Metal sheet treatment method and metal sheet treated with this method

Abstract

The invention relates to a steel substrate coated on at least one of its faces with a metallic coating based on zinc or its alloys wherein the metallic coating is itself coated with a conversion layer comprising: zincsulphate hydrate, aluminium in an amount up to 14 mg.Math.m.sup.?2, wherein the conversion layer comprises neither zinc hydroxysulphate, nor free water molecules nor any compounds having free hydroxyl groups, the surface density of sulphur in the conversion layer being greater than or equal to 5.0 mg/m.sup.2. The invention also relates to the corresponding treatment method.

Claims

1-15. (canceled)

16: A coated steel comprising: a steel substrate; and a metallic coating, the steel substrate coated on at least one face with the metallic coating, the metallic coating based on zinc or a zinc alloy wherein the metallic coating is itself coated with a conversion layer comprising: zincsulphate hydrate, aluminium in an amount up to 14 mg.Math.m.sup.?2, wherein the conversion layer comprises neither zinc hydroxysulphate, nor free water molecules nor any compounds having free hydroxyl groups, a surface density of sulphur in the conversion layer being greater than or equal to 5.0 mg/m.sup.2.

17: The coated steel as recited in claim 16 wherein the aluminium of the conversion layer is in the form of aluminium sulphate or aluminium hydroxide.

18: The coated steel as recited in claim 16 wherein the aluminium amount in the conversion layer is comprised between 5.0 and 13.0 mg.Math.m.sup.?2.

19: The coated steel as recited in claim 16 wherein the zincsulphate hydrate comprises at least one of the compounds selected from the group consisting of: zinc sulphate monohydrate (ZnSO4.Math.H.sub.2O), zinc sulphate tetrahydrate (ZnSO.sub.4.Math.4H.sub.2O) and zinc sulphate heptahydrate (ZnSO.sub.4.Math.7H.sub.2O).

20: The coated steel as recited in claim 16 wherein the surface density of sulphur in the conversion layer is between 5.0 and 22.0 mg/m.sup.2.

21: An automotive part made of the coated steel as recited in claim 16.

22: A treatment method for a moving metal strip comprising the following steps: i. coating a strip of steel on at least one face with a metallic coating based on zinc or a zinc alloy; ii. applying an aqueous treatment solution comprising at least 0.01 mol.Math.L.sup.?1 of zinc sulphate and at least 0.01 mol.Math.L.sup.?1 of aluminium sulphate to the metallic coating by simple contact so as to form a wet film, iii. subsequently drying the aqueous treatment solution with air to form, on the metallic coating, a conversion layer comprising: zincsulphate hydrate, aluminium in an amount up to 14 mg.Math.m.sup.?2, wherein the conversion layer comprises neither zinc hydroxysulphate, nor free water molecules nor any compounds having free hydroxyl groups, the surface density of sulphur in the conversion layer being greater than or equal to 5.0 mg/m.sup.2.

23: The treatment method as recited in claim 22 wherein the aqueous treatment solution contains between 10 and 140 g/L of zinc sulphate heptahydrate.

24: The treatment method as recited in claim 22 wherein the aqueous treatment solution contains between 1 and 80 g/L of aluminium sulphate octadecahydrate.

25: The treatment method as recited in claim 22 wherein the ratio in weight of zinc amount with respect to aluminium amount in the aqueous solution is from 5 to 40.

26: The treatment method as recited in claim 22 wherein the metallic coating is deposited by hot-dip coating, electrocoating deposition or physical vapor deposition.

27: The treatment method as recited in claim 22 further comprising degreasing the metallic coating before application of the aqueous treatment solution.

28: The treatment method as recited in claim 22 wherein a thickness of the wet film is between 0.5 and 4 ?m.

29: The treatment method as recited in claim 22 further comprising applying a film of oil with a coating weight of less than 2 g/m.sup.2 on the conversion layer.

