EP2006037B2 - Flat product made of a metallic substance, in particular a steel substance, use of such a flat product and roller and method for manufacturing such a flat product - Google Patents

Abstract

The flat product is made from metal, particularly steel. The frequency distribution of the high values has two distinctive maxima, which stands for distinctive highest and lowest levels of the surface. The frequency of the highest point is larger than the frequency of the lowest point. The distance between the main maxima of the frequency distribution of the high values is amount from 1 micro meter to 5 micro meters. The 99.99 percent of the topography measuring points has a minimum distance from edge of the lowest point and the highest point. Independent claims are also included for the following: (1) roller for manufacturing flat product (2) a method for manufacturing a flat product.

Description

The invention relates to a flat product made of a metal material, in particular a steel material, an advantageous use and a roller that is particularly suitable for producing such a flat product, and a method for producing such flat products. “Flat products” are understood to mean sheet metal made from a metal or a metal alloy, in particular fine sheet metal, or similarly constructed strips and other rolled products.

Components are produced from flat products of the type in question, which are then coated with one or more layers of lacquer in order to protect them against possible corrosion on the one hand and to optimize their visual appearance on the other. The quality of the visual appearance is judged, among other things, by the extent to which the surface structure of the respective metal substrate affects the surface of the paint coating.

Particularly high demands are placed on the appearance of the surfaces of automobile body parts that are visible from the outside.

In practice, the requirements placed on the paint coating of bodywork components are met by applying multi-layer paint systems. These paint systems generally include at least one so-called "filler layer", the task of which is, among other things, to level out any unevenness on the surface to be coated.

The effort involved in applying multi-layer paint systems to a metal sheet is considerable. New painting processes achieve process cost savings by eliminating the need for filler painting. These processes are increasingly used in the automotive industry. The overall layer thickness of the paint structure is significantly reduced, so that the sheet metal substrate can be seen in the top coat if the sheets are not of sufficient quality.

Another criterion when assessing the suitability of a metal flat product for the manufacture of bodywork components is its behavior during forming into the respective component. This is also decisively influenced by the surface structure of the respective flat product. For example, during deep-drawing, the indentations present on the surface of a metal sheet form pockets in which a lubricant applied to the sheet or given in the respective mold before forming can collect. The carrying capacity of the lubricating film formed by the respective lubricant depends directly on the design and distribution of these depressions.

Various attempts are known to structure the surface of metal sheets in such a way that they have an optimized appearance after painting. Examples of these attempts are in JP-A 63-50488 and the JP-A 1-293907 specified.

The regular surface structures explained in these two publications of Japanese patent applications are distinguished by cylindrical, stamp-like elevations which are surrounded in a circle by a groove-like indentation and protrude from an otherwise flat base area.

According to the JP-A 63-50488 the plateaus of the elevations should be about 2 - 10 µm above the bottoms of the valley areas between the elevations. At the same time, the combined proportion of the flat plateaus of the mountains and the flat areas of the mid-flat areas existing between the bottoms of the valleys and the mountain plateaus should be 20 - 90% of the total area.

In the JP-A 1-293907 it is also required that the proportion of flat areas between the regularly arranged and circular elevations should cover at least 85% of the sheet surface, that the depth of the valleys surrounding the elevations should be at least 4 µm starting from the flat areas and a frequency analysis of the steel sheet surface geometry, the intensity of the wavelength components of the wavelengths λ, which are in a range of 585 µm ≤ λ < 2730 µm, is a maximum of 0.6 µm ² .

The metal sheets produced according to the two Japanese patent applications are said to leave an extremely lively impression when painted. However, the requirements specified for this presuppose strictly deterministic surface structures. Especially the high ones, but after the JP-A 1-293907 permissible intensities in the wavelength components mentioned there only occur in the case of deterministic structures with strong periodicity.

In practice, however, it has been found that regular surface structures formed according to the prior art described above can only be produced with the required reliability with difficulty. This applies in particular when the substrate to be processed is galvanized sheet steel.

Against this background, the invention was based on the object of creating a flat product that offers optimized prerequisites for a paint coating that has an excellent appearance even with relatively low paint layer thicknesses in the finished painted state. In addition, a preferred use of such a flat product, a roller that is particularly suitable for producing such a flat product, and a method for producing such a flat product should be specified.

With regard to the flat product, this object has been achieved according to the invention by the teaching of claim 1.

Due to their special property profile flat products according to the invention can be used particularly for the production of components that should be provided with a layer of varnish. This applies in particular when the products according to the invention are made of steel and are provided in particular with an anti-corrosion layer, for example a galvanizing layer. Such a steel sheet can be coated, for example, with a zinc or a zinc-magnesium coating. However, the criteria specified according to the invention can also be used for flat products made from other metals.

In particular, flat products according to the invention are suitable for the production of bodywork components. After they have been shaped, they can also be provided with a coat of paint using shortened painting processes, which meets the highest requirements for their external appearance on the respective component. It is particularly noteworthy that the surface structure of such a component specified according to the invention is so delicate that optically and technically flawless coating results can be achieved even with a layer structure of the coating that is greatly simplified compared to the prior art.

With regard to the roller which is particularly suitable for the production of a flat product according to the invention, the object specified above is achieved according to the invention in claim 6 .

