Solution for the treatment of an oiled zn-coated steel sheet to improve the degreasability thereof

Abstract

The invention concerns an aqueous solution comprising at least one hydrate of zinc sulfate, and at least one P1-M1 copolymer prepared from monomers comprising a) an ethylenically unsaturated monomer P1 containing at least one phosphorus atom; and b) a monomer M1 selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid, a kit for the preparation thereof, the use of the solution for improving the degreasability of the outer surface of an oiled metallic coating based on zinc or its alloys, which coats at least one face of a steel substrate, a method for preparing an oiled Zn-coated steel substrate, a steel substrate and an automotive part made therefrom.

Description

[0001] Solution for the treatment of an oiled Zn-coated steel sheet to improve the degreasability thereof

[0002] The present invention relates to a solution for the treatment of a steel substrate coated on at least one of its faces with an oiled metallic coating based on zinc or its alloys to improve the degreasability thereof.

[0003] Steel substrates coated on at least one of its faces with a metallic coating based on zinc or its alloys are for example intended for the automotive field. Metallic coatings essentially comprising zinc are traditionally used for their good protection against corrosion. Before being used, the coated steel substrates are generally subject to diverse surface treatments to improve the properties thereof.

[0004] A film of oil or of grease is generally applied on a Zn-based coated steel sheet to protect it against corrosion (temporary protection oil) and / or to facilitate the subsequent forming thereof (lubricating oil), in particular by drawing, to form a part. The film of oil or of grease is then removed and the sheet or part undergoes supplemental steps, such as getting assembled, phosphated and / or painted. These further treatments imply aqueous solutions (for example with a phosphating solution). When the prior degreasing was incomplete, some zones of the sheet or of the part are still covered with oil and thus not treated, or not well treated, with the aqueous solution. This may lead to aesthetic defects on the surface of the sheet or part, such as apparition of spots, or to non-uniformity of properties over the whole surface (because the aqueous treatment, such as phosphating, can be incomplete on the zones wherein the film of oil or of grease remained). Incomplete degreasing is particularly an issue when a zinc-aluminum-magnesium coated steel sheets as Zn-coated steel substrate because the film or oil or grease tends to remain on the surface thereof.

[0005] Easy and complete degreasing is a requirement for any Zn-coated steel substrate. Oiled Zn-coated steel substrate able to be degreased easily and entirely by conventional degreasers (typically alkaline solutions) are thus sought.

[0006] Application EP 2 450 470 describes the use of an aqueous treatment solution containing sulfate ions in order to treat the surface of a Zn-based coated steel sheet in order to reduce the formation of metal powder or particles based on zinc or its alloys generated by the degradation of the coating while said sheet is being formed. This solution allows forming a layer based on zinc hydroxysulfate and zinc sulfate on the surface of the sheet, which improves the tribological properties of the coated steel sheet. At examples 2 and 3, the dewetting of the oil has been tested (i.e. the presence of oil on the whole surface of the oiled sheet). However, this application is silent about the ability to be degreased of a sheet coated with a layer based on zinc hydroxysulfate and zinc sulfate and further with oil.

[0007] Aqueous solutions comprising zinc sulfate are sold by ArcelorMittal under the trademark NIT (New Inorganic Treatment). Generally, the aqueous solution is applied in order to obtain a layer comprising about 20 mg / m2of sulphur on the surface of the metallic coating. Such high amounts induce costs and may be detrimental to other properties of the coated sheets.

[0008] Besides, Syensqo commercializes Addibond™ treatment for metal surfaces allowing improvement of their compatibility with adhesives. Addibond™ comprises a polymer comprising acrylic acid or derivatives thereof and phosphonic or phosphonate moieties. This treatment is not known as conferring any degreasability properties to the metal on which they are applied and which is then oiled.

[0009] An object of the invention is therefore to provide a surface treatment of an oiled Zn- coated steel substrate improving degreasability thereof.

[0010] For this purpose, according to a first object, the invention concerns an aqueous solution comprising: at least 0.01 g / L of at least one hydrate of zinc sulfate, and at least O.01 g / L of at least one P1-M1 copolymer, the P1-M1 copolymer being prepared from monomers comprising: a) an ethylen ically unsaturated monomer P1 containing at least one phosphorus atom; and b) a monomer M1 selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid.

[0011] The aqueous solution may comprise only one hydrate of zinc sulfate or mixtures thereof. The hydrate is typically zinc sulfate monohydrate, zinc sulfate tetrahydrate and zinc sulfate heptahydrate. When the aqueous solution comprises several hydrates of zinc sulfate, the cumulated weight proportions thereof in the aqueous solution is at least 0.01 g / L.

[0012] The aqueous solution may comprise only one P1-M1 copolymer or a mixture of P1-M1 copolymers. When the aqueous solution comprises several P1-M1 copolymers, the cumulated weight proportions thereof in the aqueous solution is at least 0.01 g / L.

[0013] The P1-M1 copolymer (and the monomer M1 from which it stems) preferably comprises carboxylic acid groups. In the aqueous solution, the carboxylic acid groups of the P1-M1 copolymer may be in the form carboxylic acid groups, of carboxylate salts or of mixtures thereof. Their form depends on the pH of the aqueous solution.

[0014] The P1-M1 copolymer (and the monomer P1 from which it stems) preferably comprises a phosphonic acid group or a phosphonate, preferably a group of formula -P(=O)(OR2)2, wherein R2is H or a C1-C4 alkyl, preferably methyl, ethyl or isopropyl, R2being preferably H. When R2is H, in the aqueous solution, the phosphonic acid groups of the P1-M1 copolymer may be in the form phosphonic acid groups, of phosphonate salts, or of mixtures thereof. Their form depends on the pH of the aqueous solution.

[0015] Ethylenically unsaturated monomer P1 may for instance have formula (I): wherein:

[0016] - R1is H, a C1-C4 alkyl or a group wherein R is H or C1-C4 alkyl, preferably ethyl, or t-butyl, R1being preferably H,

[0017] R2is H or a C1-C4 alkyl, preferably methyl, ethyl or isopropyl, R2being preferably H,

[0018] X is a single bond or a spacer chosen from a C1 -C4 alkyl, ether or ketone, preferably from -CH2-, -CH2-CH2-, -CH2-O-CH2-CH2, -CH2-O-CO-CH2-, -CH2-O-(CH2)n-, -O- (CH2)n- wherein n is 1 or 2, X being preferably a single bond.