30: The treatment method as recited in claim 22 wherein the drying temperature is between 20 and 200? C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive with reference to FIGS. 1a, b, c which are IRRAS (InfraRed Reflection-Adsorption Spectroscopy) spectrums of the conversion layer according to the invention and to the prior art.

DETAILED DESCRIPTION

[0052] First of all, the invention relates to a steel substrate. It can be in the form of a metal strip. It is preferably hot-rolled and then cold-rolled. It can be coiled for later use as a part for an automobile body, for example.

[0053] The steel substrate is coated on at least one of its faces with a metallic coating based on zinc, i.e. unalloyed zinc, or its alloys, i.e. zinc comprising one or more alloying elements, such as for example but not being restricted thereto, iron, aluminium, silicon, magnesium and nickel. In certain variants, a coating of this type can be present on both faces of the substrate.

[0054] The metallic coating generally has a thickness of less than or equal to 20 ?m and is intended for the purpose of protecting the substrate against perforating corrosion, in the conventional manner.

[0055] In one variant of the invention, the metallic coating comprises between 0.1% and 0.4% by weight aluminium, the rest being zinc and the unavoidable impurities resulting from the manufacturing process.

[0056] In one variant of the invention, the metallic coating comprises at least 0.1% by weight magnesium to improve the resistance to corrosion. Preferably, the metallic coating contains at least 0.5% and more preferably at least 2% by weight magnesium.

[0057] In another preferred embodiment, the metallic coating based on zinc or its alloys comprises at least one element among magnesium up to a content of 10% by weight, aluminium up to a content of 20% by weight, silicon up to a content of 0.3% by weight.

[0058] In another preferred embodiment, the metallic coating based on zinc or its alloys comprises 0.01-8.0 wt % Al, optionally 0.2-8.0 wt % Mg, the remainder being Zn and the unavoidable impurities resulting from the manufacturing process. For example, the zinc-based coating comprises 1.2 wt. % of Al and 1.2 wt. % of Mg or 3.7 wt. % of Al and 3 wt. % of Mg.

[0059] The metallic coating based on zinc or its alloys can be deposited by hot-dip coating. In this case, the bath can 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.

[0060] These different elements can, among other things, improve the ductility or the adherence of the metallic coating to the substrate. A person skilled in the art who is familiar with their effects on the characteristics of the coating will know how to use them as a function of the additional purpose sought.

[0061] Finally, the bath can contain residual elements originating from the feeding ingots melted or resulting from the passage of the substrate through the bath, such as iron in a content up to 5% by weight, preferably 3% by weight. These residual elements are partly incorporated into the metallic coating, in which case they are designated by the term unavoidable impurities resulting from the manufacturing process.

[0062] The metallic coating based on zinc or its alloys can be also be deposited by electro-coating deposition or physical vapor deposition. In this case, it is possible to deposit a metallic coating consisting of zinc, i.e. wherein the amount of zinc is above 99 wt %.

[0063] The metallic coating is at least partially covered by a conversion layer comprising zincsulphate hydrate and aluminium in an amount up to 14 mg.Math.m.sup.?2.

[0064] Zincsulphate hydrate and aluminium work in synergy. The zincsulphate hydrate provides the performances as established by the prior art while the aluminium provides conditions in which the zincsulphate hydrate is stable so that the appearance of zinc hydroxysulphate and of free water molecules is prevented.

[0065] The zincsulphate hydrate is of general formula: Zn.sub.x(SO.sub.4).sub.y.Math.zH.sub.2O where x, y and z are different from zero. Advantageously, it comprises at least one of the compounds selected from among: zinc sulphate monohydrate (ZnSO.sub.4.Math.H.sub.2O), zinc sulphate tetrahydrate (ZnSO.sub.4.Math.4H.sub.2O) and zinc sulphate heptahydrate (ZnSO.sub.4.Math.7H.sub.2O). These are stable compounds. Thanks to their presence, a later development of zinc hydroxysulphate by decomposition of unstable zincsulphate hydrates is avoided.

