RU2783513C1 - Method for processing metal sheet and metal sheet processed by this method - Google Patents

Abstract

FIELD: steel substrates.

SUBSTANCE: inventions group relates to a steel substrate, coated on at least one of its sides with a metal coating based on zinc or its alloys, having a conversion layer, an automotive part made from a steel substrate, and a processing method that ensures the formation of a conversion layer on the metal coating. The steel substrate, at least on one of the sides of which is coated with a metal coating based on zinc or its alloys, contains a metal coating coated with a conversion layer, including zinc sulfate hydrate, aluminum in an amount of 14 mg⋅m^-2, and having a surface sulfur concentration of more than or equal to 5.0 mg/m². In this case, the conversion layer contains neither zinc hydroxysulfate, nor free water molecules, nor compounds having free hydroxyl groups.

EFFECT: invention provides improved adhesion to adhesives used in the automotive industry, especially to adhesives based on epoxy resin, regardless of the drying temperature.

15 cl, 1 dwg, 1 tbl

Description

The present invention relates to a metal sheet comprising a steel substrate on at least one of the sides of which a metal coating based on zinc or its alloys is applied.

The invention relates in particular to the pre-lubrication of this coated steel substrate and its treatment with aqueous solutions containing sulfates.

This type of metal sheet is intended in particular for the manufacture of automotive parts, although it is not limited to these applications.

From US 2017260471 it is known to treat a galvanized metal sheet with an aqueous solution containing sulfate from the group consisting of aluminum sulfate, ammonium sulfate, iron sulfate and magnesium sulfate to obtain suitable tribological conditions when forming a system of flat steel products.

The present application discloses that tribologically active layers comprising the listed sulfates achieve the same effect as the conventional coatings disclosed in, for example, WO00/15878.

Indeed, it is already known from WO00/15878 to treat a galvanized metal sheet with an aqueous solution containing zinc sulfate to form a zinc hydroxy sulfate layer on a zinc-based coating. This zinc hydroxy sulfate conversion layer provides a pre-lubricated galvanized metal sheet with higher performance than those obtained by phosphating.

However, it has been observed that this zinc hydroxy sulfate conversion layer cannot provide sufficient adhesion to adhesives used in the automotive industry, especially epoxy based adhesives.

Applications WO 2019/073273 and WO 2019/073274 disclose a steel substrate on at least one of the sides of which a metal coating based on zinc or its alloys is applied, while the metal coating itself is coated with a conversion layer comprising at least one of the compounds selected from zinc sulfate monohydrate, zinc sulfate tetrahydrate and zinc sulfate heptahydrate, wherein the conversion layer contains neither zinc hydroxysulfate, nor free water molecules, nor free hydroxyl groups, while the surface concentration of sulfur in the conversion layer is greater than or equal to 0.5 mg/m ² .

These applications also disclose a processing method for the manufacture of this steel substrate, which includes the steps, according to which:

- (i) preparing a strip of steel, at least one of the sides of which is covered with a metal coating based on zinc or its alloys,

- (ii) an aqueous treatment solution comprising at least 0.01 mol/l zinc sulfate is applied to the metal coating by simple contact to form a wet film,

- (iii) the aqueous treatment solution is then dried in a dryer at a certain air temperature, and the time between application of the aqueous treatment solution to the metal coating and withdrawal from the dryer is less than 4 seconds, while the speed of the strip, the thickness of the wet film, the initial temperature of the strip, and the air flow rate is adapted to form a conversion layer on the metal coating that does not contain either free water molecules or free hydroxyl groups, while the surface concentration of sulfur in the conversion layer is higher than or equal to 0.5 mg/m ² . In WO 2019/073273, the air drying temperature is above 170°C. In WO 2019/073274, the air drying temperature is below 80°C.

In both applications, although the conversion layer does not include zinc hydroxysulfate, free water molecules, or free hydroxyl groups that impair adhesion to adhesives used in the automotive industry, the processing method includes air drying carried out at very specific temperatures. They are very limited, because outside the drying temperature range, a zinc hydroxy sulfate structure is formed that impairs the adhesion of adhesives used in the automotive industry, especially epoxy-based adhesives. All facilities cannot operate or be modified to achieve these drying temperatures. Finally, the process is complex because it requires that the time between application of the aqueous treatment solution to the metal coating and exit from the dryer be less than 4 seconds.

