EP0412073A1 - A deep drawing steel sheet with a metal coating and a method for its manufacture - Google Patents

Abstract

Deep-drawing metal sheet provided with a metal coating on at least one face and exhibiting on this face a roughness consisting of isolated and uniformly distributed wells. These wells have a depth of between 5 mu m and 25 mu m and a diameter at the base of between 80 mu m and 200 mu m; they are distributed with a density of between 50 and 150 wells per linear inch along at least two perpendicular directions in this face of the sheet. This face additionally exhibits an arithmetic mean roughness Ra at cut-off of 0.8 mm or between 0.5 mu m and 2 mu m, and the arithmetic roughness Ra of this face at cut-off of 0.8 mm does not differ by more than 0.3 mu m from the value of the arithmetic mean roughness at cut-off of 0.8 mm. The metal coating may consist of zinc, aluminium or a zinc alloy. <IMAGE>

Description

The present invention relates to a stamping sheet of steel, provided with a metallic coating. It also relates to a method of manufacturing such a sheet.

The sheets targeted by the invention include sheets coated with zinc or aluminum, both by quenching and by electrodeposition, as well as sheets coated with a zinc alloy with another metal, in particular aluminum, nickel or the iron. These sheets are intended in particular for the manufacture of visible elements of automobile bodywork. They generally have a thickness of between 0.6 mm and 1 mm.

To fix the ideas and by way of simple example intended only to illustrate the invention, the description which follows will more particularly refer to a steel sheet coated with zinc by dip galvanization, called to simplify galvanized sheet.

The objective of the present invention is to provide a sheet of this type having good seizure resistance as well as an excellent appearance after painting, this latter property concerning both the sheet in the undeformed state and the stamped part.

The applicant's tests have shown that, during stamping, the behavior of a coated sheet was significantly different from that of a bare steel sheet.

Although the reason is not clearly established at present, it seems that the difficulties encountered are due, at least in part, to the fact that these coating metals have a hardness significantly lower than that of rolled steel. Cold.

It can be seen that these sheets, and in particular galvanized sheets, are much more sensitive to seizure than sheets of steel. In addition, the stamping of a galvanized sheet gives rise to a powdering phenomenon.

Grabbing and dusting are well known per se. It will be recalled briefly that they both consist of a tearing of metal particles from the surface of the sheet by the stamping tool, under the effect of the friction forces generated by the stamping. These are therefore two phenomena of the same nature, which differ essentially in the size of the metal particles torn off. Their extent is greater with galvanized sheets than with steel sheets.

In the case of galvanized sheets, they cause an accumulation of particles at the bottom of the stamping tools, that is to say in places where there is generally no seizing with the steel sheets; this results in additional faults in the stamped parts. Finally, it is out of the question to repair by grinding certain defects in galvanized sheets as is the practice with steel, as this would cause a loss of the corrosion protection which the coating must provide.

It has already been proposed, in particular by patent BE-A-870,609, to print in the surface of the sheet a morphology made up of regularly distributed plateaus and valleys, ensuring roughness of a deterministic nature from a cylinder of finish treated appropriately. These valleys form a network in which the lubricating oil circulates, carrying away the particles torn off during stamping. The application of this method to sheets coated with metals such as zinc or aluminum, however, comes up against the difficulties which have been mentioned above, because of the much lower hardness of these metals. The particles torn off cannot be completely entrained by the oil circulating in the roughness valleys; therefore, they adhere to tools and damage the surface of the sheets.

It is also known, in particular from patent LU-A-86,784, to print in the surface of the sheets of small isolated wells, that is to say who do not communicate with each other. This arrangement makes it possible to operate with higher drawing pressures than with communicating valleys, because the small wells act as "oil pockets" preventing the too rapid expulsion of the lubricant under high pressures. However, it appeared that such an arrangement was not directly usable with sheets coated with metals such as zinc or aluminum; not only was the roughness not regularly imprinted in the coated sheet, but the sheet was also found to be particularly sensitive to galling.

The object of the present invention is to provide a sheet provided with a metallic coating, which does not have the aforementioned drawbacks and which, by an appropriate roughness, guarantees good resistance to seizing and an excellent appearance after painting.