Finally, the invention in claim 7 provides a method that allows the reliable production of metallic flat products that can be formed in a simplified manner and coated with excellent results.

The invention is based on the finding that, taking into account the criteria specified according to the invention, a metal flat product with such a delicate, stochastic surface structure can be made available in a planned manner that it is only slightly visually perceptible, if at all, after a typical automotive paint application.

At the same time, with a surface topography designed according to the invention, the change between the mountain plateaus and the valleys takes place via steep flanks. In this way it is achieved that the morphology of the sheet surface is practically independent of the actual depth of the valleys. As a result, the morphology of the sheet surface of a metal flat product according to the invention is also independent of the skin-pass degree that is used when producing the sheet texture by skin-pass rolling.

Because the valleys are present with a defined depth in the surface of a flat product according to the invention, the "empty volume" of the surface topography can be specifically estimated. From this estimation it can then be determined with high accuracy what minimum quantity of lubricant is required in practice in order to be able to form a flat product designed according to the invention with minimized forming forces and optimal preservation of the surface structure.

1. 1. Die Oberflächentopografie wird mittels eines Messsystems mit ausreichender Ortsauflösung bei einer Grundfläche von mindestens 0,8 x 0,8 mm² vermessen.
   Als geeignet für diesen Zweck haben sich Messverfahren zur Vermessung der Rauheitstopografie erwiesen, die eine Ortsauflösung ≤ 1,5 µm (lateral) und ≤ 0,05 µm (vertikal) besitzen.
2. 2. Durch geeignete mathematische Verfahren werden in an sich bekannter Weise erforderlichenfalls mögliche Schieflagen in der Topografie ausgeglichen. Ein nachträgliches Nivellieren der gemessenen Topografie (Kippen bzw. Ausrichten der gesamten Topografie), kann für die Auswertung notwendig sein, damit die Berg- bzw. Tal-Bereiche für die Auswertung möglichst auf einem Niveau liegen.
3. 3. Hochfrequenzanteile der Oberflächentopographie werden mittels Gauß'schem Tiefpassfilter (λs = 10µm) beseitigt.
4. 4. Es wird die Häufigkeitsverteilung der Höhenwerte mit einer Klassenbreite von 0,1 µm (nachfolgend kurz "Höhenverteilung" genannt) ermittelt.

In order to determine the flat products covered by the invention, the surface of the flat product under consideration is examined according to the following stipulations and the surface topography recorded is evaluated:

1. 1. The surface topography is measured using a measuring system with sufficient spatial resolution and a base area of at least 0.8 x 0.8 mm ² .
   Measurement methods for measuring the roughness topography that have a spatial resolution of ≤ 1.5 µm (lateral) and ≤ 0.05 µm (vertical) have proven to be suitable for this purpose.
2. 2. If necessary, possible imbalances in the topography are compensated by suitable mathematical methods in a manner known per se. Subsequent leveling of the measured topography (tilting or aligning the entire topography) may be necessary for the evaluation so that the mountain and valley areas are as level as possible for the evaluation.
3. 3. High-frequency components of the surface topography are eliminated using a Gaussian low-pass filter (λs = 10 µm).
4. 4. The frequency distribution of the height values with a class width of 0.1 µm (hereinafter referred to as "height distribution" for short) is determined.

1. a) Die Oberfläche besitzt besonders ausgeprägte Berg- und Tal-Niveaus und hat somit eine mindestens zweigipfelige Höhenverteilung.
2. b) Betrachtet man allein die Topografiebereiche mit geringer Neigung (Neigung ≤ 5°, d.h. ohne "Hanganteile"), so zerfällt die Höhenverteilung in mindestens zwei lokale "Hauptmaxima". Diese Hauptmaxima sind näherungsweise normal verteilt mit einer Standardabweichung (Breite) 2 · σ ≤ 2 µm für die Berge und einer Standardabweichung (Breite) 2· σ ≤ 1 µm für die Täler. Die Neigung der Flanken wird dabei wie folgt ermittelt: α = tan^-1 (| grad ( z(x, y)) |) mit z(x,y) = Höhen-/Messwerte)
3. c) Die Fläche des oberen Hauptmaximums ist bezüglich der Höhenverteilung am größten (d. h. Berge sind häufiger als Täler).
4. d) Der Abstand zwischen dem ausgeprägten Berg-Niveau und den Tal-Niveaus der Walzenoberfläche ist größer als der Abstand zwischen Berg- und Talniveau auf der erzeugten Flachproduktoberfläche.
5. e) In einer Ebene, die genau mittig zwischen Berg- und Tal-Niveau gelegen ist, beträgt die halbe Breite der Täler bzw. Berge höchstens 100 µm.

The surface topography of flat products according to the invention recorded and processed in this way then meets the following criteria:

1. a) The surface has particularly pronounced mountain and valley levels and thus has at least two peaks in height distribution.
2. b) If one considers only the topographical areas with a low inclination (inclination ≤ 5°, ie without "slope portions"), the height distribution breaks down into at least two local "main maxima". These main maxima are approximately normally distributed with a standard deviation (width) 2 · σ ≤ 2 µm for the peaks and a standard deviation (width) 2 · σ ≤ 1 µm for the valleys. The inclination of the flanks is determined as follows: α = tan ^-1 (| grad ( z(x, y)) |) with z(x,y) = height/measured values)
3. c) The area of the upper main maximum is largest with respect to the height distribution (ie mountains are more frequent than valleys).
4. d) The distance between the salient mountain level and the valley levels of the roll surface is greater than the distance between peak and valley levels on the flat product surface being produced.
5. e) In a plane that is located exactly in the middle between the peak and valley level, half the width of the valleys or peaks is at most 100 µm.

