Sheet Metal Packaging Product with Textured Surface And Method of Producing Such a Sheet Metal Packaging Product

Abstract

The invention relates to sheet metal packaging products, in particular tinplate or electrolytically chrome-plated sheet steel (ECCS), consisting of a sheet steel substrate (S) with a thickness in the region of 0.1 mm to 0.6 mm and a coating (B), in particular made of tin and/or chromium or chromium and chromium oxide, that is electrolytically deposited on at least one side of the sheet metal substrate. In addition, at least one surface of the sheet metal packaging product provided with the coating (B) has a surface profile with periodically repeating structure elements in at least one direction, wherein an autocorrelation function resulting from the surface profile has a plurality of side lobes with a height of at least 20%, preferably at least 30% of the height of the main lobe. These sheet metal packaging products have improved and novel surface properties.

Claims

1-37. (canceled)

38. A sheet metal packaging product consisting of a steel sheet substrate with a thickness in the range from 0.1 mm to 0.6 mm and a coating comprising at least one of tin, chromium and chromium oxide, the coating being deposited electrolytically on at least one side of the steel sheet substrate, wherein the coating comprises at least one of a tin layer with a coating weight in the range from 1 to 15 g/m.sup.2 tin and a chromium layer comprising at least one of metal chromium and chromium oxide with a total coating weight of chromium in the chromium layer in the range from 5 to 200 mg/m.sup.2, wherein at least one surface of the sheet packaging product provided with the coating has, in at least one direction, a surface profile with periodically recurring structural elements and an autocorrelation function, resulting from the surface profile, is comprising an absolute maximum with a given maximum height and a plurality of secondary maxima with a given height, which is at least 20% of the maximum height of the absolute maximum.

39. The sheet packaging product of claim 38, wherein the height of the secondary maxima of the autocorrelation function along a preferred direction on the sheet steel substrate is at least 40% of the maximum height of the absolute maximum.

40. The sheet packaging product according to claim 38, wherein the coating is a non-fused tin coating and wherein the height of the secondary maxima of the autocorrelation function along a preferred direction of the tin-coated steel substrate is at least 30% of the maximum height of the absolute maximum.

41. The sheet packaging product according to claim 38, wherein the sheet packaging product has a surface roughness in the range from 0.01 μm to 2.0 μm.

42. The sheet packaging product according to claim 38, wherein the topographic shape of the periodically recurring structural elements is convex or plateau-shaped and the periodically recurring structural elements have a full width at half maximum of at least 10 μm.

43. The sheet packaging product according to claim 38, wherein the coating is a tin coating with a predetermined coating weight (Sn) and in that the tin coating has a current density in the Iron Exposure Test (IET) of j.sub.IET=I/A (electric current per area) in mA/cm.sup.2 which is at most 1.4 times the arithmetic mean roughness (Ra) in μm of the coated substrate plus a constant of 0.5 divided by the coating weight (Sn) in g/m.sup.2 squared: $j_{\mathrm{IET}}(\mathrm{in}\frac{\mathrm{mA}}{{\mathrm{cm}}^2}\le (1.4.Math.R_a\left(\mathrm{in}\mathrm{\mu m}\right)+0.5/{\left(\mathrm{Sn}\right)}^2.$

44. The sheet packaging product according to claim 38, wherein the coating is a tin coating with a predetermined coating weight (Sn) of tin (m/A, mass per area in g/m.sup.2) and that the current density measured in the Iron Exposure Test (IET) is j.sub.IET=I/A (electric current per area) in mA/cm.sup.2 multiplied by the square of the coating weight (Sn) of the tin is less than 1.9 (mA/cm.sup.2).Math.(g/m.sup.2).sup.2 for an arithmetic mean roughness (Ra) of Ra≤1.0 μm and less than 3.3 (mA/cm.sup.2).Math.(g/m.sup.2).sup.2 for an arithmetic mean roughness (Ra) of 1.0 μm<Ra≤2.0 μm.

45. The sheet packaging product according to claim 38, wherein the surface structure has a regularly arranged pattern with elevations and depressions, the elevations projecting by an average height of 0.5 to 3.0 μm above an average level averaged over the entire surface of the sheet packaging product, and the depressions have an average depth of 0.5 to 3.0 μm, relative to the average level.

46. The sheet packaging product according to claim 38, wherein the periodically recurring arrangement are comprising at least one of convex and concave structural elements, web-shaped elements which are trapezoidal in a cross section, plate-shaped elements, groove-shaped depressions, plateau-shaped elevations, convex protrusions protruding above the surface of the sheet steel substrate, rectangular elements, strip-shaped element, bar-shaped element, cylindrical elements, leaf-shaped elements, crescent-shaped elements, strips extending in a longitudinal direction, ridge-shaped protrusions.

