Method for manufacturing parts with a low waviness from an electrogalvanized metal sheet, corresponding part and vehicle

Abstract

A method for manufacturing parts is provided. The method includes cold rolling a substrate with work cylinders whose work surface has a roughness Ra.sub.2.5 of less than or equal to 3.6 μm, depositing the metal coating on at least one face of the annealed substrate by electrodeposition to form the metal sheet and deforming the cut metal sheet to form the parts. The outer surface of the metal coating has a waviness Wa.sub.0.8 of less than or equal to 0.5 μm after the deformation step. A part and vehicle are also provided.

Claims

1. A part comprising a deformed metal sheet, the deformed metal sheet including a substrate having at least one face coated with a zinc-based metal coating that was deposited on the substrate by electrodeposition, an outer surface of the zinc-based metal coating having a waviness Wa.sub.0.8 of less than or equal to 0.50 μm, wherein Wa.sub.0.8 is measured according to standard SEP 1941, 1st edition dated May 2012.

2. The part according to claim 1, wherein the outer surface of the zinc-based metal coating has a waviness Wa.sub.0.8 of less than or equal to 0.45 μm.

3. The part according to claim 2, wherein the outer surface of the zinc-based metal coating has a waviness Wa.sub.0.8 of less than or equal to 0.40 μm.

4. The part according to claim 3, wherein the outer surface of the zinc-based metal coating has a waviness Wa.sub.0.8 of less than or equal to 0.35 μm.

5. The part according to 1, further comprising a paint film on the zinc-based metal coating.

6. A motorized land-based vehicle comprising: a body including a part according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be illustrated by examples provided for informational purposes, and non-limitingly, in reference to the appended FIGURE, which is a diagrammatic sectional view illustrating the structure of a metal sheet according to the invention.

DETAILED DESCRIPTION

(2) The sheet 1 comprises a steel substrate 3, each of the two faces 5 of which are covered by a metal coating 7.

(3) It will be noted that the relative thicknesses of the substrate 3 and the different layers covering it has not been respected in FIG. 1 in order to facilitate the illustration.

(4) The coatings 7 present on the two faces 5 are generally similar, and only one will be described in more detail below. As an alternative, only one of the faces 5 has a coating 7.

(5) The coating 7 generally has a thickness smaller than or equal to 25 μm and aims to protect the substrate 3 from corrosion.

(6) The coating 7 is a zinc-based coating. It may for example use zinc, or a zinc alloy, such as zinc-nickel or zinc-iron or zinc-cobalt, or a zinc-polymer composite.

(7) The following method may for example be used to manufacture the metal sheet 1.

(8) A substrate 3 in the form of a strip is used, for example obtained by hot rolling.

(9) The substrate 3 first undergoes a cold rolling step.

(10) Preferably, for the cold rolling, one begins by rolling the substrate 3 with a reduction rate generally comprised between 60 and 85%, so as to obtain a substrate 3 with a thickness for example comprised between 0.2 and 2 mm.

(11) Care will be taken so that at least the last cold rolling pass is done with work cylinders with a reduced thickness, i.e., the work surfaces of which have a thickness Ra.sub.2.5, i.e., measured with a cutoff threshold at 2.5 mm, of less than or equal to 3.6 μm, and preferably less than or equal to 3.3 μm or even 3.2 μm.

(12) It will be recalled that the work cylinders are the cylinders of the roller in direct contact with the substrate 3 to deform it. The term “work surface” designates their surfaces in contact with the substrate 3.

(13) The work cylinders with reduced roughness are present at least in the last cage(s) of the roller in the advance direction of the substrate 3 in the roller.

(14) Preferably, the work surfaces are not etched, or are etched stochastically, i.e., using a method creating a stochastic texture. Such a method may for example be an EDT (Electro Discharge Texturing) method. EBT (Electron Beam Texturing) methods are ruled out in this case, since they produce a non-stochastic texture.

(15) In one alternative, the work cylinders are so-called “smooth” work cylinders, i.e., straightened and non-etched cylinders, the work surfaces of which have a roughness Ra.sub.2.5 smaller than or equal to 0.5 μm, or even 0.4 μm.

(16) The work cylinders with reduced roughness positioned on either side of the substrate 3 will preferably have been made using the same method and will therefore have the same characteristics.

(17) The cold rolled substrate 3 can next undergo annealing carried out conventionally in an annealing furnace under an appropriate atmosphere, in order to recrystallize it after the work hardening it has undergone during the cold rolling operation.

(18) The recrystallization annealing also makes it possible to activate the faces 5 of the substrate 3 in order to favor the chemical reactions necessary for the subsequent electrodeposition operation.