Extensive tests have confirmed that flat products produced from a steel material and designed according to the invention not only have excellent suitability for painting, but can also be shaped particularly well. Roughness topographies can be adjusted in a targeted manner so that the empty volumes correspond to the amount of lubricant available. This has a positive effect on the leveling process during forming (hydrodynamic lubrication). The surface quality is uniform and optimized in such a way that a paint layer system applied to it leaves a visual impression that meets the strictest requirements, even if this paint system does not use a complex filler coating to compensate for unevenness in the surface.

1. a) Die Häufigkeitsverteilung der Höhenwerte weist zwei ausgeprägte Maxima auf, die für entsprechend ausgeprägte Berg- und Tal-Niveaus der Oberfläche stehen.
2. b) Bei Betrachtung allein derjenigen Topografiebereiche, die eine Neigung von maximal 5° gegenüber der Senkrechten aufweisen, zerfällt die Häufigkeitsverteilung der Höhenwerte in mindestens zwei lokale Hauptmaxima. Die lokalen Hauptmaxima sind für die Täler mit einer Standardabweichung (Breite) 2 σ ≤ 10 µm und für die Berge mit einer Standardabweichung (Breite) 2 σ ≤ 1 µm näherungsweise normal verteilt.
3. c) Die Häufigkeit der Täler ist auf der Walzenoberfläche größer als die Häufigkeit der Berge.
4. d) Das die Täler repräsentierende Hauptmaximum ist gleichzeitig auch ein absolutes Maximum.
5. e) Der Abstand zwischen dem ausgeprägten Berg-Niveau und den Tal-Niveaus der Walzenoberfläche ist größer als der Abstand zwischen Berg- und Talniveau auf der erzeugten Flachproduktoberfläche.
6. f) In einer Ebene, die genau mittig zwischen Berg- und Tal-Niveau liegt, beträgt die halbe Breite der Täler bzw. Berge höchstens 100 µm, wobei mindestens 99,99 % der Topografie-Messpunkte einen minimalen Abstand zum Rand der Täler bzw. Berge besitzen, der den genannten Grenzwert erfüllt.

In order to produce a flat product according to the invention, a roller with a surface structure is provided according to the invention which represents a negative image of the topography to be produced on the flat product according to the invention. Under the measurement and evaluation conditions mentioned above for the detection and evaluation of the surface of the flat product according to the invention, the following applies to the roll surface:

1. a) The frequency distribution of the height values shows two pronounced maxima, which stand for correspondingly pronounced mountain and valley levels of the surface.
2. b) When considering only those topographical areas that have a maximum inclination of 5° to the vertical, the frequency distribution of the height values breaks down into at least two local main maxima. The local main maxima are approximately normally distributed for the valleys with a standard deviation (width) 2 σ ≤ 10 µm and for the mountains with a standard deviation (width) 2 σ ≤ 1 µm.
3. c) The frequency of valleys on the roller surface is greater than the frequency of mountains.
4. d) The main maximum representing the valleys is also an absolute maximum.
5. e) The distance between the pronounced peak and valley levels of the roll surface is greater than the spacing between peak and valley levels on the produced flat product surface.
6. f) In a plane that lies exactly in the middle between the peak and valley level, the half-width of the valleys or peaks is at most 100 µm, with at least 99.99% of the topography measuring points being at a minimum distance from the edge of the valleys or peaks. Own mountains that meet the specified limit.

A roller with such a condition of its roller surface coming into contact with the respective flat product to be processed can be produced by forming a basic structure into the roller surface by means of a suitable texturing process known from practice.

One possible method of specifically adjusting the roughness of the temper rolls is texturing by means of spark erosion (Electro Discharge Texturing, EDT).

The initial state before texturing the roller should be a smoothly ground roller surface. Indentations that are as close as possible to each other are introduced into this surface by spark erosion. The "bars" remaining between the indentations already have the desired same height due to the flat initial state.

In the course of the EDT process, a defined voltage is briefly applied between the electrode and the roller, if necessary periodically. Charge carriers (ions) are accelerated out of an electrolyte towards the roll surface through the spark erosion channel. When they hit the roll surface, they loosen roll material and create an indentation. Typical diameters of the depressions are approximately 80 μm. The loosened and melted roll material is transported away via the electrode rinsing and is not able to reconnect to the roll surface due to the dielectric oil.

In practice, however, it cannot be completely avoided that melted roller material accumulates again on the originally smooth ground surface during the texturing process. This material can be removed in a manner that is also known per se by subjecting the textured roll surface to targeted machining, during which the peaks of the surface texture previously produced on the roll are removed to an exactly predetermined extent. In practice, such material removal can be accomplished, for example, by means of a fine grinding process.