47. The sheet packaging product according to claim 38, wherein the surface of the sheet packaging product has a gloss value of more than 50 gloss units and a surface roughness of less than 0.5 μm and more than 0.1 μm.

48. The sheet packaging product according to claim 38, wherein the surface of the sheet packaging product has a direction-dependent gloss value, wherein the difference in gloss value in the rolling direction and a transverse direction perpendicular thereto is less than 100 gloss units.

49. The sheet packaging product according to claim 38, wherein the sheet steel substrate has the following composition in terms of weight fractions: C: 0.01-0.1%, Si: <0.03%, Mn: 0.1-0.6%, P: <0.03%, S: 0.001-0.03%, Al: 0.002-0.1%, N: 0.001-0.12%, optional Cr: <0.1%, optional Ni: <0.1%, optional Cu: <0.1%, optional Ti: <0.09%, optional B: <0.005%, optional Nb: <0.02%, optional Mo: <0.02%, optional Sn: <0.03%, rest iron and unavoidable impurities.

50. A method of manufacturing a sheet packaging product having a textured surface, the method comprising: providing a primary cold-rolled steel sheet substrate with a thickness in the range of 0.1 mm to 0.6 mm; recrystallizing annealing of the primary cold-rolled steel sheet substrate; re-rolling or skin-passing of the recrystallization-annealed steel sheet substrate in a two-stand re-rolling mill, wherein a first stand of the re-rolling mill has at least one working roll with an unstructured roll surface, and a second stand of the re-rolling mill has at least one working roll with a surface-structured roll surface; electrolytic coating of the re-rolled or skin-passed steel sheet substrate on at least one side with a coating comprising at least one of tin, chromium and chromium oxide, the coating comprising at least one of a tin layer with a coating weight in the range from 1 to 15 g/m.sup.2 tin and a chromium layer comprising at least one of metallic chromium and chromium oxide with a total coating weight of chromium in the chromium layer in the range from 5 to 200 mg/m.sup.2; wherein the sheet packaging product has a surface profile with periodically recurring structural elements in at least one direction after coating of the steel sheet with the coating, wherein the surface profile has an autocorrelation function which has an absolute maximum with a maximum height and a plurality of secondary maxima with a height, which is at least 20% of the maximum height of the absolute maximum.

51. The method according to claim 50, wherein the surface of the at least one working roll of the second stand has been structured by a pulsed laserser.

52. The method according to claim 50, wherein the coating is a tin coating and, after the electrodeposition of the tin coating, the surface of the coating is fuzed by heating to temperatures above the melting point of tin or the surface of the coating is provided with a chromium passivation layer, comprising at least one of chromium and chromium oxide or a chromium-free passivation layer.

53. The method according to claim 50, wherein a thickness reduction of the cold-rolled and electrolytically coated steel sheet substrate to a final thickness in the range of 0.1 mm to 0.5 mm is effected during re-rolling or skin-passing, wherein during re-rolling the relative thickness reduction is in the range of more than 5% and up to 50% and during skin-passing the relative thickness reduction is in the range of 0% to 5%.

54. A steel sheet having a thickness in the range from 0.05 mm to 0.6 mm, wherein the surface of the steel sheet has in at least one direction a surface profile with periodically recurring structural elements, the surface profile having an autocorrelation function with an absolute maximum having a given maximum height and a plurality of secondary maxima having a height, which is at least 40% of the maximum height of the absolute maximum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] These and other advantages and features of the invention will be apparent from the embodiments described in more detail below with reference to the accompanying drawings. The drawings show:

[0058] FIGS. 1A and 1B: Schematic representation of sheet packaging products according to the invention in a sectional view, wherein FIG. 1A shows a sheet packaging product consisting of a sheet steel substrate and a coating and FIG. 1B shows a sheet packaging product consisting of a sheet steel substrate with a coating and a support applied thereto;

[0059] FIG. 2: Enlarged schematic sectional view in the area of the surface of the coating of a sheet packaging product according to the invention;

[0060] FIGS. 3A and 3B: Schematic sectional view in the surface area of a sheet packaging product according to the prior art (FIG. 3A) and according to the invention (FIG. 3B), in each case before (left side) and after (right side) application of the coating to the sheet steel substrate;

[0061] FIG. 4A: Microscopic view of the surface of a conventional prior art sheet packaging product with an associated surface profile (height profile), wherein the surface of the steel sheet substrate has been skin-pass rolled by a blasted or ground dressing roll prior to application of the coating;

[0062] FIGS. 4B to 4G: Height profiles of the surface of surface-structured skin pass rolls (left side in each case) and of the surfaces of packaging sheet products according to the invention skin passed with this skin pass roll before (FIGS. 4B, 4C and 4D) or after (FIGS. 4E, 4F and 4G) the electrolytic application of the coating (right side in each case);

