Method for producing a steel sheet with a znalmg coating, corresponding coated steel sheet, component and vehicle

Abstract

To provide a method for producing a ZnAlMg-coated steel sheet having small amounts of Al and Mg, in which the outer face of the coating has reduced waviness Wa0. 8.SOLUTION: From 0.80 to 1.40 wt.% Al, from 0.80 to 1.40 wt.% Mg, unavoidable impurities and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the invention relates to a method for manufacturing a steel sheet provided with a coating in which the weight content of each additional element in the coating is less than 0.3%, the remainder being Zn, the outer face of the coated steel sheet having a waviness Wa0. 8 of less than or equal to 0.50 μm before skin-passing, to a coated steel sheet obtained by this method, to a part obtained by deformation of the steel sheet, and to a land motor vehicle comprising a body including said part.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a steel sheet provided with a coating containing 0.80 to 1.40% by weight of Al, 0.80 to 1.40% by weight of Mg, unavoidable impurities, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr, and Bi, the weight content of the additional elements in the coating being less than 0.3%, the remainder being Zn, and to a coated steel sheet obtained by this method. Such steel sheet is intended in particular for making body parts for land-based motor-driven vehicles such as automobiles. [Background technology]

[0002] Typically, steel sheets are cut and deformed to form the body parts or bodies, which are then coated with a film of paint (or paint system) that ensures a good surface appearance and, together with the zinc-based coating, protects against corrosion.

[0003] Zinc-based coatings on steel sheets have what is called waviness on their outer surface, which can currently only be compensated for by a considerable thickness of paint, at the detriment of having a so-called "orange peel" appearance, which is unacceptable for bodywork parts.

[0004] The waviness W on the outer surface of the coating is a smooth, quasi-periodic geometric irregularity with a fairly long wavelength (0.8 to 10 mm) that is distinct from the roughness R, which corresponds to a geometric irregularity with a short wavelength.

[0005] The arithmetic mean Wa of the waviness profile, expressed in μm, is often used to characterize the waviness of the outer surface of a steel coating. The waviness is measured with a cut-off threshold of 0.8 mm and is expressed as Wa according to standard SEP1941. 0.8 is defined by

[0006] Wa 0.8 A reduction in the thickness of the paint film used to achieve a given characteristic of paint appearance can improve the quality of the paint appearance for a given thickness of the paint film.

[0007] Several methods are known for reducing waviness in zinc coated steel sheets.

[0008] In fact, patent application WO2014 / 135999 discloses a method for producing a steel sheet provided with a zinc coating containing 0.2 to 0.7% aluminum, the method comprising the steps of providing a steel sheet, depositing a coating on at least one surface of the steel sheet by immersing the steel sheet in a bath, and wiping the coating with wiping gas from at least one nozzle projecting through at least one outlet, the steel sheet traveling in front of the at least one nozzle, and the wiping gas being ejected from the nozzle along a main ejection direction E, and the outer surface of the coating, after solidification and before any skin pass operation, has a waviness Wa of 0.55 μm or less. 0.8 and satisfy at least one of the following formulas:

[0009]

number

[0010] The patent application also provides that the outer surface of the resulting coated steel sheet, coating, has a waviness of 0.35 μm or less before any skin pass operation. 0.8 Finally, the patent discloses that the outer surface of the coating has a waviness Wa of 0.43 μm or less. 0.8 A part obtained by deformation of said steel sheet is disclosed, having:

[0011] However, this method is only suitable for controlling waviness in coatings containing zinc and small amounts of aluminum. Indeed, it is known that the waviness of the outer surface of a coating can vary significantly depending on the properties of the coating.

[0012] Recently, new zinc-based coatings have been developed. These coatings, commonly referred to as "ZnAlMg coatings," contain aluminum, magnesium, and the remainder zinc. They are used to further improve the corrosion resistance of steel sheets.

[0013] Patent application WO2009 / 147309 discloses a method for producing a steel strip with a corrosion-protective coating, which comprises passing a steel strip through a molten bath (maintained at a temperature of 350-700°C) containing 2-8% by weight of aluminum, 0-5% by weight of magnesium, and up to 0.3% by weight of additional elements, the remainder being zinc and unavoidable impurities, to obtain a coated steel strip, which is then wiped using nozzles that spray gas on both sides of the coated steel strip, and then cooling the coating in a controlled manner until the coating is completely solidified, the cooling being carried out at a rate of less than 15°C / s between the temperature at the time of leaving the unit where the wiping occurs and the start of solidification, and then at a rate of 15°C / s or more between the start and end of solidification.

