Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
  • B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese

Definitions

• This invention relates to the casting of steel strip. It has particular application to continuous casting of thin steel strip in a twin roll caster.
• molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls.
• the term "nip" is used herein to refer to the general region at which the rolls are closest together.
• the molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip.
• This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
• the molten steel in the casting pool will generally be at a temperature of the order of 1500°C and above, and it is therefore necessary to achieve very high cooling rates over the casting surfaces of the rolls. It is particularly important to achieve a high heat flux and extensive nucleation on initial solidification of the steel on the casting surfaces to form the metal shells.
• United States Patent 5,720,336 describes how the heat flux on initial solidification can be increased by adjusting the steel melt chemistry such that a substantial proportion of the metal oxides formed as deoxidation products are liquid at the initial solidification temperature so as to form a substantially liquid layer at the interface between the molten metal and each casting surface.
• nucleation of the steel on initial solidification can be influenced by the texture of the casting surface.
• International Application AU 99/00641 discloses that a random texture of peaks and troughs can enhance initial solidification by providing potential nucleation sites distributed throughout the casting surfaces.
• nucleation is also dependent on the presence of oxide inclusions in the steel melt and that surprisingly it is not advantageous in twin roll strip casting to cast with "clean" steel in which the number of inclusions formed during deoxidation has been minimised in the molten steel prior to casting .
• Steel for continuous casting is subjected to deoxidation treatment in the ladle prior to pouring.
• the steel In twin roll casting the steel is generally subjected to silicon manganese ladle deoxidation although it is possible to use aluminum deoxidation with calcium addition to control the formation of solid Al 2 0 3 inclusions that can clog the fine metal flow passages in the metal delivery system through which molten metal is delivered to the casting pool. It has hitherto been thought desirable to aim for optimum steel cleanliness by ladle treatment to minimise the total oxygen level in the molten steel.
• Molten steel is trimmed by deoxidation in the ladle such that the total oxygen content falls within a range which ensures satisfactory solidification on the casting rolls and production of a satisfactory strip product.
• the molten steel contains a distribution of oxide inclusions (typically MnO, CaO, Si0 2 and/or Al 2 0 3 ) sufficient to provide an adequate density of nucleation sites on the roll surfaces for initial solidification and the resulting strip product exhibits a characteristic distribution of solidified inclusions.
• a assembling a pair of cooled casting rolls having a nip between them and with confining closures adjacent the ends of the nip; b. introducing molten low carbon steel having a total oxygen content of at least 100 ppm between the pair of casting rolls to form a casting pool between the casting rolls; c. counter rotating the casting rolls and solidifying the molten steel to form metal shells on the surface of the casting rolls with levels of oxide inclusions reflected by the total oxygen content of the molten steel to promote the formation of thin steel strip; and d. forming solidified thin steel strip through the nip of the casting rolls from said solidified shells.
• the total oxygen content of the molten steel in the casting pool may be between 100 ppm and 250 ppm. More specifically, it may be about 200 ppm.
• the low carbon steel may have a carbon content in the range 0.001% to 0.1% by weight, a manganese content in the range 0.1% to 2.0% by weight and a silicon content in the range 0.01% to 10% by weight.
• the steel may have an aluminum content of the order of 0.01% or less by weight. The aluminum may for example be as little as 0.008% or less by weight.
• the molten steel may be a silicon/manganese killed steel.
• the oxide inclusions are solidification inclusions and deoxidation inclusions.
• the solidification inclusions are formed during cooling and solidification of the steel in casting, and deoxidation inclusion are formed during deoxidation of the molten steel before casting.
• the solidified steel may contain oxide inclusions usually comprised of any one or more of MnO, Si0 and Al 2 0 3 distributed through the steel at an inclusion density in the range 2 gm/cm 3 and 4gm/cm .
• the molten steel may be refined in a ladle prior to introduction between the casting rolls to form the casting pool by heating a steel charge and slag forming material in the ladle whereby to form molten steel covered by a slag containing silicon, manganese and calcium oxides.
• the molten steel may be stirred by injecting an inert gas into it to cause desulphurisation, and with steels such as a silicon/manganese killed steel, then injecting oxygen, to produce steel having the desired total oxygen content of at least 100 ppm and usually less than 250 ppm.
• the desulphurisation may reduce the sulphur content of the molten steel to less than 0.01% by weight.
• the thin steel strip produced by continuous twin roll casting as described above has a thickness of less than 5 mm and is formed of a solidified steel containing solidified oxide inclusions.