[0063] FIG. 5A to 5G: Three-dimensional microscopic representations of surface profiles of the steel sheets according to FIGS. 4A and 4B to 4G (in each case at the top of the figure), together with a roughness profile of the surface (in each case at the bottom) and the autocorrelation function resulting from the roughness profile (in each case in the middle of the figure), wherein the microscopic representation of the surface, the roughness profile and the associated autocorrelation functions in each case before electrolytic deposition of a tin coating (FIGS. 5B, 5C and 5D, left), after electrolytic deposition of a tin coating (FIG. 5A, left or FIGS. 5B, 5C and 5D, center) and after (right side in each case) melting of the tin coating;

[0064] FIG. 6: Representation of the IET values measured in the “Iron Exposure Test” (IET) on tinplate according to the invention and the prior art, multiplied by the square of the tin coating as a function of the surface roughness (arithmetic mean roughness Ra in μm) of the tinplate samples;

[0065] FIG. 7: Depiction of the dependence of gloss values measured on tinplate according to the invention and the prior art (in gloss units GU) on the surface roughness (arithmetic mean roughness Ra in μm);

[0066] FIG. 8: Depiction of the dependence of the isotropy of the gloss values measured on tinplate according to the invention and the prior art (as Δgloss values in gloss units GU) on the surface roughness (arithmetic mean roughness Ra in μm);

DETAILED DESCRIPTION

[0067] FIG. 1A shows a schematic section of a packaging sheet product. The packaging sheet product consists of a sheet steel substrate S with a thickness in the ultra-thin sheet range (0.1 mm to 0.6 mm) and a coating B deposited electrolytically on the sheet steel substrate S. The sheet steel substrate is a cold-rolled steel sheet made of a steel with a low carbon content. Suitable compositions of the steel of the sheet steel substrate S are defined in the European standard DIN EN 10 202. The sheet steel substrate S preferably has the following composition in terms of the weight fractions of the alloying components of the steel: [0068] C: 0.01-0.1%, [0069] Si: <0.03%, [0070] Mn: 0.1-0.6% [0071] P: <0.03%, [0072] S: 0.001-0.03%, [0073] Al: 0.002-0.1%, [0074] N: 0.001-0.12%, preferably less than 0.07%. [0075] optionally Cr: <0.1%, preferably 0.01-0.05%, [0076] optionally Ni: <0.1%, preferably 0.01-0.05%, [0077] optionally Cu: <0.1%, preferably 0.002-0.05%, [0078] optional Ti: <0.09%, [0079] optional B: <0.005%, [0080] optional Nb: <0.02%, [0081] optional Mo: <0.02%, [0082] optional Sn: <0.03%, [0083] Residual iron and unavoidable impurities.

[0084] The coating B can be a tin coating or a coating of chromium and chromium oxide (and possibly chromium hydroxides). In the case of a tin coating, we speak of tinplate. In the case of a chromium/chromium oxide coating that has been electrolytically deposited on the steel sheet substrate, it is referred to as Electrolytic Chromium Coated Steel (ECCS). In the case of tinplate, the coating weight of the coating B is typically in the range from 1 to 15 g/m.sup.2 and, in particular, between 2 and 6 g/m.sup.2 of tin. For ECCS, the coating weight of chromium in the chromium-chromium oxide layer is typically in the range of 50 to 200 mg/m.sup.2 and in particular between 70 and 150 mg/m.sup.2.

[0085] Further coatings or overlays, for example in the form of passivation layers or organic overlays such as lacquers or polymer coatings, can be applied to the coating B in this process. This is shown schematically in FIG. 1B, where an overlay P is shown on the coating B. The overlay P can be, for example, a white lacquer coating. In the case of tinplate, for example, the overlay P can be a passivation layer. The passivation layer can be composed of metallic chromium and/or chromium oxide, as is usual for tinplate. However, the passivation layer can also be a chromium-free passivation layer applied wet-chemically onto the tin surface. In the case of tinplate, the passivation layer is intended to prevent unhindered oxide growth on the tin surface and thus ensure storage stability of the tinplate over longer periods without oxidation of the tin surface. Furthermore, in the case of tinplate, the surface of the coating or the entire tin coating can be melted after its electrolytic deposition on the steel sheet substrate by heating the tinplate to temperatures above the melting temperature of tin.

[0086] The overlay P can also be formed by an organic overlay, such as an organic lacquer or by a polymer coating made of a thermoplastic polymer, in particular PET or PP. In particular for ECCS, it is common to coat the chromium oxide surface of the ECCS with a polymer coating made of a thermoplastic polymer, for example by laminating a PET or PP film, in order to improve the corrosion resistance and the resistance of the material to acids and also the formability of the material.

