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Descaling Metal with a Laser Having a Very Short Pulse Width And Europäisches Patentamt *EP000927595B1* (19) European Patent Office Office européen des brevets (11) EP 0 927 595 B1 (12) EUROPEAN PATENT SPECIFICATION (45) Date of publication and mention (51) Int Cl.7: B23K 26/08, B23K 26/12 of the grant of the patent: 03.04.2002 Bulletin 2002/14 (21) Application number: 97122937.2 (22) Date of filing: 29.12.1997 (54) Descaling metal with a laser having a very short pulse width and high average power Entzundern von Metallen durch einen Laser mit sehr kurzer Pulsbreite und hoher Durchschnittsleistung Décalaminage de métaux par laser ayant des impulsions très courtes et une puissance moyenne élevée (84) Designated Contracting States: (74) Representative: Beetz & Partner Patentanwälte AT BE DE ES FR GB IT NL SE Steinsdorfstrasse 10 Designated Extension States: 80538 München (DE) SI (56) References cited: (43) Date of publication of application: US-A- 4 063 063 US-A- 4 368 080 07.07.1999 Bulletin 1999/27 US-A- 4 720 621 US-A- 5 736 709 (73) Proprietor: ARMCO INC. • PATENT ABSTRACTS OF JAPAN vol. 014, no. Middletown, Ohio 45044-3999 (US) 480 (C-0771), 19 October 1990 & JP 02 197588 A (SINTOKOGIO LTD), 6 August 1990, (72) Inventor: Neiheisel, Gary L. Cincinnati, Ohio 45248 (US) Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention). EP 0 927 595 B1 Printed by Jouve, 75001 PARIS (FR) 1 EP 0 927 595 B1 2 Description to descale the steel at economic line speeds. [0005] CO2, Nd:YAG, and excimer lasers represent BACKGROUND OF THE INVENTION the most common high average power industrial lasers over the wavelength range from far infrared (CO2 at 10.6 [0001] This invention relates to a process and appa- 5 microns) to the near infrared (Nd:YAG at 1.064 microns) ratus for removing oxide from metal. using a pulsed free to the ultraviolet excimer (XeCl at .308 microns, KrF at . electron laser for producing electromagnetic radiation 248 microns, and ArF at .193 microns). In order to obtain having a short pulse width, a high pulse repetition rate high descale rates, it is necessary to use very high pow- and a high average power to descale the metal by va- er lasers. Many of the high average power, e.g., greater porizing molecular layers of the oxide with each pulse. 10 than 1 KW, commercially available lasers operate in a Such a method and apparatus are disclosed in the continuous wave (CW) mode. CW operation presents US-A-4 720 621. problems for scale removal due to the absorption of the [0002] One of the more environmentally intensive op- incident laser beam by the plasma plume generated by erations in the metal making industry is the wet acid the removed elemental components. This is caused by pickling of a metal such as steel to remove oxide or scale 15 the relative long dwell times associated with CW laser formed during hot processing such as forging, rolling on processing. This has been confirmed by work done by a hot strip mill, or annealing. The techniques for remov- Schluter, et al disclosed in an article entitled Descaling ing this scale have changed little since the beginning of of Austenitic Steels by Laser Radiation, Proc. ICALEO the century. Most low carbon and electrical steel strip 94, Orlando, Fla., Oct. 17-20, 1994. This same article 20 are immersion pickled in hydrochloric acid at strip presents data showing that even pulsed CO2 lasers are speeds of about 250 m/min. Stainless steel has a more inefficient at oxide removal because the pulse duration tightly adhering scale and may require shot blasting or is long enough for the incoming laser beam to interact roller leveling to loosen or crack the scale prior to acid with the created plasma plume so that some of the in- pickling. Additionally, stainless steel pickling requires coming laser energy is absorbed by this plasma thereby more aggressive acids such as hydrofluoric, sulfuric, or 25 not removing additional oxide. The net result is a low nitric and requires longer immersion times resulting in descale rate. Wehner et al. disclose in an article entitled processing line speeds for strip of about 30-100 m/min. Ablation of Oxide Layers on Metallic Surfaces by Exci- A major motivation for improving or eliminating this type mer Laser Radiation, Proc. ECLAT 90, V2, pp. 917, that of scale removing process is the capital and environ- short pulses, e.g., 10-250 nanoseconds, from an exci- mental disposal costs of the acids associated with pick- 30 mer laser are more efficient at removing the oxide layers ling. The annual cost associated with a single production but these lasers are only available in low average power, line could be as much as $8 million dollars for disposal e.g., less than 250 W, and low repetition rates, i.e., less of hazardous acid alone. A major disadvantage of chem- than 1 KHz. A simple "ball park" estimate by applicant ical descaling processes is the environmental problems based on overcoming the heat of vaporization of the ox- related to disposal of the chemicals used for pickling. 