Method for surface texturing of a tempering roller, tempering roller and tempered steel sheet

Abstract

Method for surface texturing of a skinning roller (7) for skinning a metallic sheet, in particular a steel strip, wherein a dressing roller (7) is provided and a pattern is created on the roller surface using laser pulses (1) of a pulsed laser, wherein a sequence of overlapping laser pulses (1) is directed onto the roller surface to generate a sequence of cup-shaped depressions (8) that merge into one another, wherein two successive overlapping laser pulses have a pulse interval (d) that is between 0.75 and 1.25 of a pulse radius wherein the sequence of laser pulses (1) is repeatedly changed in its pulse energy to form a repeating pattern, wherein material structures (9, 10, 11, 12, 13, 14, 15) bounded by the pattern are subjected to a sequence of smoothing laser pulses with a smoothing laser pulse energy that is lower than a pattern pulse energy, i.e. a pulse energy of the sequence of laser pulses (1) with which the pattern is generated.

Description

[0001] In the production of metallic sheets, for example steel sheets, especially steel sheets made from steel strip, a targeted adjustment of the surface topography is relevant in many application areas.

[0002] It is known in practice that sheet metal can be tempered to achieve specific surface topographies. To obtain the desired surface topography of the tempered sheet metal, a specific texturing of the tempering rollers is applied, among other methods. This means that the outer surface of the tempering roller, which comes into contact with the sheet metal, for example, the steel strip, is given a specific surface texture.

[0003] In order for the surface topography of a sheet, for example made from a steel strip, to meet the highest standards, the intended surface texture of the tempering roller must consequently be produced as accurately and reproducibly as possible.

[0004] A typical procedure is to provide a dressing roller, the surface of which is then subjected to targeted structuring.

[0005] A known group of methods for structuring a roller surface, more precisely and equivalently within the scope of the present invention: roller shell surface, comprises methods for structuring using a pulsed laser. For example, it is known to effect material removal on the surface of a skin-treating roller using a pulsed laser, preferably a short-pulse or ultrashort-pulse laser. In methods of this type, a pattern is created on the roller surface by directing laser pulses of a pulsed laser onto the roller surface and causing a temporary melting of the material present on the surface at the point of impact. Part of the melted material evaporates, and the unevaporated part solidifies again.

[0006] EP 2 892 663 B1 describes a deterministic surface structure, meaning in particular not a stochastic surface structure, with depressions formed on a roller, wherein the depressions have a plurality of overlapping cells which are arranged, for example, in such a way as to define a double-I-shaped, H-shaped, cross-shaped, C-shaped or X-shaped material structure in the roller surface.

[0007] The JP H06-114 405 A reveals a working roller on whose surface rows of craters are formed by laser beam processing.

[0008] DE 195 03 951 A1 discloses a method and a device for gravure printing.

[0009] US patent 2014 / 0352384A1 discloses a laminating roller with a surface structure on which a multitude of craters are defined. The craters exhibit different geometries and a random distribution.

[0010] DE 29 49 953 A1 discloses a method and a device for producing surface perforations in rollers.

[0011] Against the background of the steadily increasing demands on the homogeneity of the strip surface and the reproducibility and uniformity of the roughness parameters combined with the highest possible cleanliness, the task arises to provide a process that is further developed compared to known methods, with which improved surface properties of the dressed sheets are made possible.

[0012] The problem is solved by a method for surface texturing of a tempering roller with the features of claim 1, with a tempering roller with the features of claim 10 and with a steel sheet with the features of claim 11.

[0013] The invention relates to a method for surface texturing of a tempering roller. In particular, tempering rollers for tempering a metal sheet are concerned. The metal sheet can, in particular, be a steel strip. The method initially provides that a tempering roller is supplied and a pattern is generated on the roller surface by means of laser pulses from a pulsed laser.

[0014] The outer surface of a skin-treating roller is made of steel, for example. The roller is, for example, at least on its outer surface, preferably entirely, made of a steel material, preferably a classic cold-rolled steel with a chromium content of up to 3%.