[0019] As examples of monomers P1 of formula (I), mention can be made of those of the following formulas:

[0020] Ethylenically unsaturated monomer P1 may for instance have formula (II): wherein R1and R2are as defined above and wherein X is a single bond or a spacer chosen from a C1 -C4 alkyl or ether eventually bearing hydroxyl and / or phosphate group(s) (PO4H2), preferably from -CH2-, -CH2-CH2-, -O-(CH2)4-, -O-CH2-CHOH-CH2-, -CH2-O-CH2-CHOH- CH2-, -O-CH2-C(PO4H2)-CH2-, -CH2-O-CH2-C(PO4H2)-CH2-.

[0021] Ethylenically unsaturated monomer P1 may for instance have formula (III): wherein X is a spacer chosen from C1-C4 ethers, preferably from -O-CH2- or

[0022] -CH2-O-CH2-.

[0023] Ethylenically unsaturated monomer P1 may for instance have formula (IV): defined above and Y is a spacer chosen from C1-C10 alkyl or ether or thioether eventually bearing hydroxyl and or phenyl group (Ph), preferably -CH2-CH2-, -C(CH3)2-CH2-CH2-CH2-, -(CH2-CH2)3-O-CH2-CH2-, -CH2- CHOH-CH2-O-CO-CH2-, -CH2-Ph-CH2-, CH2-CHOH-CH2-S-Ph-, CH2-CHOH-CH2-O-Ph-.

[0024] Ethylenically unsaturated monomer P1 may for instance have formula (V): where R1is H or CH3, X = -0-, -NH- or -N(CHs)-, R2is as defined above and Y is a spacer chosen from C1-C10 alkyl or alkylene oxide units, preferably ethylene oxide and / or propylene oxide units or -CH2-CH2-.

[0025] As examples of monomers P1 of formula (V), mention can be made of those of the following formulas: wherein n is an integer of from 3 to 7; wherein n is an integer of from 4 to 8; wherein m and n are integers, the sum of m+n is not greater than 15, preferably not greater than 10. Ethylenically unsaturated monomer P1 may for instance have formula (VI): wherein each occurrence of R2represents H or a C1-C3 alkyl; for example the following formula: diallyl aminophosphonic acid (DALP). Ethylenically unsaturated monomer P1 may be for instance salts of the following formula (VII): wherein each occurrence of R2represents H or a C1-C4 alkyl, An- represents anion, such as CI-; for example the following formulas: wherein each occurrence of R2represents H or a C1-C4 alkyl and at least one R2is not H.

[0026] Monomer M1 used for the preparation of the P1-M1 copolymer is selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid.

[0027] As examples of (meth)acrylates, mention can be made of C1 -C18 alkyl (meth)acrylates, preferably C1-C12 alkyl (meth)acrylates, more preferably C1-C4 alkyl (meth)acrylates, and salts of (meth)acrylic acid, preferably sodium (meth)acrylate, potassium (meth)acrylate, ammonium (meth)acrylate.

[0028] Examples of C1-C18 alkyl (meth)acrylates include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, iso-propyl (meth)acrylate, n-butyl (meth)acrylate, iso-butyl (meth)acrylate, tert-butyl (meth)acrylate, n-pentyl (meth)acrylate, iso-pentyl (meth)acrylate, n-hexyl (meth)acrylate, iso-hexyl (meth)acrylate, n-heptyl (meth)acrylate, iso-heptyl (meth)acrylate, n-octyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, iso-octyl (meth)acrylate, n-nonyl (meth)acrylate, n-decyl (meth)acrylate, n-undecyl (meth)acrylate, iso-undecyl (meth)acrylate, iso-dodecyl (meth)acrylate, n-tridecyl (meth)acrylate, n- tetradecyl (meth)acrylate, n-pentadecyl (meth)acrylate, n-hexadecyl (meth)acrylate, n- heptadecyl (meth)acrylate, and n-octadecyl (meth)acrylate.

[0029] As examples of hydroxyalkyl (meth)acrylates, mention can be made of C1-C4 hydroxyalkyl(meth)acrylates, preferably C1 -C3 hydroxyalkylacrylates, more preferably hydroxyethyl acrylate or hydroxyethyl methacrylate. Preferably, monomer M1 is selected from C1 -C2 alkyl (meth)acrylates; sodium (meth)acrylate, potassium (meth)acrylate, ammonium (meth)acrylate; C1-C4 hydroxyalkyl (meth)acrylates; and (meth)acrylic acid.

[0030] Advantageously, the P1-M1 copolymer is selected from copolymers prepared from a monomer mixture comprising: a) an ethylenically unsaturated monomer P1 having formula (I) or formula (V); and b) a monomer M1 selected from C1-C4 hydroxyalkyl (meth)acrylates, and (meth)acrylic acid.

[0031] Preferably, the P1-M1 copolymer is selected from copolymers prepared from a monomer mixture comprising: a) an ethylenically unsaturated monomer P1 of formula (I) where R1is H or a C1-C4 alkyl, R2is H or a C1-C4 alkyl, and X is a single bond, or of formula (V) where R1is H or CH3, R2is H or a C1-C4 alkyl, X is -O-, Y is -CH2-CH2-; and b) a monomer M1 selected from C1-C4 hydroxyalkyl (meth)acrylates and (meth)acrylic acid.

[0032] More preferably, the P1-M1 copolymer is selected from copolymers prepared from a monomer mixture comprising: a) an ethylenically unsaturated monomer P1 selected from vinylphosphonic acid (VPA) and ethylene glycol methacrylate phosphate; and b) a monomer M1 selected from hydroxypropyl (meth)acrylate and (meth)acrylic acid.

[0033] Advantageously, the molar ratio of units from monomer P1 to units from monomer M1 is from 1 :99 to 99:1 , preferably the molar ratio is from 5:95 to 50:50, most preferably from 20:80 to 50:50.

[0034] The copolymer usually comprises about 2-50 mole%, preferably about 4-40 mole% of units from monomer P1 .

[0035] Preferably, the P1-M1 copolymer is a copolymer prepared from a monomer mixture comprising: about 5-50 mole%, e.g., about 20-40 mole% of phosphonic acid (VPA) and / or isopropenyl phosphoric acid (IPPA) and / or diallyl aminophosphonic acid (DALP), about 50-95 mole %, e.g., about 60-85 mole % of acrylic acid (AA) and / or methacrylic acid (MAA).