Thus, the aim of the present invention is to overcome the disadvantages (devices and processes) of the prior art by providing a surface treatment method that provides more suitable adhesion to adhesives used in the automotive industry, especially epoxy based adhesives, regardless of the drying temperature.

This goal is achieved by creating a steel substrate, at least on one of the sides of which a metal coating based on zinc or its alloys is applied, while a conversion layer is applied to the metal coating itself, including:

- zinc sulfate hydrate,

- aluminum in an amount up to 14 mg⋅m ^-2 ,

in this case, the conversion layer contains neither zinc hydroxysulfate, nor free water molecules, nor any compounds having free hydroxyl groups, and the surface concentration of sulfur in the conversion layer is greater than or equal to 5.0 mg/m ² .

The steel substrate according to the invention can also be characterized by the additional features listed below, considered individually or together:

- aluminum is contained in an amount of up to 13.0 mg⋅m ² ,

- the aluminum of the conversion layer is in the form of aluminum sulfate and/or aluminum hydroxide,

- the amount of aluminum in the conversion layer is 5.0 - 13.0 mg⋅m ^-2 ,

- zinc sulfate hydrate includes at least one of the compounds selected from: zinc sulfate monohydrate (ZnSO ₄ ⋅H ₂ O), zinc sulfate tetrahydrate (ZnSO ₄ ⋅4H ₂ O) and zinc sulfate heptahydrate (ZnSO ₄ ⋅7H ₂ O) ,

- surface concentration of sulfur in the conversion layer is 5.0 - 22.0 mg/m ² ,

- a metal coating based on zinc or its alloys includes at least one element selected from magnesium with a content of up to 10 wt.%, aluminum with a content of up to 20 wt.%, silicon with a content of up to 0.3 wt.%,

- metal coating based on zinc or its alloys includes at least 0.1 wt.% magnesium.

The second object of the invention are car parts made from a steel substrate in accordance with the invention.

The third object of the invention is a method for processing a moving metal strip, comprising the steps, according to which:

i. prepare a strip of steel, at least one of the sides of which is coated with a metal coating based on zinc or its alloys,

ii. an aqueous treatment solution comprising at least 0.01 mol ^{l -1} zinc sulfate and at least 0.01 mol ^{l -1} aluminum sulfate is applied to the metal coating by simple contact to form a wet film,

iii. the aqueous treatment solution is then air-dried to form a conversion layer on the metal plating, comprising:

- zinc sulfate hydrate,

- aluminum in an amount up to 14 mg⋅m ^-2 ,

while the conversion layer does not include any zinc hydroxysulfate, or free water molecules, or any compounds having free hydroxyl groups, while the surface concentration of sulfur in the conversion layer is greater than or equal to 5.0 mg/m ² .

The processing method according to the invention can also be characterized by the additional features listed below, considered separately or together:

- aluminum is present in an amount up to 13.0 mg⋅m ² ,

- the aqueous treatment solution contains 10 - 140 g/l zinc sulfate heptahydrate,

- the aqueous treatment solution contains 1 - 80 g/l aluminum sulfate octadecahydrate,

- the mass ratio of the amount of zinc to the amount of aluminum in an aqueous solution is 5 - 40,

- the metallic coating can be applied by hot dip, electroplating or physical vapor deposition,

- the metal coating is degreased before application of the aqueous treatment solution,

- wet film thickness is 0.5 - 4 µm,

- an oil film is applied to the conversion layer with a coating weight of less than 2 g/m ² ,

- drying temperature is 20 - 200°C.

Without wishing to be bound by any theory, it is believed that the presence of zinc hydroxy sulfate itself in the conversion layer resulted in poor adhesion of the treated metal sheet to some adhesives, especially epoxy adhesives. Indeed, the hydroxyl groups of the zinc hydroxy sulfate structure react with the epoxy adhesive system and lead to adhesion problems. In particular, their presence impairs the zinc/epoxy resin interfacial bonds and also causes plasticization of the adhesive.

In addition, the inventors have found that free water molecules and/or free hydroxyl groups can be present in the conversion layer, even if it appears to be dry. These free water molecules and/or free hydroxyl groups are also highly reactive with specific adhesive compounds such as epoxy resin based compounds, leading to adhesion problems.

The inventors undertook an intensive search for a method to obtain a layer that excludes zinc hydroxysulfate and is perfectly dried, i.e. without free water molecules and free hydroxyl groups, regardless of drying conditions, to obtain a layer with good adhesion to epoxy adhesives while maintaining other properties.

