US20190176198A1

US20190176198A1 - Planishing roll, method for planishing a flat product therewith and flat product therefrom

Info

Publication number: US20190176198A1
Application number: US15/999,453
Authority: US
Inventors: Frank Beier; Martin Bretschneider; Michael Maxisch; Sven Lunke; Moritz Radszat; Björn Hildebrandt; Jürgen Spehr; Thomas Routschek; Markus Hinkel
Original assignee: Salzgitter Flachstahl GmbH
Current assignee: Salzgitter Flachstahl GmbH
Priority date: 2016-02-16
Filing date: 2017-02-13
Publication date: 2019-06-13
Also published as: KR20180113533A; WO2017140605A1; EP3416760B1; US20200368800A1; DE102016102723B3; RU2705187C1; KR102189421B1; EP3416760A1

Abstract

A planishing roll has a surface structure, in particular for producing flat products from a metallic material, in particular from a steel material. The surface structure has a material ratio of 2% at a depth of 0.2 μm to 9 μm, preferably at a depth of 0.8 μm to 5.5 μm. The depth is measured, starting from a zero line, in the direction of an axis of rotation of the planishing roll, with the zero line running parallel to the axis of rotation of the planishing roll and. Starting from the surface of the planishing roll, the zero line is displaced in the direction of the axis of rotation of the planishing roll until the material ratio of the planishing roll is 0.1%.

