SHEET STEEL HAVING A DETERMINISTIC SURFACE STRUCTURE

Abstract

The invention relates to sheet steel, more particularly a coated sheet steel, which is skin-pass rolled with a deterministic surface structure, and to a method for producing this steel.

Claims

1. A sheet steel skin-pass rolled with a deterministic surface structure, where the surface structure, is impressed into the sheet steel) starting from a surface of the sheet steel, the surface structure having a flank region which runs, starting from the surface, down to a valley region, wherein at least the valley region has a roughness Ra of less than 300 nm.

2. The sheet steel as claimed in claim 1, wherein the sheet steel is uncoated.

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

4. The sheet steel as claimed in claim 3, the sheet steel being coated with a zinc-based coating which is applied by hot-dip coating, where in the coating, as well as zinc and unavoidable impurities, there includes additional elements including at least one of aluminum with a content of up to 5 wt % and/or magnesium with a content of up to 5 wt % in the coating.

5. The sheet steel as claimed in claim 4, where the sheet steel is additionally oiled with an oil, the oil in particular being taken up with a surface weight of up to 2 g/m.sup.2 in the surface structure.

6. A method for producing a sheet steel skin-pass rolled with a deterministic surface structure, comprising the following steps: providing a sheet steel, skin-pass rolling the 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 is impressed into the sheet steel starting from a surface of the sheet steel, where the surface structure has a flank region which runs, starting from the surface, down to a valley region, and where at least the valley region has a roughness Ra of less than 300 nm.

7. The method as claimed in claim 6, where prior to the providing of the sheet steel, the sheet steel is coated by hot-dip coating.

8. The method as claimed in claim 7, where the melt for the hot-dip coating comprises, as well as zinc and unavoidable impurities, additional elements including at least one of as aluminum with a content of up to 5 wt % and/or magnesium with a content of up to 5 wt %.

9. The method as claimed in claim 8, where the sheet steel after the skin-pass rolling is additionally oiled with oil, the oil being applied in a surface weight of up to 2 g/m.sup.2.

10. The method as claimed in claim 9, where the oil is applied with a surface weight of up to 1 g/m.sup.2.

Description

IN THE DRAWING

[0022] FIG. 1a) and 1b) each show a schematic view in partial section of first and second exemplary embodiments of the invention of a coated sheet steel skin-pass rolled with a deterministic surface structure,

[0023] FIG. 2a), 2b) and 2c) show partial representations of a sheet steel coated in accordance with the prior art and skin-pass rolled with a stochastic surface structure, and

[0024] FIG. 3a), 3b) and 3c) show partial representations of a coated sheet steel of a third exemplary embodiment of the invention, skin-pass rolled with a deterministic surface structure.

[0025] FIG. 1a) and 1b) each represent a schematic view in partial section of first and second exemplary embodiments according to the invention of a coated sheet steel (1) skin-pass rolled with a deterministic surface structure (2). The surface structure (2) is impressed, starting from a surface (1.1) of the sheet steel (1), into the coated sheet steel (1), with the surface structure (2) having a flank region (2.3) which runs, starting from the surface (1.1), down to a valley region (2.2). At least the valley region (2.2) has a roughness Ra of less than 300 nm. Depending on the method of material removal used to work the corresponding skin-pass roll (not shown) for rolling the coated sheet steel (1), the valley region (2.2) may be established by means of the corresponding region on the skin-pass roll not shown (peak region/plateau). The skin-pass roll may additionally be afterworked by means of a grinding operation, where the peak region/plateau is smoothed accordingly, allowing a further reduction in the roughness Ra to be implemented in the valley region (2.2). It is readily apparent in FIGS. 1a) and 1b), furthermore, that the surface structure (2) has a flank region (2.3) which runs, starting from the surface (1.1), down to a valley region (2.2) and is configured with an angle (α) of between 1° and 89° to the perpendicular (O) of the coated sheet steel (1). Together with the valley region (2.2) attached or connected in one piece to the flank region (2.3), the flank region (2.3) which borders and configures the surface structure (2) defines a closed volume of the surface structure (2) impressed into the coating sheet steel (1) by skin-pass rolling. FIG. 1b) shows a smoother and consequently low roughness Ra in the valley region (2.2) by comparison with FIG. 1a), which has been worked by means of a ground skin-pass roll which is not shown, and so a deterministic surface structure (2) can be produced on a coated sheet steel (1), it being possible for this structure to be established at least in the valley region (2.2) of the surface structure (2) with a roughness Ra more particularly of less than 250 nm, preferably less than 200 nm, more preferably less than 150 nm, very preferably less than 100 nm. The embodiment of the surface structure in accordance with the invention may also be implemented on an uncoated sheet steel.

