Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REFINEMENT OF STEEL
Background of the Invention
[0001] This invention relates generally to refining of steel. More
particularly,
this invention relates to processes for refinement of silicon-bearing Al-
killed and Al-
Si dual killed steel to be directly cast in a continuous slab caster.
[0002] In continuous slab casting, the continuous caster is comprised of
a
tundish and an oscillating mold, in addition to a shroud and submerged entry
nozzle.
The molten steel in the ladle is poured into a tundish and then poured
vertically
through the submerged entry nozzle into a hollow water-cooled oscillating
mold, and
continuously cast slabs are withdrawn horizontally from the bottom of the
mold.
Refractory shrouds are used to transfer the molten steel from the ladle to the
tundish,
and then to the submerged entry nozzle and the mold, to avoid oxidation of the
molten
steel through contact with air. . The shroud between the tundish and the mold
feeds
through the submerged entry nozzle, and is regulated by a stopper rod.
[0003] The continuous slab caster produces wide rectangular strands of
large
cross-section, which are cut off into slabs to be hot rolled and cold rolled
for use as
material for sheet and plate. Thick slabs for flat-rolled products usually
have an as-
cast thickness of 100 to 250 mm. Thin slabs for flat-rolled products usually
have an
as-cast thickness of 30 to 100 mm. The slab caster is usually used in
conjunction with
an electric arc furnace or basic oxygen furnace, where the hot metal in
produced for
the caster.
[0004] Steel for continuous casting may be subjected to deoxidation
treatment
usually in a ladle prior to casting. Deoxidizing the molten steel in a ladle
metallurgy
furnace (LMF) or Vacuum Tank Degassed (VTD) to a desired oxygen level is
typical.
Aluminum (or a combination of Al and Si) has been widely used as a deoxidizer
and
grain size controller in the manufacture of steels. Aluminum acts as a
sacrificial
metal which combines with oxygen to form a stable aluminum oxide, which
migrates
into the slag. Aluminum is a particularly desirable material for this purpose
because it
can be safely stored, handled and transported at ambient temperature, and, it
is
reactive as an oxidizing agent with steel at steelmaking temperatures.
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[0005] Most thin slab casting and plating grades of steel are typically
Al-
killed steels. In some cases a combination of Al and Si is used to kill the
steel. While
this steel can be cast "as is" in large slab casters, further treatment is
required in thin
slab casters to avoid clogging or choking of submerged entry nozzles. One
established practice in thin slab casting is to modify alumina and spinel
inclusions by
treatment with calcium to provide for more liquidity. With proper calcium
treatment,
the majority of the solid alumina (A1203) and/or spinel (MgA1204) inclusions
are
modified to liquid inclusions and casting is performed with acceptable surface
quality
to the cast slab. For continuous casting in a thin slab caster, 600 feet
(182.9 m) of
calcium wire has been found sufficient for a 170 ton (154 tons metric) ladle
to add the
calcium to avoid nozzle clogging (about 0.134 lb/ton, 0.067 kg/ton metric).
600 feet
(182.9 m) of calcium wire contains about 22.5 lbs (10.2 kg) of calcium and is
equivalent to about 16.8 ppm effective calcium in the refined steel. The
recovery of
calcium in the steel from calcium wire is less than 100% so that the effective
calcium
will be less than the amount added.
[0006] There are two main grades of silicon-bearing steels for sheets and
plate
steels made in a thin slab caster:
= Silicon-restricted steel typically with less than 0.03% silicon
Generally ferrosilicon or silicomanganese is not added
= Silicon-bearing steel typically with about 0.1 % to 1.5% silicon
Silicomanganese and/or ferrosilicon is added to achieve the desired silicon
content.
[0007] Problems with stopper rod wear associated with excessive Ca-
addition
have been observed in silicon-bearing steels where ferrosilicon and/or
Silicomanganese have been added to achieve the desired silicon concentration
in the
finished steel. In a "Study of Casting Issues using Rapid Inclusion
Identification and
Analysis", Story, et al., AISTech 2006 Proceedings, Vol. 1, pp. 879-889, it
was
determined that ferrosilicon can contain calcium in addition to silicon and
other
alloying elements. To address stopper rod wear, Story et al. discussed using
high
purity ferrosilicon containing about 0.024% calcium.