Description

The invention relates to a planishing roll comprising a surface structure, a method for planishing a flat product consisting of a metal material, in particular consisting of a steel material, with a planishing roll, and relates to a flat product which is produced according to this method and consists of a metal material, in particular a steel material.
It is generally known that the automotive industry sets high requirements on the visual appearance of a vehicle lacquering, in particular lacquering on externally visible surfaces of a car. It is desired that the lacquering has a uniform appearance. In this case, a lowest possible proportion of wavelike reflections is to occur at a small viewing angle. These wavelike reflections are also defined as orange peel. In addition to the uniform appearance of the lacquering, cost-effective production of the lacquering is also required. Labour-intensive steps, such as polishing work and/or raw material-intensive spray lacquerings with filler layers are to be avoided.
In order to be able to meet these requirements, the fine steel sheets which are used as bodywork sheets are skin pass-rolled prior to the lacquering procedure. This skin pass rolling is also defined as planishing. The mechanical characteristic values, such as e.g. yield strength, tensile strength and expansion, as well as the surface topography e.g. characterised by roughness, number of peaks and a waviness of the fine steel sheet are set to a desired state by the skin pass rolling procedure. Fine steel sheets which are planished in this manner and have a defined surface topography simplify the subsequent lacquering procedure by avoiding or reducing the application of filler layers. These filler layers serve in particular to compensate for any unevenness present on the surface of the fine steel sheet to be lacquered, and thus to prevent this unevenness from becoming apparent in the lacquered surface. Planishing rolls which have a profiled surface are used for the skin pass rolling procedure. The profiling of the surface starting from a roll comprising a smoothly polished surface is effected by various texturing methods. Then, the surface of the fine steel sheet is textured by the skin pass rolling procedure using corresponding planishing rolls. The same also applies in general to fine steel sheets outside the field of bodywork sheets.
In general, the following methods are known as erosive texturing methods for planishing rolls: SBT (Shot Blast)—shooting angular blasting material onto the surface of the planishing rolls, EDT (Electro Discharge Texturing)—spark erosion, EBT (Electro Beam Texturing)—electron beam in a vacuum shoots defined craters into the surface of the planishing rolls, Lasertex—laser beam shoots defined craters into the surface of the planishing rolls.
Furthermore, the Salzgitter AG company literature entitled “PRETEX®”—September 2002—discloses a textured fine sheet for the most stringent requirements in bodywork construction. This fine sheet can be uncoated or surface-coated and is used in the automotive industry in bodywork construction for inner and outer skin parts with the most stringent demands on deformability behaviour and lacquering capability. The surface topography of the fine sheet significantly influences the deformability behaviour and the adhesion and the visual properties of the lacquering of the car. A defined customised surface topography of the fine sheet is produced during skin pass rolling. In contrast to the above-described, erosive texturing methods, the planishing rolls used for this purpose are produced in a positive method by applying a texture to a smooth roll. In particular, a so-called TOPOCROM® method, by which the surface of a smooth roll is electrolytically structure-chromium plated and hard-chromium plated, is suitable for this purpose. To this end, the planishing roll is coated in a reactor which is equipped with an anode cage and filled with a chromium electrolyte. The chromium ions of the electrolyte are reduced during the coating procedure and metallically deposited on the surface of the planishing roll. This coating method provides an absolutely uniform, stochastic distribution of the differently sized hard-chromium hemispheres which are deposited on the roll surface and can be adjusted in terms of their size and number per unit area in a targeted and reproducible manner by means of the process computer-controlled coating parameters to suit customer requirements. The hard-chromium hemispheres of the planishing rolls are transferred to the fine sheet during planishing in the form of corresponding spherical caps. The surface topography of the fine sheet is adapted to absorb the lubricant and to assist hydrostatic or hydrodynamic lubrication in order to reduce friction. A relatively high mean roughness index and mutually isolated lubricating pockets which serve as a lubricant reservoir improve the friction and lubrication ratios considerably. An improvement in lacquering capability with a reduction in the lacquer system layer thicknesses at the same time is achieved by high peak numbers and low long-wave and short-wave structure proportions. Long-wave structure proportions can cause the orange peel in the topcoat of lacqured car sheets.
German laid-open document DE 10 2012 017 703 A1 already discloses a flat product consisting of a metal material, in particular consisting of a steel material, a use of such a flat product and a roll and a method for producing such flat products. This flat product is said to be characterised by very good tribological properties and, after being lacquered in a manner typical of cars, the appearance of the lacquer is said to be improved. However, the lacquering is said to be achievable with curtailed, filler-free lacquering processes—and a considerably reduced layer structure. A corresponding roll for producing such a flat product is said to have a surface structure which is characterised by a peak number RPc in the range of 80 to 180 1/cm, an arithmetic mean roughness Ra in the range of 2.5 to 3.5 μm and an arithmetic mean waviness Wsa in the range of 0.08 to 1.0 μm. In one embodiment, the smooth proportion of the roll in terms of a non-textured surface is approximately 25%. The profiling of the surface starting from a roll comprising a smoothly polished surface is effected by removal of material by means of a pulsed laser.
