METAL SUBSTRATE WITH A SURFACE TEXTURE

Abstract

A metal substrate provided with a surface texture wherein the Maximum Zero Normalised Auto Correlation (MZNAC) of the surface texture is in the range of 0.2-0.8, as well as a method to apply such surface textures on a metal substrate.

Claims

1. A metal substrate provided with a surface texture, wherein Maximum Zero Normalised Auto Correlation (MZNAC) of the surface texture is in a range of 0.2-0.8, wherein the MZNAC corresponds to the highest peak in the Zero Normalised Auto Correlation (ZNAC), except for the peak at Δx=Δy=0, wherein the ZNAC of the surface can be obtained from the following equation: $\mathrm{ZNAC}={.Math.}_{i=-M}^M{.Math.}_{j=-M}^M\left\{\frac{\left[Z\left(x_i,y_i\right)-Z_m\right]\times \left[Z\left(x_i^{\prime },y_i^{\prime }\right)-Z_m\right]}{\Delta Z\Delta Z}\right\},\mathrm{with}{x}_i^{\prime }=x_i+\Delta x\mathrm{and}{y}_i^{\prime }=y_i+\Delta y.$

2. The metal substrate of claim 1, wherein the surface texture has MZNAC in a range of 0.2-0.6.

3. The metal substrate of claim 1, wherein the surface texture has MZNAC in a range of 0.4-0.8.

4. The metal substrate according to claim 1, wherein the surface texture has MZNAC in a range of 0.4-0.6.

5. The metal substrate according to claim 1, wherein the metal substrate comprises a metallic coating.

6. The metal substrate according to claim 1, wherein the surface texture comprises a plurality of features with a pseudo-deterministic spatial distribution.

7. The metal substrate according to claim 1, wherein the features have an average diameter in a range of 25-120 μm.

8. The metal substrate according to claim 1, wherein the features have an amplitude in a range of 0.2-12 μm.

9. The metal substrate according to claim 1, wherein the features have a density in a range between 100 to 10000 per square millimetre.

10. The metal substrate according to claim 1, wherein the waviness (Wsa) is <0.4 μm.

11. The metal substrate according to claim 1, wherein the metal substrate is a blank or a strip.

12. A method to obtain a metal substrate according to claim 1, comprising the steps of a. Providing a metal substrate b. Optionally temper rolling the metal substrate c. Providing a surface texture by texturing means, such as to obtain the metal substrate with MZNAC in the range of 0.2-0.8.

13. A method according to claim 12, wherein the metal substrate comprises a metallic coating.

14. A method according to claim 12, wherein the texturing means is a textured work roll, and wherein the surface texture is provided by imprinting a surface texture on the metal substrate by one or more textured work rolls.

15. A method according to claim 12, wherein the texturing means is a laser and wherein the surface texture is provided by laser ablation or laser assisted melting and redepositing.

16. The metal substrate according to claim 1, wherein the features have an amplitude in a range of 2-7 μm.

17. The metal substrate according to claim 1, wherein the features have a density in a range between 180 to 600 per square millimetre.

18. The metal substrate according to claim 1, wherein the metal substrate comprises a zinc based coating.

Description

[0032] FIG. 1. Shows an overview of the correlation of MZNAC and waviness for several surface textures, from example 1-11.

[0033] FIG. 2a. Shows four surface textures of example 1 varying from deterministic to pseudo deterministic to stochastic like (left to right), the corresponding ZNAC peaks above 0.8 (second row), the corresponding ZNAC peaks above 0.2 (third row) and the corresponding moiré patterns (last row). FIG. 2b shows the correlation of example 1 surface textures, with parameters Wsa, RPc and Ra.

[0034] FIG. 3a. Shows four surface textures of example 7 varying from deterministic to pseudo deterministic to stochastic like (left to right), the corresponding ZNAC peaks above 0.8 (second row), the corresponding ZNAC peaks above 0.2 (third row) and the corresponding moiré patterns (last row). FIG. 3b shows the correlation of the MZNAC value of example 7 surface textures, with parameters Wsa, RPc and Ra.

[0035] FIG. 4a. Shows four surface textures of example 8 varying from deterministic to pseudo deterministic to stochastic like (left to right), the corresponding ZNAC peaks above 0.8 (second row), the corresponding ZNAC peaks above 0.2 (third row) and the corresponding moiré patterns (last row). FIG. 4b shows the correlation of the MZNAC value of example 8 surface textures, with parameters Wsa, RPc and Ra.

