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EP-4314781-B1 - METHOD OF ADJUSTING THE ELEMENTAL INHOMOGENEITY OF A CONTINUOUS CAST METAL

EP4314781B1EP 4314781 B1EP4314781 B1EP 4314781B1EP-4314781-B1

Inventors

  • VAN DEN BERG, Frenk
  • SANTILLANA, Begona
  • HECHU, Kateryna

Dates

Publication Date
20260506
Application Date
20220323

Claims (15)

  1. A method of adjusting the elemental inhomogeneity of a continuously cast metal, comprising the steps of casting a metal through a mould with one or more casting parameters, determining the elemental inhomogeneity of the solidified cast metal, thereby determining if the metal cast is homogeneous or inhomogeneous, and maintaining the casting parameters to the present state when it is determined that the metal cast is homogeneous and adjusting one or more casting parameters when it is determined that the metal cast is inhomogeneous, thereby adjusting the metal cast to be more homogeneous, wherein the elemental inhomogeneity of the continuously cast metal is determined online by laser-induced breakdown spectroscopy, comprising the steps of providing a cross-section of the continuously cast metal, agitating a plurality of spots within the cross-section with a laser thereby generating a plasma and analysing the radiation emitted by the plasma of each spot to determine the elemental composition of each spot and deduce the deviation from the aimed elemental composition, and wherein the one or more casting parameters comprise primary and secondary cooling profiles, fluid flow, casting speed, superheat, mechanical changes such as soft reduction (SR) and dynamic soft reduction (DSR) and stirring practises (EMS and EMBr).
  2. The method according to claim 1, wherein the laser has a laser pulse energy of at least 10 mJ per pulse, preferably at least 22 mJ per pulse.
  3. The method according to any of the claims 1 or 2, wherein the elemental inhomogeneity of the whole area of the cross-section is determined.
  4. The method according to any of the claims 1 or 2, wherein the elemental inhomogeneity of 0.01 - 10 % of the cross-section is determined.
  5. The method according to any of the preceding claims, wherein the step size between the spots is in the range of 20 - 500 µm.
  6. The method according to any of the preceding claims, wherein each spot is agitated at least 3 times.
  7. The method according to any of the preceding claims, wherein the laser spot diameter is in the range of 100 - 300 µm.
  8. The method according to any of the preceding claims, wherein the energy density per spot is at least 60 mJ/mm 2 , the energy density being determined with the following formula: n ∗ p π ∗ d / 2 2 ¯ wherein n corresponds with the number of agitations per spot, p corresponds to the laser pulse energy and d corresponds to the laser spot diameter.
  9. The method according to any of the preceding claims, wherein the area analysis rate is at least 10 µm 2 /s, preferably at least 100 µm 2 /s, the area analysis rate being determined with the following formula: π ∗ d / 2 2 ∗ f n wherein d corresponds to the laser spot diameter, f to the laser frequency and n to the number of agitations per spot.
  10. The method according to any of the preceding claims, wherein the cross-section is prepared before laser agitation, by grinding or drilling or remachining or deburring or a combination thereof.
  11. The method according to any of the claims above, wherein the continuously cast metal has a temperature in a range of room temperature - 1000 °C.
  12. The method according to any of the preceding claims, wherein the information obtained by LIBS is analysed and presented on a human-machine interface.
  13. The method according to any of the preceding claims, wherein the cross section is transverse, i.e. perpendicular to the casting direction of the metal slab.
  14. The method according to any of the preceding claims, wherein the elemental inhomogeneity of the continuously cast metal is determined in a temperature range of 700 - 1000 °C.
  15. The method according to any of the claims above, wherein the metal is steel.

Description

The present invention relates to a method of adjusting the elemental inhomogeneity of a continuously cast metal by laser-induced breakdown spectroscopy. Continuous casting, also called strand casting, is the known process whereby molten metal, e.g. steel, is solidified into a billet, bloom, or slab. These billets blooms or slabs can be used for subsequent rolling in the finishing mills into wire, sections, plate or strip. The molten metal is tapped into a ladle (Figure 5, 60) and after undergoing any ladle treatments, such as alloying and degassing, and arriving at the correct temperature, the ladle is transported to the top of a casting machine. Usually the ladle sits in a slot on a rotating turret at the casting machine. One ladle is in the 'on-cast' position (feeding the casting machine) while the other is made ready in the 'off-cast' position, and is switched to the casting position when the first ladle is empty. From the ladle, the hot metal is transferred via a refractory pipe (55) to a holding bath called a tundish (50). The tundish allows a reservoir of metal to feed the casting machine while ladles are switched, thus acting as a buffer of hot metal, as well as smoothing out flow, regulating metal feed to the moulds and cleaning the metal (figure 5). Metal is drained from the tundish through another shroud into the top of an open-base copper mould (45). The depth of the mould can range from 0.5 to 2 metres depending on the casting speed and section size. The mould is water-cooled to solidify the hot metal directly in contact with it; this is the primary cooling process. It also oscillates to prevent the metal sticking to the mould walls. A lubricant is added to the metal in the mould to prevent sticking, and to trap any slag particles-including oxide particles or scale-that may be present in the metal and bring them to the top of the pool to form a floating layer of slag. The shroud is set so the hot metal exits it below the surface of the slag layer in the mould and is thus called a submerged entry nozzle (SEN). In the mould, a thin shell of metal next to the mould walls solidifies before the middle section, now called a strand (40), exits the base of the mould into a spray chamber. The bulk of metal within the walls of the strand is still molten. The strand is immediately supported by closely spaced, water-cooled rollers which support the walls of the strand against the ferrostatic pressure of the still-solidifying liquid within the strand. To increase the rate of solidification, the strand is sprayed with large amounts of water as it passes through the spray-chamber; this is the secondary cooling process. Final solidification of the strand may take place after the strand has exited the spray-chamber. After exiting the spray-chamber, the strand (30) passes through straightening rolls and withdrawal rolls. Finally, the strand is cut into predetermined lengths by mechanical shears or by travelling oxyacetylene torches (20), is marked for identification, and is taken either to a stockpile or to the next forming process. Inhomogeneity of a cast metal may have detrimental effects on the final properties of the product, for example, due to macrosegregation. In the steel industry, for instance, macrosegregation in a steel cast product may be detrimental to the properties of the final product. Steels with elemental inhomogeneity will be less suitable for welded tube due to worse joining performance as well as for beam blanks used in rail products and safety parts of the automotive chassis due to the lack of structural integrity. For product development, process control and quality assurance it is desired to provide an early indication of the inhomogeneity of the cast steel. However, with the currently available techniques, the assessment takes too much manpower, time and money for inspection of all slabs produced. From the article "A stochastic model of the process of sequence casting of steel, taking into account imperfect mixing" by D. Mier et al. in Applied Physics B: Lasers and Optics, vol.125:65, no.4, 01 April 2019 it is known to compare a numerical model of cast steel inhomogeneity with laser-induced breakdown spectroscopy measurements on a polished cold billet. It is therefore desirable to provide a method that can swiftly give insight in the elemental inhomogeneity of the cast metal. It is therefore an object of the invention to provide a method that rapidly assesses the inhomogeneity, preferably within 3 - 180 minutes or faster. It is also an object of the invention to provide a method that limits the preparation time of the metal sample. It is also an object of the invention to provide a method that can measure cast metals at elevated temperatures. It is also an object of the invention to provide a method that can be used online during metal casting. One or more of these objectives are reached with a method according to claim 1 - 15. In the first aspect, the invention provides a method accordin