STEEL SHEET WITH A DETERMINISTIC SURFACE STRUCTURE

Description

[0001] The invention relates to a coated steel sheet with a deterministic surface structure. Furthermore, the invention relates to a method for producing a coated steel sheet with a deterministic surface structure.

[0002] Steel sheets coated according to this type and dressed with a deterministic surface structure are known from the prior art, see for example patent specification EP2892663B1, US5532051A, US4783378A and JPH079015A.

[0003] With regard to the known state of the art, there is a need for optimization, particularly with a view to reducing the oil coating.

[0004] The task is therefore to provide a coated steel sheet with a deterministic surface structure that offers equivalent or better properties compared to the state of the art, while simultaneously reducing oil consumption.

[0005] The problem is solved using the features of claim 1.

[0006] The inventors have found that, in a coated steel sheet with a deterministic surface structure, equivalent or better properties can be provided compared to the prior art, particularly with a simultaneously reduced oil consumption, if the surface structure is embossed into the coated steel sheet starting from a surface of the coated steel sheet, wherein the surface structure has a flank region which extends from the surface to a valley region, wherein, according to the invention, the valley region has a roughness Ra of less than 300 nm, preferably to minimize the oil requirement.By limiting the roughness Ra, the measurement method for determining the Ra value is specified in DIN ISO EN 4287, to a roughness Ra of less than 300 nm, particularly less than 250 nm, preferably less than 200 nm, more preferably less than 150 nm, and further preferably less than 100 nm, in the valley region of the surface structure, the local oil distribution can be influenced. This prevents the oil from wetting the valley region, or only to a small extent, due to capillary forces acting towards the flank region, and instead causes it to accumulate along the flank region. The lower the roughness Ra in the valley region, the stronger the influence of the capillary force towards the flank region can be. Thus, the deterministic surface structure according to the invention makes it possible to produce a coated steel sheet with reduced oil requirements.

[0007] Deterministic surface structure refers to recurring surface structures that have a defined shape and / or design, cf. EP 2 892 663 B1. In particular, this also includes surfaces with a (quasi-)stochastic appearance, which are applied using a deterministic texturing process and are therefore composed of deterministic shape elements.

[0008] Steel sheet generally refers to a flat steel product which can be supplied in sheet form, in blank form or in strip form.

[0009] The steel sheet designed according to the invention is coated. The coating of the coated steel sheet comprises a metallic covering.

[0010] Further advantageous embodiments and developments will become apparent from the following description. One or more features from the claims, the description, and the drawings can be combined with one or more other features therein to form further embodiments of the invention. One or more features from the independent claims can also be combined with one or more other features.

[0011] According to one embodiment of the steel sheet according to the invention, the surface structure has a flank region which extends from the surface to a valley region and is formed at an angle between 1° and 89° to the perpendicular of the coated steel sheet. The angle can be formed, in particular, between 50° and 87°, preferably between 60° and 85°, and most preferably between 65° and 82°. The valley and flank region (negative shape) of the surface structure essentially corresponds to the surface (positive shape) of a tempering roller, which forms or imprints the surface structure by corresponding action on the coated steel sheet. The flank region surrounding and forming the surface structure, together with the valley region integrally connected to the flank region, defines a closed volume of the surface structure imprinted into the coated steel sheet by tempering.The closed volume, the so-called empty volume, can be tailored to the forming agent to be applied, especially oil, for subsequent processing using forming methods.

[0012] According to one embodiment of the steel sheet according to the invention, the steel sheet is coated with a zinc-based coating applied by hot-dip coating. In addition to zinc and unavoidable impurities, the coating may contain additional elements such as aluminum with a content of up to 5 wt.% and / or magnesium with a content of up to 5 wt.%. Steel sheets with a zinc-based coating offer very good cathodic corrosion protection and have been used in automotive manufacturing for many years. If improved corrosion protection is desired, the coating additionally contains magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt.%. Aluminum can be added as an alternative or in addition to magnesium with a content of at least 0.3 wt.%.A certain percentage of the coating must be present to improve the bond between the coating and the steel sheet, and in particular to essentially prevent the diffusion of iron from the steel sheet into the coating during heat treatment of the coated steel sheet, thus ensuring, for example, good adhesion. The coating thickness can be between 1 and 15 µm, particularly between 2 and 12 µm, preferably between 3 and 10 µm. Below the minimum limit, sufficient cathodic corrosion protection cannot be guaranteed, and above the maximum limit, joining problems may occur when connecting the steel sheet according to the invention, or a component manufactured therefrom, to another component. In particular, if the maximum coating thickness is exceeded, a stable thermal joining or welding process cannot be ensured.