[0026] FIG. 2a), 2b) and 2c) show partial representations of a sheet steel coated in accordance with the prior art and skin-pass rolled with a stochastic surface structure. The surface structure was imparted using an EDT-textured and subsequently ground skin-pass roll (not depicted). Depicted in FIG. 2a) is a detail of a sheet topography provided with a zinc coating, measured by atomic force microscopy (AFM). The roughness Ra based on an area (U) of 60×12.5 μm.sup.2 was determined in the valley region, the value found for Ra being 323 nm. Shown illustratively is a distribution of oil on a sheet topography rolled using an EDT-textured and subsequently ground skin-pass roll (not depicted) and provided with a zinc coating, with FIG. 2b) depicting a detail by means of optical microscopy and FIG. 2c) depicting the same detail but by Raman spectroscopy, with the surface oil showing as light. The valley region of the surface structure was in part filled entirely with oil, with the surface weight of oil being more than 2 g/m.sup.2.

[0027] A different outcome is apparent when looking at the partial representations of a coated sheet steel (1), skin-pass rolled with a deterministic surface structure, in a third exemplary embodiment of the invention, FIG. 3a), 3b) and 3c). The surface structure was imparted using a laser-structured and subsequently ground skin-pass roll (not shown). The deterministic surface structure (2) was started using the example of a constantly repeating I-shaped impression. Other forms of implementation are likewise conceivable and employable, and are not confined to an I-shaped impression. FIG. 3a), 3b) and 3c) show two I-shaped impressions disposed next to one another. FIG. 3a) shows a detail of a sheet topography (1, 2) provided with a zinc coating (1.2) and measured by atomic force microscopy (AFM). The roughness Ra based on an area (U) of 60×12.5 μm.sup.2 was determined in the valley region (2.2), the value found for Ra being 77 nm. Shown illustratively is a distribution of oil on a sheet topography rolled using an EDT-textured and subsequently ground skin-pass roll (not depicted) and provided with a zinc coating, with FIG. 2b) depicting a detail by means of optical microscopy and FIG. 2c) depicting the same detail but by Raman spectroscopy, with the surface oil showing as light. The valley region (2.2) of the surface structure (2) was substantially not wetted with oil, with the oil having accumulated along the flank region (2.3) and/or at the transition between flank region (2.3) and valley region (2.2), owing to the capillary effect and to the reduced roughness Ra of less than 300 nm in the valley region (2.2). The surface weight of oil was reduced to up to 1.5 g/m.sup.2, more particularly to up to 1 g/m2.

[0028] For further studies, four coated and skin-pass-rolled steel sheets (V1 to V4) were produced. The same type of coating was selected for all of the steel sheets: a zinc-based coating (zinc and unavoidable impurities) which was applied in a hot-dip coating operation and had a thickness of around 7 μm. V1 and V2 correspond to steel sheets (1) of the invention, and V3 and V4 form reference sheets, differing from V1 and V2 in that the skin-pass roll had a stochastic surface structure, the surface of the skin-pass roll having been textured by means of EDT, for example, meaning that a stochastic surface structure was also impressed into the reference sheets. Table 1 contrasts the steel sheets (1) according to the invention with the reference sheets.

TABLE-US-00001 TABLE 1 Ra [nm], Cup- Steel Ra Rz Wsa RPc valley Oil drawing sheets [μm] [μm] [μm] [1/mm] region [g/m.sup.2] test V1 0.771 3.62 0.0783 11.9 121 1 ++ V2 1.43 5.92 0.113 13.9 189 1.3 + V3 1.08 7.53 0.198 10.2 367 2 0 V4 1.08 6.81 0.198 10.1 420 1.8 −

[0029] The determination of the surface parameters Ra (arithmetic mean roughness), Rz (mean roughness depth) and RPc (peak count determined along a defined length, in the above case per mm) may be derived from DIN EN ISO 4287, and the characteristic value for the long-wave waviness Wsa (arithmetic mean waviness) in accordance with SEP1941. The data in table 1 in relation to a strip drawing test, a cup-drawing test according to DIN EN 1669, which was carried out under the same conditions for all four steel sheets V1 to V4, show surprisingly, however, that better results were achievable in comparison between V1/V2 and V3/V4. Evaluation was made according to the following criteria:

[0030] ++ means that not only the friction coefficient determined in the strip drawing test but also the thinning in the outgoing portion of the punch edge on the formed steel sheet are lower (low level of thinning, less than 5% of the original steel sheet thickness),

[0031] + means that the minimal thinning on the reshaped steel sheet is more than 5% but less than 10% of the original steel sheet thickness,

[0032] 0 indicates a markedly recognizable thinning without ruptures, which is no longer in the tolerable range (15% to 25% of the original steel sheet thickness), and [0033] means that ruptures occur.

[0034] At the same time, moreover, it was possible to reduce the surface weight of oil on the steel sheet V1 and V2, coated in accordance with the invention and skin-pass rolled with a deterministic surface structure, to below 1.5 g/m.sup.2, the quantity being sufficient to achieve an appropriately good outcome.