[0087] FIG. 2 shows a schematic representation of a sheet packaging product according to the invention in the region of the surface of the coating B. As can be seen from FIG. 2, the surface of the sheet packaging product has a plurality of elevations E (in the sectional view shown) arranged next to one another and depressions V located in between. The elevations E have a (mean) height h above an average surface level 0. The depressions V have a depth t relative to the average surface level 0. The depressions V are groove-shaped with an at least largely flat groove base c1, c2, c3. The elevations E are plateau-shaped with a substantially flat plateau surface b1, b2, b3, b4. The flanks a1, a2, a3, a4 of the depressions V and the elevations E are slightly inclined with respect to the vertical plane, as shown in FIG. 2. In the sectional view shown in FIG. 2, this results in isosceles trapeziums or truncated cones for the shape of the elevations E, which taper conically towards the surface (Gaussian or Tophat profile).

[0088] To achieve homogeneous surface properties, it is useful if the shape and arrangement of the elevations E and the depressions V are as uniform or regular as possible.

[0089] In particular, the areas of the groove bases c1, c2, c3 of the groove-shaped recesses V are as equal as possible. Deviations are preferably less than or equal to 10%. In a corresponding manner, the plateau areas (b1 to b4) of the plateau-shaped elevations are also preferably approximately the same size and the flanks a1-a4, which extend between the depressions V and the elevations E adjacent thereto, also preferably show no differences in inclination. The heights h of the elevations can vary by a maximum of 25% and the depths t of the depressions also preferably vary only slightly by about 10% or less.

[0090] The uniformity and regularity of the surface structures of the sheet packaging products according to the invention can be described mathematically with the aid of autocorrelation. The autocorrelation (also cross autocorrelation) generally describes the correlation of a signal or a profile z(x) with itself at an earlier time or at another location x.

[0091] Without shifts Ψ.sub.zz(0) represents the variance of the height values z(x) of the profile with x in the interval from 0 to l:

[00002]${\Psi }_{\mathrm{zz}}\left(0\right)=\frac{1}{l}{\int }_0^{l}z\left(x^{\prime }\right).Math.z\left(x^{\prime }+0\right){\mathrm{dx}}^{\prime }={\sigma }^2\left(\mathrm{Variance}\right)=R_q^2$

[0092] For calculating of roughness parameters, in accordance with DIN EN ISO 4288:1998-04, in the roughness range Ra=0.1 μm to 1 μm a scanning distance of l.sub.t of 4.8 mm should be used. After Gaussian high-pass filtering with cut-off wavelength L.sub.c=0.8 mm and separation of the marginal areas, this leaves for the characteristic value calculation an evaluation length of l=4 mm.

[0093] FIGS. 5A and 5B to 5G show (in each case in the bottom of the figure) surface profiles (roughness profiles) of a conventional sheet packaging product (FIG. 5A) and of sheet packaging products according to the invention (FIGS. 5B to 5G) and associated autocorrelation functions of the surface profile (in each case in the middle of the figure). Here, FIG. 5A shows the surface structure of tinplate with a conventional steel substrate with a statistically uncorrelated surface structure. FIGS. 5B to 5D show the surface structure of steel sheets according to the invention before coating with tin (left side) and after electrolytic coating on both sides with a tin coating of 2.8 g/m.sup.2 (center and right side of the figure), with the center image showing the tin surface in the molten state and the right image showing the tin surface before melting in each case. FIGS. 5E to 5G show further examples of the surface structure of tinplate according to the invention (before and after a re-melting of the tin coating).

[0094] The amplitudes of the autocorrelation are normalized with respect to the main maximum H (at x=0) (normalized autocorrelation function

[00003]$\frac{{\Psi }_{\mathrm{zz}}\left(x\right)}{{\sigma }^2}).$

The height of the secondary maxima N of the normalized autocorrelation function, which can be a maximum of 1 or 100% because of the normalization, represent a measure of the regularity and uniformity of the autocorrelation periodically along the selected direction (x) in the surface plane. Because of their symmetry property (Ψ(−x)=Ψ.sub.zz(x)), the autocorrelation function is shown in the figures only for positive x values; for negative values, it behaves as a mirror image of the ordinate. To determine the highest (secondary) maxima of the autocorrelation of the surface profile, the measuring section should, if possible, be placed in one of the preferred directions of the specimen, in particular in the longitudinal direction of the strip (or the rolling direction of the cold-rolled packaging sheet) or perpendicular to it.

