Abstract
The present disclosure relates to a process for producing a surface-treated and surface-finished sheet steel. A sheet steel having a zinc-based coating is provided, wherein zinc grains are distributed within the coating. The surface-treated sheet steel are skin-pass rolled to form embossed regions and unembossed regions on the surface of the sheet steel provided with a zinc-based coating. Skin-pass rolling is performed with a degree of skin-pass greater than 1% in such a way that due to the force exerted by the skin-pass rolling the zinc grains in the embossed region are altered in dimension relative to the zinc grains in the unembossed region.
Claims
1. A process for producing a surface-treated and surface-finished sheet steel, wherein the process comprises the steps of: providing a sheet steel having a zinc-based coating, wherein zinc grains are distributed within the coating, skin-pass rolling the surface-treated sheet steel to form embossed regions and unembossed regions on the surface of the sheet steel provided with a zinc-based coating, wherein the skin-pass rolling is performed with a degree of skin-pass greater than 1% in such a way that due to the force exerted by the skin-pass rolling the zinc grains in the embossed region are altered in dimension relative to the zinc grains in the unembossed region.
2. The process as claimed in claim 1, wherein the zinc grains in the embossed region are smaller in size than the zinc grains in the unembossed region of the surface-treated and surface-finished sheet steel.
3. The process as claimed in claim 2, wherein the zinc-based coating has the following chemical composition in % by weight: one or more alloying elements from the group (Al, Mg): Al up to 5.0; Mg up to 5.0; balance Zn and unavoidable impurities.
4. The process as claimed in claim 3, wherein the zinc-based coating comprises Al and Mg in each case in a content of at least 0.5% by weight.
5. The process as claimed in claim 4, wherein the zinc-based coating has a thickness between 2 and 20 μm.
6. The process as claimed in claim 5, wherein the skin-pass rolling introduces a deterministic surface structure into the surface-treated sheet steel.
7. The process as claimed in claim 5, wherein the skin-pass rolling introduces a stochastic surface structure into the surface-treated sheet steel.
8. The process as claimed in claim 7, wherein the surface-treated and surface-finished sheet steel is phosphated.
Description
[0024] In the drawings:
[0025] FIGS. 1 a, b) shows schematic partial sections of a provided, surface-treated sheet steel a) and a surface-treated and surface-finished sheet steel b),
[0026] FIGS. 2a, b) each show a micrograph of a subregion of a surface of a surface-treated and surface-finished sheet steel having a stochastic surface structure a) and having a deterministic surface structure b),
[0027] FIG. 3) shows a micrograph of a subregion of a surface-treated and surface-finished sheet steel in a transverse section along the line in FIG. 2a) and
[0028] FIG. 4a, b) each show a micrograph of a subregion of a surface of a surface-treated, surface-finished and phosphated sheet steel which was not subjected to skin-pass rolling according to the invention a) and according to the invention b).
[0029] FIG. 1 shows schematic partial sections before and after skin-pass rolling. FIG. 1 a) is a schematic diagram of a partial section of the upper portion of a provided, surface-treated sheet steel (10). The surface-treated sheet steel (10) comprises a sheet steel (1) having a zinc-based coating (1.1), wherein zinc grains (2) are arranged distributed within the coating (1.1). In addition to zinc and unavoidable impurities the zinc-based coating (1.1) may optionally also contain one or more alloying elements from the group (Al, Mg): Al up to 5.0, Mg up to 5.0. The thickness of the sheet steel (1) is 0.5 to 4.0 mm for example. The provided, surface-treated sheet steel (10) is supplied to a skin-pass rolling which is performed such that skin-pass rollers (not shown) comprising shaping elements act on both sides of the surface of the surface-finished sheet steel (10), wherein the skin-pass rolling forms embossed regions (3) and unembossed regions (4) on the surface of the sheet steel (1) provided with a zinc-based coating (1.1), cf. FIG. 1b). The skin-pass rolling may be used to introduce a deterministic or stochastic surface structure into the surface-treated sheet steel (10). The skin-pass rolling is performed with a degree of skin-pass greater than 1% in such a way that due to the force exerted by the skin-pass rolling the zinc grains (2.1) in the embossed region (3) are altered in dimension relative to the zinc grains (2) in the unembossed region (4) as illustrated in the schematic diagram in FIG. 1b). The zinc grains (2.1) in the embossed region (3) are smaller in size than the zinc grains (2) in the unembossed region (4) of the surface-treated and surface-finished sheet steel (11).
[0030] FIG. 2 in each case shows respective micrographs, recorded using a scanning electron microscope (SEM), of a subregion of a surface-treated and surface-finished sheet steel (11), wherein a stochastic surface structure, cf. FIG. 2a) and a deterministic surface structure, cf. FIG. 2b) have been produced. A sheet steel (1) made of a soft steel grade “CR4” was cold-rolled to a thickness of 0.7 mm and coated with a zinc-based coating (1.1) in a hot-dip coating system, wherein the coating (1.1) shown in FIG. 2a) contained 1.6% by weight of Al and 1.1% by weight of Mg and the coating (1.1) shown in FIG. 2b) contained 0.4% by weight of Al. The surface-treated sheet steel (10) was subject to skin-pass rolling with an EDT-textured skin-pass roller, not shown, (FIG. 2a)) and with an LT-textured skin-pass roller, not shown, (FIG. 2b)) in each case with a degree of skin-pass of 1.5%.
[0031] Irrespective of the type of surface structure it is apparent that from a degree of skin-pass above 1%, in particular above 1.2%, preferably above 1.4%, a change in the zinc grains (2.1) in the embossed region (3) may be effected, wherein the especially “targeted” force exertion, for example damaging or fracturing the zinc grains (2.1) in the embossed region (2) in FIG. 2b), allows advantageous microfractures (2.2) or in FIG. 2a) advantageous further microfractures (2.2) additional to those already formed in the intermetallic phase to be generated on the surface of the coating (1.1).
[0032] FIG. 3) shows a micrograph of a subregion of the surface-treated and surface-finished sheet steel (11) in a transverse section along the line (L) in FIG. 2a) which was recorded using a scanning electron microscope (SEM). The force exerted or mechanical stress in the embossed regions (4) results in damage to and/or fracturing of the zinc grains (2.1), thus altering the dimension relative to the original zinc grains or relative to the zinc grains (2) in the unembossed region (4).
[0033] In a further investigation sheet steels (1) made of a soft steel grade “CR4” were each cold-rolled to a thickness of 0.7 mm and coated with a zinc-based coating (1.1) in a hot-dip coating system, wherein the coating (1.1) contained 1.4% by weight of Al and 1.2% by weight of Mg. The surface-treated sheet steels (10) were subjected to skin-pass rolling with an EDT-textured skin-pass roller (not shown) with different degrees of skin-pass. The different surface-treated and surface-finished sheet steels (11) were then phosphated. FIG. 4) shows respective micrographs of subregions of a surface of a surface-treated, surface-finished and phosphated sheet steel subjected to skin-pass rolling with a degree of skin-pass of 0.95%, cf. FIG. 4a), and according to the invention with a degree of skin-pass of 1.25%, cf. FIG. 4b). Compared to the noninventive skin-pass rolling the inventive configuration in the right-hand image shows a more homogenous phosphating with a more uniform zinc phosphate crystal growth compared to the left-hand image, with finer/smaller zinc phosphate crystals which are especially attributable to the further advantageous “microstructures” (2.2) due to the reduction in size of the original zinc grains and the recrystallized, smaller zinc grains (2.1).
[0034] The features described are all combinable with one another insofar as this is technically possible.