[0066] The amount of aluminium is limited to 14 mg.Math.m.sup.?2, preferably 13.0 mg.Math.m.sup.?2, as it is believed that a higher amount of aluminium might decrease the adhesion bonding.

[0067] Preferably, the aluminium amount in the conversion layer is from 5 to 14 mg.Math.m.sup.?2 and more preferably from 7 to 13 mg.Math.m.sup.?2.

[0068] The form in which Aluminium is present in the conversion layer of the invention is not particularly limited. Without willing to be bound by any theory, it is believed that Aluminium is mainly present in the form of Aluminium sulphate and/or Aluminium hydroxide (Al(OH).sub.3), resulting from the combination of Aluminium with the free hydroxy groups. Preferably, the conversion layer thus comprises zincsulphate hydrate and at least one of aluminium sulphate and aluminium hydroxide.

[0069] The conversion layer also comprises neither zinc hydroxysulphate, nor free water molecules nor any compounds having free hydroxyl groups.

[0070] Zinc hydroxysulphate contains hydroxyl groups that, based on inventors' understanding, react with the epoxy system of the adhesive and lead to adhesion problems. Its absence significantly improves the adhesion of epoxy-based adhesives on metal sheets. By zinc hydroxysulphate, it is meant the compound of general formula:

[Zn.sub.x(SO.sub.4).sub.y(OH).sub.z,tH.sub.2O]

[0071] where 2x=2y+z, with y and z different from zero.

[0072] z is often higher than or equal to 6, and more often z=6 and 3?t?5. In particular, compound with x=4, y=1, z=6 and t=3 has been observed on metal sheets from the prior art.

[0073] Free water molecules and free hydroxyl groups are also very reactive with specific compounds of the adhesive such as, for example, epoxy-based compounds which leads to adhesion problems. Their absence significantly improves the adhesion of epoxy-based adhesives on metal sheets.

[0074] The presence of sulphate in the conversion coating is assessed and quantified by the measure of the surface density of Sulphur. In this case, the surface density of sulphur in the conversion layer is greater than or equal to 0.5 mg/m.sup.2. Below this value, it seems that the metallic coating deteriorates while the metal sheet is formed, which results in the formation of powder or particles of zinc or its alloys at the surface of the metal sheet. The accumulation and/or agglomeration of these particles or this powder in the forming tools may damage the formed parts, by the formation of barbs and/or constrictions.

[0075] Preferably, the surface density of sulphur in the conversion layer is between 5.0 and 22.0 mg/m.sup.2, more preferably between 10.0 and 22.0 mg/m.sup.2 and advantageously between 13.0 and 22.0 mg/m.sup.2. Without willing to be bound by any theory, it is believed that these amounts of Sulphur further improve the adhesive bonding of the steel substrate according to the present invention.

[0076] The surface density of sulphur in the conversion layer can be measured by ICP or X Ray Fluorescence (XRF).

[0077] From a process point of view, the conversion layer can be obtained by the application to the coating, possibly after degreasing, of an aqueous treatment solution comprising at least 0.01 mol.Math.L.sup.?1 of zinc sulphate and at least 0.01 mol.Math.L.sup.?1 of aluminium sulphate.

[0078] It is not possible to form such a layer when the concentration of zinc sulphate is less than 0.01 mol.Math.L.sup.?1, but it has also been found that a too high concentration does not significantly improve the rate of deposition and may even slightly reduce it. Preferably, the aqueous treatment solution contains zinc sulphate ZnSO.sub.4 in a concentration below or equal to 50 mol.Math.L.sup.?1 and aluminium sulphate Al.sub.2(SO.sub.4).sub.3 in a concentration below or equal to 50 mol.Math.L.sup.?1.

[0079] The aqueous treatment solution can be prepared by dissolving zinc sulphate and aluminium sulphate in pure water. For example, zinc sulphate heptahydrate (ZnSO.sub.4.Math.7H.sub.2O) can be used. For example, aluminium sulphate octadecahydrate (Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O) can be used. In one variant of the invention, the aqueous treatment solution consists in zinc sulphate, aluminium sulphate and water.

