Steel sheet with a surface structure having a defined structure

Abstract

The invention relates to a steel sheet (1, 1') finished with a deterministic surface structure (2) and to a method for manufacturing the same.

Description

Technical Field

[0001] This invention relates to a steel sheet finished with a deterministic surface structure. Furthermore, this invention relates to a method for manufacturing a steel sheet with a deterministic surface structure. Background Technology

[0002] This type of steel sheet with a deterministic surface structure is known from the prior art; see, for example, patent document EP 2 892 663 B1.

[0003] Regarding the known prior art, there is a need for optimization, particularly considering reducing the need for process media and / or providing process media in a demand-compliant manner. Summary of the Invention

[0004] Therefore, the objective is to provide a steel sheet with a deterministic surface structure finish that provides the same or better properties as the prior art.

[0005] This objective is achieved through a steel plate with a deterministic surface structure finish.

[0006] Providing a defined surface structure on finished steel sheets is important, especially in other processes in the further processing industries used to manufacture automotive components. During component manufacturing, particularly in forming processes, it is advantageous that the process media used, such as oil and / or lubricant, are homogeneous and present in the necessary coating at locations associated with the forming process. These locations associated with the forming process are typically the contact surfaces of the steel sheet and the forming tool, and therefore not in stamped portions within the steel sheet (where the process media preferably accumulates), but rather the surface is in the form of a raised surface on the steel sheet. The inventors have discovered that, when the surface structure is stamped from the surface of the steel sheet into the steel sheet, the same or better properties than those of the prior art can be provided in a steel sheet finished with a defined surface structure, wherein the surface structure has lateral regions extending from the surface to valley regions, wherein, according to the invention, at least the lateral regions have a roughness Ra greater than 20 nm to minimize the need for process media and / or store the process media closer to or adjacent to the locations associated with the forming process. By defining a specific roughness Ra (arithmetic mean roughness value), wherein a method for determining Ra is given in DIN ISO EN 4287, having a roughness Ra greater than 20 nm, particularly greater than 50 nm, preferably greater than 100 nm, more preferably greater than 150 nm, and even more preferably greater than 200 nm, at least in the lateral regions of the surface structure, it is possible to influence the local distribution of the process medium in a particularly targeted manner, wherein this is especially achieved through the deterministic shaping of the surface structure according to the invention. The steel sheet leading to a side region with an intentional shaping for better process-related properties of the steel sheet surface, so that the process medium is introduced purposefully close to the shaping process-related location. By means of the intentionally provided roughness Ra, a corresponding reaction surface or rather boundary layer can be provided. The roughness Ra in the side region can be limited to a maximum of 800 nm, in particular a maximum of 700 nm, preferably a maximum of 600 nm, preferably a maximum of 500 nm, particularly preferably a maximum of 400 nm, as the surface of the steel sheet, in particular in subsequent forming steps, for example in deep drawing, stretching or bending, is subjected to large shape changes, in particular on the outer fibers of the steel sheet. At this location, in particular with a deterministic surface structure, for example with strongly protruding textures, stress concentrations due to the notch effect and thus, if present, a failure of the cover layer can occur.

[0007] A deterministic surface structure can be understood as a surface structure which occurs repeatedly and which has a defined shape and / or configuration, cf. EP 2 892 663 B1. Further, belonging to this, inter alia, is a surface with a (similar) random appearance, however, which is produced by means of a deterministic texturing method and thus consists of deterministic shape elements.

[0008] A steel sheet is generally understood as a flat steel product which can be provided in the form of a sheet or in the form of a slab or in the form of a strip.

[0009] Further advantageous design proposals and refinements result from the following description. One or more features from the present invention can be combined with one or more other features to form further design proposals of the present invention. One or more features of the subject matter of the present invention can also be combined with one or more other features.