Summary of the Invention
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[0008] A method of making silicon-bearing steel comprising the steps of:
a) refining molten steel to make a silicon-bearing steel having a silicon
content between 0.1% and 1.5% by weight by addition of a calcium-containing
silicon
additive,
b) determining the amount of calcium content in the calcium-containing
silicon additive,
c) determining if the amount of calcium in the calcium-containing silicon
additive is more or less than the amount of calcium desired in the finished
steel,
d) if the amount of calcium in the calcium-containing silicon additive is
more than the amount of calcium desired in the finished steel, adding the
amount of
calcium-containing silicon additive corresponding to the excess calcium during
steel
deoxidation or early in the refining step to combine with oxygen, and sulfur
and other
impurities in the steel during the refining,
e) adding the calcium-containing silicon additive containing the total
amount of calcium desired in the finished steel after desulfurization of the
molten
steel and before casting, and
0 if the amount of calcium in the calcium-containing silicon
additive
does not provide the total amount of calcium desired in the finished steel,
adding an
additional amount of calcium using Ca wire during refining after
desulfurization of
the molten steel and before casting to the molten steel.
[0009] The calcium-containing silicon additive may be ferrosilicon and
cheap
ferrosilicon additive since the percent of calcium in the additive need not be
kept low.
The calcium-containing silicon additive may include additives having less than
about
1.8% calcium, and further includes additives with less than about 1% calcium.
[0010] The low carbon steel may have a carbon content between about
0.003% and about 0.5% by weight. The disclosed method of refining silicon-
bearing
steel includes low carbon steels.
[0011] The disclosed refining of silicon-bearing steel may occur in a
ladle
metallurgical furnace or vacuum tank degasser.
[0012] A cast steel is made by a method comprising the steps of:
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a) refining molten steel to make a silicon-bearing steel having a silicon
content between 0.1% and 1.5% by weight by addition of a calcium-containing
silicon
additive,
b) determining the amount of calcium content in the calcium-containing
silicon additive,
c) determining if the amount of calcium in the calcium-containing silicon
additive is more or less than the amount of calcium desired in the finished
steel,
d) if the amount of calcium in the calcium-containing silicon additive is
more than the amount of calcium desired in the finished steel, adding the
amount of
calcium-containing silicon additive corresponding to the excess calcium early
in the
refining to combine with oxygen, sulfur and other impurities in the steel
during the
refining;
e) adding the calcium-containing silicon additive containing the total
amount of calcium desired in the finished steel after desulfurization of the
molten
steel and before casting, and
0 if the amount of calcium in the calcium-containing silicon
additive
does not provide the total amount of calcium desired in the finished steel,
adding an
additional amount of calcium after desulfurization of the molten steel and
before
casting to the molten steel during refining; and
g) casting the molten steel into steel slabs.
[0013] Further the silicon content may be between 0.1% and 1.5% by
weight.
Brief Description of the Drawings
[0014] FIG. 1 is a diagrammatic illustration making of silicon-bearing
steel
through a refining and casting process;
[0015] FIG. 2 is a schematic side view of a portion of the continuous
slab
caster of FIG. 1;
[0016] FIGS. 3A-3C illustrate a spreadsheet showing one embodiment of
continuous casting process of the present invention.
Detailed Description
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100171 Referring now to l'IG. 1, silicon-bearing steel is refined and
casting in
process 10 as shown. Process 10 includes an electric arc furnace 12 (EAF) in
which
molten steel is produced. From the EAT 12, the molten steel is transferred by
ladle to
a ladle metallurgical furnace or vacuum tank degassed 14 (EMI' or VTD),
wherein the
refining of molten steel is completed before continuous casting into a slab.
Ladles of
molten steel suitable for casting are then transferred from LMF or VTD 14 to a
continuous slab caster 16 wherein the refined molten steel is cast into
continuous steel
slabs.
100181 The ladle 18 of unrefined molten steel is routed from the EAF 12 to
the EMI; or VTD 14 to reline the molten steel into a form suitable for casting
by the
continuous slab caster apparatus 16. In general terms, as seen in FIG. 2,
casting steel
continuously in such a slab caster involves introducing molten metal that is
supplied
during a casting operation by gravity from ladle 18 to a tundish 43, through a
slide
gate 44 and outlet nozzle 45. From tundish 43, the molten metal is supplied by
gravity through slide gate 46 and outlet nozzle 47 to a submerged entry nozzle
(SEN)
48 into continuous slab caster 16. Molten metal is introduced into the left-
hand end of
the tundish from the ladle 18 via an outlet nozzle 45 and slide gate valve 44.