[0080] Preferably, the aqueous treatment solution contains between 10 and 140 g.Math.L.sup.?1 of zinc sulphate heptahydrate, more preferably between 10 and 80 g.Math.L.sup.?1 and advantageously between 10 and 40 g.Math.L.sup.?1.

[0081] Preferably, the aqueous treatment solution contains between 1 and 80 g.Math.L.sup.?1 of aluminium sulphate octadecahydrate, more preferably between 10 and 60 g.Math.L.sup.?1 and advantageously between 10 and 30 g.Math.L.sup.?1.

[0082] Advantageously, the ratio in weight of zinc amount with respect to aluminium amount in the aqueous solution is comprised between 5 and 40, more preferably between 5 and 30 and advantageously between 10 and 25. Indeed, without willing to be bound by any theory, it is believed that when the ratio in weight of zinc amount with respect to aluminium amount in the aqueous solution is as above, there is a further improvement of adhesive bonding.

[0083] The pH of the aqueous treatment solution preferably corresponds to the natural pH of the solution, without the addition of either base or acid. The value of this pH is generally between 4 and 7.

[0084] The temperature of the aqueous treatment solution can be between 20 and 60? C.

[0085] The aqueous treatment solution can be applied to the metallic coating by simple contact and dried at air whatever the drying temperature. It is applied in the conventional manner, e.g., by dipping, roll-coating, spraying eventually followed by squeezing.

[0086] Preferably, the wet film thickness is between 0.5 and 4 ?m.

[0087] Preferably, the aqueous treatment solution is subsequently dried in a dryer at air. Preferably the dryer comprises between 6 and 12 nozzles to better distribute the air jet impingement on the metal strip. Preferably the dryer comprises nozzles positioned between 4 and 12 cm from the metal strip to avoid pressure loss in the jet without removing the wet film from the metal strip. Preferably the nozzles have openings which width is comprised between 2 mm and 8 mm so as to optimize the air velocity at the nozzle exit.

[0088] Preferably, the drying temperature is between 20 and 200? C., more preferably between 50 and 200? C. and for example below 80? C., between 80 and 150? C. or above 150? C.

[0089] Preferably, the strip velocity is between 60 and 200 m/min.

[0090] Preferably, the initial strip temperature is between 20 and 50? C.

[0091] Preferably, the air flow rate is between 5000 and 50000 Nm.sup.3/h.

[0092] After the formation of the conversion layer, a film of oil with a coating weight of less than 2 g/m.sup.2 can be applied on the conversion layer.

[0093] From a practical point of view, the absence of zinc hydroxysulphate can be controlled by infrared spectroscopy in IRRAS mode (Infrared Reflection-Adsorption spectroscopy with an incidence angle of 80?). If the conversion layer comprises zinc hydroxysulphate, the IRRAS spectrum presents multiple absorption peaks assigned to the ?.sub.3 sulphate vibrations 1077-1136-1177 cm.sup.?1 and active water bands in the OH stretching region 3000-3400 cm.sup.?1. These results match the hydroxyzincsulphate structure as indicated in the literature (?.sub.1 sulphate vibration: 1000 cm.sup.?1, ?.sub.2 sulphate vibration: 450 cm.sup.?1, ?.sub.3 sulphate vibrations: 1068-1085-1130 cm.sup.?1, ?.sub.4 sulphate vibrations: 611-645 cm.sup.?1, hydroxyl vibration: 3421 cm.sup.?1).

[0094] The presence of the zincsulphate hydrate can be controlled by infrared spectroscopy in IRRAS mode. If the conversion layer comprises zincsulphate hydrate without zinc hydroxysulphate, the IRRAS spectrum presents one single sulphate peak located around 1172 cm.sup.?1 instead of 3 peaks. More specifically, the presence of each of the stable zincsulphate hydrates mentioned above can be controlled by infrared spectroscopy in IRRAS mode coupled to Differential Scanning calorimetry (DSC) by tracking the sulphate bands and free water bands.