[0015] The plan is to create a pattern on the roller surface. Laser pulses from a pulsed laser are used to create the pattern.

[0016] A short-pulse laser, preferably an ultrashort-pulse laser, can be used as a pulsed laser. The term "short-pulse laser" refers to a pulsed laser with pulse durations in the nanosecond range and pulse widths greater than 100 picoseconds. The term "ultrashort-pulse laser" refers to a pulsed laser with pulse durations of less than 100 picoseconds.

[0017] A suitable ultrashort pulse laser is, for example, a short pulse laser which has a pulse repetition frequency between 10 kHz and 2.5 MHz and / or a wavelength between 930 and 1090 nm, preferably with a pulse duration between 0.4 ps and 100 µs, particularly preferably 1 ps and 10 µs.

[0018] For example, it may be provided that the pulsed laser has an average power output of between 10 and 150 watts, in particular between 15 and 100 watts, and especially preferably between 20 and 70 watts.

[0019] The desired pattern is generated on the roller surface using a pulsed laser. According to the invention, this is achieved by directing a sequence of laser pulses onto the roller surface. The sequence of laser pulses comprises successive, overlapping pulses. Due to the overlap of successive pulses, a sequence of cup-shaped depressions is formed, which also overlap each other. According to the invention, two successive, overlapping laser pulses have a pulse interval of between 0.75 and 1.25 times the pulse radius.

[0020] In the context of this invention, directing a sequence of laser pulses onto the roller surface is understood to mean that successive, i.e., temporally successive, pulses are to strike the roller surface one after the other, i.e., spatially one after the other, at different positions. This means that the laser pulses immediately following one another (in time) strike different positions on the roller surface. It is preferably provided that two temporally successive laser pulses are adjacent to each other and overlap. According to the invention, the two successive laser pulses overlap with a pulse interval of between 0.75 and 1.25 times the pulse radius.

[0021] It has been shown that an excessively large pulse interval between two consecutive laser pulses results in the formation of two consecutive cup-shaped depressions that do not merge into one another but are separated by a barrier of roller material, negatively impacting the roller topography. Conversely, an excessively small pulse interval leads to a very smooth, continuous groove formed from the cup-shaped depressions. While a very smooth groove forming the pattern produces good results during the dressing process, its creation is more time-consuming due to the limited penetration of the laser beam and consequently not very economical.

[0022] For the production of surface texturing of the dressing roller according to the invention, it can be provided, for example, that the pulsed laser, whether a short-pulse or ultrashort-pulse laser, is stationary and oriented towards the surface of the roller, and that the roller to be engraved rotates in one direction. During rotation, it is simultaneously possible to use focusing optics to guide the laser beam transversely in the direction of the roller axis, resulting in a helical path of the laser beam on the roller surface. The production of a deterministic structure is achieved by selectively interrupting a rapid, continuous sequence of laser pulses or by changing their pulse energy. Such an interruption can be implemented using a fast optical switch, an AOM. A corresponding procedure is described in the aforementioned document.

[0023] Alternatively, it is also possible that the movement of the roller, both rotationally and transversely in the direction of the axis of rotation, and the direction of the laser beam onto the roller surface are controlled relative to each other in another way. The essential point is that, by means of measures that are easily implemented by a person skilled in the art, the laser beam is advanced in a sequence of overlapping beams, and that, in order to selectively create a pattern on the roller surface, the laser emission is selectively interrupted (possibly by redirecting the beam path using a mirror) or the pulse energy is changed, preferably by reducing the pulse energy.