[0010] According to one design proposal of the steel sheet according to the present invention, the surface structure has a side region which extends from the surface to a valley region and is configured at an angle of between 1 ° and 89 ° to the perpendicular of the steel sheet. The angle can be configured in particular between 50 ° and 87 °, preferably between 60 ° and 85 °, particularly preferably between 65 ° and 82 °. The valley region and the side region of the surface structure (negative shape) essentially correspond to the surface on the finishing roll (positive shape), which configures or stamps out the surface structure by acting on the steel sheet accordingly. The side region surrounding and configuring the surface structure, together with the valley region integrally connected at the side region, defines a closed volume of the surface structure which is stamped into the steel sheet by means of the finishing. The closed volume, the so-called empty volume, can be matched to the process medium to be applied, in particular oil, for the subsequent processing by means of the shaping method.

[0011] The geometrical design (size and depth) of the defined surface structure (negative shape) on the finished steel sheet depends, inter alia, on how the corresponding geometrical structure (positive shape) is designed on the finishing roll. Preferably, a laser texturing method is used to enable targeted structuring (positive shape) on the surface of the finishing roll by material removal. In particular, the design of the structure can be positively influenced by targeted control of the energy, the pulse duration and the selection of a suitable wavelength of the laser beam acting on the surface of the finishing roll. With a high or higher pulse duration, the interaction time of the laser beam with the surface of the finishing roll is increased and more material can be removed from the surface of the finishing roll. The pulse leaves a substantially circular pit on the surface of the finishing roll, which pit or pits form the surface or face of the elevations on the steel sheet after the finishing process and thus the contact face between the steel sheet and the forming tool. A reduction in the pulse duration has an influence on the formation of the pit, in particular the diameter of the pit can be reduced. By reducing the pulse duration, in particular when using a short-pulse or ultrashort-pulse laser, it is possible to thus target the geometrical structure (positive shape) on the surface of the finishing roll in such a way that the surface of the steel sheet is thus textured in such a way that a defined roughness Ra can be produced in the flank region of the surface structure of the finished steel sheet. This is achieved, for example, by reducing the laser pulse duration with which the surface of the finishing roll is textured and thus a higher resolution can be produced on the roll with the geometrical structure. In particular, by the high resolution or small pit face produced by the shorter interaction of the laser and the finishing roll, it is possible to target the rougher face and the arbitrary slope (angle) of the flank region on the flank region.

[0012] The targeted setting of the roughness in the flank region and optionally the defined setting of the angle of the flank region can be used not only for the forming process. In particular, in combination with the setting of the roughness in the flank region, the angle is targeted, more degrees of freedom are achieved in the functionalization of the steel sheet surface.

[0013] Thus, by targeted setting of the roughness Ra in the flank region, it is possible to provide, for example, a defined and / or large reaction face for chemical (post-) treatment, in particular chemical (post-) treatment in the form of cleaning and / or phosphating between the finished steel sheet and the process medium. Preferably, the roughness Ra of the flank region targeted in the cleaning provides surface properties which enable, for example, relatively simple removal of interfering coatings on the boundary layer, in particular the oxide cover layer, at least partially and / or locally, in particular without negatively influencing the roughness of the surface structure in the flank region.

[0014] For example, the adhesive capacity of the steel sheet according to the application can also essentially be provided with an optimum and / or large interface by the roughness targeted in the flank region, thus providing the adhesive with a corresponding reaction face.

[0015] According to one design variant of the steel sheet according to the application, the steel sheet is coated with a metallic cover layer, in particular a zinc-based cover layer applied by hot-dip coating. Preferably, the cover layer can comprise additional elements in addition to zinc and unavoidable impurities, such as aluminum in an amount of up to 5 wt.-% and / or magnesium in an amount of up to 5 wt.-% in the cover layer. Steel sheets with a zinc-based cover layer have very good cathodic corrosion protection, which has been used in automobile construction for many years. If an improved corrosion protection is provided, the cover layer additionally has a magnesium content of at least 0.3 wt.-%, in particular at least 0.6 wt.-%, preferably at least 0.9 wt.-%. Aluminum can alternatively or additionally be present in an amount of at least 0.3 wt.-% in order to, inter alia, improve the adhesion of the cover layer to the steel sheet and, in particular, to substantially prevent diffusion of iron from the steel sheet into the cover layer when the coated steel sheet is heat treated, thereby further achieving positive corrosion properties. Here, the thickness of the cover layer can be between 1 and 15 μιη, in particular between 2 and 12 μιη, preferably between 3 and 10 μιη. Below the minimum, sufficient cathodic corrosion protection cannot be ensured, and above the maximum, problems can arise in joining the steel sheet according to the application or a component made therefrom to another component, in particular a stable process cannot be ensured when hot joining or welding at thicknesses which exceed the maximum given by the cover layer. In hot-dip coating, the steel sheet is first coated with the respective cover layer and then delivered to finishing. Finishing is carried out after hot-dip coating of the steel sheet.

