SHEET STEEL HAVING A DETERMINISTIC SURFACE STRUCTURE

Abstract

The invention relates to a steel sheet skin-pass rolled with a deterministic surface structure, and to a method for producing it.

Claims

1. A steel sheet skin-pass rolled with a deterministic surface structure, wherein the surface structure is impressed into the steel sheet starting from a surface of the steel sheet, wherein the surface structure comprises a flank region that runs from the surface to a trough region, wherein at least the flank region has a roughness Ra of greater than 20 nm.

2. The steel sheet as claimed in claim 1, wherein the flank region is formed at an angle (α) of between 1° and 89° to the perpendicular (O) of the steel sheet.

3. The steel sheet as claimed in claim 2, wherein the steel sheet comprises a metallic coat.

4. The steel sheet as claimed in claim 3, wherein the steel sheet is coated with a zinc-based coat which is applied by hot-dip coating, wherein the coat, contains in addition to zinc and unavoidable impurities, additional elements including at least one of aluminum with a content of up to 5% by weight and magnesium with a content of up to 5% by weight in the coat.

5. The steel sheet as claimed in claim 3, wherein the steel sheet is coated with a zinc-based coat which is applied by electrolytic coating.

6. The steel sheet as claimed in claim 5, wherein the steel sheet is additionally provided with a process medium, wherein in particular the process medium is included in the surface structure with an applied amount of up to 2 g/m.sup.2.

7. A method for producing a steel sheet skin-pass rolled with a deterministic surface structure, comprising the following steps: providing a steel sheet, and skin-pass rolling the steel sheet by means of a skin-pass roller, wherein the surface of the skin-pass roller acting on the surface of the steel sheet is configured with a deterministic surface structure in such a way that, after the skin-pass rolling operation, the surface structure is impressed into the steel sheet starting from a surface of the steel sheet, wherein the surface structure comprises a flank region which runs from the surface to a trough region and wherein at least the flank region has a roughness Ra of greater than 20 nm.

8. The method as claimed in claim 7, wherein the steel sheet is coated by hot-dip coating before the steel sheet is provided.

9. The method as claimed in claim 8, wherein the melt for the hot-dip coating contains, in addition to zinc and unavoidable impurities, additional elements including at least one of aluminum with a content of up to 5% by weight and magnesium with a content of up to 5% by weight.

10. The method as claimed in claim 7, wherein the skin-pass rolled steel sheet is coated by electrolytic coating after the steel sheet has been skin-pass rolled.

11. The method as claimed in claim 10, wherein the steel sheet is additionally provided with a process medium, wherein the process medium is applied with an applied amount of up to 2 g/m.sup.2.

Description

[0028] In the drawing:

[0029] FIG. 1) shows a schematic partial sectional view of an exemplary embodiment according to the invention of a steel sheet skin-pass rolled with a deterministic surface structure,

[0030] FIGS. 2a), 2b) and 2c) show respective schematic partial sectional views of different surface structures on a skin-pass rolled steel sheet according to the prior art in FIGS. 2a) and 2b) and a surface structure according to the invention on a skin-pass rolled steel sheet in FIG. 2c), and

[0031] FIGS. 3a) and 3b) show, respectively in an SEM image, a part of a coated steel sheet, skin-pass rolled with a deterministic surface structure, according to the prior art (FIG. 3a)) and as per an exemplary embodiment according to the invention (FIG. 3b)).

