Method for producing a coated metal strip having an improved appearance

The invention relates to a method for producing a metal strip having an improved appearance, in particular for use in the manufacture of external parts of land motor vehicles, but not in a limiting manner.

Steel sheets, intended for the manufacture of parts of motor land vehicles, are usually coated with a corrosion protective metal layer, based on zinc, applied either by hot dipping in a zinc-based liquid bath or by electrolytic coating in an electrolytic bath containing zinc ions. The method of hot dip coating is described in document EP1466994.

Galvanized sheets intended for the manufacture of external parts are then formed and assembled to form a bare body, which is then coated with at least one layer of paint that provides increased corrosion protection as well as good surface appearance.

To this end, vehicle manufacturers traditionally apply a cataphoresis layer to the body, then a primer coat, a base coat and possibly a coat of varnish. To achieve a satisfactory appearance of a painted surface, for example, a layer of paint with an overall thickness of between 90 and

120 μm, consisting of a cataphoresis layer with a thickness of 20 to 30 μm, a primer coat of 40 to 50 μm and a base coat of 30 to 40 μm.

To limit the thickness of paint to less than 90 μm, some vehicle manufacturers have proposed either skipping the cataphoresis step or limiting the number of paint layers to increase performance. However, at present, this reduction in paint thickness is always to the detriment of the final surface appearance of the painted part and is not used in industry.

As a result, the zinc-based coatings used as the base substrate exhibit a feature called corrugation of their surface, which can currently be compensated only by significant layers of paint, under the risk of obtaining an appearance called "orange peel", which is unacceptable on body parts.

W (waviness in English) surface folding is a mild, pseudo-periodic geometric irregularity with a fairly large wavelength (0.8 to 10 mm), which differs from the roughness R corresponding to geometric irregularities with a short wavelength (<0.8 mm) .

In the present invention, the arithmetic mean Wa of the corrugation profile, expressed in μm, was determined to characterize the corrugation of the sheet surface, and the corrugation measurements were taken at a 0.8 mm incision threshold and denoted Wa0.8.

The object of the invention is therefore to provide a method for producing a metal strip with an anti-corrosive coating whose corrugation Wa0,8 is reduced compared to the prior art tapes, thus enabling the production of painted metal parts,

- 2 requiring reduced overall paint thickness compared to prior art parts.

As a result, the first object of the invention is a method for producing a metal strip having a metal coating that protects against corrosion, comprising the steps of:

- passing the metal strip through a liquid metal bath containing between 2% by weight and 8% by weight aluminum, from 0 to 5% by weight magnesium and up to 0.3% by weight of alloying additions, with zinc and unavoidable impurities, with this bath is maintained at a temperature between 350 and 700^aboutC then

- drying the coated metal strip with nozzles blowing gas from both sides of the strip, then

- cooling the coating in a controlled manner until it solidifies completely, and this cooling is carried out at a speed of less than 15^aboutC / s between the drying exit temperature and the beginning of solidification, then at a speed greater than or equal to 15^aboutC / s between the beginning and the end of clotting.

In preferred embodiments, the method of the invention may furthermore have the following features, taken into account individually or in combination:

- cooling is carried out at a speed less than 10^aboutC / s between the drying exit temperature and the start of solidification, then at a speed greater than or equal to 15^aboutC / s between the beginning and the end of clotting,

- cooling is carried out at a speed less than 10^aboutC / s between the drying exit temperature and the start of solidification, then at a speed greater than or equal to 20^aboutC / s between the beginning and the end of clotting,

- the coated metal strip is dried in such a way that a portion of the strip located between the drying line and ending at least 10 cm higher is in contact with an atmosphere whose oxidation capacity is less than an atmosphere consisting of 4% by volume oxygen and 96% by volume nitrogen,

- metal tape is steel tape.

The invention also relates to a metal strip which is cold laminated and hot dip coated, but without smoothing rolling, which can be obtained in the process according to the invention, and whose metal coating contains from 2 to 8% by weight of aluminum, from 0 to 5% by weight magnesium, up to 0.3% by weight of alloying additions, and the restoration consists of zinc and unavoidable impurities, with this coating having a corrugation Wa₀,₈ less than or equal to 0.5 μm, preferably less than or equal to 0.45 μm.

In preferred embodiments, the metal strip according to the invention may furthermore have the following characteristics, taken into account individually or in combination:

- the metal coating does not contain magnesium,

- the metal coating contains from 1 to 4% by weight of magnesium,

- the metal band is made of steel.

The invention also relates to a metal part obtained by deforming a metal strip without smoothing rolling, the coating of which has a Wa wa 0.8 of less than or equal to 0.48 μm, preferably less than or equal to 0.43.