[0095] A comparison of the non-inventive comparative sample according to FIG. 5A with the sheet packaging products according to the invention (FIGS. 5B to 5G) shows that the sheet packaging products according to the invention have an autocorrelation function which, in addition to the main maximum at x=0, has several secondary maxima which reach an amplitude of at least 20% of the main maximum, whereas the height of the secondary maxima in the non-inventive comparative sample is (significantly) less than 20%. The sheet packaging products with a deterministic surface structure according to the invention thus have a much more uniform surface profile with periodically recurring structural elements.

[0096] The shape of the periodically recurring structural elements, in particular the elevations E and the depressions V, can be adapted in each case to the application of the sheet packaging products according to the invention or to the specifications for producing packaging therefrom. Suitable cross-sectional shapes for the elevations E and the depressions V can be essentially trapezoidal and dome-shaped.

[0097] The elevations E and depressions V can also be circular or annular in shape. For special applications, in particular to achieve homogeneous optical properties such as reflectivity and gloss, elevations E or depressions V in the form of strips or ridges have proved advantageous. The protrusions E can be convex or, in a preferred manner, have a plateau-shaped flattened upper surface which is as flat as possible. The recesses V have a largely flat recess surface.

[0098] Examples of surface structures with periodically recurring structural elements are shown in FIGS. 4B to 4G, in each case depicting (in a plan view with associated height profile) on the left-hand side the surface of the work roll with which the sheet steel substrate was cold-rolled (skin-passed) prior to the electrolytic application of the coating B, and on the right-hand side depicting in each case the resulting surface structure of the skin-passed surface of the sheet steel substrate. The data for the rolls (pairs) used in the secondary cold rolling (skin pass) of the steel sheet substrate can be taken from Table 1.

[0099] In the examples of FIGS. 4B to 4D, a re-rolling mill was used for the re-rolling (secondary cold rolling) of the steel sheet substrate, which was equipped in a first stand with a first work roll with a blasted roll surface and in a second stand with a second work roll with a structured roll surface. The surface structure of the structured roll surface of the second work roll is shown in FIGS. 4B to 4D on the left of the figure.

[0100] In the examples of FIGS. 4E to 4G, a re-rolling mill was used for secondary cold rolling of the steel sheet substrate, which was equipped in a first stand with a first work roll having a structured roll surface and in a second stand with a second work roll having a polished roll surface. The surface structure of the structured roll surface of the first work roll is shown in FIG. 4E to 4G on the left of the figure.

[0101] The surface topographies of the sheet packaging products according to the invention shown in FIGS. 3B to 3J allow a surface roughness with an arithmetic mean roughness Ra to be set which is uniformly distributed over the surface, the arithmetic mean roughness Ra preferably being in the range from 0.01 to 2.0 μm and particularly preferably in the range from 0.1 to 1.0 μm and in particular in the range from 0.1 to 0.3 μm. The surface roughness, in particular the value for the arithmetic mean roughness Ra, of the sheet packaging products according to the invention can be adapted to the particular application and specifically set by selecting the geometry and size of the periodically recurring surface structures (elevations E and depressions V).

[0102] For the production of the sheet packaging products according to the invention, the sheet steel substrate S is re-rolled with a surface-structured roll after or during (primary) cold rolling (secondary cold rolling with a degree of re-rolling in the range of 5% to 45%) or is dressed with a degree of re-rolling of less than 5%. The surface-structured work rolls used for this purpose can be, for example, rolls structured with a short-pulse laser (KPL) or with an ultrashort-pulse laser (UKPL). In Table 1, the rolls structured with an ultrashort pulse laser are designated with the abbreviation “UKPL”. It should be noted that despite this designation, the invention is not limited to the production of the deterministic surface structure by means of a roll structured with an ultrashort pulse laser (UKPL). The surface structures of the sheet packaging products according to the invention can also be imprinted by rollers structured in other ways. In any case, however, the rolls used for this purpose have a deterministically structured roll surface which is impressed into the surface of the sheet steel substrate S during rolling. It is expedient to imprint the surface structure of the roll in a secondary cold rolling step with a reduction degree (re-rolling degree) of more than 5% up to a maximum of 50% or in a skin-passing step in which the cold-rolled steel sheet is skin-passed with a low reduction degree of a maximum of 5% after primary cold rolling.

[0103] After the deterministic surface structure of the work roll (skin pass roll) has been introduced into the surface of the steel sheet substrate S, the coating B is applied according to the invention by electrolytic deposition of the coating material (e.g. tin in the case of tinplate and chromium/chromium oxide in the case of ECCS) onto the structured surface of the steel sheet substrate S. The coating B is then applied to the steel sheet substrate S by electrolytic deposition. During the electrolytic deposition of the coating B on the sheet steel substrate S, the deterministic surface structure of the sheet steel substrate is essentially retained, so that the coated sheet packaging product also has a deterministic surface structure with a uniform topography, as shown schematically in FIG. 2. The deterministic surface structure is also preserved when an additional overlay P is applied to the coating B, as shown in FIG. 1B, in particular when the overlay P is applied to the coating B in the form of a liquid, for example in the form of an aqueous passivation solution or a liquid paint or a molten polymer material. It is true that the application of the coating B reduces the (relative) height of the secondary maxima of the autorrelation function by an average of 10 to 20% compared with the uncoated steel sheet substrate. However, the process according to the invention enables to achieve surface structures of the coated sheet packaging product whose autocorrelation function has a plurality of secondary maxima with a (relative) height of at least 20% of the main maximum.