The EDT process is particularly advantageous, since repeated texturing of areas that have already been textured is almost impossible. The spark discharge most likely only takes place where the distance between the roller surface (usually the elevation) and the electrode is smallest and the electric field is therefore strongest and densest. In places where spark discharge has formed a depression further spark discharge is unlikely. This enables a high density of spark discharges and a correspondingly delicate roller surface texture. The indentations are often "shot in with an overlap". If the surface is completely covered, there are now ridges at different heights.

Due to the different bar heights, the textured skin-pass roll surfaces are subsequently ground down using a "SuperFinish" belt, referred to as SF for short. This method is the subject of the German patent application 10 2004 013 031 , one under the number EP 1 584 396 A2 published European patent application and a US patent application, serial no. 10/082,214 had received.

Strip superfinishing is the current technology for optimizing the finishing of roll surfaces. Thanks to the infinitely adjustable supply of constantly new abrasives, an even and seamless finish is created over the entire surface. Only the highest points of the roller base material are ground off. The top web heights are then at an almost uniform level.

Furthermore, the SuperFinish can be used to create steep slope angles.

In this context, it has been found to be particularly favorable with regard to the invention that, as in 1 shown schematically using a section of a section through a roll surface designed according to the invention, steep transitions U between the plateaus P of the "mountains" B and the sols O of the "valleys" T can be achieved by the targeted removal of material following the texturing, in particular by means of superfinishing belt grinding can. As already explained above, the steep slope angles β of the transitions U produced in this way have a significant influence on the properties of the surfaces of flat products according to the invention. The subsequent removal of the peaks S of the surface texture obtained after the texturing step ensures that the spatial distribution of the depressions in the subsequent thin sheet surface is practically independent of the skin-pass degree used and the distance between peak and valley levels. Steep slope angles are an essential part of the surface according to the invention, so that the spatial distribution of the depressions in the subsequent thin sheet surface is practically independent of the degree of skin pass used and the distance between peak and valley level.

The well-known in the EP 1 584 396 A2 (Belt superfinishing) has proven to be particularly advantageous in relation to the invention.

In the method according to the invention for producing a flat product according to the invention, a flat product consisting of a metal material is first made available in which at least the surface to be provided with the surface topography according to the invention has an arithmetic mean roughness of max. 1.5 μm. This flat product is then subjected to temper rolling, in which a roll designed according to claim 4 acts on the respective surface, so that a flat product is obtained whose surface topography satisfies the requirements of the invention.

It is essential that the indentations, which are introduced into the surface of the thin sheet by the tips of the roll surface during skin-passing, are as level as possible in order to reliably achieve the two-peak height distribution of the surface topography of the flat product prescribed according to the invention.

With regard to the suitability of a flat product according to the invention for forming, it proves to be particularly favorable if the surface of a flat product according to the invention is designed in such a way that, in a horizontal section through the topography with a maximum material area proportion of 80%, the empty volume below the cutting plane per measuring area is less than 0 .15 ml/m ² . At the same time, the material volume per measurement area should be less than 0.15 ml/m ² for a horizontal section through the topography with at least 20% material area above the section plane. Furthermore, it is advantageous in this connection if the void volume enclosed below a horizontal sectional plane with 20% material area proportion is at least 0.8 ml/m ² . Practical investigations on galvanized steel sheets of this type according to the invention have shown that with this distribution of the empty volume of the indentations made in the respective sheet, there is always a sufficient oil volume available for problem-free forming in a deep-drawing tool. With this configuration of the surface structure, it can thus be ensured that an oil layer of at least 0.7 g/m ² is present in the pockets formed by the depressions of a surface structure according to the invention.

The following principles apply to the metrological detection and evaluation of a surface topography according to the invention:

In the case of deterministic surface structures, simple geometric information is usually sufficient to describe the essential structures with sufficient information content. Stochastic surface structures such as those according to the invention naturally elude such an approach because the shape, width, height and arrangement of stochastic structures are not directly defined. Rather, for a comprehensive mathematical description of deterministic to stochastic surface topographies, methods of statistics or statistical image processing must be used.

a) Frequency distribution of the height values ("height distribution")

A common feature in the statistical description of surface topographies is the frequency distribution of their measured or mathematically generated height values, in short: height distribution. Another common designation for the "frequency distribution of height values" is the amplitude density curve (see DIN EN ISO 4287)

The height distribution ( Figure 2b ) indicates the frequency with which a certain height value can be found in the surface topography. It is obtained by forming the difference ("derivation") from the material content curve, also known as the Abbott-Firestone curve (DIN EN ISO 4287) ( Figure 2a ).

To determine the height resolution, the height scale is divided into discrete areas (so-called "classes"). The class width should be selected so finely that the height distribution can be reproduced with sufficient resolution. In order to be able to determine only the "main" maxima or minima in a height distribution, a correspondingly only rough class width of z. B. 0.2 microns advantageous. Because of this, negligibly small local maxima and minima are suppressed. In order to then be able to calculate the width of these essential maxima and minima as well as their exact position, a fine resolution (e.g. 0.1 µm) is advantageous, which should be three times smaller than the half-width of the maxima or minima (Nyquist -Theorem).

Various information about a surface topography initially remains hidden in a height distribution. This should be explained using the following example:

Only for simple geometric objects is it possible to directly read the slope of the "slopes" in the area of the transition from a "mountain" to a "valley" of the surface structure (or calculation of the flank angle) from the height distribution. To describe complex surface topographies, it is therefore useful to distinguish in which neighborhood a topography point is located and to classify the height value for the frequency distribution accordingly. A significant feature is the inclination of the topography in the vicinity of the peak ( Figure 2c ).