At the
bottom of tundish 43, there is an outlet 46 in the floor of the tundish to
allow molten
metal to flow from the tundish via an outlet nozzle 47 to the SEN 48. The
tundish 43
is fitted with a stopper rod 42 and slide gate valve to selectively open and
close the
tundish outlet and effectively control the flow of metal through the outlet.
From the
SEN 48, molten steel flows first through a mold 55 and then through a series
of
support rollers 53 and cooling sprays 51.
10019] In slab casting described herein, the steel is generally subjected
to
aluminum deoxidization, which results in the formation of solid A1703
inclusions in
the steel. Following in the refining process, the deoxidized molten steel in
ladle 18 is
desulfurized. After desulfurization, the steel is treated with calcium to
modify the
solid A1203 and/or spinel inclusions to liquid Ca-alumina inclusions.
Following
refining, the deoxidized, desulfurized and calcium treated molten steel in
ladle 18 is
transferred to the continuous steel slab casting apparatus 16.
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100201 In the disclosed method, the amount of calcium in the required
ferrosilicon (silicon additive) is taken into account during the refining of
the molten
steel. The following will consider FeSi as the silicon additive.
100211 First, the concentration of calcium in the source of ferrosilicon
is
determined. Next, the amount of ferrosilicon that is needed for addition to
the molten
steel to achieve the desired silicon concentration in the finished steel, and,
the
quantity of calcium in the required amount of ferrosilicon is calculated. If
the amount
of calcium is greater than the required amount (e.g., 16.8 ppm during normal
non-
startup operations), the required amount of ferrosilicon is divided into two
portions, a
early portion and a late portion. The late portion is the amount of
ferrosilicon that
contains the desired amount of calcium in the finished steel. The early
portion is the
amount of ferrosilicon containing the excess amount of calcium not wanted in
the
finished molten steel. In general, desired sources of ferrosilicon contain
less than
1.8% calcium or less than 1% calcium; although this is desired, other
concentrations,
greater than 1.8%, can also be used in this disclosed method of forming and
refining
silicon-bearing steel.
100221 The early ferrosilicon portion, FeSiearly is added early during
steel
deoxidation with Al or early during refining in the ladle metallurgical
furnace (LMF)
or vacuum tank dcgasser (VTD), typically before or during desulfurization, so
that the
calcium in the early added ferrosilicon can combine with sulfur and other
impurities,
and migrate to the slag. For example, the calcium in the early added
ferrosilicon can
react with sulfur forming CaS that migrates to and is removed as part of the
slag that
is formed during refining. The late ferrosilicon portion, FeSilate, is added
late in the
refining process, after desulfurization has completed, typically to less than
0.01% S
by weight. The calcium added to the LW' or V I'D from the FeSiiate portion
modifies
the solid alumina inclusions into liquid inclusions and reduces the incidence
of nozzle
clogging or choking in the submerged entry nozzle. Since any excess calcium
present
in the total amount of ferrosilicon added to the LMF or VTD was removed during
desulfurization by adding the excess portion, FeSiearly, during
desulfurization, the
incidence of excess stopper rod wear is reduced.
10023] Where the calcium present in the required quantity of FeSi is equal
to
or less than the required amount of calcium in the finished steel, only one
addition of
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ferrosilicon, FeSiiõ,,,, is made during refining. This single late addition of
ferrosilicon
is done after desulfurization. In the event that the calcium present in the
required
quantity of FeSi is less than the required amount, an additional amount of
calcium,
typically in the form of calcium wire, is added with the required quantity of
FeSi.
[0024] In casting campaigns using the method of forming and refining
silicon-
bearing steels described, It has been found that the casting campaigns have
been
extended to 18 heats, which is the typical limit for the submerged entry
nozzle (SEN)
before replacement. Using the early processes of adding the required amount of
ferrosilicon after desulfurization and followed by adding the required amount
of
calcium, also added after desulfurization, stopper rod wear would usually be
the
limiting factor and limited the casting campaign to 10 heats.
100251 FIGS. 3A-3C show an Excel*) spreadsheet illustrating an embodiment
of this method of refining silicon-bearing steel in accordance with the
present
invention. An initial step in this process is determining the concentration of
calcium
in the source of ferrosilicon. Five standards of ferrosilicon containing known
concentrations of calcium, 0.064%, 0.14%, 0.43 %, 0.65% and 1.8 %, were
obtained. These standards were used to calibrate an on-site slag analyzer
permitting
rapid in-house analysis of ferrosilicon when ferrosilicon was received. This
calibration permitted more rapid processing of ferrosilicon as received, so
that
ferrosilicon quantities could be readily stored and used as needed without
waiting for
off-site analysis before use.