[0095] The wet film thickness can be measured with an infrared gauge positioned before the dryer. It is composed of a light source, an infrared detector and specific filters. The measurement principle is based on infrared light absorption.

[0096] At the exit of the dryer, the absence of water in the conversion layer can be controlled notably with a hyperspectral camera. This latter is made of an infrared matrix detector coupled to a spectrometer which disperses the light into wavelengths. The measurement apparatus may be composed of a linear shape IR lamp (800 mm length) and a MWIR (Mid-Wave IR) hyperspectral camera in bidirectional reflection configuration. The detection range of the camera is 3-5 ?m which corresponds to the main absorption bands of liquid water. The measurement principle consists in measuring the intensity of light reflected off the metal strip. If water remains in the conversion layer, it absorbs a part of the light and less intensity is reflected.

[0097] In a variant, the absence of water in the conversion layer at the exit 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 hot air is spent for evaporating water and the temperature of the metal strip remains constant or even decreases due to water evaporation. Once the film is dry, the thermal energy of hot air is spent for heating the metal strip. By monitoring the temperature of the steel strip in the dryer, it is thus easy to control that the temperature of the metal strip starts to increase before the exit of the dryer.

[0098] With a view to highlighting the enhanced performance obtained by using the treatment method and steel substrate according to the invention, some concrete examples of embodiments will be detailed in comparison with coated steel sheets based on the prior art.

Example 1

[0099] As will be seen in the following non-restricting examples, which are presented exclusively by way of illustration, the inventors have shown that the invention makes it possible to improve the adhesion to adhesives used in the automotive industry, notably epoxy-based adhesives without degrading the other performances.

[0100] 10 trials were prepared by applying an aqueous treatment solution on a galvanized steel sheet or on an electrogalvanized steel sheet by roll-coating and by drying the wet film with different drying conditions.

[0101] Trials 1 and 2 were prepared according to the patent application US2017/0260471 by applying an aqueous solution comprising aluminium sulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O on a galvanized steel sheet (GI). The concentration of aluminium sulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O was of 22 g.Math.L.sup.?1 which corresponds to a concentration of Al.sub.2(SO.sub.4).sub.3 of 0,033 mol.Math.L.sup.?1. Trial 1 was subsequently dried in a dryer with air having a temperature of 100? C. for 5 seconds. Trial 2 was subsequently dried at air in a dryer having a temperature of 180? C. for 8 minutes.

[0102] Trials 3 and 4 were prepared according to the patent application WO00/15878 by applying an aqueous solution comprising zinc sulphate heptahydrate ZnSO.sub.4.Math.7H.sub.2O on a galvanized steel sheet. Trial 3 was subsequently dried in a dryer with air having a temperature of 100? C. during less than 4 seconds. Trial 4 was subsequently dried in a dryer with air having a temperature of 180? C. during 8 minutes.

[0103] For Trial 3, the strip velocity was of 120 m/min. The initial strip temperature was of 35? C.

[0104] Trial 5 was prepared according to the patent application WO2019/073273 by applying an aqueous solution comprising zinc sulphate heptahydrate ZnSO.sub.4.Math.7H.sub.2O on a galvanized steel sheet. The concentration of zinc sulphate heptahydrate was of 120 g.Math.L.sup.?1 which corresponds to a concentration of Zn.sup.2+ ions and a concentration of SO.sub.4.sup.2? of 0.42 mol.Math.L.sup.?1. The wet film had a thickness of 1.5 ?m. The wet film was subsequently dried within 4 seconds at air in a dryer having a temperature of 175? C. The strip velocity was of 120 m/min. The initial strip temperature was of 35? C.