[0104] The deterministic surface structure of the suitably produced sheet packaging products according to the invention eliminates many problems that can arise in the conventional production of sheet packaging products (especially tinplate and ECCS). Typical problems arising in the conventional manufacture of sheet packaging products are explained below with reference to FIG. 3, where FIG. 3A shows the surface of conventionally manufactured sheet packaging products before and after application of the coating B to the substrate S, and FIG. 3B shows schematically the surface of sheet packaging products according to the invention, which can eliminate the problems of the prior art.

[0105] FIGS. 3A and 3B schematically show a sectional view of a sheet steel substrate S before (left side) and after (right side) application of a coating B, where FIG. 3A shows a conventionally produced substrate or sheet packaging product with a stochastic, disordered surface structure and FIG. 3B shows a substrate treated in accordance with the invention or a sheet packaging product treated in accordance with the invention with a deterministic surface structure. As can be seen from FIG. 3A, the surface of the steel sheet substrate S and the packaging sheet product coated with the coating B has a statistical (i.e. a non-deterministically predetermined) surface topology with peaks Sp and valleys Ta. The height of the peaks Sp and the depth and/or geometry of the valleys Ta are non-uniform, i.e. inhomogeneously distributed over the entire surface of the sheet steel substrate S (FIG. 3A, left) or the coated sheet packaging product (FIG. 3A, right). This surface structure of conventional sheet packaging products leads to a variety of problems:

[0106] The sharp tips Sp can easily break off or be flattened when the sheet packaging product is subjected to mechanical stress, for example during transport or during forming into packaging. When the points Sp are broken off or flattened, the coating B is damaged or completely removed in places. This results in free, uncoated areas where the corrosion-susceptible steel sheet substrate S is exposed to the environmental influences and the filling materials of packaging made from the sheet packaging product and can thus corrode. Furthermore, this causes abrasion of the coating material, which is harmful during further processing of the sheet packaging products.

[0107] Furthermore, dirt particles and residues of oils and greases can accumulate in the pocket-shaped valleys Ta of the surface structure of conventional sheet packaging products, which can no longer be completely removed even when the coating surface of the sheet packaging products is cleaned, due to unevenly shaped valleys with undercuts. In particular, grease, cleaning agents, rolling oils or other residues from the manufacturing process of the packaging sheet product can accumulate in the deep and/or geometrically undefined valleys, which can make cleaning of the surface of the packaging sheet product more difficult and negatively affect the coating quality, such as the porosity of the tin coating in tinplate. Contamination of the surface and a formation of condensate deposited in the deep valleys also have a negative effect on corrosion resistance.

[0108] FIG. 4A shows an example of an image of a surface structure of a conventional tinplate according to the state of the art with a tin coating of 2.8 g/m.sup.2 generated with the confocal topography measuring device μSurf mobile from NanoFocus AG. A 20× objective with a resolution of approx. 1.56 μm was used for the measurement.

[0109] Prior to the electrolytic application of the tin coating, the surface of the steel sheet substrate of this tinplate was first dressed in a first stand of the re-rolling mill using a work roll with a blasted surface and then in a second stand using a work roll with a ground roll surface. Dressing has given the surface of the conventional tinplate a statistically uncorrelated structure and thus an inhomogeneous surface topography, as can be seen from the associated height profile of FIG. 4A and from the 3D representation of the surface structure, the associated roughness profile and the autocorrelation function of FIG. 5A. The surface structure of the conventional tinplate shows in particular a pronounced structure of grooves extending in the rolling direction (longitudinal direction of the strip-shaped tinplate). Dirt particles and residues of rolling oils can become lodged in the grooves, which cannot be removed by ordinary cleaning steps. Furthermore, as can be seen in particular from the 2D height profile of FIGS. 4A and 5A and the associated roughness profile of FIG. 5A, the surface structure has pronounced peaks at which damage to the coating can occur under mechanical stress.