Product studies have shown that improved adhesion to epoxy adhesives under all drying conditions was possible if the conversion layer contained zinc and aluminum sulfate hydrate in an amount up to 14.0 mg.m ^-2 .

Indeed, it is believed that the structure of the conversion layer, additionally including up to 14.0 mg.m ^-2 Al, further improves adhesion to adhesives. It appears that the aluminum binds the free hydroxyl groups resulting from the oxidation of the metal coating, which prevents the pH from rising to 7, after which the zinc hydroxy sulfate begins to precipitate on the metal coating. Moreover, because aluminum maintains a low enough pH to avoid precipitation of zinc hydroxysulfate, it is no longer necessary to choose the drying conditions carefully so that only stable zinc sulfate hydrates are formed. In this case, even if unstable hydrates are present in the conversion layer, they will not decompose into zinc hydroxysulfate. In addition, since aluminum binds free hydroxyl groups, the formation of free water molecules is also prevented.

Thus, unlike WO 2019/073273 and WO 2019/073274, no specific drying temperature is required, no specific time is required between application of the aqueous treatment solution to the metal coating and withdrawal from the dryer. The processing method of the present invention can be easily implemented in enterprises without major modifications. Moreover, the coated sheet of the present invention has better adhesion to adhesives than sheets of the prior art, as described in particular in US 2017260471 and WO 00/15878.

The invention will be better understood upon reading the following description, which is provided solely for purposes of explanation and is not intended in any way to limit the invention with reference to FIG. 1 showing the IRRAS spectra of a conversion layer according to the invention and the prior art.

First of all, the invention relates to a steel substrate. It may be in the form of a metal strip. It is preferably subjected to hot rolling and then cold rolling. It can be rolled up for later use, for example, as a car body part.

At least one side of the steel substrate is coated with a metallic coating based on zinc or its alloys, i. e. zinc, including one or more alloying elements such as, but not limited to, iron, aluminum, silicon, magnesium and nickel. In some embodiments, this type of coating may be present on both sides of the substrate.

The metal coating is typically less than or equal to 20 µm thick and is intended to protect the substrate from through corrosion in a conventional manner.

In one embodiment of the invention, the metal coating comprises 0.1-0.4 wt.% aluminum, the rest is zinc and inevitable impurities arising during the production process.

In one embodiment of the invention, the metal coating includes at least 0.1 wt.% magnesium to improve corrosion resistance. Preferably the metal coating contains at least 0.5% and more preferably at least 2 wt.% magnesium.

In another preferred embodiment, the metal coating based on zinc or its alloys includes at least one element of magnesium with up to 10 wt.%, aluminum with up to 20 wt.%, silicon with up to 0.3 wt.%.

In another preferred embodiment, the metal coating based on zinc or its alloys comprises 0.01 - 8.0 wt.% Al, optionally 0.2 - 8.0 wt.% Mg, the rest is Zn and unavoidable impurities arising during the production process. . For example, a zinc-based coating includes 1.2 wt.% Al and 1.2 wt.% Mg or 3.7 wt.% Al and 3 wt.% Mg.

A metal coating based on zinc or its alloys can be applied by hot dip. In this case, the bath may also contain up to 0.3% by weight of optional additional elements such as Sr, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.

These various elements can, inter alia, improve the ductility or adhesion of the metal coating to the substrate. One skilled in the art, familiar with their effect on coating performance, will understand how to use them depending on the additional purpose.

Finally, the bath may contain residual elements coming from the molten ingots or resulting from the passage of the substrate through the bath, such as iron in an amount of up to 5 wt.%, preferably 3 wt.% These residual elements are partially incorporated into the metal coating, and in this case they are referred to as "unavoidable impurities resulting from the manufacturing process".

A metal coating based on zinc or its alloys can also be applied by electrodeposition or physical vapor deposition. In this case, it is possible to apply a metal coating consisting of zinc, i. in which the amount of zinc exceeds 99 wt.%

The metal coating is at least partially covered with a conversion layer comprising zinc sulfate hydrate and aluminum in an amount up to 14 mg⋅m ^-2 .

Zinc sulfate hydrate and aluminum act synergistically. Zinc sulphate hydrate provides prior art performance while aluminum provides conditions in which zinc sulphate hydrate is stable so that zinc hydroxysulphate and free water molecules are prevented.

