US20030111206A1 - Casting steel strip - Google Patents

Casting steel strip Download PDF

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Publication number
US20030111206A1
US20030111206A1 US10/243,699 US24369902A US2003111206A1 US 20030111206 A1 US20030111206 A1 US 20030111206A1 US 24369902 A US24369902 A US 24369902A US 2003111206 A1 US2003111206 A1 US 2003111206A1
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Prior art keywords
steel
inclusions
casting
steel strip
ppm
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Abandoned
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US10/243,699
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English (en)
Inventor
Walter Blejde
Rama Mahapatra
Lazar Strezov
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Nucor Corp
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Individual
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Priority to US10/243,699 priority Critical patent/US20030111206A1/en
Assigned to NUCOR CORPORATION reassignment NUCOR CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: BLEJDE, WALTER N., MAHAPATRA, RAMA BALLAV, STREZOV, LAZAR
Publication of US20030111206A1 publication Critical patent/US20030111206A1/en
Priority to US10/761,953 priority patent/US7048033B2/en
Priority to US11/255,604 priority patent/US7485196B2/en
Priority to US11/419,684 priority patent/US7588649B2/en
Priority to US11/469,686 priority patent/US7690417B2/en
Priority to US12/363,896 priority patent/US8002908B2/en
Abandoned legal-status Critical Current

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    • 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.
  • twin roll casting 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.
  • 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.
  • 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 minimized in the molten steel prior to casting.
  • the molten steel contains a distribution of oxide inclusions (typically MnO, CaO, SiO 2 and/or Al 2 O 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.
  • oxide inclusions typically MnO, CaO, SiO 2 and/or Al 2 O 3
  • the total oxygen content of the molten steel in the casting pool 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 0.01% 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 inclusions 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, SiO 2 and Al 2 O 3 distributed through the steel at an inclusion density in the range 2 gm/cm 3 and 4 gm/cm 3 .
  • 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 desulphurization, 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 desulphurization 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 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, SiO 2 and Al 2 O 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. Specifically, 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 improved ductility properties.
  • FIG. 1 shows the effect of inclusion melting points on heat fluxes obtained in twin roll casting trials using silicon/manganese killed steels
  • FIG. 2 is an energy dispersive spectroscopy (EDS) map of Mn showing a band of fine solidification inclusions in a solidified steel strip;
  • EDS energy dispersive spectroscopy
  • FIG. 3 is a plot showing the effect of varying manganese to silicon contents on the liquidus temperature of inclusions
  • FIG. 4 shows the relationship between alumina content (measured from the strip inclusions) and deoxidation effectiveness
  • FIG. 5 is a ternary phase diagram for MnO SiO 2 Al 2 O 3 ;
  • FIG. 6 shows the relationship between alumina content inclusions and liquidus temperature
  • FIG. 7 shows the effect of oxygen in a molten steel on surface tension
  • FIG. 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.