[0105] Trial 6 was prepared according to the patent application WO2019/073274 by applying an aqueous solution comprising zincsulphate heptahydrate ZnSO.sub.4.Math.7H.sub.2O on a galvanized steel sheet. The concentration of zincsulphate heptahydrate was of 120 g.Math.L.sup.?1 which corresponds to a concentration of Zn.sup.2+ ions and a concentration of SO.sub.4.sup.2? of 0.42 mol.Math.L.sup.?1. The wet film had a thickness of 1.5 ?m. The wet film was subsequently dried within 4 seconds in a dryer at air having a temperature of 75? C. The strip velocity was of 120 m/min. The initial strip temperature was of 35? C.

[0106] Trials 7 and 8 were prepared by applying an aqueous solution comprising zincsulphate heptahydrate ZnSO.sub.4.Math.7H.sub.2O and aluminiumsulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O on a galvanized steel sheet. The concentration of aluminiumsulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O was of 25 g.Math.L.sup.?1 which corresponds to a concentration of Al.sup.3+ ions of 0,075 mol.Math.L.sup.?1 and 2.02 g.Math.L.sup.?1 and a concentration of SO.sub.4.sup.2? of 0.113 mol.Math.L.sup.?1. The concentration of zincsulphate heptahydrate was of 120 g.Math.L.sup.?1 which corresponds to a concentration of Zn.sup.2+ ions of 0.42 mol.Math.L.sup.?1 and 27.28 g.Math.L.sup.?1 and a concentration of SO.sub.4.sup.2? of 0.42 mol.Math.L.sup.?1. The ratio in weight of the zinc amount with respect to the aluminium amount in the aqueous solution is thus of 13.5. The wet films had a thickness of 1 to 1.5 ?m. Trial 7 was subsequently dried at air in a dryer having a temperature of 75? C. during less than 4 seconds. Trial 8 was subsequently dried at air in a dryer having a temperature of 100? C. for less than 4 seconds.

[0107] Trial 9 was prepared by applying an aqueous solution comprising zincsulphate heptahydrate ZnSO.sub.4.Math.7H.sub.2O and aluminiumsulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O on a galvanized steel sheet. The concentration of aluminiumsulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O was of 4.2 g.Math.L.sup.?1 which corresponds to a concentration of Al.sup.3+ of 0.013 mol.Math.L.sup.?1 and 0.35 g.Math.L.sup.?1 and a concentration of SO.sub.4.sup.2? of 0.019 mol.Math.L.sup.?1. The concentration of zincsulphate heptahydrate was of 32 g.Math.L.sup.?1 which corresponds to a concentration of Zn.sup.2+ ions of 0.111 mol.Math.L.sup.?1 and 7.27 g.Math.L.sup.?1 and a concentration of SO.sub.4.sup.2? of 0.111 mol.Math.L.sup.?1. The ratio in weight of the zinc amount with respect to the aluminium amount in the aqueous solution is thus of 20.77. Trial 9 was subsequently dried at air in a dryer having a temperature of 180? C. for 8 minutes.

[0108] Trial 10 was prepared by applying an aqueous solution comprising zincsulphate heptahydrate ZnSO.sub.4.Math.7H.sub.2O and aluminiumsulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O on an electrogalvanized steel sheet (EG). The concentration of aluminiumsulphate octad?cahydrate Al.sub.2(SO.sub.4).sub.3.Math.18H.sub.2O was of 4.2 g.Math.L.sup.?1 which corresponds to a concentration of Al.sup.3+ of 0.013 mol.Math.L.sup.?1 and 0.35 g.Math.L.sup.?1 and a concentration of SO.sub.4.sup.2? of 0.019 mol.Math.L.sup.?1. The concentration of zincsulphate heptahydrate was of 32 g.Math.L.sup.?1 which corresponds to a concentration of Zn.sup.2+ ions of 0.111 mol.Math.L.sup.?1 and 7.27 g.Math.L.sup.?1 and a concentration of SO.sub.4.sup.2? of 0.111 mol.Math.L.sup.?1. The ratio in weight of the zinc amount with respect to the aluminium amount in the aqueous solution is thus of 20.77. The wet film was subsequently dried at air in a dryer having a temperature of 180? C. for 8 minutes.