[0013] According to one embodiment of the steel sheet according to the invention, the coated steel sheet is additionally oiled, wherein the oil is absorbed in the surface structure in a concentration of up to 2 g / m². Due to the dimensions of the surface structure, there is only a small amount of oil required, so the concentration is limited to up to 2 g / m², in particular up to 1.5 g / m², preferably up to 1.2 g / m², and preferably up to 1 g / m².Particularly due to the strong capillary action towards the flanks and the low roughness in the valleys, the oil, after lubrication, is deposited primarily along the flanks and / or at the transition between the flanks and valleys of the surface structure. It is then available for further processes, such as forming processes, preferably deep drawing processes, to improve lubrication and reduce friction and thus wear of the forming equipment, such as forming devices, preferably (deep drawing) presses. In particular, oil accumulation in tribologically unfavorable areas that do not contribute to the oil supply to the actual contact or friction zone can be effectively suppressed.Thus, the steel sheet according to the invention with reduced oil coating has very good tribological properties and is more environmentally friendly compared to oiled steel sheets known from the prior art, especially due to lower resource consumption.

[0014] According to a second aspect, the invention relates to a method for producing a coated steel sheet with a deterministic surface structure, comprising the following steps: Providing a coated steel sheet, tempering the coated steel sheet with a tempering roller, wherein the surface of the tempering roller, which acts on the surface of the coated steel sheet, is provided with a deterministic surface structure such that after tempering the surface structure is imprinted into the coated steel sheet starting from a surface of the coated steel sheet, wherein the surface structure has a flank region which extends from the surface to a valley region and wherein the valley region has a roughness Ra of less than 300 nm.

[0015] The surface (positive form) of the skinning roller forms a surface structure on the coated steel sheet through the application of force. This structure defines a valley and flank region (negative form) and essentially corresponds to the surface (positive form) of the skinning roller itself. The skinning roller can be machined to create a deterministic surface structure using suitable methods, for example, by laser (see EP 2 892 663 B1). Furthermore, other ablation methods can be used to adjust the surface of a skinning roller, such as machining processes with geometrically defined or undefined cutting edges, chemical or electrochemical, optical, or plasma-induced processes, which are suitable for achieving a roughness Ra of less than 300 nm in the valley region of a coated steel sheet's surface structure.Alternatively or additionally, the skinning roller can also be subjected to a post-processing process, preferably a grinding process, whereby in particular the part of the skinning roller forming the valley area in a particularly coated steel sheet, in this case a corresponding mountain area or plateau on the surface of the skinning roller, is smoothed, which can optionally further reduce the roughness.

[0016] To avoid repetition, reference is made to the explanations concerning the coated steel sheet according to the invention, which is dressed with a deterministic surface structure.

[0017] According to the invention, the steel sheet is coated by hot-dip coating before being provided. Preferably, the melt for hot-dip coating can contain, in addition to zinc and unavoidable impurities, additional elements such as aluminum with a content of up to 5 wt.% and / or magnesium with a content of up to 5 wt.%.

[0018] According to one embodiment of the inventive method, the steel sheet is additionally oiled after the dressing process, wherein the oil is applied with a coating of up to 2 g / m², preferably with a coating of up to 1 g / m².

[0019] Specific embodiments of the invention are explained in detail below with reference to the drawing. The drawing and accompanying description of the resulting features are not to be interpreted as limiting to the respective embodiments, but rather serve to illustrate exemplary embodiments. Furthermore, the respective features can be used with each other as well as with features from the above description for possible further developments and improvements of the invention, especially in the case of additional embodiments not shown. Identical parts are always designated with the same reference numerals.

[0020] The drawing shows in Figures 1a) and 1b) each show a schematic partial sectional view of a first and second embodiment according to the invention of a coated steel sheet with a deterministic surface structure, Figures 2a), 2b) and 2c) show partial views of a steel sheet coated according to the prior art with a stochastic surface structure, and Figures 3a), 3b) and 3c) show partial views of a coated steel sheet with a deterministic surface structure of a third embodiment according to the invention.