  • FIG. 2 The appearance of the solidification inclusions on the strip surface, obtained from an Energy Dispersive Spectroscopy (EDS) map, is shown in FIG. 2. It can be seen that solidification inclusions are extremely fine (typically less than 2 to 3 ⁇ m) and are located in a band located within 10 to 20 ⁇ m from the surface. A typical size distribution of the inclusions through the strip is shown in FIG. 3 of our paper entitled Recent Developments in Project M the Joint Development of Low Carbon Steel Strip Casting by BHP and IHI, presented at the METEC Congress 99, Dusseldorf Germany (Jun. 13-15, 1999)
  • the comparative levels of the solidification inclusions are primarily determined by the Mn and Si levels in the steel.
  • FIG. 3 shows that the ratio of Mn to Si has a significant effect on the liquidus temperature of the inclusions.
  • 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: Carbon 0.06% by weight Manganese 0.6% by weight Silicon 0.28% by weight 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.SiO 2 .Al 2 O 3 based. These deoxidation inclusions are randomly located in the strip and are coarser than the solidification inclusions near the strip surface.
  • the alumina content of the inclusions has a strong effect on the free oxygen level in the steel.
  • FIG. 4 shows that with increasing alumina content, free oxygen in the steel is reduced.
  • MnO.SiO 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 deoxidation inclusions are much bigger, typically greater than 4 microns, whereas the solidification inclusions are generally less than 2 microns and are MnO.SiO 2 based and have no Al 2 O 3 whereas the deoxidation inclusions also have Al 2 O 3 .
  • the total oxygen content was measured by conventional procedures using the LECO TC-436 Nitrogen/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 U.S. Pat. No. 5,720,336.
  • FIG. 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 m/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.
  • 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
  • INPUTS Critical nucleation per 120 This value has unit area density no/mm2 been obtained (needed to achieve sufficient from heat transfer rates) experimental dip testing work Roll width m 1 Strip thickness mm 1.6 Ladle tonnes t 120 Steel density, kg/m3 7800 Total oxygen, ppm 75 Inclusion density, kg/m3 3000 OUTPUTS Mass of inclusions, kg 21.42857 Inclusion diameter, m 2.00E-06 Inclusion volume, m3 0.0 Total no of 1706096451319381.5 inclusions Thickness of surface 2 layer, ⁇ m (one side) Total no of 4265241128298.4536 These inclusions inclusions surface can participate only in the initial nucleation process Casting speed, m/min 80 Strip length, m 9615.38462 Strip surface area, m2 19230.76923 Total no of nucleating 2307692.30760 sites required % of available inclusions 54.10462 that need to participate in the nucleation process
  • ⁇ s density of steel, kg/m 3
  • N t total number of inclusions
  • N s total number of inclusions present in the surface (that can participate in the nucleation process)
  • a s strip surface area, m2
  • N req 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.
  • N t m i /( ⁇ i ⁇ v i ) (3)
  • N s (2.0 t s ⁇ 0.001 ⁇ N t /t ) (4)
  • N req A s ⁇ 10 6 ⁇ NC t (7)
  • N av % ( N req /N s ) ⁇ 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 ⁇ m on each side). Note that these inclusions can only participate in the initial nucleation.
  • Eq. 5 and Eq. 6 are 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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  • Chemical & Material Sciences (AREA)
  • Mechanical Engineering (AREA)
  • Engineering & Computer Science (AREA)
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  • Metallurgy (AREA)
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  • Continuous Casting (AREA)
  • Treatment Of Steel In Its Molten State (AREA)
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US10/243,699 2001-09-14 2002-09-13 Casting steel strip Abandoned US20030111206A1 (en)

