SHEET STEEL HAVING A DETERMINISTIC SURFACE STRUCTURE

Abstract

The invention relates to a sheet steel (1) coated with a zinc-based coating and skin-pass rolled with a deterministic surface structure (2), and to a method for producing it.

Claims

1. A sheet steel coated with a zinc-based coating and skin-pass rolled with a deterministic surface structure, the surface structure having peak regions and valley regions which are connected to one another via flank regions, wherein the coating, as well as zinc and unavoidable impurities, comprises additional elements such as aluminum with an amount of 0.1 up to 5 wt % and magnesium with an amount of 0.1 up to 5 wt % in the coating, the relative concentration of magnesium on the surface of the coating in the valley regions being lower than on the surface of the coating in the peak regions.

2. The sheet steel as claimed in claim 1, the relative concentration of magnesium on the surface of the coating being at least 20% lower in the valley regions than in the peak regions.

3. The sheet steel as claimed in claim 2 wherein, the relative concentration of zinc on the surface of the coating in the valley regions is greater than the relative concentration of zinc on the surface of the coating in the peak regions.

4. The sheet steel as claimed in claim 3 wherein, the flank region is configured with an angle () of between 1 and 89 to be perpendicular (O) of the sheet steel.

5. The sheet steel as claimed in claim 4 wherein, the sheet steel has a material ratio of less than 50%.

6. The sheet steel as claimed in claim 5 wherein, the sheet steel has a factor F greater than 1%.

7. The sheet steel as claimed in claim 6 wherein, the sheet steel being additionally provided with a process medium (M), the process medium (M) in particular being taken up with a surface weight of up to 2 g/m.sup.2 in the surface structure.

8. A method for producing a sheet steel coated with a zinc-based coating and skin-pass rolled with a deterministic surface structure, comprising the following steps: providing a sheet steel coated with a zinc-based coating, the coating, as well as zinc and unavoidable impurities, comprising additional elements such as aluminum with an amount of 0.1 up to 5 wt % and magnesium with an amount of 0.1 up to 5 wt % in the coating, skin-pass rolling the coated sheet steel with a skin-pass roll, where the surface of the skin-pass roll which acts on the surface of the sheet steel is furnished with a deterministic surface structure such that after the skin-pass rolling, the surface structure of the coated and rolled sheet steel has peak regions and valley regions which are connected to one another via flank regions, and the exposure to the skin-pass roll establishes a relative concentration of magnesium on the surface of the coating in the valley regions which is lower than on the surface of the coating in the peak regions.

9. The method as claimed in claim 8, wherein the degree of skin-pass rolling is being at least 0.5%.

10. The method as claimed in claim 9, wherein the sheet steel is additionally provided with a process medium (M), the process medium (M) being applied with a surface weight of up to 2 g/m.sup.2.

11. The sheet steel as claimed in claim 1, the relative concentration of magnesium on the surface of the coating being at least 40% lower in the valley regions than in the peak regions.

12. The sheet steel as claimed in claim 1, the relative concentration of magnesium on the surface of the coating being at least 60% lower in the valley regions than in the peak regions.

13. The sheet steel as claimed in claim 1, the relative concentration of magnesium on the surface of the coating being at least 90% lower in the valley regions than in the peak regions.

14. The sheet steel as claimed in claim 4 wherein, the sheet steel has a material ratio of less than 40%.

15. The sheet steel as claimed in claim 4 wherein, the sheet steel has a material ratio of less than 30%.

16. The sheet steel as claimed in claim 4 wherein, the sheet steel has a material ratio of less than 25%.

17. The sheet steel as claimed in claim 4 wherein, the sheet steel has a material ratio of less than 20%.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0046] In the drawing

[0047] FIG. 1) shows a schematic view in partial section of an exemplary embodiment of the invention of a sheet steel coated with a zinc-based coating and skin-pass rolled with a deterministic surface structure,

[0048] FIGS. 2a) and 2b) show a schematic view in partial section of a surface structure on a coated and skin-pass rolled sheet steel in FIG. 2a), and a surface structure as per FIG. 2a) in FIG. 2b), and

[0049] FIG. 3) shows ToF-SIMS data for a section of a sheet steel coated with a zinc-based coating and skin-pass rolled with a deterministic surface structure, according to one exemplary embodiment of the invention.

