used in the present invention, this temperature will therefore be between 350 and 700 ° C. It will be remembered that the initiating solidification temperature of a liquid mixture is the temperature beyond which an alloy is in a completely molten state, the melting temperature being the initiation of a mixture. binary the temperature below which an alloy is found in a completely solidified state. For some compositions, the initiating solidification temperature of a liquid mixture may be equal to the initiating melting temperature of a binary mixture depending on its composition.

After passing through the crucible, the metal band coated on its two faces is then subjected to drying by means of nozzles located on either side of the band projecting a gas, such as air or an inert gas, towards the surface from the band. This classic operation, well known to the person skilled in the art, allows to regulate the thickness of the coating precisely, while it has not yet solidified.

At the beginning of drying, the essential step of the process according to the invention is the controlled cooling of the coating until its complete solidification.

Indeed, the present inventors have observed that it was necessary to cool the coating differently before the beginning of solidification and from the beginning of it.

Thus, the cooling must be carried out at a speed of less than 15oC / s, preferably less than 10oC / s, and more particularly preferably less than or equal to 5oC / s, between the temperature at the exit of the drying and the beginning of the solidification (i.e. when the coating falls just below the temperature at which the components of a liquid mixture begin to solidify), then at a speed greater than or equal to 15oC / s, preferably greater than 20oC / s, between the beginning and the end of its solidification (that is, when the coating reaches the temperature at which the components of a mixture begin to melt).

Respecting these setpoints, there is indeed a surprising and significant improvement in the undulation of the coatings in question, as shown in the tests presented below.

The refrigeration until the beginning of the solidification can be obtained by natural convection if it is desired to refrigerate at a speed that does not exceed approximately 5oC / s. Further, it will generally be necessary to carry out forced cooling by any adapted technical means, such as a cold gas blow, for example.

Similarly, cooling from the beginning of solidification will normally be done by forced cooling.

When the coated sheet is completely refrigerated, it can undergo a treatment operation in finishing mills that allows it to be conferred a texture that facilitates its subsequent shaping. In fact, the treatment operation in finishing mills allows transfer to the surface of the sheet a sufficient roughness so that its conformation is carried out in good conditions, favoring a good retention of the oil applied on the sheet before its conformation.

This processing operation in finishing mills is generally performed for metal sheets intended for the manufacture of body parts for motorized land vehicles. When the metal sheets according to the invention are intended for the manufacture of household appliances, for example, this supplementary operation is not carried out.

The sheet or not undergoing treatment in finishing mills is then formed, for example by drawing, folding or profiling, and preferably by drawing, to form a piece that can then be painted. In the case of the parts for the appliance, this layer of paint may also be subjected to annealing by physical and / or chemical means, known per se. In this regard, the painted part can be passed through a hot air or induction furnace, or also under UV lamps or under a device that diffuses electron beams.

For the production of parts for the car, it is subjected to a quench in a cataphoresis bath, and a layer of sizing paint, a base paint layer, and possibly a finishing varnish layer is applied successively.

Before applying the cataphoresis layer on the piece, it is previously degreased and then phosphated in order to ensure adhesion of the cataphoresis. The cataphoresis layer provides the piece with a complementary protection against corrosion. The sizing paint layer, usually applied by spray gun, prepares the final appearance of the piece and protects it from splashing gravel and UV radiation. The base paint layer gives the piece its color and final appearance. The varnish layer gives the surface of the piece a good mechanical resistance, a resistance against aggressive chemical agents and a good surface appearance.

The paint layer (or paint system) used to protect and ensure an optimal surface appearance to galvanized parts, for example has a cataphoresis layer 10 to 20 µm thick, a sizing paint layer less than 30 µm, and a base paint layer less than 40 µm.

In cases, where the paint system also comprises a layer of varnish, the thicknesses of the different paint layers are generally the following:

- cataphoresis layer: less than 10 to 20 µm, - sizing paint layer: less than 20 µm, - base paint layer: less than 20 µm and advantageously less than 10 µm, and -varnish layer: preferably less than 30 µm.

The paint system may also not include a cataphoresis layer, and only have a sizing paint layer and a base paint layer and possibly a varnish layer.

On the other hand, complementary investigations have allowed to improve still the level of undulation of the plates and pieces according to the invention, taking particular measures in the zone of dried of the coating.

The present inventors have indeed observed that by so that the atmosphere of this zone has an oxidizing power less than that of an atmosphere constituted by 4% by volume of oxygen and by 96% by volume of nitrogen, the level was still reduced of corrugation of the plates that have undergone refrigeration according to the invention.