According to the present invention, a steel stamping sheet provided with a metal coating on at least one face, is characterized in that it has on said face a roughness consisting of isolated and regularly distributed wells, in that said wells have a depth between 5 µm and 25 µm and a diameter at the base between 80 µm and 200 µm, in that said wells are distributed with a density between 50 and 150 wells per linear inch in at least two perpendicular directions in said face of the sheet, in that said face has an average arithmetic roughness Ra at cut off of 0.8 mm of between 0.5 µm and 2 µm and in that the average arithmetic roughness Ra of said face at cut off of 8 mm does not deviate by more than 0.3 µm from the value of the average arithmetic roughness at cut off of 0.8 mm.

According to particular variants, the depth of the wells is preferably between 7 μm and 20 μm, and their diameter at the base is preferably between 100 μm and 150 μm.

In this regard, it should be noted that the diameter at the base considered here is the diameter of the section of the well located in the plane of the surface of the sheet. Similarly, the diameter at the base of a bead on the rolling mill cylinder is the diameter of the section of this bead located in the surface of the cylinder, this section being comparable to a planar section because of its very small dimensions compared to the diameter of the cylinder.

It will also be indicated that, in the context of the present invention, the section of the well, respectively of the bead, is not necessarily circular or directly comparable to a circle; in the case of a section of any shape, the diameter at the base will be that of the circle as best as possible circumscribed in this section.

Finally, it is recalled that the cut off expresses the cutoff wavelength used for the measurement of roughness; this expression means that, for the measurement of the roughness of a surface, no account is taken of the undulations of the surface having a wavelength greater than the value indicated, for example 0.8 mm and 8 mm in this application.

Also according to the invention, the density of the wells is advantageously between 90 and 120 wells per linear inch, in at least one of the above directions.

In addition, the average arithmetic roughness Ra at cut off of 0.8 mm is advantageously between 0.8 μm and 1.4 μm.

Still according to the invention, the metallic coating of said face preferably has a thickness of between 7 μm and 25 μm.

Also preferably, said metal coating is a zinc coating deposited by dip galvanizing.

It also appeared that the consistency of the thickness of the metal coating was an important factor in obtaining the desired roughness. For this purpose, it is appropriate, according to the invention, that the sum of the thicknesses of the metallic coatings of the two faces of the sheet, measured at the same point, should not differ by more than 30%, and preferably by more than 15%, of the sum of the nominal values of these two thicknesses.

It has also been found that the best behavior in finish rolling, with regard to compliance with the reduction rates and thicknesses of the coatings, was obtained with steels having little or no drawing level. In this regard, ultra-low carbon steels micro-alloyed with titanium have proved to be particularly interesting. Typically, these steels have a carbon content of between 0.005% and 0.030% and a titanium content of between 0.050% and 0.150% by weight.

Another aspect of the invention relates to a method of manufacturing a stamping sheet having the characteristics set out above.

According to the invention, the metal coating is deposited on at least one face of said sheet and the coated sheet is laminated by means of cylinders of which at least the cylinder corresponding to said coated face has on its surface a plurality of insulated beads, regularly distributed. with a density of between 50 and 150 beads per linear inch in at least two perpendicular directions, and preferably between 90 and 120 beads per linear inch in at least one of the above directions, said beads having the shape of spherical segments of a height between 15 µm and 30 µm and with a diameter at the base between 100 µm and 150 µm, the surface of said cylinder also having, between said beads, an average arithmetic roughness Ra at cut off of 0.8 mm less than 0.4 µm and preferably less than 0.2 µm.

According to a particular implementation of this process, said sheet is laminated with a reduction rate of less than 1%, and preferably between 0.4% and 0.8%.

The rolls used for the finishing rolling are advantageously treated by means of an energy beam, such as a laser beam or an electron beam, according to methods developed previously.

Also according to this aspect of the invention, said metal coating is deposited by immersing the sheet in a zinc bath and the thickness of said coating is adjusted to a value between 7 μm and 25 μm, the sum of the thicknesses of the metal coatings. of the two faces of the sheet, measured at the same point, not deviating by more than 30%, and preferably not more than 15%, from the sum of the nominal values of these two thicknesses.

• Fig. 1 montre, en trois vues (a),(b),(c) à des échelles diffé­rentes, la surface d'une tôle d'emboutissage galvanisée conforme à l'invention, dans son état initial; la
• Fig. 2 représente, également en trois vues (a),(b),(c) et aux mêmes échelles que la Fig. 1, cette même tôle après-­qu'elle ait subi un essai de frottement par traction entre des mâchoires planes; la
• Fig. 3 montre, toujours en trois vues (a),(b),(c) et aux mêmes échelles que les Fig. 1 et 2, la même tôle après qu'elle ait subi un essai de frottement par passage dans un ap­pareil dit "simulateur de jonc"; et la
• Fig. 4 illustre la netteté d'image d'une tôle conforme à l'in­vention après peinture par des moyens conventionnels.