[0036] In preferred embodiments, the P1-M1 copolymer polymer is a copolymer of vinyl phosphonic acid (VPA) and acrylic acid (AA). A particularly preferred P1 -M1 copolymer contains about from 20 to 40 mole% of units from vinyl phosphonic acid and from 60 to 80 mole% of units from acrylic acid. The aqueous solution according to invention advantageously allows maintaining or even improving the tribological properties of the Zn-coated steel substrate on which it is applied.

[0037] The improvement in the tribological properties may notably be shown by the reduction, or even suppression of the (“stick slip”), and / or by the reduction of the friction coefficient (p) when the aqueous solution as defined above is applied.

[0038] The most significant improvement in the tribological properties of the metallic coating of the steel sheet is observed by using an aqueous solution comprising: from 0.01 g / L to 75 g / L, in particular from 0.1 to 50 g / L, preferably from 5.0 to 30 g / L of at least one hydrate of zinc sulfate (cumulated proportions thereof when the aqueous solution comprises several hydrates), and from 0.01 g / L to 30 g / L, in particular from 5.0 to 25.0 g / L of at least one P1-M1 copolymer (cumulated proportions thereof when the aqueous solution comprises several P1-M1 copolymers).

[0039] In the aqueous solution according to the invention, the weight ratio of at least one hydrate of zinc sulfate (cumulated weights of hydrates of zinc sulfate when the aqueous solution comprises several hydrates of zinc sulfate) versus the at least one P1-M1 copolymer (cumulated weights of the P1-M1 copolymers when the aqueous solution comprises several P1-M1 copolymers) is preferably from 0.5 to 3.0, most preferably from 0.8 to 2.0. Such ratio are particularly adequate to obtain a good compatibility with adhesives.

[0040] Preferably, the weight ratio of the cumulated weights of the at least one hydrate of zinc sulfate (cumulated weights of hydrates of zinc sulfate when the aqueous solution comprises several hydrates of zinc sulfate) and of the at least one P1-M1 copolymer cumulated weights of the P1-M1 copolymers when the aqueous solution comprises several P1-M1 copolymers) versus the dry mass of the solution is at least 50%, preferably at least 80%, most preferably at least 90%. The dry mass is the weight obtained after evaporation of the water of the aqueous solution.

[0041] The aqueous solution preferably has a pH from 1 .0 to 5.0, most preferably from 1 .5 to 3.0.

[0042] In an embodiment, the aqueous solution consists in a mixture of water, of at least one hydrate of zinc sulfate and of at least one P1-M1 copolymer and optionally a base or a mixture of bases, or an acid or a mixture of acids. The base or the acid is used for adapting the pH of the aqueous solution. The addition of other compounds is not necessary.

[0043] According to a second object, the invention concerns a kit comprising: a first container comprising a volume V1 of an aqueous solution of at least one hydrate of zinc sulfate at a concentration C1 , a second container comprising a volume V2 of an aqueous solution of at least one PI- MI copolymer at a concentration C2, said P1-M1 copolymer being as prepared from monomers comprising: a) an ethylen ically unsaturated monomer P1 containing at least one phosphorus atom; and b) a monomer M1 selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid, wherein C1 and C2 are expressed in g / L and V1 and V2 are expressed in L, and C1 , C2, V1 and V2 are such that the following relations are fulfilled:

[0044] Cl x VI

[0045] - > 0.01q / L

[0046] VI + V2

[0047] C2 X V2

[0048] - > 0.01q / L

[0049] VI + V2

[0050] Mixing the aqueous solution of the first container with the one of the second container allows forming an aqueous solution according to the first object of the invention. The kit is thus useful for the preparation of the aqueous solution according to the first object of the invention.

[0051] The embodiments described above for the aqueous solution according to the first object are applicable for the kit.

[0052] Accordingly, according to a third object, the invention concerns the use of the aqueous solution according to the first object of the invention for improving the degreasability of the outer surface of an oiled metallic coating based on zinc or its alloys, which coats at least one face of a steel substrate.

[0053] Degreasability means the ability of coated steel sheet to be degreased by conventional degreaser such as alkaline solutions. The film of oil or of grease coating a metallic coating based on zinc or its alloys which has been treated with the aqueous solution according to the first object of the invention before application of said film is thus eliminated more easily and / or faster than the film of the same oil or grease coating a metallic coating based on zinc or its alloys, which has been untreated by such solution.

[0054] Moreover, the aqueous solution advantageously allows maintaining acceptable tribological properties of the outer surface of the oiled metallic coating based on zinc or its alloys, which coats at least one face of a steel substrate. Typically, the rate of reduction of the friction coefficient obtained by using them in combination is the same or even higher than the theoretical rate of reduction obtained by adding the rate of reduction conferred by using P1-M1 copolymer alone and the one obtained by using a hydrate of zinc sulfate alone. The improvement of the tribological properties of the outer surface of the metallic coating based on zinc or its alloys, which coats at least one face of the steel substrate, facilitates the subsequent forming thereof, notably by drawing.

[0055] The embodiments described above for the aqueous solution according to the first object apply to the use thereof.

[0056] The aqueous solution is used on the outer surface of a metallic coating based on zinc or its alloys, which coats at least one face of a steel substrate. The metallic coating generally comprises more than 40%, notably more than 50%, preferably more than 70%, more preferentially more than 84% by weight of zinc.

[0057] The metallic coating is preferably obtained by hot dip galvanization. The metallic coating based on zinc or its alloys preferably comprises zinc, aluminum, magnesium, and optionally silicon. Thus, the metallic coating is preferably a ZnAIMg or a ZnAIMgSi coating. The degreasability improvement is particularly observed for such coatings which are usually difficult to degrease.

[0058] The metallic coating preferably comprises: from 0.2 to 30.0% by weight, preferably from 1 .0 to 30.0% by weight, most preferably from 1 .0 to 8.0% by weight of aluminum, from 0.1 to 8.0% by weight, preferably from 1 .0 to 8.0% by weight, most preferably from 1 .0 to 5.0% by weight of magnesium, optionally up to 2.0% of silicon, most preferably up to 1.0% of silicon, most preferably up to 0,3% of silicon, optional additional elements, the balance being zinc and unavoidable impurities.