DESCRIPTION

[0050] FIG. 1) represents a schematic view in partial section of an exemplary embodiment of the invention of a sheet steel (1) coated with a zinc-based coating (1.2) and skin-pass rolled with a deterministic surface structure (2), the surface structure (2) having peak regions (2.1) and valley regions (2.2), which are connected to one another via flank regions (2.3). As well as zinc and unavoidable impurities, the coating (1.2) comprises additional elements such as aluminum with an amount of 0.1 up to 5 wt % and magnesium with an amount of 0.1 up to 5 wt %.

[0051] Depending on the method of material removal used to machine a

[0052] corresponding skin-pass roll (not represented) for skin-pass rolling the coated sheet steel (1), the flank region (2.3) and the valley region (2.2) are established by means of the corresponding region (positive shape) on the skin-pass roll not represented. It is readily apparent in FIG. 1), furthermore, that the surface structure (2) has a flank region (2.3) which, starting from the surface or from the peak region (2.1), runs down to a valley region (2.2) and which is formed with an angle () between 1 and 89 to the perpendicular (O) of the sheet steel (1). The flank region (2.3) forming the surface structure (2) and running peripherally around it, together with the valley 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 sheet steel (1) by means of skin-pass rolling.

[0053] In accordance with the invention, the relative concentration of magnesium on the surface of the coating (1.2) in the valley regions (2.2) is lower than on the surface of the coating (1.2) in the peak regions (2.1).

[0054] FIG. 2a) shows a schematic view in partial section of a deterministic surface structure, in the form of a double-I shape on a coated and skin-pass rolled sheet steel (1). The surface structure may be skin-pass rolled preferably by means of a laser-structured skin-pass roll (not represented); cf. EP 2 892 663 B1. The material ratio is less than 30%, with the material ratio reflecting the ratio of the areas of the peak regions (2.1) or the area of the peak region (2.1), dashed area of the material ratio (T), to the areas of the valley regions (2.2). FIG. 2b) shows only an I-shaped structure as an example, which can be impressed into a coated sheet steel (1). The factor F is greater than 1, expressing the ratio of skin-pass rolled area, corresponding for example to the area of the rectangle or square illustrated in FIG. 2a), minus the area of the material ratio (T), and the square of the area periphery (U); cf. FIG. 2b).

[0055] A deterministic surface structure was studied using the example of a constantly repeating I-shaped impression. Other embodiments are likewise conceivable and applicable and are not confined to an I-shaped impression. As well as zinc and unavoidable impurities, the zinc-based coating contained magnesium with an amount of 1.1 wt % and aluminum with an amount of 1.6 wt %. The thickness of the coating was established at 7 m in the course of hot-dip coating on a steel sheet/strip 1.2 mm thick. The degree of skin-pass rolling was 0.8%. This skin-pass rolled sheet steel was studied more closely, in respect of the surface composition in particular. FIG. 3) shows ToF-SIMS data for a section of this skin-pass rolled steel sheet, in a high lateral resolution in a region of 200200 m, to characterize the chemical surface composition. Represented on the right are images of single-signal distributions of the alloy elements Mg, Al and Zn, and illustrated on the left is a superimposition of the individual signal distributions. It is readily apparent that magnesium-rich oxide layers have formed on the surface of the coating (1.2) in the course of the hot-dip coating operation. Because the peak regions (2.1) interact only insubstantially, if at all, with the shaping elements of a skin-pass roll and therefore are not subject to any substantial action of force, the magnesium-rich oxide layers are substantially retained on the surface, represented with light coloring, of the coating (1.2) in the peak regions or in the peak region (2.1); see FIG. 3, left-hand illustration. The situation is different for the relative concentration of magnesium in the valley regions (2.2) and in the flank regions (2.3). The relative concentration of magnesium (in metallic and/or oxidic form and/or hydroxidic form and/or as magnesium carbonate) present on the surface of the coating (1.2) in the valley regions (2.2) is lower than on the surface of the coating (1.2) in the peak regions (2.1) or in the flank regions (2.3). For example, the relative concentration of magnesium on the surface of the coating (1.2) in the valley regions (2.2) is at least 20% lower than on the surface of the coating (1.2) in the peak regions (2.1), more particularly at least 40%, preferably at least 40%, more preferably at least 90%.