Priority Applications (6)

Application Number Priority Date Filing Date Title
US10/243,699 US20030111206A1 (en) 2001-09-14 2002-09-13 Casting steel strip
US10/761,953 US7048033B2 (en) 2001-09-14 2004-01-21 Casting steel strip
US11/255,604 US7485196B2 (en) 2001-09-14 2005-10-20 Steel product with a high austenite grain coarsening temperature
US11/419,684 US7588649B2 (en) 2001-09-14 2006-05-22 Casting steel strip
US11/469,686 US7690417B2 (en) 2001-09-14 2006-09-01 Thin cast strip with controlled manganese and low oxygen levels and method for making same
US12/363,896 US8002908B2 (en) 2001-09-14 2009-02-02 Steel product with a high austenite grain coarsening temperature

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US32226101P 2001-09-14 2001-09-14
US10/243,699 US20030111206A1 (en) 2001-09-14 2002-09-13 Casting steel strip

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EP (1) EP1439926B1 (pt)
JP (1) JP4495455B2 (pt)
CN (1) CN1277634C (pt)
AT (1) ATE509716T1 (pt)
AU (2) AU2002331433A2 (pt)
BR (1) BRPI0212499B1 (pt)
CO (1) CO5560594A2 (pt)
HR (1) HRP20040234B1 (pt)
IS (1) IS7168A (pt)
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US20030154819A1 (en) * 2002-02-15 2003-08-21 Rama Mahapatra Model-based system for determining process parameters for the ladle refinement of steel
US20040144519A1 (en) * 2003-01-24 2004-07-29 Blejde Walter N. Casting steel strip
US20050145304A1 (en) * 2003-01-24 2005-07-07 Blejde Walter N. Casting steel strip
US20060144553A1 (en) * 2001-09-14 2006-07-06 Nucor Corporation Steel product with a high austenite grain coarsening temperature, and method for making the same
US20060196630A1 (en) * 2001-09-14 2006-09-07 Nucor Corporation Casting steel strip
US20070079950A1 (en) * 2001-09-14 2007-04-12 Nucor Corporation Thin cast strip with controlled manganese and low oxygen levels and method for making same
US20080219879A1 (en) * 2005-10-20 2008-09-11 Nucor Corporation thin cast strip product with microalloy additions, and method for making the same
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AT504225B1 (de) * 2006-09-22 2008-10-15 Siemens Vai Metals Tech Gmbh Verfahren zur herstellung eines stahlbandes
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US7690417B2 (en) 2001-09-14 2010-04-06 Nucor Corporation Thin cast strip with controlled manganese and low oxygen levels and method for making same
US7588649B2 (en) 2001-09-14 2009-09-15 Nucor Corporation Casting steel strip
US20090191425A1 (en) * 2001-09-14 2009-07-30 Nucor Corporation Steel product with a high austenite grain coarsening temperature, and method for making the same
US8002908B2 (en) 2001-09-14 2011-08-23 Nucor Corporation Steel product with a high austenite grain coarsening temperature
US20060144553A1 (en) * 2001-09-14 2006-07-06 Nucor Corporation Steel product with a high austenite grain coarsening temperature, and method for making the same
US7485196B2 (en) 2001-09-14 2009-02-03 Nucor Corporation Steel product with a high austenite grain coarsening temperature
US20060196630A1 (en) * 2001-09-14 2006-09-07 Nucor Corporation Casting steel strip
US20070079950A1 (en) * 2001-09-14 2007-04-12 Nucor Corporation Thin cast strip with controlled manganese and low oxygen levels and method for making same
US20030154819A1 (en) * 2002-02-15 2003-08-21 Rama Mahapatra Model-based system for determining process parameters for the ladle refinement of steel
US6808550B2 (en) * 2002-02-15 2004-10-26 Nucor Corporation Model-based system for determining process parameters for the ladle refinement of steel
US20050223850A1 (en) * 2002-02-15 2005-10-13 Bleide Walter N Model-based system for determining process parameters for the ladle refinement of steel
US7211127B2 (en) 2002-02-15 2007-05-01 Nucor Corporation Model-based system for determining process parameters for the ladle refinement of steel
US20060032557A1 (en) * 2003-01-24 2006-02-16 Blejde Walter N Casting steel strip with low surface roughness and low porosity
US7299856B2 (en) 2003-01-24 2007-11-27 Nucor Corporation Casting steel strip with low surface roughness and low porosity
US20080032150A1 (en) * 2003-01-24 2008-02-07 Nucor Corporation Casting steel strip with low surface roughness and low porosity
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US7281569B2 (en) * 2003-01-24 2007-10-16 Nucor Corporation Casting steel strip with low surface roughness and low porosity
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US20060157218A1 (en) * 2003-01-24 2006-07-20 Nucor Corporation Casting steel strip with low surface roughness and low porosity
US20050145304A1 (en) * 2003-01-24 2005-07-07 Blejde Walter N. Casting steel strip
US20040177944A1 (en) * 2003-01-24 2004-09-16 Blejde Walter N. Casting steel strip with low surface roughness and low porosity
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AU2008249238B2 (en) 2011-03-24
WO2003024644A1 (en) 2003-03-27
HRP20040234B1 (hr) 2013-02-28
NO20041500L (no) 2004-06-10
NO342646B1 (no) 2018-06-25
MY134786A (en) 2007-12-31
AU2002331433A2 (en) 2003-04-01
RU2004111292A (ru) 2005-05-20
EP1439926A1 (en) 2004-07-28
AU2008249238A1 (en) 2008-12-18
MXPA04002374A (es) 2004-11-22
BRPI0212499B1 (pt) 2015-12-08
CN1277634C (zh) 2006-10-04
BR0212499A (pt) 2004-12-28
ATE509716T1 (de) 2011-06-15
CO5560594A2 (es) 2005-09-30
UA77001C2 (en) 2006-10-16
HRP20040234A2 (en) 2004-08-31
EP1439926A4 (en) 2004-11-03
RU2297900C2 (ru) 2007-04-27
CN1553836A (zh) 2004-12-08
JP4495455B2 (ja) 2010-07-07
EP1439926B1 (en) 2011-05-18
JP2005501741A (ja) 2005-01-20
IS7168A (is) 2004-03-03

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