The invention also has as its object a metal part obtained by deforming the metal strip after smoothing before deformation, the coating of which has a corrugation Wa₀,₈ less than or equal to 0.35 μm, preferably less than or equal to 0.32 and even 0.31 μm.

The features and benefits of the present invention will better emerge from the following description, by way of example, without limitation.

The first step of the method of the invention consists in continuously passing a metal strip, such as a steel strip, through a coating bath containing molten metal placed in the crucible.

The belt passing speed on industrial lines is usually in the range between 40 m / min and 200 m / min, for example, and is preferably greater than 120 m / min and even greater than 150 m / min.

The composition of the coating bath for use in the process of the invention is based on zinc and contains from 2% to 8% by weight of aluminum, which is therefore an essential element in the coating. The inventors have found that the coating containing less than 2% by weight of aluminum did not allow for improved corrugation in the method of the invention. Similarly, a coating containing more than 8% by weight of aluminum no longer achieves this desired effect of the invention. This element also allows you to improve corrosion resistance.

The bath may also contain up to 5% by weight magnesium to improve the corrosion resistance of the galvanized coating and in particular its resistance to red rust. To achieve a significant corrosion resistance effect, it is preferred to add it at least 0.1% by weight and even 0.2% by weight, and more preferably at least 1% by weight. Its content in the bath is limited to 5% and even 4% by weight, as the obtained coating could have a problem of sensitivity and adhesion during subsequent molding, at higher concentrations. In addition, the inventors have found that the addition of this element does not undermine the results obtained with regard to corrugation by the addition of aluminum.

The bath composition may also contain up to 0.3% by weight of elements optionally added, such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. These individual elements may allow, among other things, to improve the corrosion resistance of the coating or even for example its sensitivity or adhesion. A specialist who knows their effect on the coating characteristics will be able to apply them depending on the desired additional purpose. It was also checked that these elements are not contradictory to controlling the folding obtained in the method according to the invention. In some circumstances, however, it is more preferable to limit the titanium content to less than 0.01% and even less than 0.005%, since this element may generate problems of degreasing bath contamination and phosphating problems by vehicle manufacturers.

Finally, the bath may contain unavoidable impurities, either from the ingots supplying the crucible or from the passage of the belt through the bath. You can replace iron, etc. in this way ...

The bath is held at a temperature between the liquidus temperature of +10^aboutC a 700^aboutC, liquidus temperature varies depending on its composition. Therefore, for the range of coatings used in the present invention, this temperature will be between 350 and 700^aboutC. We remind you that liquidus is the temperature above which the alloy is in a completely liquid state, solidus is the temperature below which the alloy is in a completely solidified state. In some compositions, the liquidus temperature may be equal to the solidus temperature.

After passing through the crucible, the metal strip coated on both sides is then subjected to drying by means of nozzles located on both sides of the strip, which blow gas, such as air or inert gas, towards the surface of the strip. This traditional operation, which is well known to the skilled person, allows the thickness of the coating to be adjusted accurately when it is not yet solidified.

After drying, an important step of the process according to the invention is the controlled cooling of the coating until it solidifies completely.

As a result, the inventors found that it was necessary to cool the coating in a differentiated manner before and from the start of solidification.

Therefore, cooling must be carried out at a speed of less than 15^aboutC / s, preferably less than 10^aboutC / s, and particularly preferably less than or equal to 5^aboutC / s, between the temperature at the exit from drying and the beginning of solidification (i.e. when the coating drops just below the liquidus temperature), then at a speed greater than or equal to 15^aboutC / s, preferably greater than 20^aboutC / s, between the beginning and the end of solidification (i.e. when the coating reaches the solidus temperature).

Taking these tips into account, the effect of a significant and surprising improvement in the corrugation of these coatings is observed, as shown in the tests below.

Cooling to the start of solidification can be obtained by natural convection if you want to cool at a speed not exceeding about 5^aboutC / s. Above, it will usually be necessary to proceed with forced cooling by any adapted technical means, such as blowing with cold gas.

In the same way, cooling from the onset of solidification will usually take place by forced cooling.

When the coated sheet is completely cooled, it can be subjected to a smooth rolling operation that allows it to be structured to facilitate subsequent forming. As a result, the smoothing operation allows the surface to pass a rough surface sufficient for its formation to take place in good conditions, promoting good retention of oil applied to the sheet before it is formed.

This smoothing rolling operation is usually carried out on metal sheets intended for the manufacture of parts of land vehicle bodies

- 5 engine. If the metal sheets according to the invention are intended for the production of household electrical appliances, for example, this additional operation is not carried out.

Sheet metal, rolled or not, for smoothing is then formed, for example by stamping, bending or profiling, and preferably by stamping to form a part which can then be painted. In the case of parts for household electrical appliances, it is also possible to optionally subject this paint layer to annealing by physical and / or chemical means generally known. For this purpose, it is possible to pass this painted part through a hot air or induction furnace, or under UV lamps or an electron beam emitting device.

