Method and arrangement for the liquid-assisted laser texturing of moving steel strip

Abstract

A method and arrangement for the texturing of a moving steel strip wherein a texture is applied to a surface of a moving steel strip by ablation by means of a single laser beam or a plurality of laser beams directed at the surface of the moving steel strip and wherein a liquid is supplied on the moving steel strip over a surface area on the moving steel strip that covers the working area of the single laser beam or the plurality of laser beams on the moving steel strip.

Claims

1. A method for the texturing of a moving steel strip, wherein a texture is applied to a surface of a moving steel strip by ablation of the surface of the moving steel strip by a single laser beam or a plurality of laser beams directed at the surface of the moving steel strip to contact the surface of the moving steel strip, the method comprising supplying a liquid on the moving steel strip over a surface area on the moving steel strip that covers the working area of the single laser beam or the plurality of laser beams on the moving steel strip, wherein the liquid is supplied by spraying, wherein the layer thickness of the liquid is controlled by controlling the spray volume in dependence of the velocity of the moving steel strip.

2. The method according to claim 1, wherein the supplied liquid is formed into a predefined thickness of a liquid film.

3. The method according to claim 1, wherein the thickness of the supplied liquid is controlled by guiding the liquid between the moving steel strip and guide means provided at a distance with respect of the surface of the moving steel strip.

4. The method according to claim 3, wherein the distance between the guide means and the surface of the moving steel strip is controlled.

5. The method according to claim 1, wherein the liquid is supplied on the moving steel strip in two or more stacked layers.

6. The method according to claim 1, wherein the thickness of the liquid film is in the range of 0.1 mm to 10 mm.

7. The method according to claim 1, wherein the line speed of the moving steel strip is in the range of 0.5 m/s to 15 m/s.

8. The method according to claim 1, wherein the single laser beam is a pulsed laser beam, wherein the pulse duration is in the range of 1 fs to 100 ms, the electromagnetic radiation of the laser beam has a wavelength between 200 nm and 11 μm, and with an energy density of the laser beam in the range of 1 nJ/cm.sup.2 to 100 J/cm.sup.2.

9. The method according to claim 1, wherein the line speed of the moving steel strip is in the range of 1 m/s to 5 m/s.

10. The method according to claim 1, wherein the plurality of laser beams are pulsed laser beams, wherein the pulse durations are in the range of 1 fs to 100 ms, the electromagnetic radiation of each laser beam has a wavelength between 200 nm and 11 μm, and with an energy density of each laser beam in the range of 0.1 J/cm.sup.2 to 10 J/cm.sup.2.

11. The method according to claim 1, wherein the single laser beam or the plurality of laser beams are directed downwardly at the surface of the moving steel strip.

12. The method according to claim 1, wherein the thickness of the supplied liquid is controlled by guiding the liquid between the moving steel strip and guide means provided at a distance with respect of the surface of the moving steel strip, wherein the guide means has an upward directed edge connected to a flat portion to define a declining slope to guide the liquid under the flat portion and the guide means is of transparent material.

13. A method for the texturing of a moving steel strip wherein a texture is applied to a surface of a moving steel strip by ablation by a single laser beam or a plurality of laser beams directed at the surface of the moving steel strip, the method comprising supplying a liquid on the moving steel strip over a surface area on the moving steel strip that covers the working area of the single laser beam or the plurality of laser beams on the moving steel strip, wherein the moving steel strip is guided through a container with the liquid and wherein focusing optics of the single laser beam or the plurality of laser beams are submerged in the liquid at a predefined distance of the surface of the moving steel strip.

14. A method for the texturing of a moving steel strip, wherein a texture is applied to a surface of a moving steel strip by ablation of the surface of the moving steel strip by a single laser beam directed at the surface of the moving steel strip to contact the surface of the moving steel strip, the method comprising supplying a liquid on the moving steel strip over a surface area on the moving steel strip that covers the working area of the single laser beam on the moving steel strip, wherein the liquid is supplied by spraying; wherein the single laser beam is a pulsed laser beam, wherein the pulse duration is in the range of 1 fs to 100 ms, the electromagnetic radiation of the laser beam has a wavelength between 200 nm and 11 μm, and with an energy density of the laser beam in the range of 1 nJ/cm.sup.2 to 100 J/cm.sup.2.

15. The method of claim 1, wherein a liquid supply means supplies the liquid on the surface of the moving steel strip where the texture is to be applied.