[0024] To create the repeating pattern, the pulse energy of the laser pulse sequence is varied according to the invention. Alternatively, in a further development, the sequence of laser pulses can be selectively interrupted to create the repeating pattern. This means that sequentially overlapping laser pulses are applied to the roller, and this sequence of laser pulses, while keeping other parameters constant, in particular the pulse repetition frequency, optionally the roller rotation speed and / or the feed rate of the axial axis of the focusing optics, is either interrupted at predetermined positions, so that no laser pulse occurs at a position where a laser pulse would normally be expected, or the pulse energy is changed so that the pattern is created.For example, it can be provided that the laser pulses of the pulsed laser generate the pattern with a first laser pulse energy, and that the pulse energy assumes a second value to define the pattern. In particular, it can be provided that the sequence of laser pulses is set to form a deterministically repeating pattern, that is, that the pattern is always present on the surface in a regular and uniform sequence, for example, arranged according to the arrangement principle of a checkerboard pattern, namely with a square pattern that is repeatedly arranged alternating in 0-degree rotation and adjacent in 90-degree rotation.

[0025] Preferably, at least in one region of the pattern, i.e., in a region of the roller surface to be structured by the laser, preferably in the entire surface of the pattern, and particularly preferably in the entire roller surface, each pair of sequentially overlapping laser pulses has the same pulse spacing. This means that the pulse spacing not only meets the requirement defined above for two consecutive pulses, but also meets the requirement that the pulse spacing is identical for all overlapping pulses.

[0026] The deterministic pattern to be set can, for example, be a double-I-shaped, H-shaped, cross-shaped, C-shaped, or X-shaped pattern defining the material structure. According to the invention, the pulse energy of the sequence of laser pulses is selectively changed such that material structures bounded by the pattern are subjected to a pattern pulse energy that produces the defined pattern, and the material structure bounded by the pattern is subjected to a sequence of smoothing laser pulses with a smoothing pulse laser energy that is lower than the pattern pulse energy.In other words, unlike in the aforementioned publication, the material structures that remain unprocessed or are not subjected to overlapping laser pulses are not delimited by the deterministically repeating pattern; rather, even the material structures bounded by the pattern are subjected to processing with ultrashort or short-pulse laser pulses. For example, the material structures in the roller surface, which can be double-I-shaped, H-shaped, cross-shaped, C-shaped, or X-shaped, are also processed in this case with laser pulses that sequentially strike the bounded material structure. These laser pulses have the same pulse spacing as those defined for the pulses that initially created the pattern, but differ in their smoothing laser pulse energy.It is particularly preferred that the smoothing laser pulse energy be 60 percent or less of the pattern pulse energy.

[0027] For example, the pulse energy of a pattern pulse may be between 0.1 and 1.0 mJ, and the pulse energy of a smoothing pulse may be less than 60% of this value, for example, 50% of this value or less.

[0028] In particular, it can be provided that an entire tempering area of ​​the tempering roller, preferably the entire cylindrical surface of the tempering roller, is processed sequentially with overlapping pulses. This includes either subjecting the entire area to a laser pulse to create the pattern or subjecting it to a smoothing pulse as the area to be delimited by the pattern, i.e., as the material structure. This is particularly preferably carried out in such a way that all adjacent pulses overlap each other in both a circumferential direction and an axial direction of the roller. This means, in particular, that each pattern pulse is overlapped by at least four, preferably exactly four, pulses, each of which is either a pattern pulse or a smoothing pulse.Since the pulses are approximately circular, small areas without direct laser pulse exposure are conceivable; rather, the described procedure should be understood as meaning that no area remains unexposed that has such a large surface area that it can accommodate the cross-section of the pulse without intersecting with a neighboring pulse.

[0029] The preferred sequential formation of the pattern and smoothing of the material structures includes, in particular, that all pulses, i.e., both the pattern pulses and the smoothing laser pulses, are generated sequentially. This means that the laser beam is moved sequentially from position to position across the area of ​​the roller shell to be structured, and at each position in the sequence, a pulse is applied that is either a pattern pulse or a smoothing laser pulse with a modified energy, until the structuring is complete. That is to say, preferably, instead of first applying a first sequence of pattern pulses and a second sequence of smoothing pulses, only one sequence of pulses is applied in which all two adjacent pulses overlap. Depending on the pulse's position, the pulse energy is either maintained as pattern pulse energy to form the pattern or reduced to smooth the limited material structure.