European patent application EP 0 606 473 A1 describes a roll comprising a laser-textured surface structure for planishing steel sheets. The surface structure of the roll has a multiplicity of protruding hemispheres consisting of chromium, of which 40 to 100% are transferred to the steel sheet during planishing. This produces in the steel sheet impressions having a diameter d of 50 to 500 μm and a height h of 2 to 40 μm. The mutual spacing between the individual impressions is between 1.0 d and 4.0 d.
The translation of European patent document DE 694 23 784 T2 also mentions a method for structuring the surface of a working roll by means of an electron beam. The structured surface consists of a two-dimensional, determined pattern of dots. Each dot is in the form of a crater comprising a predetermined edge. After structuring, the roll surface has a roughness Ra between 0.4 to 8.0 μm.
The patent U.S. Pat. No. 5,532,051 already discloses chromium plating of a surface-structured roll for the cold-rolling of steel materials. The chromium plating is said to considerably prolong the service life of the roll treated in this manner.
The object of the invention is to provide an improved planishing roll comprising a surface structure, an improved method for planishing a flat product consisting of a metal material, in particular consisting of a steel material, with a planishing roll and to an improved flat product which is produced according to this method and consists of a metal material, in particular a steel material.
In accordance with the invention, an improved planishing roll, in particular for producing flat products consisting of a metal material, in particular consisting of a steel material, comprising a surface structure is achieved by virtue of the fact that the surface structure has a material proportion of 2% at a depth of 0.2 μm to 9 μm, preferably at a depth of 0.8 μm to 5.5 μm, the depth is measured starting from a zero line in the direction of an axis of rotation of the planishing roll, the zero line extends in parallel with the axis of rotation of the planishing roll and the zero line is displaced starting from the surface of the planishing roll in the direction of the axis of rotation of the planishing roll to the extent until its material proportion is 0.1%. In qualitative terms, the surface structure of the planishing roll thus has a low material proportion so that, during planishing, the largest possible ratio is achieved between the change in topography of a flat product to be planished and the lengthening of the flat product.
The low material proportion can also be determined by virtue of the fact that the surface structure has a material proportion of 5% at a depth of 0.7 μm to 12 μm, preferably at a depth of 1.1 μm to 6.5 μm.
The material proportion is further determined by virtue of the fact that the surface structure has a material proportion of 10% at a depth of 1.0 μm to 15 μm, preferably at a depth of 1.4 μm to 7.4 μm.
In an advantageous manner, provision is made that the surface structure of the planishing roll is electrolytically structure-chromium plated and hard-chromium plated.
In a particularly advantageous manner, provision is made that the surface structure thereof has a roughness Ra=0.3-5 μm and a peak number RPc=50-300 1/cm.
In accordance with the invention, a method for planishing a flat product consisting of a metal material, in particular consisting of a steel material, is improved by virtue of the fact that the flat product is rolled with the planishing roll in accordance with the invention.
In an advantageous manner, the flat product is rolled with a degree of planishing in the range of 0.1 to 2.0%.
In accordance with the invention, an improved flat product consisting of a metal material, in particular consisting of a steel material, produced according to the method in accordance with the invention, is characterised in that the flat product has a planished surface topography with a roughness Ra=0.9-1.4 μm, preferably 0.9-1.2 μm, a peak number RPc>90 1/cm, preferably RPc>95/cm and a waviness of the surface described by the parameter Wsa (1-5) according to VDEh SEP1941 of less than 0.28 μm, preferably less than 0.25 μm, particularly preferably less than 0.22 μm.
In an advantageous manner, provision is made that the thickness of the flat product is in the range of 0.35-2.0 mm.
Preferably, the flat product can be provided with coats consisting of zinc or a zinc-aluminium alloy or a zinc-iron alloy or zinc-aluminium-magnesium alloy.
The flat products, in particular fine steel sheets, which are produced in conjunction with the invention, are used preferably in motor vehicles, domestic appliances—so-called white goods—and steel sheet furniture. In this case, the flat products can be electrolytically galvanised or hot-dip galvanised. The flat products can also be formed into components by e.g. deep-drawing and stretch-drawing. Preferably, the flat products are intended for subsequent lacquering and are used as lacquered visible parts. The flat products are particularly suitable for forming processes, in particular deep-drawing.
In conjunction with the present invention, flat products are understood to be sheets, in particular fine sheets, which are produced from metal, metal alloys, in particular steel.
The resistance provided by the flat product against the change in topography must be compensated for in the planishing process. The surface structure of the planishing rolls is more deeply impressed than the impression remaining in the surface of the flat product after the pressure is removed. By reason of this material-dependent resilience, surface structures of planishing rolls comprising a lower material proportion have a particularly positive effect during processing of flat products comprising a high deformation resistance.
Flat products comprising soft metallic coats also permit an improved transfer of the surface structure of the planishing roll than stronger uncoated flat products e.g. consisting of steel.
Flat products which are planished by means of a planishing roll in accordance with the invention are characterised by the following advantages:

- uniform, reproducible and defined roughness characteristic values over the entire length and width of the flat product by reason of the homogeneous roughness structure of the planishing roll,
- improved forming properties by reason of the excellent tribological behaviour during deep-drawing, established by finely distributed, mutually isolated hydrostatic lubricating pockets.
- excellent lacquering capability even during vertical application by reason of high peak numbers and the stochastically distributed roughness structure,
- environmentally sound coating methods of the rolls by reason of the closed reactor process, developed specifically for this intended use, without any accumulating flush water.

The invention will be explained in greater detail hereinafter with the aid of an exemplified embodiment and associated drawings. In the figures:

FIG. 1 shows a graph including a schematic exemplary progression of a surface structure of a roll in accordance with the invention,

FIG. 2 shows an illustration of a zero line selected for measuring the roll material proportion,

FIG. 3 shows a schematic enlarged sectional view of a surface structure of a roll in accordance with the invention with an associated material proportion graph,

FIG. 4 shows a sectional view according to FIG. 2 for a conventional roll with an associated material proportion graph, and

FIG. 5 shows a projection of a section through a flat product with PRETEX® topography in accordance with the invention.

FIG. 1 shows a Cartesian coordinate system with a depth in μm of approximately 15 to 0 μm as the y-axis and a material proportion of approximately 0 to approximately 10% as the x-axis. An exemplary curve of a surface structure of a planishing roll in accordance with the invention is indicated in the coordinate system and is designated by A.
The material proportion in % describes the presence of material of the planishing roll within a surface which is representative of the entire upper peripheral surface of the planishing roll. Therefore, a proportion of roughness valleys can be considered to be the opposite of the material proportion in %. The proportion of material in the surface is ascertained for this purpose by means of a 3D measurement starting from the surface in the direction of a central axis of rotation of the roll. The 3D measurement relates to a representative surface of approximately 2.5 mm². The material proportions in % ascertained by the 3D measurement then relate to a depth given in μm starting from the surface of the planishing roll. The material proportions can be determined using the roughness profile of the roll. The material proportions illustrated in FIG. 2 have been ascertained according to ISO 25178 from an extensive 3D roughness measurement on the corresponding rolls. To ensure that the ascertained material proportions are comparable, a reference plane must be defined for these measurements, from which the height or depth values are counted. In general, in order to produce material proportion curves a zero line displacement of 5% is used in this case, i.e. starting from the surface of the roll in the direction of the axis of rotation of the roll. Since the roll profiles already have apparent differences in the region of the first 5% of the material proportion, a zero line displacement of only 0.1% material proportion is used, i.e. the zero line for measuring or determining the depth in μm is displaced only until the material proportion is 0.1% of the roughness peaks of the surface present in this region. The depth in μm is then determined starting from the zero line in the direction of the axis of rotation of the roll. The depth indicated in FIG. 2 is also averaged over the surface part of the roll and is representative of the entire peripheral surface of the roll because the planishing of the roll is uniform.
FIG. 2 shows an illustration of the above-described displacement of the zero line N starting from the surface of the planishing roll 1 radially in the direction of an axis of rotation D of the planishing roll 1. In this case, the zero line N extends in parallel with the axis of rotation D and in the region of the surface of the planishing roll 1. The axis of rotation D extends in the direction of the longitudinal extension of the planishing roll 1 and centrally in the planishing roll 1. The zero line N is displaced into schematically indicated structure elements 2 of a surface structure of the planishing roll 1 and radially in the direction of the axis of rotation D until the desired material proportion of 0.1%, based on the structure elements 2, is achieved.
FIG. 1 shows that the exemplary progression A of a planishing roll in accordance with the invention starting from the depth of 0 μm, based on the 0.1% material proportion, extends very flatly and only has a material proportion of 10% in the region of approximately 3.5 μm. In qualitative terms, this can therefore be described as a planishing roll comprising a low material proportion at low depths. Furthermore, the Topocrom® method produces a topography of hard chromium hemispheres with steep flanks.
Surface structures of planishing rolls which have a specific progression in the depth direction in the region of the material proportions of 2% to 10% are particularly favourable. This progression can be described as a sequence' of selected percentage material proportions of the roll. A sequence of material porportions of 2%, 5% and 10% is selected hereinafter. Rolls, of which the material proportions and depths are in the following ranges enable production of an advantageous flat product topography:

material proportion 2%-depth 0.2 μm to 9 μm—preferably depth 0.8 μm to 5.5 μm
material proportion 5%-depth 0.7 μm to 12 μm—preferably depth 1.1 μm to 6.5 μm
material proportion 10%-depth 1.0 μm to 15 μm—preferably depth 1.4 μm to 7.4 μm