[0014] This patent also discloses a cold rolled steel strip that has been hot-dip galvanized but not skin-passed, the coating of which contains 2-8 wt. % aluminum, 0-5 wt. % magnesium, and up to 0.3 wt. % additional elements, the balance consisting of zinc and unavoidable impurities, the coating having a waviness of 0.5 μm or less. 0.8 It has.

[0015] Finally, this patent application describes a steel part obtained by deformation, the coating of which has a waviness Wa of 0.48 μm or less. 0.8 and a steel part obtained by deformation, which has been further subjected to a skin pass process before deformation, and the coating has a waviness Wa of 0.35 μm or less. 0.8 A steel component having:

[0016] However, in this application, the ZnAlMg coating contains a large amount of aluminum, and as shown in the examples, when the amount of aluminum is less than 2%, the application of this method does not have the effect of flattening the waviness. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] International Publication No. 2014 / 135999 [Patent Document 2] International Publication No. 2009 / 147309 Summary of the Invention [Problem to be solved by the invention]

[0018] Therefore, the object of the present invention is to provide a ZnAlMg coated steel sheet having a small amount of Al and Mg, the outer surface of the coating having reduced waviness. 0.8 The present invention provides a method for producing a steel sheet having a [Means for solving the problem]

[0019] For this purpose, the subject of the invention is a method according to claim 1.

[0020] The method may also have the features of claims 2 to 7 taken individually or in combination.

[0021] The object of the invention is also a steel sheet according to claim 8.

[0022] The steel plate may also have the features set forth in claim 9.

[0023] The subject of the invention is also a component according to claim 10.

[0024] The part may also comprise the features of claims 11 to 13 taken individually or in combination.

[0025] The subject of the invention is also a vehicle according to claim 14.

[0026] The present invention will now be illustrated by way of example, given by way of guidance and not by way of limitation, and with reference to the accompanying figures, in which: [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 2 is a schematic side view for explaining the method of the present invention. [Figure 2] FIG. 2 is a partial schematic enlarged view of the circled portion I in FIG. [Figure 3] 2 along the arrow II in FIG. 2 and illustrates the shape of the output of the nozzle in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0028] The object of the present invention is a method for producing a coated steel sheet comprising a steel sheet coated with a coating containing 0.80-1.40% by weight of Al, 0.80-1.40% by weight of Mg, unavoidable impurities and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the weight content of each additional element in the coating being less than 0.3%, the remainder being Zn, the method comprising the following successive steps: A. providing the steel plate; B. cold rolling the steel sheet, wherein at least the last pass has a roughness Ra of 0.5 μm or less on the working surface. 2.5 a step performed on a straightened, unetched work roll having C. annealing the steel sheet in a continuous annealing line; D. depositing the coating by immersing the steel sheet in a molten bath; E. running the coated steel sheet through a containment zone including wiping nozzles that inject wiping gas through at least one outlet onto both sides of the sheet along a main jetting direction (E), the wiping satisfying at least one of the following formulas:

[0029]

number

[0030] Without wishing to be bound by any theory, the method according to the present invention has been shown to provide a steel sheet with a coating containing 0.80-1.40 wt. % Al, 0.80-1.40 wt. % Mg, the remainder Zn, that has a sufficiently low waviness (Wa) leading to a highly improved surface appearance, particularly a painted appearance. 0.8 It is believed that this method makes it possible to obtain an outer surface having a waviness of 0.01 mm or less. In fact, for these ZnAlMg coated steel sheets, the conventional methods of the prior art do not appear to result in such low waviness. The inventors have discovered that the applied method, as well as the chemical elements of the coating and the amounts of elements in this coating, affect the waviness. In order to obtain the lowest possible waviness for ZnAlMg coated steel sheets having the above-mentioned specific amounts of Al and Mg, it appears that the method according to the present invention is necessary to control the surface of the above-mentioned ZnAlMg coating and to obtain waviness values ​​that have not been achieved in the prior art.

[0031] In a preferred embodiment, the wiping step of the method according to the invention is such that at least one of the following formulas is further satisfied:

[0032]

number

[0033] It has been observed that, in addition to satisfying at least one of formulas (1) or (2), satisfying at least one of formulas (3) or (4) can further reduce the waviness of the coated steel sheet.