• the distribution of the inclusions may be such that at the two the surface regions of the strip to a depth of 2 microns from the outer faces contain solidified inclusions to a per unit area density of at least 120 inclusions/mm 2 .
• the solidified steel may be a silicon/manganese killed steel and the oxide inclusions may comprise any one or more of MnO, Si0 and Al 2 0 3 inclusions.
• the inclusions typically may range in size between 2 and 12 microns, so that at least a majority of the inclusions are in that size range.
• the method described above produces a unique steel high in oxygen content distributed in oxide inclusions.
• the combination of the high oxygen content in the molten steel and the short residence time of the molten steel in the casting pool results in a thin steel strip with an improved ductility properties.
• Figure 1 shows the effect of inclusion melting points on heat fluxes obtained in twin roll casting trials using silicon/manganese killed steels
• Figure 2 is an energy dispersive spectroscopy (EDS) map of Mn showing a band of fine solidification inclusions in a solidified steel strip
• Figure 3 is a plot showing the effect of varying manganese to silicon contents on the liquidus temperature of inclusions
• Figure 4 shows the relationship between alumina content (measured from the strip inclusions) and deoxidation effectiveness
• Figure 5 is a ternary phase diagram for MnO.Si0 2 .Al 2 0 3 ;
• Figure 6 shows the relationship between alumina content inclusions and liquidus temperature
• Figure 7 shows the effect of oxygen in a molten steel on surface tension
• Figure 8 is a plot of the results of calculations concerning the inclusions available for nucleation at differing steel cleanliness levels.
• the oxide inclusions formed in the solidified metal shells and in turn the thin steel strip comprise inclusions formed during cooling and solidification of the steel, and deoxidation inclusions formed during refining in the ladle.
• the free oxygen level in the steel is reduced dramatically during cooling at the meniscus, resulting in the generation of solidification inclusions near the surface of the strip.
• These solidification inclusions are formed predominantly of MnO.Si0 2 by the following reaction:
• Mn+Si+30 MnO.Si0 2
• EDS Spectroscopy
• a manganese silicon killed steel having a carbon content in the range of 0.001% to 0.1% by weight, a manganese content in the range 0.1% to 2.0% by weight and a silicon content in the range 0.1% to 10% by weight and an aluminum content of the order of 0.01% or less by weight can produce such oxide inclusions during cooling of the steel in the upper regions of the casting pool.
• the steel may have the following composition, termed M06:
• Aluminium 0.002% by weight.
• Deoxidation inclusions are generated during deoxidation of the molten steel in the ladle with Al, Si and Mn.
• the composition of the oxide inclusions formed during deoxidation is mainly MnO.Si0 2 .Al 2 0 3 based. These deoxidation inclusions are randomly located in the strip and are coarser than the solidification inclusions near the strip surface.
• alumina content of the inclusions has a strong effect on the free oxygen level in the steel.
• Figure 4 shows that with increasing alumina content, free oxygen in the steel is reduced.
• MnO.Si0 2 inclusions are diluted with a subsequent reduction in their activity, which in turn reduces the free oxygen level, as seen from the reaction below:
• the effect of inclusion composition on liquidus temperature can be obtained from the ternary phase diagram shown in Figure 5.
• Analysis of the oxide inclusions in the thin steel strip has shown that the MnO/Si0 2 ratio is typically within 0.6 to 0.8 and for this regime, it was found that alumina content of the oxide inclusions had the strongest effect on the inclusion melting point (liquidus temperature), as shown in Figure 6.
• deoxidation inclusions are much bigger, typically greater than 4 microns, whereas the solidification inclusions are generally less than 2 microns and are MnO. Si0 2 based and have no Al 2 0 3 whereas the deoxidation inclusions also have Al 2 0 3 It has been found in casting trials using the above
• M06 grade of silicon/manganese killed steel that if the total oxygen content of the steel is reduced in the ladle refining process to low levels of less than 100 ppm, heat fluxes are reduced and casting is impaired whereas good casting results can be achieved if the total oxygen content is at least above 100 ppm and typically on the order of 200 ppm.
• the total oxygen content may be measured by a "Leco” instrument and is controlled by the degree of "rinsing" during ladle treatment, i.e. the amount of argon bubbled through the ladle via a porous plug or top lance, and the duration of the treatment.
• the total oxygen content was measured by conventional procedures using the LECO TC-436 Nitroge /Oxygen Determinator described in the TC 436 Nitrogen/Oxygen Determinator Instructional Manual available from LECO (Form No. 200-403, Rev. Apr. 96, Section 7 at pp. 7-1 to 7-4.