[0059] Magnesium is preferably present in the metallic coating in the following proportions: from 0.1 to 8.0% by weight of magnesium, preferably from 0.2 to 2.0% of magnesium, most preferably from 1 .0 to 2.0% of magnesium, for an aluminum content greater than or equal to 0.2% and less than 2.0% by weight (and thus also for an aluminum content greater than or equal to 1 .0% and less than 2.0% by weight), or more than 1.0% and less than or equal to 8.0% by weight of magnesium, preferably more than 2.0% and less than or equal to 5.0% by weight of magnesium, for an aluminum content greater than or equal to 2.0% and less than or equal to 30.0% by weight (and thus also for an aluminum content greater than or equal to 2.0% and less than 8.0% by weight). Additionally to these magnesium proportions or independently, silicon is preferably present in the metallic coating in the following proportions: less than 1.0%, preferably less than 0.3% of silicon, for an aluminum content greater than or equal to 0.2% and less than 2.0% by weight (and thus also for an aluminum content greater than or equal to 1 .0% and less than 2.0% by weight), or less than 2.0%wt of silicon, for an aluminum content greater than or equal to 2.0% and less than or equal to 30.0% by weight (and thus also for an aluminum content greater than or equal to 2.0% and less than 8.0% by weight).

[0060] Optional additional elements are preferably selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi, the content by weight of each additional element in the metallic coating being less than 0.3%. These different additional elements may notably give the possibility of improving the ductility or the adhesion of the metallic coating on the steel substrate. One skilled in the art, which is aware of their effects on the properties of the metallic coating, will know how to use them depending on the sought complementary purpose.

[0061] The measurement of the composition of a coating is generally achieved by chemical dissolution of the coating. The given result corresponds to an average content in the whole of the metallic coating.

[0062] According to a fourth object, the invention concerns a method for preparing an oiled Zn-coated steel substrate comprising: providing a steel substrate coated on at least one of its faces with a metallic coating based on zinc or its alloys, applying on the outer surface of the metallic coating the aqueous solution according to according to the first object of the invention so as to form a wet film, drying the wet film in order to form, on the outer surface of the metallic coating, a layer comprising the at least one P1-M1 copolymer and a zinc sulfate compound selected from zinc hydroxysulfate, zinc sulfate in hydrated form or not, and mixtures thereof, after having dried the wet film, applying a film of grease or oil on the layer coating the outer surface of the metallic coating coating the steel substrate.

[0063] The embodiments described above for the aqueous solution and for the steel substrate coated on at least one of its faces with a metallic coating based on zinc or its alloys are applicable for the method.

[0064] In the method according to the invention, the aqueous solution may be applied at a temperature comprised between 5 and 70°C, preferably between 15 and 50°C. The duration of application of the aqueous solution may be between 0.1 s and 40 s, preferably between 0.5 s and 20 s. The aqueous solution may be applied by dipping, by roller coating or by spraying, optionally followed by squeezing. Roller coating is preferred since it gives the possibility of more easily controlling the amount of applied aqueous solution while ensuring a homogeneous distribution of the aqueous solution on the outer surface of the metallic coating. Generally, the wet film thickness consisting of the applied aqueous solution on the outer surface of the metallic coating is from 0.2 to 5.0 pm, typically between 0.5 and 3.0 pm 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 is based on infrared light absorption.

[0065] The drying step of the method may be carried out by subjecting the Zn-coated steel substrate to a temperature comprised between 70 and 180°C, for example between 80 and 120°C, generally for 1 to 30 seconds, notably 1 to 10 seconds, for example 2 s.

[0066] When the outer surface of the metallic coating based on zinc or its alloys is treated by the aqueous solution according to the invention, a layer forms on the outer surface, which comprises at least one P1-M1 copolymer and the zinc sulfate compound. Such a layer does not form when the concentration of the at least one zinc sulfate hydrate or the concentration of the at least one P1-M1 copolymer is less than 0.01 g / L in the aqueous solution. Too high concentrations thereof do not substantially improve the properties of the obtained sheet (neither improvement of degreasability, nor of compatibility with adhesives), can even be detrimental for the properties thereof (difficulties to degrease and / or to apply subsequent treatments such as phosphating, welding...) and for the electrodeposition line, which can get polluted.

[0067] After the drying step, a film of grease or oil is applied on the layer as defined above in order to protect the Zn-coated steel substrate against corrosion and / or to facilitate its subsequent forming. The oil is thus preferably a temporary protective oil or a lubricating oil. The grease or oil is generally applied with a density of less than 3 mg / m2.

[0068] The method can comprise additional steps.

[0069] For example, the method may comprise a prior step of preparing a steel substrate coated on at least one of its faces with a metallic coating based on zinc or its alloys, preferably by hot dipping.

[0070] The method may comprise a prior step of preparing the aqueous solution according to the first object of the invention either by mixing the aqueous solution of the first container of the kit according to the second object of the invention with the aqueous solution of the second container, or by adding zinc sulfate or a hydrate thereof and at least one P1-M1 copolymer in water. The method may comprise or be free of surface treatment step(s) other than the one consisting of applying the aqueous solution according to the invention (for example a chemical conversion treatment). The aim of this(these) surface treatment(s) is generally to increase the paint adhesion and / or corrosion resistance. When this(these) surface treatment step(s) lead to the formation of a layer on the metallic coating, this(these) other surface treatment step(s) is(are) carried out simultaneously or after the step for application of the aqueous solution according to the invention, so that there is no intermediate layer between the outer surface of the metallic coating based on zinc or its alloys and the aqueous solution according to the invention. In particular, the method may comprise, after having dried the wet film, a phosphating step or a step of applying a zirconium-based conversion treatment.

[0071] These optional surface treatment steps may comprise other rinsing, drying sub-steps.

[0072] Preferably, the method is free of any surface treatment step between the step of applying the aqueous solution according to the invention and the step of applying a film of grease or of oil.

[0073] If the steel substrate is a steel strip, the strip may optionally be wound before being stored and then being sent to a pre-lacquering line.

[0074] Typically, before forming the part, the strip is cut out. In order to remove the impurities deposited on the steel sheet from this cutting out, a washing / re-oiling treatment may be applied before forming.