Other features and advantages of the present invention will be revealed by the examples of implementation which are described below and which are illustrated by the appended drawings, in which the

• Fig. 1 shows, in three views (a), (b), (c) at different scales, the surface of a galvanized stamping sheet according to the invention, in its initial state; the
• Fig. 2 also shows in three views (a), (b), (c) and on the same scales as FIG. 1, this same sheet after it has undergone a traction friction test between flat jaws; the
• Fig. 3 shows, still in three views (a), (b), (c) and on the same scales as FIGS. 1 and 2, the same sheet after it has undergone a friction test by passing through an apparatus called a "rod simulator"; and the
• Fig. 4 illustrates the image clarity of a sheet according to the invention after painting by conventional means.

Figs. 1, 2 and 3 present photographs using a scanning electron microscope.

The sheet used in this example is made of steel for deep drawing, of the St 14 type. It has been galvanized by dipping on both sides in a lead-free bath, at a rate of 140 g of zinc per square meter on each side. The zinc layer was 8 µm to 10 µm thick on each side. Finishing rolling was made by means of cylinders which had been ground so as to have a cut off roughness of 0.8 mm of less than 0.2 µm; the cylinders were then textured by means of a laser beam so that their surface has beads having the shape of spherical segments with a height between 16 µm and 20 µm and with a diameter at the base between 100 µm and 120 µm. The beads were distributed according to a predetermined scheme, with a density K _L = 90 beads per linear inch in the direction of the periphery of the cylinders and a density K _d = 120 beads per linear inch in the axial direction of the cylinders. The finish rolling was carried out with a reduction rate of 0.8%; after this operation, the sheet had a total thickness of 0.76 mm.

Fig. 1 represents the surface of the sheet in the receiving state, that is to say after galvanization and finishing rolling. Fig. 1 (a) shows the regularity of the surface distribution of the patterns, each of which consists of a well and a plate. Fig. 1 (b) shows a portion of the surface of this sheet, on a larger scale: the wells correspond to the darker part of each pattern; the lighter part of these patterns indicates the plate, the height of which does not exceed 5 µm. Finally, FIG. 1 (c) shows the shape of a well, comparable to a circle in the present case, as well as the diameter at the base D of this well.

In this state, the sheet has a practically flat surface, in which wells are formed with a depth of approximately 12 μm and a diameter at the base of approximately 100 μm. The distribution density of these wells is respectively 120 and 90 per linear inch in the transverse direction and in the longitudinal direction of the sheet. This sheet has an average arithmetic roughness Ra, measured at the cut off of 0.8 mm, of the order of 1.2 μm. An interesting feature is that this roughness is little influenced by the level of cut off. Thus, at the 8 mm cut off, it does not exceed 1.5 µm.
The drawing ability of this sheet was evaluated according to the most critical problem encountered with galvanized products, namely the friction behavior and the tendency to seize.

The friction behavior was assessed on the basis of a tensile test between two flat jaws enclosing the sheet; it was poorly lubricated with protective oil. The sheet showed very satisfactory behavior: the depth and the diameter of the wells were not modified, the height of the plates became less than 3 µm. Fig. 2 shows the surface of the sheet after this test.

We also conducted a test in a device called a rush simulator. This test consists of pulling the sheet through a set of three successive rollers so as to apply a double bending in the opposite direction, in tension and with friction. This test did not cause any change in the shape or dimensions of the wells, as shown in FIG. 3. The average arithmetic roughness Ra, at the cut off of 0.8 mm, fell to 0.9 µm.

With regard to the tendency to seize, this sheet was subjected to a U-shaped test, that is to say stamping, commonly applied by the applicant. It was thus possible to successively stamp 15 blanks without any trace of seizure, while a conventional galvanized sheet allows only 5 to 6 blanks.

This sheet was further subjected to a paint test, carried out under conditions typical of the process for manufacturing automobile body sheets: deposition of a layer of paint with a thickness of approximately 30 μm by cataphoresis followed by annealing. at 180 ° C for 30 min, then deposition of a layer of surface with a thickness of approximately 20 μm followed by annealing at 150 ° C for 30 min and finally deposition of a layer of enamel with a thickness of about 45 µm followed by annealing at 125 ° C for 30 min.