16. The method according to claim 15, wherein the liquid supply means comprise one or more nozzles supplying the liquid across the width of the surface area of the moving steel strip where the texture is to be applied.

17. The method according to claim 15, wherein guide means are provided parallel to a path of the moving steel strip and downstream of the liquid supply means to guide the liquid over the surface of the moving steel strip.

18. The method according to claim 17, wherein distance between the guide means and the path of the moving steel strip is in a range of 1 mm to 5 mm.

19. The method according to claim 17, wherein the guide means comprise a transparent plate.

20. The method according to claim 19, wherein the transparent plate has at the upstream side an upward directed edge.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be further explained on hand of the example shown in the drawing, in which: FIG. 1 shows a diagram with the thickness of the liquid film in relation to the ablation rate;

(2) FIG. 2 shows schematically an arrangement with supply nozzles to supply a liquid on a moving steel strip, a laser device and guide means to guide the liquid over the steel strip;

(3) FIG. 3 shows schematically an arrangement with spray nozzles to spray a liquid film on a moving steel strip, a laser device and cleaning nozzles, and

(4) FIG. 4 shows schematically an arrangement with a container filled with a liquid through which a steel strip is moved and a laser device projecting into the container.

DETAILED DESCRIPTION OF THE DRAWINGS

(5) In FIG. 1 a diagram is shown with the ablation rate as a function of liquid layer or liquid film thickness. A liquid film must have a thickness above a critical value t.sub.critical and below t2 to ensure optimal material removal. In practice, the values t1 and t2 are between 0.5 mm and 10 mm. Preferably the thickness of the liquid film is kept in a more limited range from 1 mm up to and including 5 mm which creates a much more stable focus regime for the laser beam or laser beams.

(6) In the schematic FIGS. 2, 3 and 4, the strip is processed from left to right.

(7) FIG. 2 shows schematically a set-up with one or more supply nozzles 1 supplying a liquid 2 on a moving steel strip 3, wherein guide means 4 are provided to guide and/or force the supplied liquid 2 into a liquid film of a predefined thickness between strip 3 and guide means 4. Directly above the guide means a laser device 5 is mounted providing one or more laser beams to apply a texture on the moving steel strip 3.

(8) The guide means comprise a flat portion 6 of a transparent material which has low to negligible refraction for the incoming laser light. For the transparent material glass or hardened glass could be used which should preferably be of high quality with no internal defects. The glass itself can also be textured to alleviate turbulent flow at the water/glass interface. The contact side of the glass surface may be hydrophobic in character to prevent turbulent flow being introduced through the liquid adhesion to the contact surface. If required, the glass surface can be made hydrophobic in a wide variety of ways including surface texturing by laser, etching, lithography, selective deposition or by chemical modification through surfactants.

(9) The distance between the flat portion of the guide means and the surface of the moving steel strip is within tens of millimetres, and preferably between 1 and 5 millimetres.

(10) Control means (not shown in the drawing) are provided for the adjustment of the distance between the guide means and the moving steel strip and to keep the guide means within a predefined distance range above the moving steel strip. The predefined distance can be controlled for instance by jacks, motors, or a pulley system.

(11) Further means are provided to measure the distance between the guide means and the moving steel strip (also not shown in the drawing) which is used in the distance control of the guide means. Such means could for instance be based on ultrasound distance measurements, but of course many alternatives exists that could be used for the measurement of said distance.

(12) The guide means are provided with an upward directed edge 7 connected to the flat portion 6 of the guide means. The declining slope of the edge 7 guides the supplied liquid under the flat portion 6 of the guide means and ensures a conical flow of liquid from the nozzle or nozzles 1 to the working area of laser device 5 below flat portion 6.

(13) It is important to prevent that the liquid film affects the steel strip surface in any way. In many cases water can be used without that the steel strip, for instance galvanised steel strip, is affected in any way. If necessary demi-water or other liquids can be used, or liquids with additives for instance water or demi-water with alcohol, ammonia (up to 5%) and the like. An important requirement is that the liquid or the liquid with additives has negligible effect on the refraction of the incoming laser light.

(14) A support roll 8 is provided for the steel strip guiding the steel strip 3 in a downward direction. With this configuration the used liquid can easily be collected downstream of the guide means 4. A cleaning nozzle 10 is provided to remove texturing debris from the steel strip with a cleaning liquid. A collection container 9 is provided to collect the used liquid and the texturing debris.