[0109] Surface Characterization

[0110] After drying, the surface of the conversion layer was characterized by IRRAS. The amount of sulphur in the layer was determined by ICP-MS.

[0111] Adhesion Test

[0112] The adhesion of epoxy-based adhesives on the conversion layer formed in all trials was evaluated by a single lap shear test. At first, test pieces 100 mm long and 25 mm wide were re-oiled using Anticorit Fuchs 3802-39S (1 g/m.sup.2) without being degreased. Two test pieces, one treated with the aqueous treatment solution and one untreated, were then assembled with the epoxy-based adhesive Teroson? 8028 GB from Henkel? by overlapping them on 12.5 mm long using teflon shims in order to maintain a homogeneous thickness of 0.2 mm between the two pieces. The whole assembly was cured in the oven for 20 minutes at 190? C. The samples were then conditioned for 24 h before adhesion test and ageing test. For each test condition, 5 assemblies were tested.

[0113] The adhesion has been assessed according to DIN EN 1465 standard. In this test, each bonded assembly is fixed in the clamping jaws (gripping 50 mm of each test piece in each clamp and leaving 50 mm of each test piece free) of a tensile machine using cell force of 50 KN. The samples are pulled at a rate of 10 mm/min, at room temperature. The maximal shear stress values are recorded in MPa and the failure pattern is visually classified as: [0114] superficial cohesive failure if the tear appears in the bulk of the adhesive close to the strip/adhesive interface, [0115] adhesive failure if the tear appears at the strip/adhesive interface.

[0116] The test is not passed if the percentage of adhesive failure is high.

[0117] The ageing of the adhesion has been assessed by cataplasm test. In this test, each bonded assembly (5 specimens each time) is wrapped in cotton (weight of 45 g+/?5) with deionized water (10 times the weight of cotton), put in polyethylene bag which is then sealed. The sealed bag is kept in the oven at 70? C., 100% HR for 7 days. Once the cataplasm test has been performed, the adhesion is reassessed according to DIN EN 1465 standard.

[0118] Loss Factor Evaluation

[0119] Before and after the adhesion ageing, the tensile strength is measured for each trial using a tensile sensor. The mechanical loss factor, corresponding to the loss of tensile strength after the adhesion ageing, defined in percent was then determined. It is calculated using this formula:

[00001]$\mathrm{Loss}\mathrm{factor}\left(\%\right)=\frac{\begin{array}{c}\mathrm{tensile}\mathrm{strength}\mathrm{before}\mathrm{adhesion}\mathrm{ageing}-\\ \mathrm{tensile}\mathrm{strength}\mathrm{after}\mathrm{adhesion}\mathrm{ageing}\end{array}}{\mathrm{tensile}\mathrm{strength}\mathrm{before}\mathrm{adhesion}\mathrm{ageing}}?100$

[0120] Friction Test

[0121] Test pieces of trials 1, 3 and 7 were placed and clamped in a friction tool comprising two flats tools made of Tungsten Carbide reproducing a stamping tool. The end of the test pieces were then pulled using a pulling clamp. The pulling force of the pulling clamp, called Fp, varies from 10 to 80 MPa. The resulting normal force, called Fn, which is perpendicular to the direction of Fp increases during the pulling. The higher the tensile force Fp, the higher the contact pressure in the friction tool. Fp and Fn were measured during the test. Then, the friction coefficient, called ?, was calculated with the following formula:

[00002]$?=\frac{\mathrm{Fp}}{2*\mathrm{Fn}}$

for 3 pulling forces (10, 40 and 80 MPa). The friction coefficient is expected to be between 0.07 and 0.15.

[0122] Results are in the following Table 1.