The area that must be maintained at a low level of oxidation is at least the one that begins immediately above the drying line and stops at least 10 cm above and that surrounds the band as it passes through its two faces. Drying line is understood here as the shortest segment that connects the nozzle and the sheet, corresponding to the minimum path made by the drying gas.

This regulation of the level of oxidation may be carried out by any adapted means, such as, for example, a confinement box covering the drying zone and fed with neutral gas such as nitrogen. It will also be possible to use an oxygen-poor gas as a drying gas and not particularly to feed the box with inert gas, which is then only fed by the flow of drying gas.

To determine the oxidizing power of the atmosphere surrounding the band, the partial pressure of oxygen equivalent to equilibrium will be evaluated.

When the only oxidizing gas present is O2, mixed with an inert gas (nitrogen, argon), this pressure is then equal to the volume content in O2 that can be measured in real time by means of an adapted sensor.

When other oxidizing gases, such as H2O or CO2 are present in the mixture with a reducing gas such as H2 or CO, for example, the equivalent partial pressure of oxygen is calculated by the law of mass action at the temperature of the gases considered .

For example, for the H2 / H2O pair, the reaction is written as follows:

H2 + 1/2 O2 2O

In thermodynamic equilibrium, the partial pressures of the gases obey the following relationship:

where R is the constant of the perfect gases, T the temperature of the gases in Kelvin, and fG the variation

Free energy associated with the reaction, found in the thermodynamic tables, in calories per mole or in Joules per mole according to the value taken for the constant R.

From this relationship, the value of pO2, partial pressure of oxygen equivalent to equilibrium for the gas mixture considered, is extracted.

In the context of the invention, the value of pO2 must be between 0.0015 and 0.04.

On the other hand, it may be desirable to prolong the eventual confinement box to the surface of the bath or to an intermediate position between the bath and the drying line in order to limit oxidation before drying. Indeed, when the surface of the sheet is exposed in the open air, such a layer is systematically formed but is most often removed and sent back to the coating bath under the impact of the drying jet. A confinement of this type therefore allows to reduce the amount of bath oxides that can be carried along the band during its passage and thus create redhibitory defects.

However, it has the disadvantage of favoring the vaporization of zinc from the bath, vapors that can subsequently dirty the confinement box.

Although all kinds of drying nozzles can be used for carrying out the process according to the invention, it will be more particularly preferred to select nozzles whose outlet opening is in the form of a slat whose width exceeds that of the strip to be coated. This type of nozzle makes it possible to make a good confinement of the lower part of the drying zone.

essays

The tests were carried out from a cold rolled IF-Ti type metal sheet, which was passed through a crucible containing a zinc-based metal bath comprising varying proportions of aluminum and magnesium. It was maintained at a temperature of 70 ° C beyond the temperature at which the components of a liquid mixture of the composition begin to solidify.

At the exit of the bath, the coating obtained is dried with nitrogen by means of two classic nozzles in order to obtain a coating thickness of the order of 7 µm.

Above these drying nozzles, a series of cooling boxes were placed, on either side of the belt, boxes that allow air to be blown to solidify the coating by forced convection. Depending on the flow rate, the temperature of the blowing gas and the number of boxes put into operation, the cooling rate can be adjusted between 5 and 50oC / s.

Playing with these parameters and with the position of the boxes in operation, a series of tests were carried out in order to obtain controlled refrigerations at different refrigeration rates, before reaching the temperature at which the components of a liquid mixture begin to solidify and between this temperature and that of the beginning of the fusion of the components of a mixture. When all the boxes are disconnected, the coating only undergoes a natural convection at a speed of approximately 5oC / s.

An assay was also carried out (referenced with 17), carrying out a nitrogen-containing drying in a confined box with a limitation of the oxygen content to 4% by volume in the area that begins at the level of the drying line and ends 10 cm higher.

Finally, the samples obtained according to a 3.5% equibiaxial deformation mode (Marciniak) were subjected to drawing. A part of the samples was previously subjected to a treatment operation in finishing mills with an elongation percentage of 1.5%.

As the progress of the trials took place, the Wa0.8 ripple values ​​were measured. This measurement consisted of acquiring by mechanical palpation, without skid, a profile of the sheet with a length of 50 mm, measured at 45o of the rolling direction. The approximation to its general shape was deduced by the signal obtained by a polynom of degree at least 5. The Wa wave was then separated from the roughness Ra by a Gaussian filter with a cut-off threshold of 0.8 mm.