In the production of vehicle parts, they are immersed in a cataphoresis bath and a layer of primer paint, base paint layer and possibly a finishing coat layer are applied.

Before applying the cataphoresis layer to the part, it is first degreased and then phosphated to ensure adhesion of the cataphoresis. The cataphoresis layer provides parts with additional corrosion protection. A primer paint layer, usually applied with a gun, prepares the final appearance of the part and protects against gravel impacts and UV rays. The base paint layer gives the part its color and final appearance. The varnish layer gives the surface of the parts good mechanical resistance, resistance against aggressive chemical agents and good surface appearance.

The paint (or paint) layer used to protect and ensure the optimal surface appearance of the galvanized parts has, for example, a cataphoresis layer 10 to 20 μm thick, a primer layer thinner than 30 μm and a base coat thinner than 40 μm.

In cases where the paint means also contain a varnish layer, the thicknesses of the individual paint layers are usually as follows:

- cataphoresis layer: below 10 to 20 μm,

- a primer coat: below 20 μm,

- a base paint layer: below 20 μm and preferably below 10 μm and

- a varnish layer: preferably less than less than 30 μm.

Paints may also not contain a cataphoresis layer, but contain only one layer of primer paint and one layer of base paint and possibly a layer of varnish.

In addition, additional tests have further improved the level of corrugation of the sheets and parts of the invention by taking special measures in the coating drying zone.

As a result, the present inventors have found that, by proceeding in such a way that the atmosphere of this zone has a lower oxidation capacity than in an atmosphere consisting of 4% oxygen by volume and 96% nitrogen by volume, the corrugation level of the sheets subjected to cooling according to the invention is further reduced.

- 6 The zone to be kept at a low level of oxidation is at least the one that starts directly above the drying line and ends at least 10 cm higher and surrounds the moving belt on both sides. By the drying line, here is meant the shortest segment connecting the nozzle and the sheet metal, corresponding to the minimum distance covered by the drying gas.

This regulation of the level of oxidation can be done by any adapted means, such as, for example, a stop chamber enclosing a drying zone and fed with a neutral gas, such as nitrogen. It will also be possible to use oxygen-poor gas as the drying gas and not to supply the inert gas specifically to the chamber, which would be supplied in this way only by the drying gas stream.

In order to determine the oxidizing capacity of the atmosphere surrounding the tape, we will proceed to assess its equivalent oxygen partial pressure in equilibrium.

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

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

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

H2 + 1/2 O2 θ H2O

In thermodynamic equilibrium, the partial gas pressures are subject to the following equation:

PH 2O

P^H 2 p /^p°2

ΔG

RT where R is the ideal gas constant, T is the gas temperature in Kelvin, and ΔG is the change in free energy associated with the reaction that we find in the thermodynamic tables, in calories per mole or in joules per m according to the value chosen for the constant R.

From this equation, the pO2 value of the equivalent oxygen partial pressure in equilibrium is drawn for a given gas mixture.

Within the scope of the invention, pO2 must be between 0.0015 and 0.04.

In addition, it may be beneficial to extend the optional retention chamber to the surface of the bath or to an intermediate position between the bath and the drying line to reduce oxidation prior to drying. As a result, when the sheet surface is exposed to free air, such a layer forms systematically, but is most of the time eliminated and put into the coating bath under the influence of drying. This retention thus reduces the amount of bath oxides that can be caused by the belt when it is moved and thus creates excessive defects.

However, it is an inconvenience by promoting evaporation of the zinc starting from the bath, the vapors can then contaminate the holding chamber.

Although all types of drying nozzles can be used to apply the method of the invention, we will prefer to choose nozzles whose output aperture is in the form of a plate whose width exceeds the width of the coated strip. As a result, this type of nozzle allows you to properly stop the bottom of the drying zone.

tests

Tests were carried out based on cold laminated steel sheet metal IF-Ti, which is carried out through a crucible containing a zinc-based metal bath containing varying proportions of aluminum and magnesium. It is kept at 70^aboutC above liquidus composition.

After leaving the bath, the obtained coating is dried with nitrogen using two classic nozzles in such a way as to obtain a coating thickness of 7 μm.

Above these drying nozzles there is a series of cooling chambers, on both sides of the belt, the chambers allow air blowing so that the coating solidifies as a result of forced convection. Depending on the flow, the temperature of the gas blown out and the number of chambers used, the cooling rate can be adjusted between 5 and

50^aboutC / s.

Affecting these parameters and the position of the chambers used, a series of tests are carried out in such a way as to obtain controlled cooling at different cooling speeds, before the liquidus temperature is reached and between this temperature and the solidus temperature. When all chambers are disconnected, the coating is only subject to natural convection at a speed of about 5^aboutC / s.