An additional distinguishing criterion is the curvature of the surface topography, in that it also separates local maximum ("mountains"), saddle (turning points) and minimum ("valleys") portions from each other (in the Figures 2a - 2c however, this is not shown). By differentiating the height values according to the slope, it is possible to show in the height distribution whether e.g. B. Mountain and valley parts (with an inclination ≤ 5°) are each on the same level or not.

In metrological practice, there is always a certain "uncertainty" in the height values. In particular, this blurriness can also be caused by a misalignment in the topography. In order to be able to derive meaningful information about the topography from the height distribution, it is therefore important to minimize possible tilting by aligning the overall topography beforehand. The inaccuracy in determining the peak and valley levels can be approximately described by a normal distribution. For the surface topography according to the invention, the standard deviation σ of the corresponding normal distribution should not exceed an upper limit ( 3 ).

In 3 a line profile is shown as an example with its corresponding height distribution (at a small angle of inclination). The distance between the two local maxima in the height distribution is labeled "T". Accordingly, "T/2" is half the distance.

b) distribution in the area

The areal distribution of the topographic components, such as mountains or valleys, can be described using a contour section. By means of a threshold operation, a distinction is made as to whether a measuring point "z" is above or below a certain level (threshold z _h ). A binary pattern ("light", "dark") then arises accordingly, as in 4 shown. Height levels commonly used in practice are the arithmetic mean, the median (median average, height values are in equal proportions above and below the threshold) and half the maximum or minimum values. The latter are used to determine so-called half-widths (FWHM = Full Width at Half Maximum/Minimum).

The contour line results directly from the edges of the light-dark pattern. This means that more delicate surface structures have large contour lengths. This characteristic value is similar to the peak number RPc according to DIN EN 10049, which, however, uses two threshold value operations (two height levels at a distance | C _s | = 0.5 µm from the arithmetic mean value). However, both parameters do not provide complete information about the arrangement and size of the light-dark pattern.

A sheet metal flat product according to the invention is distinguished by a characteristic height distribution with two distinct maxima, which are also referred to here as peak and valley levels. An excellent cutting level is the middle level between the peak and valley levels.

A simple operation to determine the "half width" of the mountains or valleys (HWHM = Half Width at Half Maximum) is to calculate the minimum distance r _min to the nearest edge (contour line) ( Figure 4a ). The distance to the contour line r _min is defined as negative here if it is determined in areas below the threshold value ("dark pattern", valley area). became. This enables a simultaneous display and evaluation of all r _min values ( Figure 4b ).

In the case of stochastic surfaces of the type according to the invention, it does not make sense to set the permissible upper and lower limits for r _min absolutely because of the statistical fluctuations that are present. Instead, it makes more sense to consider the frequency distribution of r _min ( figure 5 ).

The frequency distribution of r _min can be found here ( figure 5 ) can be approximately described by an asymmetric normal distribution. This means that the standard deviations σ ₁ and σ ₂ are different "to the right" and "to the left" of the maximum (most common value, also called "mode"). The most frequent value of the accumulation distribution does not necessarily have to coincide with the ordinate.

The distance of the mode from the ordinate is denoted here as "m". 3σ ₁ - m and 3σ ₂ + m are good measures for the left and right limits of r _min in the frequency distribution. This means that more than 99.99% of the calculated r _min values (with asymmetric normal distribution) are within these limits.

In the Figures 6 to 9 are typical examples of the "height distributions" ( figures 6 , 7 ), "Distributions of height values in the area" ( Figures 8a (elevation plot),8b (line profile)) and an example of a typical distance mapping ( 9 ) played back.

Each of the in the Figures 6 - 9 reproduced measurement and evaluation results were determined on steel sheet samples that have been subjected to skin-pass rolling with a roll whose corresponding surface structure in the above-described, from the EP 1 584 396 A2 has been produced in a known manner by an electroerosion process (“EDT” for short) with a subsequent fine grinding process. The degree of skin pass at the in 6 example shown was 0.6%, while in the Figures 7 to 9 examples shown was 0.9% in each case.

Figure 8a shows the recorded surface in a height representation, whereas Figure 8b represents the line profile corresponding to this representation.

The effects of a composition according to the invention on the forming behavior and the appearance after painting are explained in detail below:

The surface finish of a flat product according to the invention is characterized by indentations that are distributed very evenly and finely and have a clearly defined maximum depth in a surface that is otherwise as smooth as possible. These indentations serve as a lubricant reservoir during the forming of a metal sheet according to the invention into a component during tribological contact between the tool and the metal sheet. Particularly deep crater structures, which would only show an effect with a correspondingly strong leveling of the surface, are avoided in a flat product according to the invention, since they would only form superfluous lubricant sinks.

When it comes to painting, too, deep and wide craters in sheet metal can only be leveled out with a great deal of effort using a multi-layer paint system. In contrast, the indentations introduced into a thin sheet surface according to the invention are almost completely on one level and reduce long-wave structures that already exist in advance, such as those found in e.g. B. can be caused by a metallic coating, drastically.