[0026] Once the concentrations of calcium and silicon in the ferrosilicon
are
known, the concentrations are entered into the spreadsheet at 101 and 103,
respectively. The desired concentration of silicon in the finished steel is
entered at
105. A total quantity 107 of required ferrosilicon is then calculated. The
total
quantity 107 of ferrosilicon required, FeSi i based on the
heat size 102, multiplied
-req, .S
by the target % ferrosilicon 105 and adjusted to account for the silicon
concentration
103 in the ferrosilicon and the recovery factor 121 for ferrosilicon as
follows:
Ileat Size* 'A Si target , .
(1.0) FeSireq % FeSi recovery
% Si in FeSi
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100271 The total ferrosilicon required, FeSireq, is then divided into a
first or
early ferrosilicon addition 111, FeSicariy, and a second or late ferrosilicon
addition
109, FeSilate= The late ferrosilicon addition, FeSibte, is the amount of
ferrosilicon that
contains the target quantity Caturget, 123, of calcium from the total
ferrosilicon
required, FeSireq. The target quantity of calcium, Catarget, is that amount of
calcium
which results in 16.8 ppm calcium continuous operation, (22,4 ppm calcium
startup),
in the refined metal at the time of casting. lithe calcium available, Caavaii,
in the total
ferrosilicon required. FeSireq, is equal to or less than the target quantity
of calcium,
Catõrget, then FeSiiõ,,, is equal to FeSireq and there is no early addition of
ferrosilicon. If
the calcium available, Caavail, in the total ferrosilicon required is greater
than the target
quantity of calcium, Catarget, then FeSilate is that amount of FeSi that
contains the target
quantity of calcium, Catarget. Specifically, this amount can be calculated by
dividing
the target calcium, Cathrget by the calcium available, Caayaii, multiplied by
the total
ferrosilicon required, FeSi RN =
(2.0) If Caavaii < Catargeb
FeShate FeSireq; FeSieõriy = 0
(2.1) Caavaii = FeSireq * concentration of Ca in ferrosilicon * % Ca
ferrosilicon recovery
(3.0) If Caavaii >Larget
Catargeb
Ca
FeSitate - * l'eSireq; -early ¨ FeSireq - FeSikite
CA
100281 In the event that the calcium, Caõvaii, present in the total
ferrosilicon
required, FeSireq, is less than the amount of calcium required, Catarget, 123,
additional
calcium is added, usually in the form of calcium wire with the FeSiiate
portion
ferrosilicon. For convenience, the additional calcium required 113, Caadd, is
calculated in feet of calcium wire, because atypical way of adding any
additional
calcium is by adding calcium wire. Other units of measurement, such as pounds,
kilograms, etc. could also be used.
(4.0) Caadd ¨ Catarget Caa, ail
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100291 FIG. 3A illustrates a situation where the calcium available, Caavad,
is
greater than the calcium required Catõrget. In this situation, the
ferrosilicon required,
FeSireq is divided into a late portion, FeSiiõte of 1226 lbs (556 kg) and an
early portion,
eS
icar.y, o1252 pounds (114.3 kg). FIG. 313 illustrates a situation where the
calcium
available from the ferrosilicon, Caavait, is less that the calcium required,
Catargei. In this
situation, there is no early portion, FeSi early of ferrosilicon, and
additional calcium,
Caadd, of 118 feet (35.97 in) of calcium wire is required. This additional
calcium is
added to the molten metal when the late portion of ferrosilicon, FeSitate is
added. FIG.
3C shows a situation where the calcium available in the total ferrosilicon
required,
Caavad, is equal to the calcium required, Catõrg,. In this situation, no
additional
calcium. Caadd, is required, and the early portion of ferrosilicon, FeSieariõ
is zero.
100301 The disclosed methods of making silicon-bearing steel reduce the
cost
of making the steel by replacing calcium wire with calcium containing
ferrosilicon
and by extending the length of a casting campaign to about 18 heats. It has
been
estimated the cost savings per ton of steel using the disclosed methods is
about $2 per
ton, about half due to reduced calcium wire usage and about half due to
extending the
length of the casting campaign.
100311 Although preferred embodiments of the invention have been disclosed
for illustrative purposes, those skilled in the art will appreciate that many
additions,
modifications, and substitutions are possible and that the scope of the claims
should
not be limited by the embodiments set forth herein, but should be given the
broadest
interpretation consistent with the description as a whole.