[0021] In the Figures 1a) and 1bFigures 1 and 2 are schematic partial sectional views of a first and second embodiment of a coated steel sheet (1) with a deterministic surface structure (2). The surface structure (2) is embossed into the coated steel sheet (1) starting from a surface (1.1) of the steel sheet (1), wherein the surface structure (2) has a flank region (2.3) which extends from the surface (1.1) to a valley region (2.2). The valley region (2.2) has a roughness Ra of less than 300 nm. Depending on the ablation method used to process the corresponding skinning roller (not shown) for skinning the coated steel sheet (1), the valley region (2.2) can be adjusted by the corresponding area on the skinning roller (not shown) (peak region / plateau).Additionally, the dressing roller can be reworked by means of a grinding process, whereby the peak / plateau area is smoothed accordingly, so that a further reduction of the roughness Ra in the valley area (2.2) can be achieved. Furthermore, in the . Figures 1a) and 1b It is clearly visible that the surface structure (2) has a flank region (2.3) which extends from the surface (1.1) to a valley region (2.2) and is formed at an angle (α) between 1° and 89° to the perpendicular (O) of the coated steel sheet (1). The flank region (2.3) surrounding and forming the surface structure (2), together with the valley region (2.2) integrally connected to the flank region (2.3), defines a closed volume of the surface structure (2) embossed into the coated steel sheet (1) by means of a skin forming process. Figure 1b ) shows in the valley area (2.2) a difference compared to Figure 1a) smoother and therefore with a low roughness Ra, which was processed by means of a ground dressing roller (not shown), so that a deterministic surface structure (2) can be produced on a coated steel sheet (1), which in the valley region (2.2) of the surface structure (2) can be set with a roughness Ra of in particular less than 250 nm, preferably less than 200 nm, preferably less than 150 nm, more preferably less than 100 nm.

[0022] In the Figures 2a), 2b) and 2c Figures 1 and 2 show partial representations of a steel sheet coated with a stochastic surface structure according to the prior art. The surface structure was created using an EDT-structured and subsequently ground dressing roller (not shown). Figure 2aFigure 1 shows a section of a zinc-coated sheet topography measured using atomic force microscopy (AFM). The roughness Ra, based on an area (U) of 60 x 12.5 µm², was determined in the valley region, yielding a value of Ra = 323 nm. An example of oil distribution on a zinc-coated sheet topography treated with an EDT-structured and subsequently ground treatment roller (not shown) is also shown, where in Figure 2b ) a section using light microscopy and in Figure 2c The same section, however, is imaged using Raman spectroscopy, with the oil layer shown in bright light. The valley region of the surface structure was partially or completely filled with oil, with the oil layer exceeding 2 g / m².

[0023] The result was different when considering the partial representations of a coated steel sheet (1) with a deterministic surface structure of a third embodiment according to the invention. Figures 3a), 3b) and 3c ) from. The surface structure was created using a laser-structured and subsequently ground dressing roller (not shown). The deterministic surface structure (2) was investigated using the example of a recurring I-shaped indentation. Other embodiments are also conceivable and applicable and are not limited to an I-shaped indentation. The Figures 3a), 3b) and 3c ) show two adjacent I-shaped indentations. In Figure 3aFigure 1 shows a section of a zinc-coated sheet topography (1, 2) measured using atomic force microscopy (AFM). The roughness Ra, based on an area (U) of 60 x 12.5 µm², was determined in the valley region (2.2), yielding a value of Ra = 77 nm. An example of an oil distribution on a zinc-coated sheet topography (1, 2) treated with a laser-structured and subsequently ground tempering roller (not shown) is also shown. Figure 2b ) a section using light microscopy and in Figure 2cThe same section, however, is imaged using Raman spectroscopy, with the oil coating shown in bright light. The valley region (2.2) of the surface structure (2) was essentially not wetted with oil, with the oil accumulating along the flank region (2.3) and at the transition between the flank region (2.3) and the valley region (2.2) due to capillary action and the reduced roughness Ra of less than 300 nm in the valley region (2.2). The oil coating could be reduced to as low as 1.5 g / m², and in particular to as low as 1 g / m².