When forming thin sheets into components, defined friction conditions in the forming tool are essential. The lowest possible friction and thus an unimpeded flow of material are required in critical areas such as the edges of dies or punches, since high surface pressures and high relative speeds between the tool and sheet metal surfaces can usually occur here at the same time. A reduction in friction at these points allows in particular higher process speeds and better utilization of production capacities.

In contrast to this, high friction is required in those areas in which hardly any material flow or thinning of the material is desired (e.g. deep drawing under the punch).

Possibilities for adjusting these tribological states are offered by an appropriate selection of the material pairing (such as coating of forming tools), lubricant and the process parameters (such as hold-down forces).

In the past, attempts were made to set the process window as precisely as possible by setting the narrowest possible limits for the production of the sheet metal. Characteristic values for characterizing the sheet metal surface were in particular the arithmetic mean roughness Ra and the peak number RPc (see ISO EN 10049). In most cases, fine sheet metal surfaces with a high roughness Ra were required in order to achieve the best possible forming results.

Practical experience has shown, however, that surfaces can behave very differently despite similar characteristic values Ra and RPc. Subsequent adjustment of the process parameters (such as oiling) to production-related fluctuations in the roughness of the flat product is therefore rarely used in practice.

Due to their clearly defined topography of the flat product and morphology, flat products with a surface quality according to the invention now allow forming processes to be adjusted in a more targeted manner.

A comparison of the ACTUAL and TARGET surface topographies of the flat product can be used to optimally set the process parameters. Critical formed parts in particular can be produced longer and with a lower failure rate.

The structural elements of the roughness fine shape act in particular as a reservoir for the lubricant (empty volume, 10 ) and thus enable its storage and distribution during the transformation. Due to the tool contact (surface pressure locally >300 MPa in some cases), the sheet metal surface topography is leveled during the forming process. This reduces the original void volume ( 10 ). Thus, the lubricant trapped in the topography is either compressed or displaced, and hydrostatic or hydrodynamic lubrication of the contact surface then occurs.

The problem is when the void volume is not filled with sufficient lubricant. Then the effect turns negative. The lubricant is pushed out of the contact areas between the tool and the thin sheet into the valleys that are not yet sufficiently filled. Under strong tribological stress, the lubricant film then tears and forming failure occurs due to dry friction or cold welding (zinc abrasion on sheet metal in the press shop). 11 shows a typical forming behavior (stick-slip) with insufficient oil coating.

Depending on the tool geometry (areas with high and low surface pressure) and the tribological stress (such as relative speeds), both open and closed empty spaces must be sufficiently filled with lubricant.

Many years of experience have shown that a lack of lubricant is one of the most common causes of forming problems. From this practical experience, the finding on which the invention is based is based, that the valleys of the surface structure according to the invention should have a depth that is as uniform as possible (and also smaller). The surface, on the other hand, should be more load-bearing. Furthermore, the empty volume provided for the lubricants should be limited.

In the past, the quality of a paint finish was judged solely on the basis of subjective standards. Later, painted boundary pattern panels were used to characterize different paint finishes.

For some years now, however, the wavescan DOI measuring device from Byk-Gardner has established itself as the "appearance standard" that is used by all European and almost all automobile manufacturers worldwide for the characterization and qualitative assessment of series paint finishes. The wavescan-DOI device measures e.g. the following values:

DOI ("DOI" = Distinction of Image, which means the sharpness of an image reflected by the paint), Shortwave (SW) and Longwave (LW) as well as the waviness parameters du, Wa, Wb, Wc, Wd and We.

For DOI, the higher the determined value, the better the quality of the painted surface. For all other values, on the other hand, the lower the better.

The appearance of a paint consists of brilliance, DOI and waviness. The latter can present itself as so-called "orange peel" which is seen when looking at the paint surface itself.

Shortwave structures are best noticed at a distance of 40 cm. These structures (fine-grained, crinkled) are recorded with a shortwave (SW) parameter. 40 cm corresponds approximately to the distance between the eyes when washing a car by hand.

Long-wave structures, on the other hand, can best be recognized at a distance of 3 m. These structures (orange peel skin, long wave) are recorded with the Longwave (LW) parameter. The distance of 3 m corresponds to the view in the exhibition room (showroom distance).

The wavescan-DOI device records an optical profile of the surface with a laser and a sensor. This is separated into wavelength ranges by mathematical filters. State of the art is the division into six waviness parameters: du (< 0.1 mm, "dullness"), Wa (0.1-0.3 mm), Wb (0.3-1 mm), Wc (1-3 mm), Wd (3-10 mm) and We (10-30 mm).

The measuring range extends from 0 (smooth) to 100 (strong structure). The determined values are dimensionless.

The measured values are plotted against the wavelength ranges, which results in a structure spectrum, as is shown for a high-quality surface in 12 is shown.

The invention is based on the finding that the quality of the painted surface can be positively influenced by a targeted setting of the surface structure. Structures of <0.1 mm (du) produce a lower contrast of the painted surface due to light refraction. Structures from 0.1 to 1 mm (Wa, Wb) lead to disruption of the outlines of an image reflected in the paint.

An automotive finish that satisfies the usual requirements has a DOI value of at least 85. For very good finishes, the DOI value is in the range of 90-95. In the case of qualitatively good painting of a metal sheet according to the invention, this range can also be reached if a greatly reduced paint layer thickness (fillerless process) is set compared to the prior art. For example, DOI values of at least 94 were achieved for sheet metal coated according to the invention without a filler coating being required for this.