[0016] According to one alternative design variant of the steel sheet according to the application, the steel sheet is coated with a metallic cover layer, in particular a zinc-based cover layer, which is applied by electrolytic coating. Here, the thickness of the cover layer can be between 1 and 10 μιη, in particular between 1.5 and 8 μιη, preferably between 2 and 5 μιη. In contrast to hot-dip coating, the steel sheet can first be finished and then electrolytically coated. Depending on the cover layer thickness, it is also possible to substantially maintain the roughness in the side regions after electrolytic coating. Alternatively, it is also conceivable to first electrolytically coat and then finish.

[0017] It is also conceivable not to provide a cover layer, for example not to provide a metallic cover layer. It is also conceivable that the steel sheet is coated with a non-metallic cover layer, for example in a strip coating installation, wherein the steel sheet is finished before or after coating with the non-metallic cover layer.

[0018] According to one design variant of the steel sheet according to the application, in particular the coated steel sheet is additionally provided with a process medium, in particular with oil, wherein in particular the process medium with a cover layer of up to 2 g / m 2 of the steel sheet is accommodated in the surface structure. Due to the dimensions of the surface structure, there is only a small demand for process medium, so that the cover layer is limited to at most 2 g / m 2in particular at most 1.5 g / m2 2 preferably at most 1 g / m2 2 preferably at most 0.6 g / m2 2 further preferably at most 0.4 g / m2 2 In particular by the roughness and in combination with the corresponding reaction surface in the side region, the process medium after application is substantially accumulated in the side region and optionally at the transition between the side region and the valley region of the surface structure and provides for a further process, for example for a forming process, preferably for a deep drawing process which is closer or adjacent to the location relevant to the surrounding forming process, in order to improve lubrication and reduce friction and thus wear of the forming device, for example the forming apparatus, preferably the deep drawing press. In particular, the deposition of the process medium on areas which are disadvantageous from the tribological point of view can be effectively suppressed, which is disadvantageous for the introduction of the process medium into the actual contact or friction zone. Thus, the steel sheet according to the application with less process medium requirement has very good tribological properties and is more environmentally friendly, in particular due to the less resource use, compared to the steel sheets known from the prior art, in particular the oiled steel sheets.

[0019] According to a second aspect, the application relates to a method for manufacturing a steel sheet finished with a deterministic surface structure, the method comprising the following steps:

[0020] providing a steel sheet,

[0021] finishing the steel sheet with a finishing roll, wherein the surface of the finishing roll acting on the surface of the steel sheet is provided with a deterministic surface structure, such that after finishing, the surface structure is stamped into the steel sheet from the surface of the steel sheet, wherein the surface structure has a side region which extends from the surface to a valley region, and wherein at least the side region has a roughness Ra of more than 20 nm.

[0022] The surface (positive shape) of the finishing roll configures the surface structure by the action of force on the surface of the steel sheet, which surface structure defines the valley region and the side region (negative shape) and essentially corresponds to the surface (positive shape) of the finishing roll. The finishing roll for configuring the deterministic surface structure can be machined with suitable means, for example by means of a laser, see also EP 2 892 663 B1. Furthermore, other removal methods can also be used to provide the surface at the finishing roll, for example chip removal methods with a geometrically determined or undetermined blade, chemical or electronic, optical or plasma-induced methods which are suitable for being able to be carried out on the steel sheet to be finished with the surface structure, which steel sheet has a roughness Ra of more than 20 nm at least in the side region.