[0036] FIG. 5a shows a close up of the surface texture of example 1, 5b shows the surface texture of example 1 for a sample of 2 mm×2 mm and 5c shows the corresponding ZNAC values, clearly showing the 0,0 peak (Δx=Δy=0), and the MZNAC of 0.490, corresponding to the highest satellite peak in the ZNAC spectrum.

[0037] FIG. 1. Shows a summary of computational results for surface textures according to the invention and according to the prior art. Several surface textures were modelled and analysed for surface appearance properties such as waviness (Wa), peak count (RPc) and roughness (Ra), moiré effect and related MZNAC. For each example, the spatial distribution of features was either in a hexagonal or square raster with different levels of dislocation based on a superposition of a Gaussian shaped and rectangular shaped distribution. Consequently, the textures varied from deterministic, to pseudo deterministic to stochastic like. The shape of the features as seen on the surface was either circular or square. On average, the features had an amplitude of 7 μm. Example 1 (see also FIG. 2) had depressions with a depth of 7 μm in a hexagonal raster. Example 6 had depressions with rims, which is typically obtained if the surface texture is applied by laser. Example 7-10 show a metal substrate with superposed surface textures. Example 7 (see also FIG. 3) shows a (pseudo)deterministic surface texture on a rough surface, with a depression depth relative to the rough surface, which would be obtained via a method with a laser as texturing means. Example 8 (see also FIG. 4) shows a (pseudo)deterministic surface texture on a rough surface with depressions with a constant minimum, as typically obtained from a process with a work roll as texturing means. For all examples, the ZNAC of the surface of 2 mm×2 mm was obtained from the Zero Normalised Cross Correlation (ZNCC) as described by Pan et.al, Meas. Sci. Technol. 20 (2009) 062001, doi:10.1088/0957-0233/20/6/062001, in table 1, by adjusting the formula for a single surface (f=Z and g=Z). Thus, the ZNAC was obtained from the following equation:

[00002]$ZNAC={.Math.}_{i=-M}^M{.Math.}_{j=-M}^M\left\{\frac{\left[Z\left(x_i,y_i\right)-Z_m\right]\times \left[Z\left(x_i^{\prime },y_i^{\prime }\right)-Z_m\right]}{\Delta Z\Delta Z}\right\},\mathrm{with}{x}_i^{\prime }=x_i+\Delta x\mathrm{and}{y}_i^{\prime }=y_i+\Delta y$

[0038] The ZNAC was determined for a sample of at least 2 mm×2 mm, with a spacing of at most 2 μm. Δx and Δy was determined up to at least 0.3 mm. An example of the obtained ZNAC values is given for example 1, with a MZNAC value of 0.49 in FIG. 5. The MZNAC was determined from the ZNAC and relates to the highest peak in the ZNAC, except for the peak at Δx=Δy=0, also referred to as the 0,0 peak. The waviness, roughness and peak count was also analysed.

[0039] In addition the moiré effect was visualised by calculating a digital 4K (3840×2160 pixels) photographic image of the examples. A 250 mm×141 mm surface texture was analysed. Each pixel covered a part of the surface of 250/3840 by 141/2160 mm.sup.2, roughly corresponding to 65 by 65 μm.sup.2. In the calculations, the height of the surface was taken as the original and the intensity of each pixel was calculated as the average height of that piece of the surface that is captured by that pixel. Due to the camera grid, the image is not the same as the original. A 150 by 150 pixels part of the 4K grey scale image was calculated and is shown as a black/white image where the threshold lies halfway the highest and the lowest intensity.

[0040] For all examples, a higher waviness was obtained for a lower MZNAC. It was found that a MZNAC below 0.2 resulted in a too large increase in waviness. Therefore the metal substrate provided with a surface texture accorded to the invention should have a MZNAC of at least 0.2 to have good waviness, and hence good paint appearance. At the other hand, a clear moiré pattern was observed for all samples if the MZNAC was above 0.8. The moiré pattern was acceptable for all examples at a MZNAC of at most 0.8. This can be clearly seen in FIG. 2., FIG. 3 and FIG. 4. Furthermore, it was observed from the experiments that a MZNAC between 0.4-0.6 resulted in a significant suppression of the moiré pattern, while maintaining acceptable waviness.