Good-quality coatings have SW values (short ripple) of < 25 when coated horizontally. Their LW values (long waves) are < 8 when painted horizontally.

The gloss of an automobile paint finish is measured at an angle of 20° to the normal to the surface and, almost independently of DOI and waviness parameters, achieves the same high values for good and bad paint finishes. The gloss mainly depends on the paint system and the paint process conditions and does not allow any conclusions to be drawn about a good or bad paint finish.

• Kein Welligkeitsmesswert größer als 30.
• Zielwert für Wb/ Wd von 1,5 ("Longwave coverage", Überdeckung der Langwelligkeit)
• Zielwert für Wd / Wc von > 1 ("Wet Look")
• Die Kurvenform sollte einen Doppelhöcker ("camel back") aufweisen.
• du und Wa können leicht erhöht sein, um Orangenhaut zu verschleiern.

A finish is generally considered good if it conforms to the in 12 corresponds to the master curve shown. The following guidelines apply in general:

• No ripple reading greater than 30.
• Target value for Wb/ Wd of 1.5 ("longwave coverage")
• Target value for Wd / Wc of > 1 ("wet look")
• The curve shape should have a double hump ("camel back").
• du and Wa may be slightly elevated to disguise orange peel.

Textured sheet metal surfaces mainly affect the Wb value. This is typically the ripple parameter with the highest numerical value and should be as low as possible ( 13 ). For good finishes, the Wb value should be less than 30.

The condition of the sheet metal surface also has less of an influence on the Wa parameter. Very rough sheet metal can negatively influence the Wc and even Wd parameters. In the case of such flat products, the measured values obtained are too high and are more difficult to correct in terms of painting.

The coating can also influence the waviness parameters. The clear coat or its application has an influence on the waviness values du (clear coat too milky, dry spraying of the clear coat), Wc, Wd (clear coat film thickness too low). KTL and filler layers can increase the Wb value significantly if the application is rough or there is no sanding. The Wc value is increased by sanding marks or dry spraying of the filler.

The general rule is that sheet metal to be painted should be used with an optimized texturing that is as constant as possible and specified within narrow tolerances. The painting process, with its numerous parameters and options for intervention, must be kept as constant as possible by the OEM in order to achieve the same or very similar effect from body to body in terms of quality, color matching and, in the case of modern paints, in particular with effect pigments.

A low Wb value is an important factor in painted sheet metal, especially with regard to plastic add-on parts. Plastic parts only have a very low roughness, so that very low Wb values and very flat structure spectra are obtained. This is particularly noticeable when plastic parts on the body that are painted too smooth are adjacent to a sheet metal surface that is painted too rough. When you look over the body, there is an "optical break" in the paintwork, which is not desired. On the part of the plastic part manufacturer, the waviness of the painted plastic components is adapted to the waviness of the painted sheet metal components by surface roughness or waviness already applied during the shaping injection molding.

Here, the texturing of the sheet metal surface according to the invention offers the possibility of producing sheets with a lower Wb value after painting, which can be installed optically better next to painted plastic components. In high-quality automobiles in particular, there has always been a trend toward so-called "piano finishes". This means a reflective coating with a very good DOI value and very low waviness, which is based on a glossy black lacquered concert grand piano.

Such a finish can usually only be achieved by repeated sanding and painting. A trend towards the use of large glass roofs can also be observed in luxury, upper and middle-class automobiles. These are e.g. Partly dark tinted and usually painted black on the edge to hide the adhesive from the back. Due to the extreme, reflective smoothness of a dark glass roof, it is particularly difficult here to paint the adjacent sheet metal components such as the roof frame or roof cover in a way that makes them look similar. This task can also be solved by using flat products according to the invention.

An ideal painting surface is flat and shows no roughness or bumps. This is technically difficult to achieve with sheet metal, since the surface usually has to be reshaped in order to obtain a component. For the forming, an oil holding capacity is required for the lubrication, but this requires a certain roughness/surface topography of the flat sheet.

In 13 the measured values determined for the paint appearance as a function of the surface topography are plotted for a sheet with too coarse texturing (dashed line), for a sheet with a standard texturing (dash-dotted line) and a sheet according to the invention (solid line). It can be seen that with unfavorable roughness texturing, the value for Wb increases sharply and thus causes poorer painting or increased sanding effort after KTL and filler painting. It is also clearly recognizable that the texturing according to the invention enables improved painting with reduced values for Wb for the forming process.

With a surface structure according to the invention, an optimal compromise has been found, since there are large flat areas at a height level both on the plateaus and in the depressions, which are only separated from one another by short but steep flanks. In a flat product according to the invention, the proportion of non-planar portions on the surface that negatively influence the overall impression is thus reduced to a minimum.

The coating forms the substrate, e.g. T. and builds on bumps. The mutual dependence of sheet metal structure <-> paint structure is in 14 clarified.

15 shows poor (dotted line) painted including a coat of filler, common (dashed line) and good (dash-dotted) automotive finish compared to a thin sheet of the invention (solid line) painted without a layer of filler paint. The waviness Wb, which is greatly reduced compared to the usual automotive coating, can be clearly seen here, which leads to improved brilliance and higher DOI values.