Zinc sulfate hydrate has the general formula: Znx(SO ₄ ) _y ⋅zH ₂ O, where x, y, and z are nonzero. Preferably, it includes at least one compound selected from zinc sulfate monohydrate (ZnSO ₄ ⋅H ₂ O), zinc sulfate tetrahydrate (ZnSO ₄ ⋅4H ₂ O) and zinc sulfate heptahydrate (ZnSO ₄ ⋅7H ₂ O). These are stable connections. Due to their presence, the subsequent formation of zinc hydroxy sulfate is prevented by the decomposition of unstable zinc sulfate hydrates.

The amount of aluminum is limited to 14 mg⋅m ^-2 , preferably 13.0 mg⋅m ^-2 , since it is believed that more aluminum can reduce the adhesive bond.

Preferably, the amount of aluminum in the conversion layer is 5-14 mg.m ^-2 and more preferably 7-13 mg.m ^-2 .

The form in which aluminum is present in the conversion layer of the invention is not particularly limited. Without wishing to be bound by any theory, it is believed that aluminum is mainly present in the form of aluminum sulfate and/or aluminum hydroxide (Al(OH) ₃ ), formed by the reaction of aluminum with free hydroxyl groups. Preferably the conversion layer thus comprises zinc sulfate hydrate and at least one of aluminum sulfate and aluminum hydroxide.

The conversion layer also does not include zinc hydroxysulfate, free water molecules, or any compounds having free hydroxyl groups.

Zinc hydroxysulfate contains hydroxyl groups which, according to the inventors, react with the epoxy adhesive system and lead to adhesion problems. Their absence significantly improves the adhesion of epoxy-based adhesives to metal sheets. By zinc hydroxysulfate is meant a compound of the general formula:

[Zn _x (SO ₄ ) _y (OH) _z , tH ₂ O]

where 2x=2y+z, where y and z are non-zero.

z is preferably greater than or equal to 6 and more preferably z=6 and 3≤t≤5. In particular, compounds with x=4, y=1, z=6 and t=3 have been observed on prior art metal sheets.

Free water molecules and free hydroxyl groups are also highly reactive with specific adhesive compounds such as epoxy resin based compounds, leading to adhesion problems. Their absence significantly improves the adhesion of epoxy-based adhesives to metal sheets.

The presence of sulfate in the conversion coating is assessed and quantified by measuring the surface concentration of sulfur. While the surface concentration of sulfur in the conversion layer is greater than or equal to 0.5 mg/m ² . Below this value, it appears that the metal coating is destroyed during the forming of the metal sheet, resulting in the formation of powder or particles of zinc or its alloys on the surface of the metal sheet. Accumulation and/or agglomeration of these particles or this powder in molding tools can lead to damage to molded parts due to the formation of nicks and/or constrictions.

Preferably, the surface concentration of sulfur in the conversion layer is 5.0-22.0 mg/m ² , more preferably 10.0-22.0 mg/m ² and preferably 13.0-22.0 mg/m ² . Without wishing to be bound by any theory, it is believed that these amounts of sulfur further improve the adhesive bond of the steel substrate in accordance with the present invention.

The surface concentration of sulfur in the conversion layer can be measured using ICP or X-ray fluorescence (XRF).

From a technological point of view, the conversion layer can be obtained by applying to the coating, possibly after degreasing, an aqueous treatment solution containing at least 0.01 mol ^{l -1} zinc sulfate and at least 0.01 mol l ^-1 aluminum sulfate.

It is impossible to form such a layer at a zinc sulfate concentration of less than 0.01 mol⋅l ^-1 , but it has also been found that too high a concentration does not significantly improve the deposition rate and may even reduce it somewhat. Preferably, the aqueous treatment solution contains zinc sulfate ZnSO ₄ at a concentration less than or equal to 50 mol ^{l -1} and aluminum sulfate Al ₂ (SO ₄ ) ₃ at a concentration less than or equal to 50 mol ^{l -1} .

An aqueous treatment solution can be prepared by dissolving zinc sulfate and aluminum sulfate in clean water. For example, zinc sulfate heptahydrate (ZnSO ₄ ⋅7H ₂ O) can be used. For example, aluminum sulfate octadecahydrate (Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O) can be used. In one embodiment of the invention, the aqueous treatment solution consists of zinc sulfate, aluminum sulfate and water.

Preferably, the aqueous treatment solution contains 10-140 g/l zinc sulfate heptahydrate, more preferably 10-80 g/l and preferably 10-40 g/l.

Preferably, the aqueous treatment solution contains 1-80 g/l aluminum sulfate octadecahydrate, more preferably 10-60 g/l and preferably 10-30 g/l.