The test (marked 17) is also carried out by performing nitrogen drying in a holding chamber with oxygen content reduced to 4% by volume in the zone starting at the drying line level and ending 10 cm higher.

Finally, the obtained samples are extruded according to the biaxial tensile compression method at 3.5% (Marciniak). Some samples have previously been subjected to smoothing rolling operations with an elongation rate of 1.5%.

As the tests progress, the Wa0.8 fold values ​​are measured. This measurement consists in obtaining by mechanical touch, without slipping, a 50 mm sheet metal profile measured at 45^about from the direction of lamination. Cut out at the received mark, approximation of its general form by a polynomial of at least grade 5. The folding of Wa is thus isolated from the roughness Ra by a Gaussian filter at a 0.8 mm intersection.

The results obtained are summarized in the following table:

<td rowspan="2">Test</td><td colspan="3">Coating composition (in% by weight)</td><td rowspan="2">Liquidussolidus (^aboutC)</td><td rowspan="2">VR before liquidus (+ C / s)</td><td rowspan="2">VR between liquidus and solidus (^aboutC / s)</td><td colspan="4">Corrugation Wa₀,₈ (Μιη)</td>

<td>Zn</td><td>Al</td><td>mg</td><td>Without smoothing rolling and deformation</td><td>No smoothing after deformation</td><td>With smoothing rolling, before deformation</td><td>With smoothing rolling, after deformation</td>

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

<td> 2</td><td> 93</td><td> 4</td><td> 3</td><td> 357-340</td><td> 5</td><td> 10</td><td> 0,92</td><td>AD</td><td>AD</td><td>AD</td>

<td> 3*</td><td> 93</td><td> 4</td><td> 3</td><td> 357-340</td><td> 5</td><td> 15</td><td> 0,43</td><td>AD</td><td>AD</td><td>AD</td>

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

<td> 5*</td><td> 93</td><td> 4</td><td> 3</td><td> 357-340</td><td> 10</td><td> 20</td><td> 0,47</td><td>AD</td><td>AD</td><td>AD</td>

<td> 6</td><td> 93</td><td> 4</td><td> 3</td><td> 357-340</td><td> 15</td><td> 20</td><td> 3,01</td><td>AD</td><td>AD</td><td>AD</td>

<td> 7</td><td> 99,7</td><td> 03</td><td> 0</td><td> 416-413</td><td> 5</td><td> 20</td><td> 0,71</td><td> 0,62</td><td> 0,41</td><td> 0,63</td>

<td> 8</td><td> 97</td><td> 15</td><td> 1,5</td><td> 380-340</td><td> 5</td><td> 20</td><td> 0,84</td><td>AD</td><td>AD</td><td>AD</td>

<td> 9</td><td> 86</td><td> 11</td><td> 3</td><td> 428-340</td><td> 5</td><td> 20</td><td> 0,68</td><td>AD</td><td>AD</td><td>AD</td>

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

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

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

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

<td> 14</td><td> 95</td><td> 5</td><td> 0</td><td> 381-381</td><td> 15</td><td> 20</td><td> 3,36</td><td>AD</td><td>AD</td><td>AD</td>

<td> 15</td><td> 95</td><td> 5</td><td> 0</td><td> 381-381</td><td> 5</td><td> 5</td><td> 1,37</td><td> 1,14</td><td> 0,43</td><td> 0,93</td>

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

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

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

- 9 Based on tests 1 to 3, we find that starting with a cooling rate when solidification of greater than or equal to 15^aboutC / s, a spectacularly reduced undulation is obtained in the smooth state without rolling.

In addition, the inventors found that cooling too fast in the first cooling phase, before liquidus was achieved, was also detrimental to the appearance of the coating, as shown in tests 4 to 6, where we observe that exceeding threshold 15^aboutC / s significantly worsens the folding, which passes from 0.47 to 3.01 μm when it passes under cooling from 10 to 15^aboutC / s.

It is therefore concluded that this first cooling step is also important for obtaining the desired coating appearance, which leads to the conclusion that the entire cooling process must be controlled.

Tests 7 to 12 also show that the effect of reducing corrugation is not obtained for any galvanized coating, but only for coatings whose composition corresponds to the ranges defined in the present invention.

As far as tests 4, 10 to 13 and 16 are concerned, including sheets after smoothing and extrusion rolling, a surprising improvement in the level of corrugation is found, while at the same time comparative tests 1, 7 and 15 show deterioration of the results obtained after extrusion, not allowing to go below threshold 0 , 35 μm corrugations for the part based on sheet metal after smoothing rolling.

Finally, comparing test 17 results with test 4 results, carried out under identical conditions but without particular care for the level of drying, a reduction in the level of folding in all cases of the figures is found.

Prepared and verified

Grażyna Palka

Patent Attorney