[0075] After the step of applying a film of grease or of oil, the method may comprise a step of forming, typically by bending, profiling or drawing, preferably by deep drawing or cold drawing. The obtained formed steel sheet corresponds to a part, for example an automotive part.

[0076] The method then generally comprises a degreasing step, typically achieved by applying a basic aqueous solution on the outer surface of the metallic coating. As explained above, the oiled Zn-coated steel substrate is advantageously easily degreased.

[0077] The method may then comprise (or be free of):

[0078] - other surface treatment steps, for example a phosphating step, and / or

[0079] - a cataphoresis step, and / or

[0080] - a painting step.

[0081] The invention is also directed to a steel substrate obtainable by the method described above, and to the part obtainable by the method when it comprises a forming step. According to a fifth object, the invention concerns a steel substrate coated on at least one of its faces with a metallic coating based on zinc or its alloys, wherein the outer surface of the metallic coating is itself coated with a layer comprising: a zinc sulfate compound selected from zinc hydroxysulfate, zinc sulfate in hydrated form or not, and mixtures thereof, and at least one P1-M1 copolymer, the P1-M1 copolymer being as defined above, wherein the layer is coated by a film of grease or oil, typically by a temporary protective oil or a lubricating oil.

[0082] Preferably, the density of sulphur in the layer is from 1.0 to 15.0 mg / m2, preferably from 2.0 to 10.0 mg / m2, most preferably from 3.0 to 7.0 mg / m2and / or the density of phosphorus in the layer is from 1.0 et 20.0 mg / m2, preferably from 3.0 to 15.0 mg / m2, most preferably from 4.0 to 9.0 mg / m2.

[0083] Advantageously, thanks to the synergistic effect between the zinc sulfate compound and the P1-M1 copolymer, it is possible to decrease the sulphur and / or phosphorous densities compared to the sulphur density usually obtained when zinc sulfate is used alone (typically from 15 to 30 mg / m2) and when the P1-M1 copolymer is used alone. The densities of sulphur and of phosphorus may be determined by X-ray fluorescence (XRF).

[0084] Preferably, the ratio of the density of sulphur in the layer versus the density of phosphorus therein is from 1 / 3 to 2 / 1 , notably from 2 / 5 to 3 / 2, preferably in particular from 1 / 2 to 1 / 1 , most preferably from 5 / 8 to 1 / 1 . A ratio lower than 5 / 8 may lead to a degradation of the compatibility of the coated steel substrate with adhesives.

[0085] Generally, the ratio of the cumulated weights of the at least one P1-M1 copolymer (cumulated weights of the P1-M1 copolymers when the aqueous solution comprises several P1-M1 copolymers) and of the zinc sulfate compound compared to the weight of the layer is from 50 to 100%, notably from 75 to 100%, typically from 90 to 100%.

[0086] The layer is coated by a film of grease or of oil. The Zn-coated steel substrate coated by the layer defined above and by a film of oil or grease may be easily and entirely degreased by conventional degreasers (typically alkaline solutions). This is advantageous because the parts obtained after forming are usually degreased before further treatment with aqueous solutions (for example with a phosphating solution). The efficiency of the subsequent treatment with aqueous solutions depends on the prior degreasing.

[0087] The steel substrate is preferably the one obtainable by the method described above.

[0088] According to a sixth object, the invention concerns an automotive part made of the steel substrate as defined above. The invention will now be illustrated with examples given as an indication, and not as a limitation, and with reference to the appended figure, which is a schematic sectional view illustrating the structure of a steel sheet 1 according to the invention.

[0089] The steel sheet 1 of Fig. 1 comprises a steel substrate 3 covered on each of both of its faces 5 with a metallic coating 7 based on zinc or its alloys. The coatings 7 are covered with a layer 9 comprising the at least one P1-M1 copolymer and the zinc sulfate compound selected from zinc hydroxysulfate, zinc sulfate in hydrated form or not, and mixtures thereof. Preferred hydrate forms are zinc sulfate monohydrate, zinc sulfate tetrahydrate and zinc sulfate heptahydrate. As the steel sheet 1 is not necessarily covered with such a layer 9 on both sides, there would be no departure from the scope of the invention if the upper layer 9 and / or the lower layer 9 were absent (alternatives not shown).

[0090] The steel sheet 1 of the figure comprises a steel substrate 3 covered on each of its two faces 5 with a metallic coating 7. It will be observed that the relative thicknesses of the substrate 3 and of the coatings 7 covering it have not been observed on the figure in order to facilitate the illustration.

[0091] The metallic coating 7 generally has a thickness of less than or equal to 25 pm and conventionally aims at protecting the steel substrate 3 against corrosion.

[0092] The coatings 7 present on both faces 5 are analogue and a single one will be described in detail subsequently. Alternatively (not shown), only one of the faces 5 has a metallic coating 7.

[0093] The metallic coating generally comprises more than 40%, notably more than 50%, preferably more than 70%, more preferentially more than 84% by weight of zinc.

[0094] The metallic coating 7 preferably comprises zinc, aluminum, magnesium, and optionally silicon. Thus, the metallic coating is preferably a ZnAIMg or a ZnAIMgSi coating. The embodiments described above apply, notably as regards the aluminum, magnesium and silicon proportions, as regards the natures and proportions of optional additional elements.

[0095] In order to manufacture this steel sheet 1 , it is for example possible to proceed as follows.

[0096] The method may comprise a preliminary step of preparing the steel substrate 3 having two faces 5, at least one of which is coated with a metallic coating 7 based on zinc or its alloys. A steel substrate 3 is used, for example obtained by hot and then cold rolling.

[0097] The metallic coating 7 may be deposited on the substrate 3 by any known deposition method, notably by electro-zinc-plating, by physical vapor deposition (PVD), by jet vapor deposition (JVD) or hot dip galvanization, preferably by hot dip galvanization. Generally, the substrate 3 is then in the form of a strip which is run in a bath for depositing the metallic coating 7 by hot dipping. The composition of the bath is generally close to, or even the same as the composition of the desired metallic coating 7.