[0110] These problems of the prior art can be eliminated with the packaging sheet products according to the invention. Due to the deterministic and homogeneous surface structure of the sheet packaging products according to the invention, they are free of sharp Sp peaks with radii of curvature greater than 0.2 mm, at which damage to the coating and increased abrasion of the coating material could occur, resulting in increased susceptibility of the sheet packaging product to corrosion. Thus, both the susceptibility to corrosion and the adverse effects of abrasion can be avoided. Furthermore, the surfaces of the sheet packaging products according to the invention are also free of deep and/or geometrically undefined valleys in which dirt particles and residues, such as residual greases and rolling oils, could accumulate. In the case of the sheet packaging products according to the invention, this facilitates cleaning of the surface and thus improves corrosion resistance, because both before and after coating of the substrate S the surface of the uncoated or coated steel sheet can be largely completely freed from oil residues, dirt and deposits.

[0111] FIGS. 4B to 4G and 5B to 5G show examples of packaging sheet products with specific surface structures according to the invention. In FIGS. 5A to 5G, the parameters of the roughness profile are listed below the roughness profile, with the abbreviations used in the table of FIGS. 5A to 5G representing the following parameters: [0112] Ra: Mean roughness value or arithmetic mean roughness (arithmetic mean value of the amounts of all profile values of the roughness profile). [0113] Rq: root mean square of all profile values of the roughness profile [0114] Rsk: is a measure of the asymmetry of the amplitude density curve

[0115] FIG. 4D shows an exemplary top view of a hexagonal surface structure with cylindrical elevations arranged in a hexagonal structure, each elevation E being cylindrical with an at least substantially flat or convex top surface (as shown in the surface profile of FIG. 5D). The cylindrical elevations have an average height h and an average diameter (FWHM ø) at half height and are spaced apart (on average) by a distance d.

[0116] The average height h of elevations is generally (irrespective of the geometric shape) preferably in the range from 0.1 to 8 μm, in particular between 0.5 and 4.0 μm. The diameter ø preferably has average values in the range of at least 10 μm and preferably from 60 to 250 μm and in particular between 30 and 80 μm. The distance d between adjacent elevations can be, for example, between 30 and 300 μm and in particular in the range of 60 to 250 μm.

[0117] The hexagonal basic structure with a plurality of elevations E shown in FIG. 4D can be arranged uniformly over the entire surface of a sheet packaging product according to the invention, resulting in a uniform, deterministic surface structure with hexagonal arrangements of elevations E.

[0118] The proportion Mrl of the plateau area of the elevations to the total area of the surface of the sheet packaging product (which can be referred to as the “load-bearing proportion”) is preferably between 5% and 50%. The number of elevations E with radii of curvature greater than 0.2 mm is preferably less than 50 per cm.sup.2 and is in particular less than 20 per cm.sup.2.

[0119] Further examples of such surface structures with a hexagonal structure of elevations E are shown in FIGS. 5E and 5G, each showing the surface of embodiments of a sheet packaging product according to the invention in a plan view with an associated roughness profile (height profile) and the resulting autocorrelation.

[0120] The embodiments of sheet packaging products according to the invention shown in FIGS. 5D, 5F and 5G are particularly suitable for increasing the corrosion resistance of the sheet packaging products due to the selected surface structure with a hexagonal arrangement of protrusions E. This results in particular from the fact that the deterministic surface structure of these examples has no sharp peaks and no deep or geometrically undefined valleys, but instead has a uniform arrangement of elevations with an at least substantially planar plateau on the upper side of the elevations, and with substantially planar valleys between the elevations E. The elevations E also withstand strong mechanical stresses due to the plateau-shaped design of the upper side, whereby abrasion and damage to the coating B can be avoided. Furthermore, no dirt or residues can settle in the valleys formed between adjacent elevations. To increase corrosion resistance, it is advantageous if the average distance between adjacent structural elements (“peak to peak distance”) is between 60 and 250 μm.

[0121] One parameter for the quantitative description of the corrosion properties of tinplate is the so-called IET value, which is measured in the standardized “Iron Exposure Test” and describes the tin porosity of the tin coating. Under constant conditions of the manufacturing process, such as pretreatment (cleaning), total tin coating and constant process parameters, the tin porosity (IET value) essentially depends on the surface roughness (arithmetic mean roughness Ra) and the tin coating (in g/m.sup.2).