[0034] 1 includes a steel plate having two surfaces coated with the above-described ZnAlMg coating. Preferably, the steel plate is a low-carbon steel, such as an interstitial-free steel (IF steel), a bake-hardenable steel, or an Al-killed steel.

[0035] The coating generally has a thickness of 25 μm or less and is intended to protect the steel plate 1 from corrosion.

[0036] To produce the steel plate 1, for example, the following procedure can be carried out.

[0037] For example, a sheet such as a steel sheet obtained by hot rolling and then cold rolling is used.

[0038] Preferably, cold rolling begins with cold rolling the sheet with a reduction generally comprised between 30 and 85%, to obtain a sheet 1 with a thickness comprised between 0.2 and 2 mm. It is necessary to ensure that at least the last cold rolling pass is carried out with so-called smooth or bright work rolls, i.e., rolls that are not leveled and are not etched. For this purpose, the working surface must have a roughness Ra of 0.5 μ or less. 2.5 , i.e., with roughness measured with a cutoff threshold at 2.5 mm.

[0039] It is recalled that the work rolls are the rolls of the rolling mill that come into direct contact with the sheet 1 to ensure its deformation. The term working surface refers to the surface that is in contact with the sheet 1.

[0040] Smooth work rolls will be present at least in the last stand of the rolling mill, taking into account the running direction of the plate in the rolling mill.

[0041] By using smooth work rolls at least for the last rolling pass, the waviness of the steel sheet 1 subsequently obtained by coating the sheet on the one hand and the part that can be produced by deforming the steel sheet 1 on the other hand can be reduced. 0.8 This allows for better control of the

[0042] In particular, such cold rolling reduces waviness compared to rolling that relies solely on rolls with more severe roughness etched, for example, by either shot blasting or electrical discharge (so-called electron discharge textured (EDT) rolls). 0.8 This allows for a reduction in

[0043] In step C), the cold-rolled sheet 1 is annealed in a continuous annealing line, preferably in a reducing atmosphere, with the aim of recrystallizing after the work hardening suffered during the cold rolling operation.

[0044] The recrystallization anneal also provides the possibility of activating the surface of the sheet to promote the chemical reactions necessary for the subsequent dip coating operation.

[0045] Depending on the steel grade, the recrystallization annealing can be carried out at a temperature comprised between 650 and 1200°C, preferably between 650 and 900°C, for a time necessary for recrystallization and surface activation of the steel.

[0046] The plate is then cooled to a temperature close to that of the molten bath 2 contained in the crucible 3 .

[0047] In step D), the steel sheet is coated by hot dip in such bath 2. The composition of bath 2 is based on zinc and contains 0.8-1.4 wt. % aluminum and 0.8-1.4 wt. % magnesium. Preferably, the coating contains 1.0-1.40 wt. % Al and 1.0-1.40 wt. % Mg. Indeed, without wishing to be bound by any theory, it is believed that these amounts of Al and Mg in the coating further improve the waviness of the ZnAlMg coating, while maintaining improved corrosion resistance compared to Zn coatings.

[0048] Bath 2 may also contain up to 0.3 wt. % of any additional element such as Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.

[0049] These different elements may, for example, make it possible to improve the corrosion resistance of the coating, or its brittleness or its adhesion.

[0050] Those skilled in the art, aware of their effect on the properties of the coating, will know how to use them according to the additional purpose sought, and it has been determined that these elements do not interfere with the waviness control obtained by the method of the present invention.

[0051] Finally, bath 2 may contain unavoidable impurities from the ingots for feeding the tank or even from the passage of plate 1 through bath 2. Thus, in particular, it is possible to mention, for example, up to 5% by weight of iron.

[0052] During hot dip coating, the aluminum present in the bath first reacts with the steel to produce a so-called inhibitor layer consisting of intermetallic elements made of aluminum and iron. Such inhibitor layers are usually composed of FeAl3 and have a thickness of 20-80 nm. As mentioned above, a coating layer containing 0.8-1.4 wt. % aluminum and 0.8-1.4 wt. % magnesium is formed on this inhibitor layer.