(15) The configuration as shown in FIG. 2 can also be used at an angle up to and including a vertical position.

(16) As an alternative the guide means as described above could be replaced by a knife device which defines a passageway with respect to the moving steel strip, wherein the width of the passageway is a key factor in the thickness of the liquid film downstream of the knife device. With such a knife device there is no need for a transparent plate or the like, but the thickness of the liquid film can not be controlled as well as with the embodiment according to FIG. 2 and ripples and or waves on the surface of the liquid can not be prevented. Such a knife device can also be used at an angle up to and including a vertical position.

(17) In the configuration shown in FIG. 3 spray nozzles 11 are used to spray a liquid film on moving steel strip 3. The thickness of the liquid film is controlled by the pressure of the spray system and size(s) of the spray nozzle or nozzles 11, in combination with the line speed of the steel strip. The created layer thickness is in the range of sub-millimeters, preferably in the range t.sub.critical to t1, see FIG. 1, which is in the range of about 0.1-0.5 mm.

(18) In FIG. 3 a number of successive spray nozzles are shown, where each of the shown spray nozzles 11 may in fact be a series of spray nozzles divided over the width or part of the width of the steel strip. With the successive spray nozzles 11 a number of thin layers of sprayed liquid are stacked to get a thickness within the range of about 0.1-0.5 mm. In this embodiment the adhesion between the steel strip and the liquid film with limited thickness is the essential factor in providing a largely constant thickness of the liquid film.

(19) Within this thickness range of the liquid film the ablation rate is not as constant as with a thicker liquid film but is nevertheless within a certain range, see FIG. 1, which will result in a texturing of the steel strip that will be sufficient for many applications.

(20) The applied liquid film will be evaporated and/or reduced at least partly within the working area of the laser device 5. Downstream of the working area of laser device 5 a hot air device 12 is mounted with which hot air is blown on the moving steel strip. With the hot air the remaining part of the liquid film is dried out and dust resulting from the ablation by the laser beam or beams is blown away from the finished surface of the steel strip.

(21) During laser processing under water film, the ablation of strip material is accompanied by the formation of bubbles, as well as a cavitation. The cavitation bubble(s) decay in a time t.sub.x, during which laser processing is not desirable in the same location. In addition, bubbles might stick to the window, flat portion 6 and negatively affect the laser beam. To ensure that bubbles generated by the process escape efficiently from the laser-material interaction zone and do not stick to the window 6, the set-ups as depicted in FIG. 2 and FIG. 4 could be rotated around the axis of the support roll at any angle. Preferably the chosen angle is counter clockwise around the axis of the support roll at any angle in the set-ups as depicted in FIG. 2 and FIG. 3.

(22) In the embodiment shown in FIG. 4 a container 13 is provided filled with a liquid, wherein the container is provided with a watertight entry slot and exit slot for the moving steel strip 3 passing through the liquid 2 in the container. Laser beam focusing optics 14 are submerged in the liquid 2 in container 13. The distance between the focusing optics 14 and the steel strip surface determines the focal length of the laser beam. Since the laser beam optics 14 and the steel strip 3 are in same medium, a wavy nature of the liquid surface does not play any role in the texturing of the steel strip.

(23) This embodiment with the focusing optics 14 submerged in the liquid reduces the texturing process from a two medium processing system as in the embodiments according to FIG. 2 and FIG. 3 respectively, to a single medium processing system, thus eliminating the need of liquid layer surface variation control.

(24) The container is further provided with an inlet and outlet port to change and/or replenish the liquid 2 in container 13, which could be done either continuously or intermittently. Temperature control means, comprising heater and/or cooler devices, are provided to keep the liquid within a predefined temperature range.

(25) Instead of a container 13 with watertight entry slot and exit slot for the moving steel strip 3 a container with an at least partly open topside can be used. In such a configuration a number of guide rolls are needed to guide the steel strip into the container, through the liquid in the container, out of the container and back again in the overall direction of the processing line.

(26) The liquid is the key component in all set-ups. The liquid may comprise solvent with additives resulting in a desired dynamic viscosity in the range of 0.01-1000 mPa.Math.s, preferably in the range 5-500 mPa.Math.s, more preferably in the range 50-250 mPa.Math.s and even more preferably in the range 75-125 mPa.Math.s and has negligible refractive index contribution in the beam path of the used laser light of wavelength (200 nm-11 μm) in a temperature range of 5-50° C.