[0122] In order to take into account the (quadratic) dependence of the tin porosity (IET value) on the tin coating Sn (tin weight in g/m.sup.2) in tinplate, it is useful to multiply the IET value measured on a tinplate sample (in mA/cm.sup.2) by the square of the tin coating Sn (in g/m.sup.2). FIG. 6 shows the product of the measured IET value and the tin coating squared (Sn.sup.2) for various tinplate samples, including conventional tinplate and tinplate according to the invention, and plots it against the mean roughness Ra of the samples. From the graph in FIG. 6, it can be calculated that the current density measured in the Iron Exposure Test (IET) is j.sub.IET=I/A (electric current per area in mA/cm.sup.2) for the samples according to the invention is at most 1.4 times the arithmetic mean roughness Ra (in μm) plus a constant of 0.5 divided by the tin coating Sn (weight coating of tin, m/A, mass per area in g/m.sup.2) squared:

[00004]$j_{\mathrm{IET}}(\mathrm{in}\frac{\mathrm{mA}}{{\mathrm{cm}}^2}\le (1,4.Math.R_a\left(\mathrm{in}\mathrm{\mu m}\right)+0,5/{\left(\mathrm{Sn}\right)}^2.$

[0123] The examples 1, 2 and 3 lying below the straight line (y=1.4 x+0.5) in the diagram of FIG. 6 fulfill this condition. Examples 1, 2 and 3 of FIG. 6 are specimens with a surface structure according to FIGS. 5D, 5F and 5G.

[0124] Through the IET value, a positive influence of the deterministic surface structures of the tinplate samples according to the invention on their corrosion resistance of the tinplates can be quantitatively demonstrated.

[0125] The packaging sheet products according to the invention can also be used to optimize the gloss properties. FIGS. 7 and 8 show diagrams showing the dependence of the gloss values measured on packaging sheets according to the invention (tinplate with a weight coating of 2.8 g/m.sup.2) and the isotropy of the gloss values (measured as delta gloss values (Δgloss), which represent the difference in gloss values in the rolling direction and perpendicular to it) on the surface roughness (arithmetic surface roughness Ra). As shown in FIG. 7, the gloss value (in gloss units GE) decreases (inversely proportional) with increasing roughness (Ra). With roughness Ra in the range of less than 0.4 μm, gloss values of more than 200 and, with Ra≤0.1 μm, up to approx. 1400 gloss units (GE) can be achieved with the packaging sheets according to the invention.

[0126] With a surface roughness of, for example, Ra=0.1 μm, gloss values of more than 670 can be achieved, and with surface roughnesses of Ra≤0.05 μm, gloss values of more than 1000 can be achieved.

[0127] FIG. 8 shows that homogeneous gloss properties with delta gloss values of Δgloss<100 can be achieved with the packaging sheets according to the invention, whereas conventional packaging sheets (designated “standard material” in FIG. 8) with otherwise identical coating parameters (in particular the same coating material with the same weight layer, the same process parameters in the electrolytic coating process and the same pretreatment) exhibit substantially more inhomogeneous gloss values with Δgloss>100. Preferably, the Δgloss value for the packaging sheet products according to the invention is 70 gloss units (GU) or less.

[0128] For the same surface roughness, the ΔGloss value of the packaging sheets according to the invention with deterministic surface structure is at least a factor of 4 smaller than that of the conventional packaging sheets with a statistically uncorrelated surface structure. This improvement in the homogeneity of the gloss can be explained by the uniform surface structures of the packaging sheets according to the invention with the same height or depth of the surface structures (elevations or depressions) both longitudinally and transversely to the rolling direction (longitudinal direction of the strip).

[0129] Furthermore, it can be seen from the diagram of FIG. 8 that the “double I structures” with a surface structure according to FIGS. 4B and 4E show the best results in terms of the Δgloss value with respect to isotropic and homogeneous gloss effects, respectively.

TABLE-US-00001 TABLE 1 Topography left FIG. Roll arrangement Re-rolling mill and right image 4B Surface according to the invention, Left: Topography designation: double I structure work roll Rolling Re-rolling mill: Right: Topography Stand 1: 2 work rolls with blasted surface sheet without coating Stand 2: 2 work rolls with UKPL structure 4C Surface according to the invention, Left: Topography designation: stone finish structure work roll Rolling Re-rolling mill: Right: Topography Stand 1: 2 work rolls with blasted surface sheet without coating Stand 2: 2 work rolls with UKPL structure 4D Surface according to the invention, Left: Topography designation: hexagonal structure work roll Rolling Re-rolling mill: Right: Topography Stand 1: 2 work rolls with blasted surface sheet without coating Stand 2: 2 work rolls with UKPL structure 4E Surface according to the invention, Left: Topography designation: double I structure work roll Rolling Re-rolling mill: Right: Topography Stand 1: 2 work rolls with UKPL structure sheet with coating Framework 2: 2 work rolls with polished surface structure 4F Surface according to the invention, Left: Topography designation: stone finish structure work roll Rolling Re-rolling mill: Right: Topography Stand 1: 2 work rolls with UKPL structure sheet with coating Framework 2: 2 work rolls with polished surface structure 4G Surface according to the invention, Left: Topography designation: hexagonal structure work roll Rolling Re-rolling mill: Right: Topography Stand 1: 2 work rolls with UKPL structure sheet with coating Framework 2: 2 work rolls with polished surface structure