These ranges relating to the depths and material proportions are indicated in FIG. 1 as polygons. The polygon with a dotted line shows the aforementioned further depth ranges and the polygons with a broken line show the preferred depth ranges.
A metallic flat product, in particular a steel strip, comprising an advantageous flat product topography, can be produced with such planishing rolls with, an arbitrarily produced surface structure and the progression of material proportions described in relation to FIG. 1. For this purpose, a corresponding flat product is rolled in the longitudinal direction in a typical manner with planishing rolls which are deployed using external force, in particular hydraulic pressure. By virtue of the force transmitted linearly to the flat product volume located between the planishing rolls, the flat product is lengthened and thereby reduced in thickness and the surface structure of the planishing rolls is formed on the surface of the flat product. The material displacement follows the principle of least resistance, depicting the surface structure and lengthening the flat product. If the surface structure of the planishing rolls is completely filled, an increase in force acts upon the lengthening of the flat product. The maximum lengthening of the flat product is defined by the change in the mechanical characteristic values of the flat product.
The ideal surface structure of a planishing roll for producing a topography of the flat product in accordance with specifications is configured in such a way that it can be depicted in the surface of the flat product before the maximum permissible strip lengthening is achieved.
A planishing roll comprising a surface structure as shown in FIG. 1 approximates an idealised structure. This planishing roll allows a high transmission of a specific rolling force which corresponds to a force per length of the line contacting the flat product, a change in the topography of the flat product and a low transmission of the specific rolling force in favour of a larger lengthening of the flat product. A typical specific rolling force is in the region of 1.9 kN/mm. The change in topography can be described by the parameters of roughness Ra, peak number RPc or more generally by displaced volume. The lengthening of the flat product is expressed by the degree of planishing. Since the degree of planishing is generally defined by the change in the mechanical properties, the largest possible ratio between the change in topography and lengthening of the flat product is favourable in order to achieve a low waviness and small lengthening of the flat product. The waviness can be described by the Wsa (1-5) value according to SEP1941. In this case, a minimum degree of planishing is certainly required in order to achieve a desired change in topography before a desired degree of planishing is exceeded. The degree of planishing is approximately in the range of 0.1 to 2.0%.
A material proportion, which is low in accordance with the invention, on the planishing roll results in the contact force—i.e. high local pressures—being distributed over a small area—and thus results primarily in a change in topography instead of the lengthening of the flat product. By reason of the change in topography, a locally defined, lateral volume redistribution is effected on the surface of the flat product. The low material proportion of the planishing roll in accordance with the invention has a positive effect such that it hinders the volume redistribution to a lesser extent than planishing rolls comprising a higher material proportion. Accumulations of redistributed volumes at points on the surface of the flat product are avoided. The accumulations at points result in an undesirably high waviness Wsa (1-5) according to SEP1941.
The surface structures of planishing rolls comprising a low material proportion and the surface structure which are produced by methods which allow a surface structure with a high flank steepness are preferred. Such surface structures on planishing rolls can be achieved preferably by the hard-chromium plating method known as Topocrom®.
FIG. 3 schematically illustrates an enlarged sectional view of a surface structure of a roll in accordance with the invention with an associated material proportion graph. By way of example, the surface structure has elevations in the form of a rectangle, a parabola and a triangle. These elevations are separated by intermediate spaces which are defined in depth by a planar surface of the roll. Next to the schematic illustration, the graph known from FIG. 1 is illustrated in qualitative terms with the depth above the material proportion. It is apparent that, as known from the graph in FIG. 1, the material proportion of the elevations is low overall and, as the depth increases to a complete surface of the roll, increases initially only slightly and greatly in the region of maximum depth. During planishing of a flat product, a roll comprising this type of surface structure with a low material proportion over the entire height or entire depth of the elevations results preferably in a change in topography of the planished flat product prior to the lengthening of its strip. A use of the rolls in accordance with the invention results in a small waviness in the topography of the flat product and a small lengthening of the strip of the flat product.
FIG. 4 corresponds substantially to FIG. 3 but the surface structure is inverse to the surface structure shown in FIG. 3. Accordingly, the elevations—shown by way of example in FIG. 3—in the form of a rectangle, parabola and triangle placed upside down each form the intermediate spaces. The surface structure shown in FIG. 4 is found in conventional rolls. Also, next to the schematic illustration, the graph known from FIG. 1 is again illustrated in qualitative terms with the depth above the material proportion. In comparison with the curve progression in FIG. 3, it is apparent that the material proportion of the elevations is high overall and, as the depth increases to a complete surface of the roll, increases initially greatly and only slightly in the region of maximum depth. During planishing of a flat product, a roll comprising this type of surface structure with a high material proportion over the entire height or entire depth of the elevations results preferably in a lengthening of the strip of the planished flat product prior to its change in topography. Since the length of the strip increases to a greater extent, configuration of the topography of the flat product will be possible only to a limited extent.
The surface structures shown in FIGS. 3 and 4 differ significantly in terms of their material proportions and their progression over the depth of the elevations of the surface structures. However, these surface structures cannot be distinguished in terms of the roughness index Ra but, when used on planishing rolls for transferring the roughness from the roll to the flat product, provide different results in terms of the increase in waviness until the surface topography required by the customer and described by Ra and RPc of the sheet is achieved. The roughness index Ra is in the range of 0.9 to 1.4 μm and the peak number RPc is greater than 75 1/cm.
FIG. 5 shows a projection of a section through a flat product comprising an inventive PRETEX® topography which has been produced with a planishing roll comprising a material proportion which is low in accordance with the invention. The y-axis plots a height/depth in each case in μm based on a zero line and the x-axis plots a length of the section in μm. The inventive PRETEX® topography shows a pronounced sequence of elevations and—valley-shaped intermediate spaces and has the characteristic values Ra=1.04 μm, RPc=106 cm-1 and waviness Wsa (1-5)=0.208 μm. A high flank steepness at the edge of the spherical cap impression can also be seen. The material proportion of the associated planishing roll ascertained from a 3D roughness measurement is 2% at a depth of 2.0 μm, 5% at a depth of 2.8 μm and 10% at a depth of 4.3 μm. This corresponds approximately to the progression indicated in FIG. 1 as A.
By using an inventive planishing roll comprising a low material proportion, it is possible to achieve a sheet surface which with a low roughness (0.9 μm<Ra<1.2 μm) and high peak number (RPc>95/cm) has a low waviness Wsa (1-5) of less than 0.22 μm. The material proportion of the sheet surface ascertained via a 3D roughness measurement is 2% at a depth of 1.5 μm, 5% at a depth of 2.3 μm and 10% at a depth of 2.8 μm.