[0053] 1 and 2, in step E), after leaving the bath 2, the steel sheet 1 enters a containment zone which is arranged on both sides of the steel sheet 1 and which includes wiping nozzles 4 which inject a wiping gas, for example air or an inert gas, towards the outer surface of the coating. The containment zone can be constructed, for example, according to WO2010 / 130883, namely: - At the bottom, by the wiping line (shown by the dotted line in Figure 2), and the upper outer surface of the wiping nozzle 4 - at the top, by the tops of two containment boxes 5, located on either side of the plate, directly above the nozzles 4 and having a height of at least 10 cm relative to the wiping line; - on the sides by the lateral parts of the containment box 5 The boundaries are indicated.

[0054] The wiping gas is ejected from each nozzle 4 along a main ejection direction E.

[0055] In the illustrated example, the direction E is horizontal and perpendicular to the steel sheet 1 and follows the wiping line. In other embodiments, the direction E may have other inclinations relative to the steel sheet 1.

[0056] The running speed V of the plate 1 on the production line used is generally between 60 m / min and 200 m / min, preferably between 80 m / min and 120 m / min.

[0057] Alternatively, the nozzles 4 may be configured differently, positioned differently, and / or adjusted differently. It is also possible to provide nozzles on only one side of the steel plate 1.

[0058] The nozzle 4 has an outlet 6 through which the wiping gas is emitted towards the outer surface of the coating located on the opposite side. Various shapes of the nozzle 4 are conceivable.

[0059] The outlet 6 of the nozzle 4 is located at a distance Z from the steel sheet 1 along the main injection direction E. As shown in Figure 3, the outlet 6 generally appears as an elongated gap extending perpendicular to the running direction S and the plane of Figure 3 over a width L at least equal to the width of the steel sheet 1.

[0060] The height of the outlet 6, i.e. the dimension parallel to the running direction S of the steel sheet 1 at the front face of the nozzle 4, is preferably constant as shown in Figure 3. In this case, in a particular alternative, this height may vary over the width of the discharge opening 6. Thus, the outlet 6 may, for example, have a shape that widens slightly towards its ends (bowtie shape).

[0061] To take into account these possible height variations and different possible embodiments, the average height d of the outlet 6 over its width L will now be considered.

[0062] The nozzles 4 inject gas onto both sides of the steel sheet, the gas preferably having an oxidizing power lower than that of an atmosphere consisting of 4% by volume of oxygen and 96% by volume of nitrogen. In particular, it may be advantageous to use pure nitrogen or pure argon, or a mixture of nitrogen or argon with an oxidizing gas, such as oxygen, a CO / CO2 mixture, or an H2 / H2O mixture. It is also possible to use a CO / CO2 mixture or an H2 / H2O mixture without the addition of an inert gas. Preferably, the wiping gas consists of nitrogen.

[0063] Then, in step F), the coating is allowed to cool in a controlled manner so that it solidifies.

[0064] In addition to this solidification step, a step G) can be carried out consisting of a skin-pass operation to give texture to the outer surface 23 of the coating 7 and to facilitate the subsequent forming process of the steel sheet 1.

[0065] In fact, the skin-pass operation gives the possibility of transferring a sufficient roughness to the outer surface of the coating of the steel sheet 1 for the forming method to be carried out properly, while promoting good retention of the oil applied to the steel sheet 1 before it is formed. The elongation of the steel sheet 1 during the skin-pass operation is generally comprised between 0.5 and 2%.

[0066] The work rolls have a surface with a roughness of less than 5 μm, so the skin-pass operation has low waviness. 0.8 This gives the possibility of preserving

[0067] The skin pass operation is preferably carried out such that the working surface has a roughness Ra comprised between 1.70 and 2.95 μm. 2.5 If the elongation during the skin pass operation is 1.1% or less, the roughness of the working surface of the EDT work roll is 2.5 is preferably between 2.50 and 2.95 μm. When the elongation during the skin pass operation is 1.1% or more, the roughness Ra of the working surface of the EDT work roll is 2.5 is preferably between 1.70 and 2.50 μm.

[0068] The skin-pass operation is generally carried out on steel sheets 1 intended for producing automotive body parts.

[0069] If the steel sheet 1 is intended for the manufacture of a household appliance, for example, this additional operation is not carried out. In the case of components for household appliances, it is also possible to subject the paint film to a baking operation by physical and / or chemical means known per se.