The image distinctness index DOI (= Distinctness of Image) was determined according to a standard procedure well known by those skilled in the art under the name Ford test. This index varies in particular according to the undulation of the surface before the application of the paint. This undulation can in particular be expressed by the magnitude W = (Ra ^{co 8mm} - Ra ^{co 0.8mm} )
which represents the difference of the average arithmetic roughnesses Ra at the respective cut offs of 8 mm and 0.8 mm, these roughnesses being measured on the unpainted galvanized sheets.

Fig. 4 illustrates the evolution of the Ford index of image sharpness of the painted sheet as a function of the ripple W of the surface of the sheet before painting. The two lines (a) and (b) delimit a zone of dispersion of the values corresponding to a series of conventional sheets; we see that, in the best of cases, the image sharpness index is between 8 and 9 for an extremely weak ripple (W = 0.2).

The points obtained with sheets in accordance with the teachings of the present invention have been reported on this diagram.

In its receiving state, that is to say galvanized and laminated, the sheet has a corrugation 4 = 0.25 and, after painting by the procedure recalled above, its Ford index of image sharpness is equal to 10 (point I).

A paint test was also carried out with a sheet of the invention, deformed by 10% in biaxial expansion to simulate the drawing. The corrugation of the sheet surface increased slightly, going to W = 0.4; the Ford image sharpness index nevertheless remained above 9 (point II).

The galvanized sheet according to the invention therefore exhibits remarkable friction behavior as well as a very low tendency to seize up; in addition, it has a very high Ford image sharpness index, both in the undeformed state and in the deformed state. These properties make it particularly advantageous for the manufacture of visible parts of vehicles, in particular of automobile body sheets.

Claims (12)

1. Stamping sheet provided with a metal coating on at least one face, characterized in that it has on said face a roughness consisting of isolated and regularly distributed wells, in that said wells have a depth of between 5 μm and 25 µm and a diameter at the base between 80 µm and 200 µm, in that said wells are distributed with a density between 50 and 150 wells per linear inch in at least two perpendicular directions in said face of the sheet, in that said face has an arithmetic roughness Ra at cut off of 0.8 mm between 0.5 µm and 2 µm and that the arithmetic roughness Ra of said face at cut off of 8 mm does not deviate from more than 0.3 µm from the value of the average arithmetic roughness at cut off of 0.8 mm. 2. Stamping sheet according to claim 1, characterized in that said metallic coating consists of zinc, aluminum or a zinc alloy. 3. Stamping sheet according to claim 2, characterized in that said metal coating is deposited by immersion of the sheet in a bath of zinc or a zinc-aluminum alloy respectively. 4. Stamping sheet according to either of claims 1 to 3, characterized in that said metallic coating consists of zinc and iron and in that it is formed by hot diffusion. 5. Stamping sheet according to either of claims 1 to 4, characterized in that the thickness of the metallic coating of said face is between 7 µm and 25 µm. 6. Stamping sheet according to either of claims 1 to 5, characterized in that the sum of the thicknesses of the metallic coatings of the two faces of the sheet, measured at the same point, does not deviate from more than 30% of the sum of the nominal values of these two thicknesses. 7. Stamping sheet according to either of claims 1 to 6 characterized in that it consists of a steel having no or virtually no drawing bearing. 8. A method of manufacturing a stamping sheet according to either of claims 1 to 7, characterized in that a metal coating is deposited on at least one face of said sheet and in that the 'said coated sheet is subjected to a finishing rolling by means of cylinders of which at least the cylinder corresponding to said coated face has on its surface a plurality of isolated beads, regularly distributed with a density between 50 and 150 beads per linear inch following at least two perpendicular directions, said beads having the form of spherical segments with a height of between 15 μm and 30 μm and with a diameter at the base of between 100 μm and 150 μm, the surface of said cylinder also having, between said beads, an average arithmetic roughness Ra at cut off of 0.8 mm less than 0.4 μm. 9. Method according to claim 8, characterized in that said coated sheet is laminated with a reduction rate of less than 1%. 10. Method according to either of claims 8 and 9, characterized in that a coating is deposited on at least one face of said sheet by immersion of the sheet in a bath made of zinc, aluminum or a zinc-aluminum alloy. 11. Method according to either of Claims 8 to 10, characterized in that a coating having a thickness between 7 µm and 25 µm is deposited on said sheet. 12. Method according to either of claims 8 to 11, characterized in that a coating is deposited on said sheet, the sum of the thicknesses on the two faces of said sheet, measured at the same point, does not deviates no more than 30% from the sum of the nominal values of these two thicknesses.

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