[0056] The zinc(oxide) and/or aluminum(oxide) alloy elements situated directly beneath the magnesium-rich oxide layer reach the surface owing to the mechanical action of the shaping elements of the skin-pass roller, and so the relative concentration of zinc on the surface of the coating (1.2) in the valley regions (2.2) is greater than the relative concentration of zinc on the surface of the coating (1.2) in the peak regions (2.1). The relative concentration of zinc on the surface of the coating (1.2) in the valley regions (2.2) is at least 20% greater than the relative concentration of zinc on the surface of the coating (1.2) in the peak regions (2.1), more particularly at least 30%, preferably at least 40%, more preferably at least 50%. The relative concentration of magnesium, zinc and/or aluminum within the valley regions (2.2) can be influenced by the action of the shaping elements of the skin-pass roll, in particular as a function of the degree of skin-pass rolling, which can be between 0.5% and 2.5%.

[0057] For further studies, six coated, skin-pass rolled steel sheets (V1 to V6) were employed. The nature of the coating was chosen the same for all of the steel sheets: a zinc-based coating (zinc and unavoidable impurities) with a magnesium content of 1.6 wt % and an aluminum content of 1.1 wt %, which was applied in a hot-dip coating operation and had a thickness of around 7 m. V1 to V3 were skin-pass rolled with a stochastic surface structure, and V4 to V6 with a deterministic surface structure, in the form of a repeating I-shape.

TABLE-US-00001 TABLE 1 Degree of skin- Steel Oil surface Cup-drawing pass rolling [%]/ Material sheet weight [g/m.sup.2] test Factor F [%] ratio T [%] V1 2 ++ 0.8/* * V2 1.5 + 0.9/* * V3 1 0 1.0/* * V4 1 +++ 0.8/>2 <25 V5 0.8 ++ 0.9/>2 <20 V6 0.6 + 1.0/>2 <15 The data in table 1 relating to a strip-drawing test, a cup-drawing test according to DIN EN 1669, which was carried out under identical conditions for all six steel sheets V1 to V6, show essentially a positive outcome. For V1 to V3, it was not possible to determine the factor F and the material ratio, since a stochastic surface structure does not exhibit any reasonably detectable size and was therefore marked *. Evaluation was made according to the following criteria: +++ means that no thinning was perceptible; ++ means that both the friction coefficient determined in the strip-drawing test and the thinning at the runout of the die edge on the formed steel sheet were lower (low thinning below 5% of the original steel sheet thickness), + means that the minimum thinning on the formed steel sheet was more than 5% but less than 10% of the original steel sheet thickness, 0 means that there was a marked, perceptible thinning without ruptures, but this thinning was not longer within the tolerable range (15% to 25% of the original steel sheet thickenss).

[0058] As a result of the establishmentmore particularly, targeted establishmentof the relative concentration of magnesium, and more particularly through the reduction in the magnesium-rich oxide layers on the surface of the coating in the valley regions, it was possible to reduce the surface weight of oil on the steel sheets V4 to V6, coated in accordance with the invention and skin-pass rolled with a deterministic surface structure, to below 1 g/m.sup.2, the amount being sufficient to achieve an appropriately good outcome.