Preferably, the weight ratio of zinc to aluminum in aqueous solution is 5-40, more preferably 5-30 and preferably 10-25. Indeed, without wishing to be bound by any theory, it is believed that when the weight ratio of zinc to aluminum in an aqueous solution such as above, there is an additional improvement in the adhesive bond.

The pH of the aqueous treatment solution preferably corresponds to the solution's own pH without the addition of a base or acid. This pH value is usually 4 - 7.

The temperature of the aqueous treatment solution may be 20-60°C.

The aqueous treatment solution can be applied to the metal coating by simple contact and air dried at any drying temperature. It is applied in the usual way, for example by dipping, rolling, spraying followed by compaction.

Preferably, the wet film thickness is 0.5-4 µm.

Preferably, the aqueous treatment solution is then dried in an air dryer. Preferably, the dryer includes 6 to 12 nozzles for better distribution of the shock air jet over the metal strip. Preferably, the dryer includes nozzles located 4 to 12 cm from the metal strip in order to avoid pressure loss in the jet without removing the wet film from the metal strip. Preferably the nozzles have openings that are 2 to 8 mm wide in order to optimize the air velocity at the outlet of the nozzle.

Preferably the drying temperature is 20-200°C, more preferably 50-200°C and for example below 80°C, between 80-150°C or above 150°C.

Preferably the strip speed is 60-200 m/min.

Preferably the starting temperature of the strip is 20-50°C.

Preferably the air flow is 5000 - 50000 Nm ³ /h.

After the formation of the conversion layer, an oil film with a coating weight of less than 2 g/m ² can be applied to the conversion layer.

From a practical point of view, the absence of zinc hydroxysulfate can be monitored using infrared spectroscopy in the IRRAS mode (reflectance-adsorption infrared spectroscopy with an angle of incidence of 80°). If the conversion layer includes zinc hydroxysulfate, the IRRAS spectrum presents several absorption peaks related to sulfate ν ₃ vibrations 1077-1136-1177 cm ^-1 and active water bands in the OH stretching region 3000-3400 cm ^-1 . These results are consistent with the structure of zinc hydroxysulfate as reported in the literature (v ₁ sulfate oscillation: 1000 cm ^-1 , ν ₂ sulfate oscillation: 450 cm ^-1 , ν ₃ sulfate oscillation: 1068 - 1085 - 1130 cm ^-1 , ν ₄ sulfate oscillation : 611-645 cm ^-1 hydroxyl vibrations: 3421 cm ^-1 ).

The presence of zinc sulfate hydrate can be monitored using infrared spectroscopy in IRRAS mode. If the conversion layer contains zinc sulfate hydrate without zinc hydroxysulfate, the IRRAS spectrum is one single sulfate peak located at about 1172 cm ^-1 instead of 3 peaks. More specifically, the presence of each of the above-mentioned stable zinc sulfate hydrates can be monitored using infrared spectroscopy in IRRAS mode in combination with differential scanning calorimetry (DSC) detection of sulfate bands and free water bands.

Wet film thickness can be measured using an infrared sensor located in front of the dryer. It consists of a light source, an infrared detector and special filters. The measuring principle is based on the absorption of infrared light.

At the outlet of the dryer, the absence of water in the conversion layer can be monitored, in particular, using a hyperspectral camera. The latter consists of an infrared array detector connected to a spectrometer, which distributes light over wavelengths. The measuring device may consist of a linear IR lamp (800 mm long) and a MWIR (Mid-IR) hyperspectral camera in a bidirectional reflection configuration. The detection range of the camera is 3 - 5 µm, which corresponds to the main absorption bands of liquid water. The measurement principle is to measure the intensity of light reflected from a metal strip. If water remains in the conversion layer, it absorbs some of the light and is reflected at a lower intensity.

Alternatively, the absence of water in the conversion layer at the outlet of the dryer is controlled by monitoring the temperature of the steel strip in the dryer. As long as there is water in the film, the thermal energy of the hot air is spent on the evaporation of water, and the temperature of the metal strip remains constant or even decreases due to the evaporation of water. After the film has dried, the thermal energy of the hot air is used to heat the metal strip. Thus, by controlling the temperature of the steel strip in the dryer, it is easy to control that the temperature of the metal strip begins to rise before leaving the dryer.

To emphasize the improved performance obtained by using the treatment method and the steel substrate in accordance with the invention, some specific embodiments will be described in detail in comparison with prior art coated steel sheets.