[0032] FIG. 1) illustrates a schematic partial sectional view of an exemplary embodiment according to the invention of a steel sheet (1, 1′) skin-pass rolled with a deterministic surface structure (2). It is possible for the steel sheet (1, 1′) to be an uncoated steel sheet (1), i.e. it does not have an in particular metallic coat or non-metallic coat, or a steel sheet (1′) coated with a metallic coat (1.2). The surface structure (2) is impressed into the steel sheet (1, 1′) starting from a surface (1.1) of the steel sheet (1), wherein the surface structure (2) comprises a flank region (2.3) which runs from the surface (1.1) to a trough region (2.2). At least the flank region (2.2) has a roughness Ra of greater than 20 nm. Depending on the material removal method used to process the corresponding skin-pass roller (not illustrated) for skin-pass rolling the steel sheet (1, 1′), the flank region (2.3) and the trough region (2.2) are set by the corresponding region (positive shape) on the skin-pass roller, which is not illustrated. Furthermore, it can be clearly seen in FIG. 1) that the surface structure (2) has a flank region (2.3) which runs from the surface (1.1) to a trough region (2.2) and is formed at an angle (α) of between 1° and 89° to the perpendicular (O) of the steel sheet (1, 1′). The flank region (2.3) forming and running peripherally around the surface structure (2), together with the trough region (2.2) connected or bonded in one piece to the flank region (2.3), defines a closed volume of the surface structure (2) impressed into the steel sheet (1, 1′) by means of skin-pass rolling.

[0033] FIGS. 2a), 2b) and 2c) respectively show schematic partial sectional views of different surface structures on a skin-pass rolled steel sheet.

[0034] FIG. 2a) shows a schematic partial sectional view of an in particular coated steel sheet skin-pass rolled with a stochastic surface structure, the surface structure having been skin-pass rolled by means of an EDT-structured skin-pass roller (not illustrated). The surface structure is substantially completely filled or covered with a process medium (M), for example oil. The requirement for process media (M) is higher in comparison with the other two embodiments (FIGS. 2b) and 2c)), since the surface structure in the case of EDT is not realized as a closed structure but as an open structure.

[0035] FIG. 2b) shows a schematic partial sectional view of an in particular coated steel sheet skin-pass rolled with a deterministic surface structure, the surface structure having been skin-pass rolled by means of a laser-structured skin-pass roller (not illustrated); cf. EP 2 892 663 B1. In comparison with FIG. 2a), it is possible to use less process media (M) since the surface structure is closed.

[0036] The configuration according to the invention of an in particular coated steel sheet (1, 1′) skin-pass rolled with a deterministic surface structure (2) is illustrated schematically in FIG. 2c) in a partial sectional view, the surface structure (2) having been skin-pass rolled by means of a laser-structured skin-pass roller (not illustrated): cf. also EP 2 892 663 B1, but with the difference that the roughness in the positive shape on the surface of the skin-pass roller has been set in a defined manner in the flank region (2.3) to be produced that acts on the steel sheet (1, 1′), with the result that, on the skin-pass rolled steel sheet (1, 1′), a deterministic surface structure (2) is set in the flank region (2.3) having a roughness Ra of greater than 20 nm, in particular greater than 50 nm, preferably greater than 100 nm, preferably greater than 150 nm, more preferably greater than 200 nm. This makes it possible, in comparison with the other embodiments (FIGS. 2a) and 2b)), to further minimize the need for process media (M) and to store it closer to or adjacent to the locations (1.1) that are relevant to the forming process.

[0037] A deterministic surface structure has been analyzed using the example of a constantly recurring I-shaped impression. Other embodiments are likewise conceivable and applicable and are not restricted to an I-shaped impression. FIG. 3a) illustrates an SEM image of a sheet topography provided with a zinc-based coat, the surface structure having been impressed by means of a skin-pass roller (not illustrated), the surface of the skin-pass roller having been structured by means of a laser; cf. EP 2 892 663 B1. FIG. 3b) illustrates an SEM image of the topography or deterministic surface structure (2) of a steel sheet (1′) skin-pass rolled with a zinc-based coat (1.2), the surface structure (2) having been impressed by means of a skin-pass roller (not illustrated), the surface of the skin-pass roller having been structured by means of a laser: cf. EP 2 892 663 B1, but with the difference that the roughness Ra in the positive shape on the surface of the skin-pass roller has been set in a defined manner in the flank region (2.3) to be produced that acts on the coated steel sheet (1′). The differently formed flank regions of the respective I-structure are clearly visible.