[0024] For further investigations, four coated and tempered steel sheets (V1 to V4) were produced. The type of coating was the same for all steel sheets: a zinc-based coating (zinc and unavoidable impurities) applied by hot-dip coating, with a thickness of approximately 7 µm. V1 and V2 correspond to steel sheets (1) according to the invention, while V3 and V4 serve as reference sheets. The difference between V1 and V2 is that the tempering roller had a stochastic surface structure. This surface was structured, for example, by EDT, and thus a stochastic surface structure was also imprinted on the reference sheets. Table 1 provides a comparison of the steel sheets (1) according to the invention and the reference sheets. Table 1 steel sheets Ra [µm] Rz [µm] Wsa [µm] RPc [1 / mm] Ra [nm], valley area Oil [g / m²< ] cupping 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 -

[0025] The surface parameters Ra (arithmetic mean roughness), Rz (average roughness depth), and RPc (number of peaks determined along a defined length, in the above case per mm) can be determined from DIN EN ISO 4287, and the characteristic value related to the long-wave waviness Wsa (arithmetic mean waviness) can be derived according to SEP1941. However, the data in Table 1 relating to a strip drawing test, a cup drawing test according to DIN EN 1669, which was carried out on all four steel sheets V1 to V4 under identical conditions, surprisingly show that better results were achieved in the comparison between V1 / V2 and V3 / V4. The evaluation was based on the following criteria: ++ means that both the coefficient of friction determined in the strip drawing test and the thinning at the exit of the die edge on the formed steel sheet are lower (low thinning below 5% of the original steel sheet thickness), + means that the minimum thinning on the formed steel sheet is above 5% but below 10% of the original steel sheet thickness, 0 indicates a clearly recognizable thinning without tearing, which is no longer within the tolerable range (15% to 25% of the original steel sheet thickness) and - means that tearing occurs.

[0026] In addition, the oil coating on the steel sheet V1 and V2 coated according to the invention with a deterministic surface structure could simultaneously be reduced to below 1.5 g / m², with the amount being sufficient to achieve a correspondingly good result.

Claims

1. Steel sheet (1) coated by hot-dip coating with a deterministic surface structure (2), the surface structure (2) being embossed into the coated steel sheet (1) starting from a surface (1.1) of the coated steel sheet (1), the surface structure (2) having a flank region (2.3) which extends from the surface (1.1) to a valley region (2.2), characterized in that the valley region (2.2) has a roughness Ra of less than 300 nm, measured as in the description.

2. Steel sheet according to claim 1, wherein the flank region (2.3) is formed at an angle (α) of between 1° and 89° to the perpendicular (O) of the coated steel sheet (1).

3. Steel sheet according to claim 1 or 2, wherein the steel sheet is coated with a zinc-based coating (1.2), wherein in addition to zinc and unavoidable impurities, the coating (1.2) may contain additional elements such as aluminum with a content of up to 5% by weight and / or magnesium with a content of up to 5% by weight in the coating (1.2).

4. Steel sheet according to one of the preceding claims, wherein the coated steel sheet (1) is additionally oiled with an oil, wherein in particular the oil is absorbed in the surface structure (2) with a coating of up to 2 g / m2.

5. A method of manufacturing a coated steel sheet (1) dressed with a deterministic surface structure (2) comprising the following steps: - Providing a coated steel sheet, whereby the steel sheet is coated by hot-dip coating before the steel sheet is provided, - skin-passing the coated steel sheet with a skin-pass roller, the surface of the skin-pass roller, which acts on the surface of the coated steel sheet, being set up with a deterministic surface structure in such a way that, after skin-passing, the surface structure (2) is impressed into the coated steel sheet (1) starting from a surface (1.1) of the coated steel sheet (1), wherein the surface structure (2) has a flank region (2.3) which extends from the surface (1.1) to a valley region (2.2) and wherein the valley region (2.2) has a roughness Ra of less than 300 nm, measured as in the description.

6. The process according to claim 5, wherein the melt for hot-dip coating may contain, in addition to zinc and unavoidable impurities, additional elements such as aluminum with a content of up to 5% by weight and / or magnesium with a content of up to 5% by weight.

7. Method according to claim 5 or 6, wherein the steel sheet (1) is additionally oiled with oil after skin-passing, the oil being applied in a layer of up to 2 g / m2.

8. The method according to claim 7, wherein the oil is applied with a coating of up to 1 g / m2.