(19) Depending on the grade of the steel, the recrystallization annealing is done at a temperature comprised between 650 and 900° C. for the time necessary to recrystallize the steel and activate faces 5.

(20) The substrate 3 next undergoes a skin-pass operation to give the faces 5 a texture facilitating subsequent shaping of the metal sheet 1.

(21) In fact, the skin-pass operation makes it possible to transfer a sufficient roughness to the faces 5, and therefore the outer surfaces 21 of the coatings 7 of the metal sheet 1, so that it can subsequently be shaped under good conditions, while favoring good retention of the oil applied to the metal sheet 1 before it is shaped.

(22) The elongation rate of the substrate 3 during the skin-pass operation is generally comprised between 0.5 and 2%.

(23) In one alternative, the skin-pass operation will be carried out with EDT work cylinders whose work surfaces have a roughness Ra.sub.2.5 comprised between 1.65 and 2.95 μm, preferably between 1.65 and 2.30 μm.

(24) The coatings 7 are next made by electrodeposition. To that end, the substrate 3 passes through an electrolyte bath.

(25) The progression speed of the substrate 3 in the electrolyte bath is generally comprised between 20 and 200 m/mn.

(26) The electrolyte is for example an aqueous acetyl electrolyte of the sulfate, chloride or chloride-sulfate mixture type, suitable for electrolytic deposition on the substrate 3 of either a layer of zinc, or a layer of zinc alloy, or a layer of zinc-polymer composite.

(27) Thus, in order to deposit a zinc coating on the substrate 3, it is possible to use a chloride-based electrolyte bath comprising 50 to 150 g/l of zinc in the form of zinc chloride (ZnCl.sub.2), 250 to 400 g/l of potassium chloride (KCl) and having a pH comprised between 4 and 5. The bath can also comprise additives, for example grain refiners with a concentration comprised between 1 and 1.5 ml/l. In order to deposit the layer of zinc on the substrate 3, the current density is preferably adjusted to a value comprised between 30 and 150 A/dm.sup.2, and the temperature of the bath is preferably adjusted to a value comprised between 40 and 80° C., and preferably below 60° C. to limit evaporation of the bath.

(28) It is also possible to use a sulfate-based electrolyte bath comprising 50 to 150 g/l zinc in the form of zinc sulfate (ZnSO.sub.4), and having a pH below 5. The bath can also comprise additives, for example NaCO.sub.3. Preferably, the current density is adjusted to a value comprised between 10 and 150 A/dm.sup.2, and the temperature of the bath to a value comprised between 40 and 80° C., preferably below 60° C. to limit the evaporation of the bath.

(29) If one wishes to deposit a zinc alloy coating, such as zinc-nickel, zinc-iron or zinc-cobalt, nickel, iron or cobalt ions are added to the sulfate-, chloride- or chloride-sulfate mixture-based electrolyte bath previously described.

(30) Likewise, to deposit a zinc-polymer composite coating, 0.1 to 2 wt % of a polymer, for example polyethylene glycol or polyacrylamide, is added to the sulfate-, chloride- or sulfate-chloride mixture-based electrolyte bath previously described. The obtained composite coating imparts excellent corrosion resistance and also makes it possible to avoid the toxic chromating or phosphating treatments necessary for the paint to catch.

(31) The surfaces 21 of the coatings 7 will preferably have references Ra.sub.2.5 comprised between 0.9 and 1.8 μm, and still more preferably between 0.9 and 1.5 μm.

(32) The metal sheet 1 thus obtained can next be cut, then shaped, for example by stamping, bending or profiling, to form parts that can next be painted to form a paint film (or paint system), not shown, on each coating 7.

(33) In the event of parts for household appliances, the paint films may also be subject to annealing by physical and/or chemical means, known in themselves.

(34) To that end, the painted part may be passed through a hot air or induction oven, or under UV lamps or under a device diffusing electron beams.

(35) After deformation, the outer surfaces 21 of the coatings 7 of the part have a waviness Wa.sub.0.8 of less than or equal to 0.50 μm, preferably less than or equal to 0.45 μm, still more preferably less than or equal to 0.40 μm, or even 0.35 μm.

(36) The use of work cylinders with a reduced roughness Ra.sub.2.5 at least for the last cold rolling pass makes it possible to better control the waviness Wa.sub.0.8 of the metal sheet 1 subsequently obtained by coating the substrate 3 on the one hand, and of the parts that can be produced by cutting and deforming the metal sheet 1 on the other hand.

(37) In particular, such cold rolling makes it possible to reduce the waviness Wa.sub.0.8 relative to rolling only using cylinders with a higher roughness.