The results obtained are indicated in the following table:

<dl><dt>Test </dt><dd>Coating composition (% by weight) Liquidus-Solidus (oC) VR before liquidus (oC / s) VR between liquidus and solidus (oC / s) Ripple Wa0.8 (µm) </dd></dl>

<dl><dt>Zn </dt><dd>To the Mg No treatment in finishing mills or deformation No treatment in finishing laminators and after deformation With treatment in finishing laminators before deformation With treatment in finishing laminators and after deformation </dd></dl>

<dl><dt>1 </dt><dd> 93 4 3 357-340 5 5 1,21 1,08 0,42 0,87 </dd></dl>

<dl><dt>2 </dt><dd>93 4 3 357-340 5 10 0.92 ne ne ne </dd></dl>

<dl><dt>3* </dt><dd>93 4 3 357-340 5 fifteen 0.43 ne ne ne </dd></dl>

<dl><dt>4* </dt><dd> 93 4 3 357-340 5 20 0,39 0,34 0,32 0,30 </dd></dl>

<dl><dt>5* </dt><dd>93 4 3 357-340 10 twenty 0.47 ne ne ne </dd></dl>

<dl><dt>6 </dt><dd>93 4 3 357-340 fifteen twenty 3.01 ne ne ne </dd></dl>

<dl><dt>7 </dt><dd> 99,7 0,3 0 416-413 5 20 0,71 0,62 0,41 0,63 </dd></dl>

<dl><dt>8 </dt><dd>97 1.5 1.5 380-340 5 twenty 0.84 ne ne ne </dd></dl>

<dl><dt>9 </dt><dd>86 eleven 3 428-340 5 twenty 0.68 ne ne ne </dd></dl>

<dl><dt>10* </dt><dd> 95,5 3 1,5 369-340 5 20 0,42 0,39 0,34 0,33 </dd></dl>

<dl><dt>11* </dt><dd> 91 6 3 371-340 5 20 0,43 0,40 0,35 0,32 </dd></dl>

<dl><dt>12* </dt><dd> 98 2 0 401-381 5 20 0,48 0,46 0,36 0,35 </dd></dl>

<dl><dt>13* </dt><dd> 95 5 0 381-381 5 20 0,43 0,39 0,35 0,34 </dd></dl>

<dl><dt>14 </dt><dd>95 5 0 381-381 fifteen twenty 3.36 ne ne ne </dd></dl>

<dl><dt>15 </dt><dd> 95 5 0 381-381 5 5 1,37 1,14 0,43 0,93 </dd></dl>

<dl><dt>16* </dt><dd> 92 8 0 410-381 5 20 0,47 0,44 0,37 0,34 </dd></dl>

<dl><dt>17* </dt><dd> 93 4 3 357-340 5 20 0,36 0,33 0,30 0,28 </dd></dl>

ne: not evaluated, *: according to the invention, VR: cooling rate

From tests 1 to 3, it was observed that from a cooling rate in solidification greater than or equal to 15oC / s, a spectacularly reduced undulation was obtained, in the state without being subjected to finishing laminators.

On the other hand, the present inventors observed that cooling too quickly in the first

5 The cooling phase, before reaching the temperature at which the components of a liquid mixture begin to solidify, was equally harmful for the appearance of the coating, as shown in tests 4 to 6 where it is observed that the threshold exceeds 15oC / s considerably degrades the undulation that goes from 0.47 to 3.01 µm when it passes from a cooling of 10 to 15oC / s.

It was therefore observed that this first stage of cooling is equally important for obtaining the desired surface aspect, which leads to the conclusion that the whole of the cooling process must be controlled.

In the reading of tests 7 to 12, it was also observed that the waving leveling effect was not obtained for any galvanized coating, but only for the coatings whose composition 15 respects the ranges defined by the present invention.

With regard to tests 4, 10 to 13 and 16 concerning sheets that have been subjected to finishing and sausage mills, a surprising improvement in the level of undulation was observed, while at the same time, comparative tests 1, 7 and 15 show a degradation of the results obtained after

twenty of the drawing, not allowing to lower below the threshold of 0.35 µm of undulation for a piece made from a sheet subjected to finishing mills.

Finally, comparing the results of test 17 with those of test 4, performed under identical conditions but without particular caution at the level of drying, a decrease in the level of undulation was observed in all cases of figures.