35 ide layer reveals that to remove a 5 micron thick alumi- [0003] It is known to remove or assist in the removal num oxide layer from one side of a 1 m wide aluminum of scale from steel using a laser. For example, US-A- surface moving at 31 m/min requires an average laser 4,063,063 relates to a process for descaling metal prod- power of 100 KW. Although a 45 KW CO2 laser is com- ucts by irradiating a metal surface with a CO2 laser mercially available from Trans Tec/Convergent Energy, beam of sufficient intensity to produce a rapid and in- 40 it operates in a CW mode. Thus, even if two of these 45 tense local heating of an oxide film. The laser described KW lasers were used to cover a 1 m wide aluminum sur- in this patent may have a beam power up to 10 KW. face, the desired descale rate will not be achieved be- However, subsequent work has revealed that it is not cause of the plasma absorption problems associated possible to completely remove the oxide scale using with CW or long pulse width lasers just mentioned. To 45 pulsed or continuous wave CO2 lasers at comparable obtain this kind of power from short pulse width excimer line speeds to that achieved by acid pickling. Only an lasers is possible only by using a large number, e.g., unreasonably large and costly number of lasers could 400, of lasers each treating some small fraction of the achieve comparable descale rates. desired full width material. Thus, conventional industrial [0004] The JP-A-2-197588 relates to a method for re- lasers are not suited for economic removal of steel oxide moving scale or rust from steel. Scale or rust on the steel 50 layers. is irradiated with a laser beam in a UV wavelength region [0006] A major disadvantage of these prior art laser of the electromagnetic spectrum such as an excimer la- descaling processes is complete scale removal from a ser beam having a 100-400 nm wavelength for ≤ 200 steel strip traveling at a high speed was not possible. nsec pulse duration time to cause fine cracking of the Alternatively, the slow strip speeds required for com- scale or rust. Since the power of an excimer laser is lim- 55 plete scale removal by conventional laser technology ited to about 300 Watts and a pulse repetition rate less does not economically justify using this type of scale re- than 1 KHz, it also would be necessary to have an un- moval. reasonably large and costly number of excimer lasers [0007] US 4 720 621 discloses a method and an ap- 2 3 EP 0 927 595 B1 4 paratus for removing oxide from a metal surface, in par- with the metal surface. The focused radiation beam ex- ticular from the surface of a metal strip, using a laser, e. tends transversely completely across the oxide covered g. a CO2 laser, for producing coherent radiation with a surface of the metal to remove the oxide by vaporization high pulse repetition rate of 13.5 to 27 Mhz, a high av- by one or more laser pulses thereby forming an oxide erage power of 25 kW and a surface power density of 5 free surface. 10 MW/cm2 at the contact point with the metal surface. [0014] Another feature of the invention is for the afore- [0008] Accordingly, there remains a need for a metal said laser radiation having a photon wavelength in the oxide descaling process that does not require the use ultraviolet range. of an acid which causes an environmental disposal [0015] Another feature of the invention is for the afore- problem. There remains a need for a descaling process 10 said optical element being a lens, a mirror or a combi- wherein an oxidized metal strip traveling at a high speed nation thereof. does not have to be given a shot blasting pre-treatment [0016] Another feature of the invention is for the afore- to loosen the scale to insure complete removal. A laser said optical element being a combination of aligned el- process is needed that could economically remove the ements.
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