[0098] The bath thus preferably comprises zinc, aluminum and magnesium. The bath preferably comprises: from 0.2 to 30.0% by weight, preferably from 1 .0 to 30.0% by weight, most preferably from 1 .0 to 8.0% by weight of aluminum, from 0.1 to 8.0% by weight, preferably from 1 .0 to 8.0% by weight, most preferably from 1 .0 to 5.0% by weight of magnesium, optionally up to 2.0% of silicon, most preferably up to 1.0% of silicon, most preferably up to 0.3% of silicon, optional additional elements, the balance being zinc and unavoidable impurities.

[0099] To obtain an appreciable effect on the corrosion resistance, it is preferred to add at least 0.1% by weight, or even 0.2% by weight, of magnesium. The maximum content thereof in the bath is limited to 8.0% by weight since the coating obtained could have severe cracking problems during the subsequent forming, especially a drawing operation.

[0100] Magnesium is preferably present in the bath in the following proportions: from 0.1 to 8.0% by weight of magnesium, preferably from 0.2 to 2.0% of magnesium, most preferably from 1 .0 to 2.0% of magnesium, for an aluminum content greater than or equal to 0.2% and less than 2.0% by weight (and thus also for an aluminum content greater than or equal to 1 .0% and less than 2.0% by weight), or more than 1.0% and less than or equal to 8.0% by weight of magnesium, preferably more than 2.0% and less than or equal to 5.0% by weight of magnesium, for an aluminum content greater than or equal to 2.0% and less than or equal to 30.0% by weight (and thus also for an aluminum content greater than or equal to 2.0% and less than 8.0% by weight).

[0101] Additionally to these magnesium proportions or independently, silicon is preferably present in the bath in the following proportions: less than 1.0%, preferably less than 0.3% of silicon, for an aluminum content greater than or equal to 0.2% and less than 2.0% by weight (and thus also for an aluminum content greater than or equal to 1 .0% and less than 2.0% by weight), or less than 2.0%wt of silicon, for an aluminum content greater than or equal to 2.0% and less than or equal to 30.0% by weight (and thus also for an aluminum content greater than or equal to 2.0% and less than 8.0% by weight). Optional additional elements are preferably selected from Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi, the content by weight of each additional element in the bath being less than 0.3%.

[0102] The bath may finally contain residual elements stemming from supply ingots, or resulting from the passage of the substrate 3 in the bath, a source of unavoidable impurities in the metallic coating 7.

[0103] After deposition of the metallic coating 7, the substrate 3 is for example wiped by means of nozzles spraying a gas on either side of the substrate 3.

[0104] The metallic coating 7 is then left to cool in a controlled way so that it solidifies.

[0105] Alternatively, the wiping operation may be adapted for removing the metallic coating 7 deposited on one face 5 so that only one of the faces 5 of the steel sheet 1 is definitively coated by a metallic coating 7.

[0106] The controlled cooling of the, or of each, metallic coating 7 is ensured at a rate preferably greater than or equal to 15°C / s between the beginning of the solidification (i.e. when the metallic coating 7 falls just below the temperature of the liquidus) and the end of solidification (i.e. when the metallic coating 7 attains the temperature of the solidus). More preferably, the cooling rate of the, or each, metallic coating 7 between the start of the solidification and the end of solidification is higher than or equal to 20° C / s.

[0107] The thereby treated strip may then be subject to a so called skin-pass step which allows it to be work hardened and gives it roughness facilitating its subsequent forming, by promoting good retention of the oil applied to the sheet 1 prior to the forming thereof. The rate of elongation of the sheet 1 during the skin-pass operation is generally between 0.5% and 2%. Skin-pass is generally carried out for a sheet 1 intended for use in the manufacture of body work parts for motor vehicles. When the sheet 1 is intended for use in the manufacture of household electrical appliances, for example, skin-pass is not required to be performed.

[0108] The outer surface 15 of the metallic coating 7 is subject to a surface treatment step which consists of applying to them the aqueous solution comprising at least 0.01 g / L of at least one hydrate of zinc sulfate, and at least 0.01 g / L of a at least one P1-M1 copolymer as described above.

[0109] The aqueous solution preferably has a pH from 1 .0 to 5.0, most preferably from 1 .5 to 3.0.

[0110] Typically, the method comprises, after the step for application on the outer surface 15 of the metallic coating 7 of the aqueous solution, a drying step, which allows obtaining, on the outer surface 15 of the metallic coating 7, a layer 9 comprising (or consisting of) at least one P1-M1 copolymer as defined above and a zinc sulfate compound as defined above. The drying may be carried out by subjecting the steel sheet 1 to a temperature comprised between 70 and 180°C, for example between 80 and 120°C, generally for 1 to 30 seconds, notably 1 to 10 seconds, for example 2 s.

[0111] The metallic coating of the steel sheet obtained is then typically coated with a layer 9 wherein density of sulphur is from 1.0 to 15.0 mg / m2, preferably from 2.0 to 10.0 mg / m2, most preferably from 3.0 to 7.0 mg / m2, and / or the density of phosphorus is from 1 .0 et 20.0 mg / m2, preferably from 3.0 to 15.0 mg / m2, most preferably from 4.0 to 9.0 mg / m2.

[0112] Generally once the steel sheet 1 has been prepared (whether or not skin-passed), it may be cut out into blanks. In order to remove the impurities deposited on the steel sheet from this cutting out, a washing / re-oiling treatment may be applied. The latter consists of applying on the surfaces of the Zn-coated steel sheet an oil with a low viscosity, and then of brushing, and then applying an oil with a greater viscosity.

[0113] The sheet 1 can subsequently be formed, for example, by means of drawing, bending or profiling, preferably by deep drawing or cold drawing, in order to produce a part that can then be subjected to painting so as to form on each metallic coating 7, a film of paint (or paint system).

[0114] In the case of parts for household electrical appliances, it is also possible to eventually subject the paint films to an annealing process by physical and / or chemical means, known per se.

[0115] The examples hereafter and the figure illustrate the invention.

[0116] In the examples, the P1-M1 copolymer used is Addibond™ 021 sold by Syensqo, which is a copolymer prepared from vinylphosphonic acid (P1 ) and acrylic acid (M1 ) at a molar ratio of VPA / AA of 30 / 70.

[0117] An aqueous solution according to the invention and comprising 10 g / L of this copolymer and 30 g / L of heptahydrate of ZnSC was used. The natural pH thereof was 2.2. For some of the experiments described hereafter, the pH thereof has been adjusted to 1 .5 by adding an acid.