[0070] For this purpose, the painted parts can be passed through a hot air or induction oven or even under a UV lamp or a device that diffuses electron beams.

[0071] Using the method according to the invention, it is possible to obtain a waviness (Wa) of 0.50 μm or less, preferably 0.45 μm or less, and even better 0.40 μm or less or 0.35 μm or less before the skin pass. 0.8 It is possible to obtain a steel sheet having an outer surface having the following characteristics.

[0072] The skin-passed steel sheet 1 is then cut and then subjected to a forming method, for example by pulling, bending or profiling, to form a part, which can then be painted to obtain a film of paint (or paint system) on both sides.

[0073] After deformation, the outer surface of the part has a waviness Wa of 0.50 μm or less, or even 0.45 μm or less, or 0.40 μm or less, or even 0.38 μm or less. 0.8 It has.

[0074] This waviness can be measured after 5% equibiaxial tension using a Marciniak tool. Conventional methods measure waviness after 3.5% equibiaxial tension. Generally, a difference in waviness value of 0.03 can be expected from tensions between 3.5 and 5%.

[0075] For automotive applications, after phosphate coating, each part is immersed in a cataphoretic bath and sequentially coated with a primer coat, a base coat, and optionally a finish varnish.

[0076] Before applying the electrophoretic layer to the part, the latter is pre-degreased and then coated with phosphate to ensure electrophoretic adhesion.

[0077] The electrophoretic layer provides the part with additional protection against corrosion. The primer paint layer is typically applied with a gun and provides the final appearance of the part and protection against stone chips and UV. The base paint layer gives the part its color and final appearance. The varnish layer gives the part's surface good mechanical strength, resistance to aggressive chemicals and a good surface appearance.

[0078] Generally, the weight of the phosphate coating layer is 1.5-5 g / m 2 is included between.

[0079] The paint film applied to ensure protection and optimal surface appearance of the part comprises, for example, a 15-25 μm thick electrophoretic layer, a 35-45 μm thick coat of primer paint, and a 40-50 μm thick base coat of paint.

[0080] If the paint film further comprises a varnish layer, the thicknesses of the different paint layers are generally as follows: Electrophoretic layer: between 15 and 25 μm, preferably less than 20 μm; Primer coating layer: less than 45 μm, Base coating layer: less than 20 μm, and Varnish layer: less than 55 μm.

[0081] Preferably, the total thickness of the paint film is less than 120 μm, or even less than 100 μm.

[0082] Finally, the subject of the invention relates to a land vehicle comprising a bodywork, the bodywork comprising a part according to the invention.

[0083] The invention is illustrated herein by tests given by way of guidance and not by way of limitation. [Example]

[0084] All specimens were made of cold-rolled normal IF steel, with the working surface having a roughness Ra of 0.35 μm. 2.5 A final rolling pass was performed with straightened, unetched work rolls having a thickness of 1.2 μm. The samples were then annealed at a temperature of 765°C and hot-dip coated in a molten bath containing 1.2 wt% Al, 1.2 wt% Mg (samples 2-38) or 1.5 wt% Al, 1.5 wt% Mg (sample 1), the balance being Zn. They were then sent to a containment zone and wiped with nitrogen. After solidification of the coating, the coated steel sheets were polished to a roughness Ra of 2.1 μm. 2.5 The workpiece was skin-passed with a roll having a working surface having a thickness of 1 / 2 mm.

[0085] All specimens were deformed using a Marciniak tool. They were tensioned in a 5% equibiaxial tension mode. The waviness before skin pass (SKP), after skin pass, and after skin pass and deformation (DEF) was measured for each specimen.

[0086] Wa 0.8 The measurement procedure consists of mechanically probing (skidless) a steel sheet profile with a length of 50 mm in the rolling direction, according to a protocol in accordance with the SEP 1941 standard. From the signal obtained by probing, an approximation of its general shape with a polynomial of degree 5 is subtracted. The waviness Wa and the arithmetic mean roughness Ra are then separated by a Gaussian filter, applying a cutoff of 0.8 mm. In the case of deformed steel sheets, this procedure is applied to the deformed and undeformed zones of the sheet.

[0087] The process parameters and waviness values ​​for Tests 1 to 15 are summarized in Table 1. All of the tests according to the present invention satisfy formula (1) or formula (2).