Example 1

As will be seen from the following non-limiting examples, which are provided by way of illustration only, the inventors have shown that the invention improves adhesion to adhesives used in the automotive industry, especially epoxy adhesives, without compromising other characteristics.

10 samples were prepared by applying an aqueous treatment solution to a galvanized steel sheet or an electro-galvanized steel sheet by roller coating and drying the wet film under various drying conditions.

Samples 1 and 2 are prepared in accordance with US 2017260471 by applying an aqueous solution containing Al ₂ (SO ₄ ) ₃ ·18H ₂ O aluminum sulfate octadecahydrate to galvanized galvanized steel sheet (GI). The concentration of aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O is 22 g⋅l ^-1 , which corresponds to an Al ₂ (SO ₄ ) ₃ concentration of 0.033 mol⋅l ^-1 . Sample 1 is then dried in an air dryer at 100° C. for 5 seconds. Sample 2 is then air dried in a dryer at 180° C. for 8 minutes.

Samples 3 and 4 are prepared according to WO 00/15878 by applying an aqueous solution containing zinc sulfate heptahydrate ZnSO ₄ ⋅7H ₂ O to a galvanized steel sheet. Sample 3 is then dried in an air dryer at 100° C. for less than 4 seconds. Sample 4 was then dried in an air dryer at 180° C. for 8 minutes.

For Sample 3, the strip speed was 120 m/min. The initial temperature of the strip is 35°C.

Sample 5 is prepared according to WO 2019/073273 by applying an aqueous solution comprising zinc sulfate heptahydrate ZnSO ₄ ⋅7H ₂ O to a galvanized steel sheet. The concentration of zinc sulfate heptahydrate is 120 g⋅l ^-1 , which corresponds to the concentration of Zn ^{2+ ions} and the concentration of SO ₄ ^2- 0.42 mol·l ^-1 . The wet film thickness is 1.5 µm. Then the wet film is dried for 4 seconds in air in a dryer at a temperature of 175°C. The strip speed is 120 m/min. The initial temperature of the strip is 35°C.

Sample 6 was prepared according to WO 2019/073274 by applying an aqueous solution containing zinc sulfate heptahydrate ZnSO ₄ ⋅7H ₂ O to a galvanized steel sheet. The concentration of zinc sulfate heptahydrate is 120 g⋅l ^-1 , which corresponds to the concentration of Zn ^{2+ ions} and the concentration of SO ₄ ^2- 0.42 mol⋅l ^-1 . The wet film thickness is 1.5 µm. The wet film is then dried for 4 seconds in a dryer at an air temperature of 75°C. The strip speed is 120 m/min. The initial temperature of the strip is 35°C.

Samples 7 and 8 are prepared by applying an aqueous solution comprising zinc sulfate heptahydrate ZnSO ₄ ⋅7H ₂ O and aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O onto a galvanized steel sheet. The concentration of aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O is 25 g/l, which corresponds to the concentration of Al ³⁺ ions of 0.075 mol⋅l ^-1 and 2.02 g/l and the concentration of SO ₄ ^2- 0.113 mol⋅ l ^-1 . The concentration of zinc sulfate heptahydrate is 120 g⋅l ^-1 , which corresponds to the concentration of Zn ^{2+ ions} of 0.42 mol⋅l ^-1 and 27.28 g⋅l ^-1 and the concentration of SO ₄ ^2- 0.42 mol⋅l ^-1 . Thus, the mass ratio of the amount of zinc to the amount of aluminum in aqueous solution is 13.5. The wet film thickness is 1 - 1.5 µm. Sample 7 is then air dried in a dryer at 75° C. for less than 4 seconds. Sample 8 is then air dried in a dryer at 100° C. for less than 4 seconds.

Sample 9 is prepared by applying an aqueous solution comprising zinc sulfate heptahydrate ZnSO ₄ ⋅7H ₂ O and aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O onto a galvanized steel sheet. The concentration of aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O is 4.2 g/l, which corresponds to the concentration of Al ³⁺ 0.013 mol⋅l ^-1 and 0.35 g/l and the concentration of SO ₄ ^2- 0.019 mol ⋅l ^-1 . The concentration of zinc sulfate heptahydrate is 32 g⋅l ^-1 , which corresponds to a concentration of Zn ^{2+ ions} of 0.111 mol⋅l ^-1 and 7.27 g⋅l ^-1 and a concentration of SO ₄ ^2- 0.111 mol⋅l ^-1 . Thus, the mass ratio of the amount of zinc to the amount of aluminum in aqueous solution is 20.77. Sample 9 is then air dried in a dryer at 180° C. for 8 minutes.