(38) Thus, the use of work cylinders with a roughness Ra.sub.2.5 of less than or equal to 3.6 μm makes it possible to achieve a waviness Wa.sub.0.8 of less than or equal to 0.50 μm after the coating deposition step or after any deformation step.

(39) The use of work cylinders with a roughness Ra.sub.2.5 of less than or equal to 3.3 μm, or even 3.2 μm, for example makes it possible to achieve a waviness Wa.sub.0.8 of less than or equal to 0.45 μm after the coating deposition step or after any deformation step.

(40) The use of smooth work cylinders with a roughness Ra.sub.2.5 of less than or equal to 0.5 μm for example makes it possible to achieve a waviness Wa.sub.0.8 of less than or equal to 0.40 μm, or even 0.35 μm after the coating deposition step or after any deformation step.

(41) For automobile applications, after phosphating, each part is soaked in a cataphoresis bath, and a primer layer, a base paint layer, and optionally a finishing varnish layer are successively applied.

(42) Before applying the cataphoresis layer on the part, the latter is first degreased, then phosphated so as to ensure adherence of the cataphoresis.

(43) The cataphoresis layer then provides additional corrosion protection for the part. The primer layer, generally applied using a gun, prepares the final appearance of the part and protects it from grit and UV rays. The base paint layer gives the part its final color and appearance. The layer of varnish gives the surface of the part good mechanical strength, the ability to withstand aggressive chemical agents and a good surface appearance.

(44) Generally, the weight of the phosphating layer is comprised between 1.5 and 5 g/m.sup.2.

(45) The paint films used to protect and guarantee an optical surface appearance for the parts for example comprise a cataphoresis layer with a thickness of 15 to 25 μm, a primer layer with a thickness of 35 to 45 μm, and a base paint layer with a thickness of 40 to 50 μm.

(46) In the event the paint film layers further comprise a varnish layer, the thicknesses of the different paint layers are generally as follows: cataphoresis layer: between 15 and 25 μm, preferably less than 20 μm, primer layer: less than 45 μm, base paint layer: less than 20 μm, and varnish layer: less than 55 μm.

(47) The paint films may also not comprise a cataphoresis layer, and only comprise a primer layer and a base paint layer, and optionally a varnish layer.

(48) Preferably, the total thickness of the paint films will be less than 120 μm, or even 100 μm.

(49) The invention will now be illustrated by trials provided for information and non-limitingly.

(50) The trials done seek to show the positive influence of cold rolling done with work cylinders with a reduced roughness Ra.sub.25, relative to rolling done with cylinders whose work surface has a greater roughness.

(51) To that end, grade DC-06 steel substrates undergo cold rolling to reach a thickness of 0.8 mm, using either EDT etched work cylinders, the work surfaces of which have roughnesses Ra.sub.2.5 of 3.1 μm, 3.5 μm and 3.75 μm, or smooth work cylinders whose work surfaces have a roughness Ra.sub.2.5 of 0.3 μm.

(52) After recrystallization annealing at 720° C., the substrates 3 next undergo a same skin-pass operation done with EDT etched work cylinders, the work surfaces of which have a roughness Ra.sub.2.5 of 1.85 μm.

(53) The substrates 3 are next coated with zinc by electrodeposition in a sulfate-based electrolyte bath. The metal sheets thus obtained are cut and shaped by 3.5% equi-biaxial deformation using a Marciniak tool.

(54) The waviness Wa.sub.0.8 and roughness Ra.sub.2.5 values of the outer surfaces 21 of the coatings 7 are noted at the end of the electrodeposition step (EG) and the deformation step (DEF).

(55) The measurement results for Wa.sub.0.8 and Ra.sub.2.5 are provided in table I. Trials 2 to 4 correspond to different embodiments of the invention and are identified by asterisks.

(56) TABLE-US-00001 TABLE 1 Difference Ra.sub.2.5(μm) Wa.sub.0.8 cylinders Wa.sub.0.8(μm) Ra.sub.2.5(μm) Wa.sub.0.8(μm) Ra.sub.2.5(μm) after DEF and Trials for LAF work after EG after EG after DEF after DEF after EG 1 3.75 0.47 1.12 0.55 1.24 +17% 2 3.5* 0.49 1.39 0.47 1.27 −4.1%  3 3.1* 0.36 1.27 0.36 0.96  0% 4 0.3* 0.40 1.05 0.35 0.86 −13%

(57) It is in particular observed that the waviness Wa.sub.0.8 after deformation of the metal sheets according to the invention is close to or below the waviness level before deformation. This effect is particularly clear for trials 2 and 4.