[0023] For the avoidance of repetitions, reference is made accordingly to the embodiments relating to the steel sheet finished with a deterministic surface structure according to the application.

[0024] According to one embodiment of the method of the invention, the steel sheet is coated by hot-dip coating before being provided. Preferably, the melt used for hot-dip coating may contain additional elements, such as aluminum at a content of up to 5% by weight and / or magnesium at a content of up to 5% by weight, in addition to zinc and unavoidable impurities.

[0025] According to another embodiment of the method of the present invention, the finished steel sheet is coated by electrolytic coating after finishing.

[0026] According to one embodiment of the method of the present invention, after finishing, the steel plate is additionally provided with a process medium, preferably oil, wherein the process medium is at most 2 g / m³. 2 The coating is further preferably up to 0.4 g / m 2 The coating is applied. Attached Figure Description

[0027] The specific design of the present invention will now be described in detail with reference to the accompanying drawings. The drawings and accompanying descriptions of the features should not be construed as limiting to any particular design, but rather as illustrative of exemplary designs. Furthermore, the features can be used interchangeably or in conjunction with the features described above for possible further extensions and improvements to the invention, particularly in additional design embodiments not shown. Identical components are always provided with the same reference numerals.

[0028] In the attached image:

[0029] Figure 1 This shows a schematic partial cross-sectional view of a steel plate finished with a defined surface structure according to an embodiment of the present invention.

[0030] Figure 2 In a), 2b) and 2c), respectively, Figure 2 Schematic partial cross-sectional views of different surface structures on finished steel plates according to the prior art are shown in a) and 2b). Figure 2 c) illustrates the surface structure of the finished steel plate according to the invention, and

[0031] Figure 3 a) and 3b) respectively show the prior art ( Figure 3 a)) and according to embodiments of the present invention ( Figure 3 b)) A REM photograph of a portion of a coated, determinately surface-finished steel plate. Detailed Implementation

[0032] Figure 1A schematic partial sectional view of a steel sheet (1, 1') finished with a deterministic surface structure (2) is shown in accordance with an embodiment of the present application. The steel sheet (1, 1') can be an uncoated steel sheet (1), that is to say without a cover layer, in particular of metal or of a non-metal, or can be a steel sheet (1') coated with a metal cover layer (1.2). The surface structure (2) is stamped into the steel sheet (1, 1') 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) up to a valley region (2.2). At least the flank region (2.2) has a roughness Ra of more than 20 nm. The flank region (2.3) and the valley region (2.2) are set by a corresponding region (positive shape) on a not shown finishing roll, by means of which a corresponding finishing roll (not shown) for finishing the steel sheet (1, 1') is machined, in accordance with a removal method. Furthermore, it can be clearly seen in Figure 1 that the surface structure (2) has a flank region (2.3) which extends from the surface (1.1) up to a valley region (2.2) and is configured at an angle (a) of between 1° and 89° with respect to the perpendicular 0 of the steel sheet (1, 1'). The flank region (2.3) which surrounds and configures the surface structure (2) together with the valley region (2.2) which is coupled or connected on the flank region (2.3) in one piece defines a closed volume of the surface structure (2) which is stamped into the steel sheet (1, 1') by means of finishing.

[0033] In Figure 2 a), 2b) and 2c) a schematic partial sectional view of different surface structures on a finished steel sheet is shown respectively.

[0034] Figure 2 a) shows a schematic partial sectional view of a steel sheet, in particular coated, finished with a random surface structure, wherein the surface structure is finished by means of an EDT structured finishing roll (not shown). The surface structure is substantially completely filled or covered by a process medium (M), for example oil. In comparison with the other two design options Figure 2 b) and 2c), the demand for process medium (M) is higher, since in the EDT the surface structure is not embodied as a closed structure, but as an open structure.

[0035] Figure 2 b) shows a schematic partial sectional view of a steel sheet, in particular coated, finished with a deterministic surface structure, wherein the surface structure is finished by means of a laser structured finishing roll (not shown), see EP 2 892 663 B1. In comparison with Figure 2 a), less process medium (M) can be used, since the surface structure is closed.