Sample 10 is prepared by applying an aqueous solution comprising zinc sulfate heptahydrate ZnSO ₄ ⋅7H ₂ O and aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O onto electrogalvanized steel sheet (EG). The concentration of aluminum sulfate octadecahydrate Al ₂ (SO ₄ ) ₃ ⋅18H ₂ O is 4.2 g/l, which corresponds to the concentration of Al ³⁺ 0.013 mol⋅l ^-1 and 0.35 g/l and the concentration of SO ₄ ^2- 0.019 mol ⋅l ^-1 . The concentration of zinc sulfate heptahydrate is 32 g⋅l ^-1 , which corresponds to a concentration of Zn ^{2+ ions} of 0.111 mol⋅l ^-1 and 7.27 g·l ^-1 and a concentration of SO ₄ ^2- 0.111 mol⋅l ^-1 . Thus, the mass ratio of the amount of zinc to the amount of aluminum in aqueous solution is 20.77. The wet film is then air dried in a dryer at 180° C. for 8 minutes.

Surface characteristic

After drying, the surface of the conversion layer was characterized by the IRRAS method. The amount of sulfur in the layer is determined by ICP-MS.

Adhesion strength test

Adhesion of epoxy adhesives to the conversion layer formed in all samples was evaluated by a lap shear test. First, specimens 100 mm long and 25 mm wide are recoated with Anticorit Fuchs 3802-39S (1 g/m ² ) without degreasing. Two test specimens, one treated with an aqueous treatment solution and the other untreated, are then bonded using Teroson® 8028GB epoxy resin adhesive supplied by Henkel® by overlapping them by 12.5 mm using Teflon spacers to maintain uniform thickness 0, 2 mm between two parts. The entire assembly is cured in an oven for 20 minutes at 190°C. The samples are then conditioned for 24 hours before testing for adhesive strength and aging. For each test condition, 5 assemblies are tested.

Adhesion is evaluated according to DIN EN 1465. In this test, each bonded assembly is clamped in the jaws (grabbing 50 mm of each test piece in each clamp and leaving 50 mm of each test piece free) of a tensile machine with a force of 50 kN. The samples are stretched at a rate of 10 mm/min at room temperature. The maximum shear stress values are recorded in MPa, and the nature of the destruction is visually classified as:

- surface cohesive failure if a break occurs in the mass of adhesive near the strip/adhesive interface,

- failure of adhesion if a break appears at the strip/adhesive interface.

The test is not considered passed if the percentage of adhesive failure is high.

Adhesion aging is assessed using a cataplasmic test. In this test, each bonded assembly (5 samples each time) is wrapped in cotton (weight 45 g +/- 5) with deionized water (10 times the weight of cotton), placed in a plastic bag, which is then sealed. The sealed bag is kept in an oven at 70°C, 100% HR (relative humidity) for 7 days. After the cataplasmic test, adhesion is reassessed according to DIN EN 1465.

Loss factor estimation

Before and after adhesive aging, the tensile strength is measured for each test using a tensile gauge. Then determine the coefficient of mechanical loss corresponding to the loss of tensile strength after adhesive aging, defined as a percentage. It is calculated using this formula:

Friction test

Test specimens 1, 3 and 7 are placed and clamped in a friction sensor consisting of two flat elements made of tungsten carbide, simulating a punching tool. Then the ends of the test specimens are pulled together with a pull clamp. The traction force of the traction clamp, denoted by Fp, varies between 10 - 80 MPa. The resulting normal force, denoted Fn, which is perpendicular to the Fp direction, increases during stretching. The higher the tensile force Fp, the higher the contact pressure in the friction sensor. Fp and Fn will be measured during the test. The coefficient of friction, denoted µ, is then calculated using the following formula: µ=Fp/(2⋅Fn) for 3 tensile forces (10, 40 and 80 MPa). The coefficient of friction is expected to be in the range of 0.07 to 0.15.

The results are presented in the following table 1.

As shown in the IRRAS spectra in FIG. Sample 1a shows a single sulfate peak at about 1180 cm ^-1 associated with the presence of aluminum sulfate. As shown in FIG. 1c, the spectrum of sample 4 shows several absorption peaks related to ν ₃ sulfate vibrations in the zinc hydroxysulfate structure. In addition, sample 4 includes free water corresponding to peaks near 1650 cm ^-1 and free hydroxyl groups corresponding to peaks located at a peak near 3600 cm ^-1 . As shown in FIG. 1b spectrum of sample 9 according to the invention shows a single sulfate peak at about 1170 cm ^-1 related to zinc sulfate hydrates. The structure of zinc hydroxysulfate, free water and free hydroxyl groups are not observed in FIG. 1b.