TABLE-US-00001 TABLE 1 Coated steel sheets obtained Surface characterization of the zincsulphate Adhesion Test based layer 0 day Treatment method ICP Adhesive Cohesive Drying S amount Al amount Failure Failure Trials Coating Aqueous solution conditions IRRAS (mg .Math. mg?2) (mg .Math. mg?2) (%) (%) 1 GI Al2(SO4)318H2O 100? C., 5 s 17 14 5 95 (US2017260471) 2 GI Al2(SO4)318H2O 180? C., 8 FIG. 1 a: 15 13 5 95 (US2017260471) min 1 single sulphate peak: 1180 cm.sup.?1 3 GI ZnSO47H2O 100? C., <4 s 18 0 5 95 (WO00/15878) 4 GI ZnSO47H2O 180? C., 8 FIG. 1c: 15 0 25 75 (WO00/15878) min 3 sulphate peaks: 1080-1130- 1172 cm.sup.?1 1 hydroxyl peak: 3600 cm.sup.?1 1 water peak: 1650 cm.sup.?1 5 GI ZnSO47H2O 175? C., <4s 17 0 0 100 (WO2019/073273) 6 GI ZnSO47H2O 75? C., < 4s 17 0 0 100 (WO2019/073274) 7* GI ZnSO47H2O and 75? C., < 4 s 16 12 0 100 Al2(SO4)318H2O 8* GI ZnSO47H2O and 100? C., <4 s 16 12 0 100 Al2(SO4)318H2O 9* GI ZnSO47H2O and 180? C., 8 FIG. 1b: 15 13 0 100 Al2(SO4)318H2O min 1 single sulphate peak: 1170 cm.sup.?1 10* EG ZnSO47H2O and 180? C., 8 18 7 Al2(SO4)318H2O min Coated steel sheets obtained Adhesion Test 16 days Loss Adhesive Cohesive factor Friction behavior Trials Failure (%) Failure (%) (%) 10 MPa 40 MPa 80 MPa 1 80 20 46 0.11 0.09 0.07 (US2017260471) 2 80 20 56 (US2017260471) 3 70 30 37 0.11 0.11 0.07 (WO00/15878) 4 100 0 65 (WO00/15878) 5 40 60 (WO2019/073273) 6 50 50 (WO2019/073274) 7* 50 50 29 0.11 0.11 0.08 8* 50 50 31 9* 30 70 10* *according to the present invention

[0123] As illustrated in the IRRAS spectrums of FIG. 1a, Trial 2 presents a single sulphate peak around 1180 cm.sup.?1 assigned to the presence of aluminium sulphate. As illustrated in FIG. 1c, Trial 4 presents multiple absorption peaks assigned to the ?.sub.3 sulphate vibrations of the hydroxyzincsulphate structure. Moreover, Trial 4 comprises free water corresponding to the peaks located around 1650 cm.sup.?1 and free hydroxyl groups corresponding to the peaks located with a peak located at 3600 cm.sup.?1. As illustrated in FIG. 1b, Trial 9 according to the invention presents a single sulphate peak around 1170 cm.sup.?1 assigned to zincsulphate hydrates. Hydroxyzincsulphate structure, free water and free hydroxyl groups were not detected in FIG. 1b.

[0124] As shown by the ICP-MS analysis, the amount of Sulphur of all Trials is above 0.5 mg.Math.m.sup.?2. Trials 7 to 10 have an amount of aluminium above 0 and below or equal to 13 mg.Math.m.sup.?2 according to the present invention.

[0125] The adhesive bonding of Trials 7 to 10 is significantly improved compared to Trials 1 to 4. Trials 7 to 10 have similar behavior adhesion to adhesives than Trials 5 and 6. Nevertheless, the treatment methods of Trials 5 and 6 are difficult to manage and implement compared to the treatment method of Trials 7 to 10.

[0126] The loss factor of Trials 7 and 8 is significantly better than Trials 1 to 4.

[0127] The friction behavior of Trials 1, 3 and 7 is similar.

[0128] Thus, the coated steel substrate of the present invention allows for an improvement of the adhesive bonding compared to the prior art without degrading the other performances and an easy-to-implement and easy-to-manage treatment method.