[0118] Comparative solutions were also prepared: an aqueous solution comprising 10 g / L of this copolymer (but free of ZnSC ), and another solution comprising 30 g / L of ZnSC (but free of copolymer).

[0119] The steel substrate coated by a metallic coating based on zinc or its alloys is DX56 steel which has been hot dip coated either with a ZnAI3.7Mg3.0 coating (called “ZMP” hereafter) or with a AI1.2Mg1.2 coating (called “ZMS” hereafter). Example 1 : Ability of a ZnAIMg coated steel sheet treated by the solution and then oiled to be degreased

[0120] Samples having dimensions of 100 mm x 100 mm x 0.7 mm thickness were cut out in the ZnAIMg steel sheets treated by one of the aqueous solutions as defined above except for the reference steel sheets (control) not having been subject to any treatment with an aqueous solution.

[0121] Fuchs® 3802-39S oil (in an amount of 0.5 g / m2) was applied on both faces of the samples in order to re-oil them.

[0122] The samples were then degreased by one of the following methods: method A: Dipping the samples during 6 minutes in an ARSA bath (bath comprising Gardoclean 5176 at 15 g / l and Gardobond H7352 at 1 ,5 g / l), having an alkalinity of 7.3 and a pH of 11 .6, and method B: Dipping the samples during 6 minutes in an BMW bath (bath comprising Gardoclean 5176 at 20 g / l and Gardobond H7352 at 2,0 g / l, the pH of the bath being adjusted at 10.5).

[0123] The wettability of the surface was then determined by waterbreak evaluation as described in the VDA 230-213. More precisely, after test plates are removed from the cold rinsing bath, they are immediately checked to ensure the test oil was removed without residue using the so-called water breakage test. If the water film draws back from the side edges (approx.. 1 cm) or if the water film breaks at the top edge, this should not be seen as a negative result. The tested product is considered removable of the water film on then test side does not break apart or run together within 10 seconds. The area of wetted surface is assessed visually. The wettability is calculated as being 100 x the ratio of the area of wetted surface versus the one of the total surface. The measure is carried out on three different samples and the given result in the table below is an average of the three tests. A wettability above 85% indicates a satisfying level of oil degreasing ability.

[0124] Table 1 provides the results, which were the same no matter whether ZMP or ZMS coated steel sheet was used as ZnAIMg coated steel sheet.

[0125] Table 1 : Wettability of de-oiled ZnAIMg coated steel sheets

[0126] As illustrated by the table 1 above, it was observed that the application of a solution according to the invention allows a satisfying oil degradability at various pH grades.

[0127] Example 2: Tribological properties

[0128] In order to illustrate the invention, measurement tests of the friction coefficient (p) according to the contact pressure (MPa) were conducted and are described as non-limiting examples.

[0129] Samples having dimensions of 450 mm x 35 mm x 0.7 mm thickness were cut out in the ZMP coated steel sheets treated by one of the aqueous solutions as defined above except for the reference steel sheets (control) not having been subject to any treatment with an aqueous solution.

[0130] Fuchs® 3802-39S oil (in excess) was applied on both faces of the samples.

[0131] The tribometer consists mainly of two hydraulic cylinders:

[0132] - a clamping cylinder applies force to the test specimen via two tools;

[0133] - a traction cylinder forces the specimen to move.

[0134] The equipment is equipped with measuring instruments and a data acquisition system. The friction test consists in rubbing the specimen over a length of approximately 135 mm, between two tools with a flat surface. The friction is carried out crosswise to the rolling direction of the specimen. The specimen is moved at a constant speed (typically 0.1 m / min), while the clamping force increases linearly from 5 to 800 daN. Given the friction surface, this results in an apparent contact pressure increasing from 2 MPa up to 80 MPa (800 bar).

[0135] A computer system enables the tool clamping force and the specimen tensile force to be acquired. The result is the force measured at the collet. It represents the sum of the tensile forces of each of the 2 rubbing surfaces. Recording it enables to calculate the coefficient of friction (p) according to Coulomb's principle. It's an arithmetic average of the 2 faces of the specimen.

[0136] The friction coefficient was thus measured (p) according to the contact pressure (MPa) by varying the contact pressure from 0 to 80 MPa: on the sample of the ZMP coated steel sample treated with the aqueous solution according to the invention comprising both the copolymer and ZnSC , on a sample of ZMP coated steel sample but not treated with the aqueous solution (control), on a sample of ZMP coated steel sample treated with the aqueous solution comprising only the copolymer (comparative example), on a sample of ZMP coated steel sample treated with the aqueous solution comprising only ZnSC (comparative example).

[0137] The results are provided at table 2. Table 2: Tribological properties (Observation of stick slip and friction coefficient (p) according to the exerted pressure) for the tested ZMP coated steel sheets samples

[0138] As illustrated by the table 2 above, it was observed that the application of a solution according to the invention allows reducing the friction coefficient relatively to an untreated coated steel sheet (control), relatively to a coated steel sheet treated by P1-M1 copolymer alone (comparative) and relatively to a coated steel sheet treated by hydrate of zinc sulfate alone (comparative).

[0139] The other properties of the ZMP coated sheets obtained by the method according to the invention (mechanical properties, compatibility with one of the subsequent steps for cataphoresis and / or phosphating and / or painting) were not degraded.

Claims

CLAIMS1 . Aqueous solution comprising: at least 0.01 g / L of at least one hydrate of zinc sulfate, and at least 0.01 g / L of at least one P1-M1 copolymer, the P1-M1 copolymer being prepared from monomers comprising: a) an ethylenically unsaturated monomer P1 containing at least one phosphorus atom; and b) a monomer M1 selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid.

2. Aqueous solution according to claim 1 , wherein the ethylenically unsaturated monomer P1 comprises a phosphonate or a phosphonic acid group.

3. Aqueous solution according to claim 2, wherein the ethylenically unsaturated monomer P1 has formula (I):wherein:OÂR1is H, a C1-C4 alkyl or a group wherein R is H or C1-C4 alkyl, R1being preferably H,R2is H or a C1-C4 alkyl, R2being preferably H,X is a single bond or a spacer chosen from a C1-C4 alkyl, ether or ketone, preferably from -CH2-, -CH2-CH2-, -CH2-O-CH2-CH2, -CH2-O-CO-CH2-, -CH2- O-(CH2)n-, -O-(CH2)n- wherein n is 1 or 2, X being preferably a single bond.