As shown by the analysis of ICP-MS, the sulfur content in all samples exceeds 0.5 mg.m ^-2 . In samples 7 to 10, the aluminum content is above 0 and below or equal to 13 mg.m ^-2 according to the present invention.

The adhesive bond in samples 7 - 10 is significantly improved compared to samples 1 - 4. In samples 7 - 10, the adhesive characteristics with respect to adhesives are the same as for samples 5 - 6. However, the processing methods in the case of samples 5 - 6 difficult to control and apply compared to the processing method in the case of samples 7 - 10.

The loss factor of samples 7 - 8 is significantly better than that of samples 1 - 4.

The friction characteristics of specimens 1, 3, and 7 are similar.

Thus, the coated steel substrate of the present invention can improve adhesive bonding over the prior art without degrading other characteristics, and is easy to implement and easily controllable in a processing method.

Claims (24)

1. Steel substrate, at least on one of the sides of which a metal coating based on zinc or its alloys is applied, while the metal coating is coated with a conversion layer, including: - zinc sulfate hydrate, - aluminum in an amount up to 14 mg⋅m ^-2 , wherein the conversion layer contains neither zinc hydroxysulfate, nor free water molecules, nor any compounds having free hydroxyl groups, while the surface concentration of sulfur in the conversion layer is greater than or equal to 5.0 mg/m ² . 2. The steel substrate of claim 1, wherein the aluminum of the conversion layer is in the form of aluminum sulfate and/or aluminum hydroxide. 3. The steel substrate according to claim 1 or 2, wherein the amount of aluminum in the conversion layer is 5.0-13.0 mg⋅m ^-2 . 4. Steel substrate according to any one of paragraphs. 1-3, wherein the zinc sulfate hydrate comprises at least one of the compounds selected from: zinc sulfate monohydrate (ZnSO ₄ ⋅H ₂ O), zinc sulfate tetrahydrate (ZnSO ₄ ⋅4H ₂ O), and zinc sulfate heptahydrate (ZnSO _{4 ⋅7H2O} ). 5. Steel substrate according to any one of paragraphs. 1-3, in which the surface concentration of sulfur in the conversion layer is 5.0-22.0 mg/m ² . 6. Automotive part made from a steel substrate according to any one of paragraphs. 1-5. 7. A method for processing a moving metal strip, which includes the steps, according to which: ensure the presence of a steel strip on at least one of the sides of which is coated with a metal coating based on zinc or its alloys, an aqueous treatment solution comprising at least 0.01 mol ^{l -1} zinc sulfate and at least 0.01 mol ^{l -1} aluminum sulfate is applied to the metal coating by simple contact to form a wet film, the aqueous treatment solution is then air-dried to form a conversion layer on the metal plating, comprising: - zinc sulfate hydrate, - aluminum in the amount of 14 mg⋅m ^-2 , wherein the conversion layer contains neither zinc hydroxysulfate, nor free water molecules, nor any compounds having free hydroxyl groups, while the surface concentration of sulfur in the conversion layer is greater than or equal to 5.0 mg/m ² . 8. The treatment method according to claim 7, wherein the aqueous treatment solution contains 10-140 g/l zinc sulfate heptahydrate. 9. The treatment method according to claim 7 or 8, wherein the aqueous treatment solution contains 1-80 g/l aluminum sulfate octadecahydrate. 10. Processing method according to any one of paragraphs. 7-9, in which the mass ratio of the amount of zinc to the amount of aluminum in an aqueous solution is 5-40. 11. Processing method according to any one of paragraphs. 7-10, in which the metallic coating is applied by hot-dip, electrodeposition, or physical vapor deposition. 12. Processing method according to any one of paragraphs. 7-11 wherein the metal coating is degreased prior to application of the aqueous treatment solution. 13. Processing method according to any one of paragraphs. 7-12, in which the wet film thickness is 0.5-4 µm. 14. Processing method according to any one of paragraphs. 7-13, in which an oil film with a coating weight of less than 2 g/m ² is applied to the conversion layer. 15. Processing method according to any one of paragraphs. 7-14, in which the drying temperature is 20-200°C.

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