The structural spectrum of the thin sheet according to the invention is in 15 In the example shown, the Wb value is slightly above the curve for a good automotive finish and shows lower values for the Wd value. This results from the paint structure chosen for the texturing according to the invention. In order to allow the texturing/structure of a metal sheet to stand out as much as possible to the value Wb, no filler (approx. 35 µm layer thickness) was applied at all. Neither was a special filler-free painting concept used instead, nor was the KTL layer sanded.

Despite these more stringent conditions, the sheet metal produced according to the invention shows a painting result which is comparable to a good automobile painting.

By applying an increased clear coat thickness to the thin sheet of the invention, the influence of the coating on the waviness parameters Wd could be completely suppressed (low clear coat thickness results in an increased Wd value). This also makes the differences in texturing stand out clearly. In the structure spectrum, a lower value can be seen for Wd here than for a good automotive finish. The thin sheet according to the invention thus lowers the Wd value compared to the Wd value that can be determined for a standard texturing. In order to achieve a desired painting result with Wd/Wc ratios as in the master curve Fig. 12 to achieve this, only the clear coat thickness needs to be adjusted.

The texturing of a flat product according to the invention thus leads to a good coating result with good values for Wb and DOI even if the filler layer is omitted. At the same time, it lowers the value for the long waviness Wd compared to standard textures, which minimizes the formation of orange peel.

Sheets made according to the invention are therefore preferably suitable for the use of paint concepts in which the application of a filler and the regrinding of the filler layer are dispensed with. The invention thus satisfies the need, particularly in the field of automobile construction, for sheet metal substrates which permit a shortened painting process while at the same time having excellent functional properties and appearance.

Claims (7)

1. Flat product made of a metal material, in particular a steel material, with a stochastic surface structure, for whose surface, over a basic area of at least 0.8 x 0.8 mm², after removing a possible slope in its topography, filtering out high frequency portions by means of a Gaussian low-pass filter (λs = 10 µm) and determining the frequency distribution of the height values with a class size of 0.1 µm, the following applies:

   a) The frequency distribution of the height values has two pronounced maxima, which equate to correspondingly pronounced peak and valley levels of the surface.

   b) When just those topography regions, which have a slope of 5° at the most in relation to the horizontally oriented basic area are observed, the frequency distribution of the height values resolve into at least two local main maxima. The local main maxima are approximately normally distributed with a standard deviation (width) of 2 σ ≤ 2 µm for the peaks and with a standard deviation of 2 σ ≤ 1 µm for the valleys.

   c) The frequency of the peaks is greater than the frequency of the valleys.

   d) The upper main maximum representing the peaks at the same time is also an absolute maximum.

   e) The distance between the maxima of the frequency distribution of the height values amounts to 1 µm - 5 µm.

   f) On a level, which lies exactly midway between peak and valley level, half the width of the valleys or peaks amounts to 40 µm or 100 µm at the most, wherein at least 99.99 % of the topography measurement points possess a minimum distance to the edge of the valleys or peaks, which fulfils this condition.
2. Flat product according to Claim 1, characterized in that it is a steel sheet or strip.
3. Flat product according to Claim 2, characterized in that the steel sheet or strip is provided with a corrosion protective layer.
4. Flat product according to Claim 3, characterized in that the corrosion protective layer is a coating based on zinc.
5. Use according to any one of Claims 1 to 4 of a flat product formed for producing components, which are coated with a paint finish.
6. Roll for producing flat products formed according to any one of Claims 1 to 4, having a stochastic surface structure created by means of Electro Discharge Texturing ("EDT") followed by a fine grinding process, wherein for the surface of the roll, over a basic area of at least 0.8 x 0.8 mm², after removing a possible slope in its topography, filtering out high frequency portions by means of a Gaussian low-pass filter (λs = 10 µm) and determining the frequency distribution of the height values with a class size of 0.1 µm, the following applies:

   a) The frequency distribution of the height values has two pronounced maxima, which equate to correspondingly pronounced peak and valley levels of the surface.

   b) When just those topography regions, which have a slope of 5° at the most in relation to the vertical are observed, the frequency distribution of the height values resolve into at least two local main maxima. The local main maxima are approximately normally distributed with a standard deviation (width) of 2 σ ≤ 10 µm for the valleys and with a standard deviation (width) of 2 σ ≤ 1 µm for the peaks.

   c) The frequency of the valleys on the roll surface is greater than the frequency of the peaks.

   d) The distance between the pronounced peak level and the valley levels of the roll surface is greater than the distance between peak and valley level on the flat product surface obtained.

   e) On a level, which lies exactly midway between peak and valley level, half the width of the valleys or peaks amounts to 100 µm at the most, wherein at least 99.99 % of the topography measurement points possess a minimum distance to the edge of the valleys or peaks, which fulfils this condition.
7. Method for producing a flat product formed according to any one of Claims 1 to 4, wherein

   - a flat product consisting of a metal material is made available, wherein at least the surface to be provided with the surface topography constituted according to Claim 1 has an arithmetic roughness average of 1.5 µm at the most, and

   - the flat product made available is subjected to skin-pass rolling, wherein a roll constituted according to Claim 5 acts on the surface to be provided with the surface topography according to Claim 1, so that a flat product with a surface constituted according to Claim 1 is obtained.

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