4. Aqueous solution according to anyone of claims 1 to 3, having a pH from 1.0 to 5.0, most preferably from 1 .5 to 3.0.

5. Aqueous solution according to anyone of claims 1 to 4, wherein the weight ratio of the at least one zinc sulfate hydrate versus the at least one P1-M1 copolymer is from 0.5 to 3.0, most preferably from 0.8 to 2.

06. Kit comprising: a first container comprising a volume V1 of an aqueous solution of at least on hydrate of zinc sulfate at a concentration C1 , a second container comprising a volume V2 of an aqueous solution of at least one P1-M1 copolymer at a concentration C2, said P1-M1 copolymer being as prepared from monomers comprising: a) an ethylenically unsaturated monomer P1 containing at least one phosphorus atom; and b) a monomer M1 selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid, wherein C1 and C2 are expressed in g / L and V1 and V2 are expressed in L, and C1 , C2, V1 and V2 are such that the following relations are fulfilled:Cl x VI- > 0.01q / LVI + V2C2 X V2- > 0.01q / LVI + V27. Use of the aqueous solution according to anyone of claims 1 to 5 for improving the degreasability of the outer surface (15) of an oiled metallic coating (7) based on zinc or its alloys, which coats at least one face (5) of a steel substrate (3).

8. Use according to claim 7, wherein the metallic coating (7) based on zinc or its alloys comprises zinc, aluminum, magnesium, and optionally silicon.

9. Method for preparing an oiled Zn-coated steel substrate comprising: providing a steel substrate (3) coated on at least one of its faces with a metallic coating (7) based on zinc or its alloys,Applying, on the outer surface (15) of the metallic coating (7), the aqueous solution according to any one of claims 1 to 5 so as to form a wet film, drying the wet film in order to form, on the outer surface (15) of the metallic coating (7), a layer (9) comprising the at least one P1-M1 copolymer and a zinc sulfate compound selected from zinc hydroxysulfate, zinc sulfate in hydrated form or not, and mixtures thereof,after having dried the wet film, applying a film of grease or oil on the layer (9) coating the outer surface (15) of the metallic coating (7) coating the steel substrate (3).

10. Method according to claim 9, wherein the metallic coating (7) based on zinc or its alloys comprises zinc, aluminum, magnesium, and optionally silicon.11 . Method according to claim 10, wherein the metallic coating (7) comprises: from 0.2 to 30.0% by weight, preferably from 1.0 to 30.0% by weight, most preferably from 1 .0 to 8.0% by weight of aluminum, from 0.1 to 8.0% by weight, preferably from 1.0 to 8.0% by weight, most preferably from 1 .0 to 5.0% by weight of magnesium, optionally up to 2.0% of silicon, most preferably up to 1.0% of silicon, most preferably up to 0.3% of silicon, optional additional elements, the balance being zinc and unavoidable impurities.

12. Method according to claim 11 , wherein magnesium is present in the metallic coating (7) in the following proportions: from 0.1 to 8.0% by weight of magnesium, preferably from 0.2 to 2.0% of magnesium, most preferably from 1.0 to 2.0% of magnesium, for an aluminum content greater than or equal to 0.2% and less than 2.0% by weight, or more than 1 .0% and less than or equal to 8.0% by weight of magnesium, preferably more than 2.0% and less than or equal to 5.0% by weight of magnesium, for an aluminum content greater than or equal to 2.0% and less than or equal to 30.0% by weight.

13. Method according to anyone of claims 9 to 12, comprising forming the Zn-coated steel substrate after having applied the film of grease or oil, whereby a part is formed.

14. Method according to claim 13, wherein the forming is by drawing, most preferably by cold drawing or deep drawing.

15. Steel substrate coated on at least one of its faces (5) with a metallic coating (7) based on zinc or its alloys, wherein the outer surface (15) of the metallic coating (7) is itself coated with a layer (9) comprising:a zinc sulfate compound selected from zinc hydroxysulfate, zinc sulfate in hydrated form or not, and mixtures thereof, at least one P1-M1 copolymer, the P1-M1 copolymer being prepared from monomers comprising: a) an ethylenically unsaturated monomer P1 containing at least one phosphorus atom; and b) a monomer M1 selected from (meth)acrylates, hydroxyalkyl (meth)acrylates, and (meth)acrylic acid, wherein the layer is coated by a film of grease or oil.

16. Steel substrate according to claim 15, wherein the metallic coating (7) based on zinc or its alloys comprises zinc, aluminum, magnesium, and optionally silicon.

17. Steel substrate according to claim 16, wherein the metallic coating (7) comprises: from 0.2 to 30.0% by weight, preferably from 1 .0 to 30.0% by weight, most preferably from 1 .0 to 8.0% by weight of aluminum, from 0.1 to 8.0% by weight, preferably from 1.0 to 8.0% by weight, most preferably from 1 .0 to 5.0% by weight of magnesium, optionally up to 2.0% of silicon, most preferably up to 1.0% of silicon, most preferably up to 0.3% of silicon, optional additional elements, the balance being zinc and unavoidable impurities.

18. Steel substrate according to claim 17, wherein magnesium is present in the metallic coating (7) in the following proportions: from 0.1 to 8.0% by weight of magnesium, preferably from 0.2 to 2.0% of magnesium, most preferably from 1.0 to 2.0% of magnesium, for an aluminum content greater than or equal to 0.2% and less than 2.0% by weight, or more than 1 .0% and less than or equal to 8.0% by weight of magnesium, preferably more than 2.0% and less than or equal to 5.0% by weight of magnesium, for an aluminum content greater than or equal to 2.0% and less than or equal to 30.0% by weight.

19. Steel substrate according to anyone of claims 15 to 18, wherein the density of sulphur in the layer (9) is from 1.0 to 15.0 mg / m2, preferably from 2.0 to 10.0 mg / m2, most preferably from 3.0 to 7.0 mg / m2and the density of phosphorus in thelayer (9) is from 1.0 et 20.0 mg / m2, preferably from 3.0 to 15.0 mg / m2, most preferably from 4.0 to 9.0 mg / m2.

20. Steel substrate according to anyone of claims 15 to 19, obtainable by the method according to anyone of claims 9 to 14.

21. Automotive part made of a steel substrate according to any one of claims 15 to 20.

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