[0036] The design concept of the especially coated steel sheet (1, 1') finished with a deterministic surface structure (2) according to the present application is in Figure 2 c) schematically shown in a partial cross-sectional view, wherein the surface structure (2) is finished by means of a laser-structured finishing roll (not shown), see also EP 2 892 663 B1, however with the difference that the roughness in the positive shape provided at the surface of the finishing roll acting into the steel sheet (1, 1') and the lateral area (2.3) to be generated is limitedly defined, so that at the finished steel sheet (1, 1') in the lateral area (2.3) a deterministic surface structure (2) is provided having a roughness Ra of more than 20 nm, especially more than 50 nm, preferably more than 100 nm, preferably more than 150 nm, further preferably more than 200 nm. Thereby, compared to the other embodiments Figure 2 a) and Figure 2 b) the need for process media (M) can be further minimized and the process media can be stored close to or adjacent to the location (1.1) related to the forming process.

[0037] The deterministic surface structure is for example investigated for a total recurring I-shaped stamping. Other embodiments are also conceivable and applicable and are not limited to I-shaped stampings. In Figure 3 a) a REM photo of the plate topography provided with a zinc-based overlay is shown, wherein the surface structure is stamped by means of a finishing roll (not shown), wherein the surface of the finishing roll is structured by means of a laser, see EP 2 892 663 B1. In Figure 3 b) a REM photo of the topography or the deterministic surface structure (2) of a finished steel sheet (1') having a zinc-based overlay (1.2) is shown, wherein the surface structure (2) is stamped by means of a finishing roll (not shown), wherein the surface of the finishing roll is structured by means of a laser, see EP 2 892 663 B1, however with the difference that the roughness Ra in the positive shape at the surface of the finishing roll acting into the coated steel sheet (1') and the lateral area (2.3) to be generated is limitedly defined. The differently configured lateral areas of the individual I-structures can be clearly seen.

[0038] In an example of the embodiment according to Figure 3 b) two uncoated and two hot dip coated steel sheets (1, 1') are finished with a deterministic surface structure. The lateral areas of the plate topography are investigated by means of an atomic force microscope (AFM). The scan area of the atomic force microscope has an area of 90 x 90 pm 2 , wherein a 20 x 2 pm 2Roughness Ra in the side area of the area. Values of Ra = 45.99 nm and Ra = 51.48 nm were determined for the two uncoated, finished steel sheets (1) and values of Ra = 131.07 nm and Ra = 205.40 nm were determined for the two coated, finished steel sheets (1’), respectively.

[0039] For further investigations, four coated, finished steel sheets (V1 to V4) were considered. The type of coating was chosen to be the same for all steel sheets, a zinc-based coating (zinc and unavoidable impurities) applied in a hot dip coating process, with a thickness of about 7 pm. V1 and V2 correspond to the steel sheets (1’) of the present application and V3 and V4 form reference sheets. V3 and V4 differ from V1 and V2 in that V3 and V4 were finished with a finishing roll which determines a defined surface structure and an undefined side area, see embodiment Figure 3 a). Table 1 gives an overview of the comparison of the steel sheets (1’) according to the present application and the reference sheets.

[0040] Table 1

[0041]

[0042] The determination of the roughness Ra (arithmetic mean roughness) was determined by means of the method given in DIN EN ISO 4287 and in the case of the values in the table relates to an area of 20 x 2 pm 2 which only takes into account the side area. The roughness Ra in the side area of the steel sheets V3 and V4 is very small. The data in Table 1 with regard to the strip tensile test, the cupping test according to DIN EN 1669, which was carried out under the same conditions in all four steel sheets V1 to V4, essentially shows positive results. The evaluation was carried out according to the following scale:

[0043] +++ means, no noticeable thinning,

[0044] ++ means, the friction values determined in the strip tensile test and the thinning at the exit of the punch edge of the shaped steel sheet are small (slight thinning of less than 5% of the initial steel sheet thickness),

[0045] + means, the minimum thinning on the shaped steel sheet exceeds 5% of the initial steel sheet thickness but is less than 10% of the initial steel sheet thickness.