[0030] The desired pulse overlap according to a further development of the invention can be achieved, for example, by always setting both the circumferential and axial overlap of the roller with the aforementioned requirement of a pulse spacing between 0.75 and 1.25 of a pulse radius, regardless of whether the pulse is a smoothing laser pulse or a pattern pulse. The pattern pulses create a material structure exhibiting a pattern, for example, one of those mentioned above such as double-I-shaped, H-shaped, etc., and the limited structure overlaps all adjacent pulses in both the axial and circumferential directions, differing only in energy, namely, the lower energy of the smoothing pulse used to process the limited material structures.

[0031] It may be provided that the cup-shaped depressions are between 6 µm and 14 µm deep and / or that the cup-shaped depressions have a diameter between 20 µm and 80 µm.

[0032] One aspect of the invention relates to a dressing roller which is manufactured using one of the methods mentioned above.

[0033] In particular, a tempered steel sheet is provided, which is produced from a cold-rolled or hot-rolled strip of steel, for example a steel for outer skin applications in vehicle construction, by tempering with a tempering roller as described above.

[0034] It has been shown that by changing the pulse energy according to the invention during the texturing of the roller surface, existing topographic defects in the area of ​​the material structures bounded by the pattern can be corrected even during the production of the structure itself. This has the advantage that a surface topography with excellent properties can be produced during the tempering of a sheet without an additional process step, so that by integrating the post-treatment of the generated pattern-bounded structures into the conventional texturing process, an improved topography of the roller, and consequently also of the tempered sheets, is obtained in a very economical manner.In particular, often necessary superfinishing processes, such as grinding operations known from practice for EDT texturing, can be saved without compromising the expected homogeneous, uniform shape on the roller. Fig. Figure 1 shows a schematic representation of the texturing of a dressing roller using a pulsed laser; Fig. 2, Fig. 3 and Fig. Figure 5 shows a schematically represented section of the surface structure of a roller. Fig. 2 and Fig. 3 according to procedures known from practice, Fig. 5 according to the further development according to the invention. Fig. Figure 4 clarifies the concept of pulse interval.

[0035] The surface structures according to the invention, with a deterministic distribution of the shape features (structural features), are produced, for example, using a roller texturing process that employs a pulsed laser, preferably a short-pulse or ultra-short-pulse laser, for material removal on the surface of a rotating roller. For this purpose, the textured roller 7 to be engraved is rotated in a rotary device. While the roller rotates, a focusing optic 5, which focuses the laser beam onto the roller surface, is moved transversely in the direction of the roller axis at a relatively low speed. The laser beam thus describes a helical path on the roller surface, see [reference]. Fig. 1. By selecting the parameters pulse frequency, pulse energy, spacing of the tracks on the rotating roller surface, laser impact point diameter (spot diameter), laser intensity profiles, and / or roller rotation speed, the distribution and geometry of the pits on the roller surface are determined. Overlapping individual pits engrave or create valley-shaped, continuous areas. The rapid, continuous sequence of laser pulses is precisely controlled, for example, with an optical switch 2. The laser used (pulsed fiber laser) can, for example, have a maximum average power of 500 W and a pulse repetition rate of up to 100 kHz at a wavelength of approximately 1070 nm with a pulse duration of up to 10 µs. After passing through the AOM (artificial optical module), the laser beam 1 is guided to the focusing optics 5 via an optical fiber cable 3. The arrow 6 indicates the direction of movement of the focusing optics 5.The shape of the structures is controlled by a specially designed image processing program. Depending on the pulse repetition frequency, this program enables the determination of the roller rotation speed and the feed rate of the axial axis of the focusing optics 2, as well as the control of the AOM 10 in such a way that the density of the cells and their deterministic distribution are generated according to the structure design specifications.