• Oxygen levels in Ca-Si grades were lower, typically 20 to 30 ppm compared to 40 to 50 ppm with M06 grades.
• Oxygen is a surface active element and thus reduction in oxygen level is expected to reduce the wetting between molten steel and the casting rolls and cause a reduction in the heat transfer rate.
• oxygen reduction from 40 to 20 ppm may not be sufficient to increase the surface tension to levels that explain the observed reduction in the heat flux.
• the thickness of this layer can be measured at points throughout its area to map variations in the solidification rate and therefore the effective rate of heat transfer at the various locations. It is thus possible to produce an overall solidification rate as well as total heat flux measurements. It is also possible to examine the microstructure of the strip surface to correlate changes in the solidification microstructure with the changes in observed solidification rates and heat transfer values and to examine the structures associated with nucleation on initial solidification at the chilled surface.
• a dip testing apparatus is more fully described in United States Patent 5,720,336
• the relationship of the oxygen content of the liquid steel on initial nucleation and heat transfer has been examined using a model described in Appendix 1.
• This model assumes that all the oxide inclusions are spherical and are uniformly distributed throughout the steel.
• a surface layer was assumed to be 2 ⁇ m and that only inclusions present in that surface layer could participate in the nucleation process on initial solidification of the steel.
• the input to the model was total oxygen content in the steel, inclusion diameter, strip thickness, casting speed, and surface layer thickness.
• the output was the percentage of inclusions of the total in the steel required to meet a targeted nucleation per unit area density of 120/mm 2 .
• Figure 8 is a plot of the percentage of oxide inclusions in the surface layer required to participate in the nucleation process to achieve the target nucleation per unit area density at different steel cleanliness levels as expressed by total oxygen content, assuming a strip thickness of 1.6 mm and a casting speed of 80/min. This shows that for a 2 ⁇ m inclusion size and 200 ppm total oxygen content, 20% of the total available oxide inclusions in the surface layer are required to achieve the target nucleation per unit area density of 120/mm 2 . However, at 80 ppm total oxygen content, around 50% of the inclusions are required to achieve the critical nucleation rate and at 40 ppm total oxygen level there will be an insufficient level of oxide inclusions to meet the target nucleation per unit area density.
• the oxygen content of the steel can be controlled to produce a total oxygen content in the range 100 to 250 ppm and typically about 200 ppm.
• This will have the result that the two micron deep layers adjacent the casting rolls on initial solidification will contain oxide inclusions having a per unit area density of at least 120/mm 2 .
• These inclusions will be present in the outer surface layers of the final solidified strip product and can be detected by appropriate examination, for example by energy dispersive spectroscopy (EDS) .
• EDS energy dispersive spectroscopy
• w roll width
• m t strip thickness
• mm m s steel weight in the ladle
• tonne p s density of steel
• kg/m 3 pi density of inclusions
• kg/m3 O t total oxygen in steel
• ppm d inclusion diameter
• m vi volume of one inclusions
• m3 mi mass of inclusions
• N s total number of inclusions present in the surface (that can participate in the nucleation process)
• u casting speed
• L s strip length
• a s strip surface area, m2
• N re q Total number of inclusions required to meet the target nucleation density
• NC t target nucleation per unit area density, number/mm2 (obtained from dip testing)
• N av % of total inclusions available in the molten steel at the surface of the casting rolls for initial nucleation process.
• Mn-Si killed steel 0.42kg of oxygen is needed to produce 1 kg of neclusions with a composition of 30% MnO, 40% Si0 2 and 30% Al 2 0 3 .
• N r ⁇ q A S x 10 6 x NC t
• N av % (N r ⁇ q /N s ) x 100 . 0
• Eq. 1 calculates the mass of inclusions in steel.
• Eq. 2 calculates the volume of one inclusion assuming they are spherical .
• Eq. 3 calculates the total number of inclusions available in steel.
• Eq. 4 calculates the total number of inclusions available in the surface layer (assumed to be 2 um on each side) . Note that these inclusions can only participate in the initial nucleation.
• Eq. 5 and Eq. 6 used to calculate the total surface area of the strip.
• Eq. 7 calculates the number of inclusions needed at the surface to meet the target nucleation rate.
• Eq. 8 is used to calculate the percentage of total inclusions available at the surface which must participate in the nucleation process. Note if this number is great than 100%, then the number of inclusions at the surface is not sufficient to meet target nucleation rate.

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