[0046] The data in Table 1 with regard to the tensile shear test according to DIN EN 1465, which was carried out under the same conditions on all four steel sheets V1 to V4, shows different results with regard to the adhesive capacity. The evaluation of the fracture behavior was carried out according to DIN EN ISO 10365, wherein the values given below were determined by means of empirical values. The evaluation was carried out according to the following scale:

[0047] ++ means that the share of cohesive fracture surfaces (which exist in the scope of the tensile shear test as fracture surfaces in the adhesive) is at least 85%,

[0048] + means that the share of cohesive fracture surfaces (which exist in the scope of the tensile shear test as fracture surfaces in the adhesive) is 60% to less than 85%,

[0049] 0 means that the share of cohesive fracture surfaces (which exist in the scope of the tensile shear test as fracture surfaces in the adhesive) is 40% to less than 60%.

[0050] Furthermore, the coating of the process medium (M) at the coated, finished steel sheet V1 and V2 with a defined surface structure according to the application is reduced to below 1 g / m 2 wherein this amount is sufficient to obtain a corresponding good result.

Claims

1. Steel sheet (1, 1') finished with a deterministic surface structure (2), wherein, The surface structure (2) is stamped into the steel sheet (1, 1') from the surface (1.1) of the steel sheet (1, 1') by means of a finishing roll, wherein the laser pulse duration for texturing the surface of the finishing roll is reduced and thereby a geometry is produced on the roll with a higher resolution, wherein the surface structure (2) has a flank region (2.3) which extends from the surface (1.1) to a valley region (2.2), characterized in that at least the flank region (2.3) has a roughness Ra of more than 20 nm up to at most 400 nm.

2. The steel sheet according to claim 1, wherein, The flank region (2.3) is configured at an angle (a) of between 1° and 89° with respect to a perpendicular (0) of the steel sheet (1, 1').

3. Steel sheet according to any one of the preceding claims, wherein, The steel sheet (1') has a metallic cover layer (1.2).

4. The steel sheet according to claim 3, wherein, The steel sheet (1') is coated with a zinc-based cover layer applied by hot-dip coating.

5. The steel sheet according to claim 4, wherein, Preferably, the cover layer (1.2) can contain additional elements in addition to zinc and unavoidable impurities.

6. The steel sheet according to claim 5, wherein, In the cover layer (1.2) there is aluminum in a content of up to 5% by weight and / or magnesium in a content of up to 5% by weight.

7. Steel sheet according to claim 3, wherein the steel sheet (1') is coated with a zinc-based cover layer (1.2) applied by electrolytic coating.

8. The steel sheet according to claim 1, wherein The steel sheet (1, 1') is additionally provided with a process medium (M).

9. The steel sheet according to claim 8, wherein, Process media (M) having a coating of up to 2 g / m 2 are accommodated in the surface structure (2).

10. Method for producing a steel sheet (1, 1') finished with a deterministic surface structure (2), comprising the following steps: providing a steel sheet, finishing the steel sheet with a finishing roll, wherein the surface of the finishing roll acting on the surface of the steel sheet is provided with a deterministic surface structure, so that after finishing the surface structure (2) is stamped into the steel sheet (1, 1') from the surface (1.1) of the steel sheet (1, 1'), wherein the laser pulse duration for texturing the surface of the finishing roll is reduced and thereby a geometry is produced on the roll with a higher resolution, 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 at least the flank region (2.3) has a roughness Ra of more than 20 nm up to at most 400 nm.

11. Method according to claim 10, wherein before providing the steel sheet, the steel sheet is coated by hot-dip coating.

12. Method according to claim 10, wherein the melt for hot-dip coating can contain additional elements in addition to zinc and unavoidable impurities.

13. Method according to claim 12, wherein the melt contains aluminum in a content of up to 5% by weight and / or magnesium in a content of up to 5% by weight.

14. Method according to claim 10, wherein after finishing the steel sheet, the finished steel sheet is coated by electrolytic coating.

15. The method of any one of claims 10 to 14, wherein, The steel sheet (1, 1') is additionally provided with a process medium (M), wherein the process medium (M) is applied in a coating of up to 2 g / m 2 .

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