[0038] Using the example of the embodiment according to FIG. 3b), two uncoated and two hot-dip coated steel sheets (1, 1′) were skin-pass rolled with a deterministic surface structure. The flank regions of the sheet topography were analyzed by atomic force microscopy (AFM). The scanning area of the atomic force microscopy had a surface area of 90 × 90 .Math.m.sup.2, the roughness Ra in the flank region having been determined on a surface area of 20 × 2 .Math.m.sup.2 within the scanning area. For the two uncoated, skin-pass rolled steel sheets (1), the respective values Ra=45.99 nm and Ra=51.48 nm were determined, and for the two coated, skin-pass rolled steel sheets (1′), the respective values Ra=131.07 nm and Ra=205.40 nm were determined.

[0039] Four coated, skin-pass rolled steel sheets (V1 to V4) were used for further analysis. The type of coating was selected to be the same for all of the steel sheets: a zinc-based coat (zinc and unavoidable impurities), which was applied in the hot-dip coating process and was approx. 7 .Math.m thick. V1 and V2 correspond to steel sheets (1′) according to the invention and V3 and V4 are reference sheets. V3 and V4 are different with respect to V1 and V2 in that V3 and V4 were skin-pass rolled with a skin-pass roller having a deterministic surface structure and an undefined flank region (cf. embodiment in FIG. 3a). Table 1 reports a comparison of the steel sheets (1′) according to the invention and reference sheets.

TABLE-US-00001 Steel sheets Ra [nm] flank region Oil [g/m.sup.2] Cup-drawing test Adhesion test V1 131.07 < 0.8 ++ ++ V2 205.40 < 0.8 +++ + V3 < 20 1.0 + 0 V4 < 20 1.3 + 0

[0040] The roughness Ra (arithmetic mean roughness value) was determined using the method specified in DIN EN ISO 4287 and the numerical values in the table relate to a surface area of 20 × 2 .Math.m.sup.2, which only included the flank region. The roughness Ra of the steel sheets V3 and V4 was very small in the flank region. The information in Table 1 relating to a strip-drawing test, the cup-drawing test according to DIN EN 1669, which was carried out under the same conditions for all four steel sheets V1 to V4, essentially shows a positive result. The evaluation was based on the following criteria: [0041] +++ means that no thinning-out is visible, [0042] ++ means that both the coefficient of friction determined during the strip-drawing test and the thinning-out of the formed steel sheet at the run-out of the edge of the stamp are lower (slight thinning-out of less than 5% of the initial thickness of the steel sheet), [0043] + means that the minimum thinning-out of the formed steel sheet is more than 5% but less than 10% of the initial thickness of the steel sheet.

[0044] The information in Table 1 relating to the lap shear test on the basis of DIN EN 1465, which was carried out under the same conditions for all four steel sheets V1 to V4, shows different results in terms of the suitability for adhesion. The fracture behavior is evaluated on the basis of DIN EN ISO 10365, the numerical values specified below having been determined using empirical values. The evaluation was based on the following criteria:

[0045] ++ means that the proportion of the cohesive fracture surface area that was present as a fracture surface area in the adhesive in the course of the lap shear test was at least 85%, [0046] + means that the proportion of the cohesive fracture surface area that was present as a fracture surface area in the adhesive in the course of the lap shear test was between 60% and 85%, [0047] 0 means that the proportion of the cohesive fracture surface area that was present as a fracture surface area in the adhesive in the course of the lap shear test was between 40% and 60%.

[0048] In addition, at the same time it was possible to reduce the amount of process medium (M) applied to the steel sheet V1 and V2, that is coated according to the invention and skin-pass rolled with a deterministic surface structure, to below 1 g/m.sup.2, and the amount was sufficient to achieve a correspondingly good result.