[0088] Subsequently, additional tests 17-20, 22-25, 27, 28, and 30-37 with improved waviness values ​​were conducted, and the corresponding process parameters and waviness values ​​are summarized in Table 2. All such tests satisfy equation (3) or equation (4) in addition to equation (1).

[0089] [Table 1]

[0090] [Table 2]

Claims

1. 1. A method for producing a steel sheet provided with a coating comprising 0.80 to 1.40 wt. % Al, 0.80 to 1.40 wt. % Mg, unavoidable impurities, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr, and Bi, wherein the weight content of each additional element in the coating is less than 0.3%, with the remainder being Zn, The following successive steps A. Providing a steel plate; B. cold rolling the steel sheet, wherein at least the last rolling pass has a roughness Ra of 0.5 μm or less on the working surface. ２．５ a step performed on a straightened, unetched work roll having C. Annealing the steel sheet in a continuous annealing line; D. Depositing the coating by immersing the steel sheet in a molten bath; E. Running the coated steel sheet through a containment zone including wiping nozzles that inject wiping gas through at least one outlet onto both sides of the sheet along a main jetting direction (E), wherein the wiping satisfies at least one of the following formulas: [Equation 1] [In the formula, V is the running speed of the steel plate in front of the nozzle, and V is m s －１ is expressed as P is the pressure of the wiping gas in the nozzle, P is expressed in Pa, Z is the distance between the steel plate and the nozzle along the main jetting direction (E), Z is expressed in mm; d is the average height of the nozzle outlet along the running direction (S) of the steel sheet in front of the nozzle, d is expressed in mm; p.o. ２ is the oxygen partial pressure in the containment zone.] F. solidifying the coating; G. The coated steel sheet is polished to a roughness Ra of less than 5 μm. ２．５ a step of skin-passing the work roll having the A method comprising:

2. The skin pass of the coated steel sheet has a roughness Ra of 1.70 μm to 2.95 μm. ２．５ 10. The method of claim 1, wherein the method is performed on an EDT work roll having a

3. Further satisfying at least one of the following formulas: [Equation 2] [In the formula, V is the running speed of the steel plate in front of the nozzle, and V is m s －１ is expressed as P is the pressure of the wiping gas in the nozzle, P is expressed in Pa, Z is the distance between the steel plate and the nozzle along the main jetting direction (E), and Z is expressed in mm. d is the average height of the nozzle outlet along the running direction (S) of the steel sheet in front of the nozzle, d is expressed in mm; p.o. ２ is the oxygen partial pressure in the containment zone.] 3. The method according to claim 1 or 2.

4. The method of any one of claims 1 to 3, wherein the coating comprises 1.0 to 1.40 wt% Al and 1.0 to 1.40 wt% Mg.

5. The method of any one of claims 1 to 4, wherein the wiping gas consists of nitrogen.

6. A coated steel sheet obtainable by the method of any one of claims 1 to 5, the steel sheet having a coating containing 0.80 to 1.40% Al, 0.80 to 1.40% Mg, unavoidable impurities, and optionally one or more additional elements selected from Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, Zr or Bi, the weight content of each additional element in the coating being less than 0.3%, the remainder being Zn, and the outer surface of the coated steel sheet has a waviness (Wa) of 0.5 μm or less before skin pass. ０．８ wherein the waviness is measured in a Marciniak tool in 5% equibiaxial tension mode.

7. The outer surface of the coated steel sheet before skin pass has a waviness Wa of 0.4 μm or less. ０．８ 7. The steel sheet of claim 6, wherein the waviness is measured in a Marciniak tool in a 5% equibiaxial extension mode.

8. 8. The steel sheet according to claim 6, wherein the coating contains 1.0 to 1.40 wt. % Al and 1.0 to 1.40 wt. % Mg.

9. A part obtained by deforming the coated steel sheet according to any one of claims 6 to 8, wherein the outer surface of the coated steel sheet has a waviness Wa of 0.5 μm or less. ０．８ wherein the waviness is measured in a Marciniak tool in a 5% equibiaxial extension mode.

10. The outer surface of the coated steel sheet has a waviness Wa of 0.45 μm or less. ０．８ 12. The part of claim 11, wherein the waviness is measured in a Marciniak tool in a 5% equibiaxial extension mode.

11. The component of claim 10 further comprising a film of paint on the coated steel sheet.

12. 12. The part of claim 11, wherein the paint film has a thickness of 120 μm or less.

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