LIST OF REFERENCE SIGNS

1 planishing roll
2 structure element
A example progression
D axis of rotation
N zero line
V direction of the zero line displacement

Claims

What is claimed is:

1.-10. (canceled)

11. A planishing roll comprising a surface structure, in particular for producing a flat product of a metal material, in particular a steel material, said surface structure having a material proportion of 2% at a depth of 0.2 μm to 9 μm, with the depth being measured starting from a zero line in a direction of an axis of rotation of the planishing roll, said zero line extending in parallel relation to the axis of rotation of the planishing roll and being displaced starting from the surface of the planishing roll in the direction of the axis of rotation of the planishing roll until the material proportion is 0.1%.

12. The planishing roll of claim 11, wherein the depth is 0.8 μm to 5.5 μm.

13. The planishing roll of claim 11, wherein the surface structure has a material proportion of 5% at a depth of 0.7 μm to 12 μm, preferably at a depth of 1.1 μm to 6.5 μm.

14. The planishing roll of claim 11, wherein the surface structure has a material proportion of 10% at a depth of 1.0 μm to 15 μm, preferably at a depth of 1.4 μm to 7.4 μm.

15. The planishing roll of claim 11, wherein the surface structure is electrolytically structure-chromium plated and hard-chromium plated.

16. The planishing roll of claim 11, wherein the surface structure has a roughness Ra=0.3-5 μm and a peak number RPc=50-300 1/cm.

17. A method for planishing a flat product of a metal material, in particular a steel material, comprising:

rolling the flat product with a planishing roll having a material proportion of 2% at a depth of 0.2 μm to 9 μm, with the depth being measured starting from a zero line in a direction of an axis of rotation of the planishing roll, said zero line extending in parallel relation to the axis of rotation of the planishing roll and being displaced starting from the surface of the planishing roll in the direction of the axis of rotation of the planishing roll until the material proportion is 0.1%.

18. The method of claim 17, wherein the depth is 0.8 μm to 5.5 μm.

19. The method of claim 17, wherein the surface structure has a material proportion of 5% at a depth of 0.7 μm to 12 μm, preferably at a depth of 1.1 μm to 6.5 μm.

20. The method of claim 17, wherein the surface structure has a material proportion of 10% at a depth of 1.0 μm to 15 μm, preferably at a depth of 1.4 μm to 7.4 μm.

21. The method of claim 17, further comprising electrolytically structure-chromium plating and hard-chromium plating the surface structure.

22. The method of claim 17, wherein the surface structure has a roughness Ra=0.3-5 μm and a peak number RPc=50-300 1/cm.

23. The method of claim 17, wherein the flat product is rolled with a degree of planishing in a range of 0.1 to 2.0%.

24. A flat product of a metal material, in particular a steel material, said flat product including a planished surface topography with a roughness Ra=0.9-1.4 μm, preferably 0.9-1.2 μm, a peak number RPc>90 1/cm, preferably RPc>95 1/cm, and a waviness of the surface described by a waviness parameter Wsa (1-5) according to VDEh SEP1941 of less than 0.28 μm, preferably less than 0.25 μm, particularly preferably less than 0.22 μm.

25. The flat product of claim 24, wherein the flat product has a thickness in a range of 0.35-2.0 mm.

26. The flat product of claim 24, further comprising a coat of a material selected from the group consisting of zinc, a zinc-aluminium alloy, a zinc-iron alloy, and a zinc-aluminium-magnesium alloy.