[0036] In Fig. Figure 2 shows overlapping cells 8 arranged in a linear pattern, defining a multitude of double-I-shaped material structures on the roller surface. Furthermore, the double-I-shaped material structures are arranged in a checkerboard pattern, meaning that each square of cell patterns contains a double-I 9, 10, 11, 12, with each pair of adjacent squares defining double-Is tilted 90 degrees relative to each other. The overlapping cells form open voids on the roller surface. For this process to achieve excellent typography comparable to laser texturing, a finishing process such as grinding and / or hard chrome plating would be necessary. Fig. Figure 3 illustrates the principle of the H-pattern, for which all explanations to Fig. 2 apply with the additional proviso that the in Fig. The two pulse positions shown with dashed lines are not subjected to a pattern pulse, but contain material limited by the pattern. Instead of two I's, a number of H's are generated: 13, 14, 15.

[0037] In the improved method, laser pulses are also applied to the freestanding material structures, i.e., to the area bounded by the pattern. Specifically, the sequence of laser pulses for forming the repeating pattern is not determined at the positions of the material structures, as in [previous method]. Fig. 2 and Fig. 3. As shown, the pulses are not selectively interrupted, but their pulse energy is modified such that a lower pulse energy than the pattern pulse energy, namely the smoothing laser pulse energy, is used for processing the isolated areas. All areas of the material structures bounded by the pattern are subjected to smoothing laser pulses. The smoothing laser pulses are applied to the areas bounded by the pattern, which is shown in Fig. 5 based on the H-shaped material structures of the Fig. Figure 3 shows that 16 smoothing laser pulses are applied to all positions shown in gray, with a smoothing laser pulse energy that is lower than the pattern pulse energy.

[0038] Based on Fig. Figure 4 illustrates the concept of pulse interval d, which is described in Fig. 4 is 1.0 times the pulse radius.

Claims

[1] Method for surface texturing of a skinning roller (7) for skinning a metallic sheet, in particular a steel strip, wherein a dressing roller (7) is provided and a pattern is created on the roller surface using laser pulses (1) of a pulsed laser, wherein a sequence of overlapping laser pulses (1) is directed onto the roller surface to generate a sequence of cup-shaped depressions (8) that merge into one another, wherein two successive overlapping laser pulses have a pulse interval (d) that is between 0.75 and 1.25 of a pulse radius wherein the sequence of laser pulses (1) is repeatedly changed in its pulse energy to form a repeating pattern, wherein material structures (9, 10, 11, 12, 13, 14, 15) bounded by the pattern are subjected to a sequence of smoothing laser pulses with a smoothing laser pulse energy that is lower than a pattern pulse energy, i.e. a pulse energy of the sequence of laser pulses (1) with which the pattern is generated. [2] Method according to claim 1, wherein the pulsed laser is a short pulse laser, preferably an ultrashort pulse laser. [3] Method according to claim 1 or according to claim 2, wherein the sequence of laser pulses (1) is repeatedly and selectively changed in its pulse energy to form a deterministically repeating pattern. [4] Method according to one of the preceding claims, wherein at least in one area of ​​the pattern, preferably in the entire area of ​​the pattern, each pair of two successive overlapping laser pulses has the same pulse spacing (d). [5] Method according to any of the preceding claims, wherein the smoothing laser pulse energy is 60 percent or less of the pattern pulse energy. [6] Method according to one of the preceding claims, wherein an entire dressing area of ​​the dressing roller (7) is processed sequentially with overlapping pulses, wherein the smoothing pulse energy is set for processing an area (9, 10, 11, 12, 13, 14, 15) to be delimited by the pattern and the pattern pulse energy is set for processing the pattern. [7] Method according to one of the preceding claims, wherein an entire dressing area of ​​the dressing roller (7) is completely processed during the processing with a sequence of overlapping pulses such that within a pattern boundary each pattern pulse is brought into overlap with at least four, preferably exactly four, pulses, each of which is either a pattern pulse or a smoothing pulse. [8] Method according to any of the preceding claims, wherein the cup-shaped depressions (8) are between 6 µm and 14 µm deep. [9] Method according to any of the preceding claims, wherein the cup-shaped depressions (8) have a diameter between 20 µm and 80 µm. [10] Dressing roller (7) having a surface texture, wherein the surface texture is produced by a method according to one of the preceding claims. [11] Tempered steel sheet produced from a cold strip or a hot strip by tempering with a tempering roller (7) according to claim 10.

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