Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND
This invention relates, in general, to a process
for refining steel, and more specifically, to an improve-
ment in the basic oxygen process wherein molten steel con-
tained in a vessel is refined by top blowing oxygen into
the melt, i.e. :Erom above the melt surface.
One problem frequently encountered in making basic-
oxygen steel is the limited life of the vessel's refractory
lining. It is periodically necessary therefore to reline
the vessel, incurring loss of production an~ relining
expense
Prior methods of improving lining life have been
~o add dolomitic lime to the slag, see for example
Kristiansen et al, 'tThe Effects of Operating Variables On
Sulfur Performance in a BOF Shop," 1976 Open Hearth Pro-
ceedin~ ISS-AIME, pp. 28-41~ However, too much dolomitic
l~me renders the slag too viscous for efficient sulfur re-
moval.
OBJECTS
Accordingly, it is an object of this invention to
increase the life of refractory linings for basic oxygen
vessels.
It is another object of this invention to increase
the life of refractory linîngs for basic oxygen vessels
without interfering with the ability to make steel having
a low sulfur content.
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S~MMARY OF THE INVENTION
The above and other objects, which will readily
be apparent to those skilled in the art, are achieved by the
present invention, which comprises:
a method for increasing the life of the refractory
lining of a basic-refractory-lined vessel for the production
o steel by blowing oxygen into a ferrous melt from above
the surface of the melt, comprising:
(a) introducing into the vessel~ slag forming
ingredients, including dolomitic lime, such that the amount
of dolomitic lime exceeds the amount normally used, and
(b) introducing inert gas into the melt in
such manner as to cause intensive interaction between the
slag and the melt.
The term "inert gas" as used throughout the present
specification and claims is intended to mean a gas other than
oxygen having as many as possible of the following character-
istics: low reactivity, low specific heat, absence o~ ob
jectionable contaminents, and high density. The preferred~
~O inert gas is argon. However, if nitrogen contamination of
the melt is not a problem, nitrogen or air may be used.
Other possible inert gases for use in practicing the inven-
tion include helium, neon, krypton, xenon, carbon dioxide,
steam, a~nmonia, and mixtures thereof. However, argon, which
may be either commercially pure or crude argon is by far the
most preferable inert gas.
The preferred method of introducing inert gas is
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through the oxygen lance admixed with oxygen.
DETAILED DESCRIPTION OF T~E INVENTION
The iron charged to a basic oxygen furnace typically
contains carbon, silicon, sulfur, and other impurities. The
main purpose of the oxygen is to remove carbon and silicon
from the melt. The silicon is oxidized to silicon dioxide
which floats on the surface of the melt. The carbon is
oxidized to carbon monoxide gas which escapes from the mouth
of the vessel. Slag forming ingredients, typically including
high-calcium lime, are added to the melt to form a basic slag.
The high-calcium lime, normally containing at least 90 percent
by weight of CaO, also removes sulEur by reacting with it to
form calcium sul~ide. Dolomitic lime, i.e. lime containing ~ -
at least 30% by weight magnesium o~ide, is known to improve
the life of a vessel's lining, but in addition, it increases
the viscosity of the slag, thereby reducing the amount of
interaction between the lime in the slag and the melt. The
reduced interaction makes it difficult for the lime to remove
sulfur from the melt. Since the conventional solution to
the problem of obtaining long vessel lining life makes it
difficult to make steel having a sufficiently low sulfur
content, the amount of dolomitic lime charged to the vessel
for conventional oxygen blowing must be limited.
The normal amount of dolomitic lime used for con-
ventional processes varies from zero to about 40% of the total
slag-forming ingredients. Typical slag-forming ingredients
include high-calcium lime, dolomitic lime, lime stone, and
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flurospar. In accordance with the present invention, dolomitic
lime is introduced to the slag in an amount exceeding that
normally used. That is, each basic oxygen refining system
will have a normal amount of dolomitic lime added to the
vessel for each grade of steel produced. To practice the
present invention the amount of dolomitic lime introduced must ~ -
exceed the normal amount. Of course, the slag forming com-
pounds should be introduced in an amount sufficient to reduce
the sulfur content of the melt to a desired level, based upon
the s~oichiometric and thermodynamic aspects of the reaction
of sulfur with the compounds.
The use of increased amounts of dolomitie lime over
that normally used is essential to increasing the life of the
vessel's refractory lining. Preferably, if high-calcium li~e
is used to remove sulfur, the amount of dolomitic lime will
be at least equal to that of the lime introdueed, as shown ln
the examples to follow. The sulfur content of the steel can
be reduced to meet the speci~ication, even i~ the amount of
h . ~
dolomitic lime is equal to 2 or 3 times the amount of lime
introduced
~ xtra agitation or mixing is required in the vessel
because of the increased slag viscosity caused by the larger
amount of dolomitic lime. The extra agitation is provided
by introducing inert gas into the vessel in sueh manner as
to cause intensive interaction between the slag and the melt.
The present invention may be practiced in conjunc-
tion with the method of Thokar et al for using argon in the
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BOF or make low nitrogen and low oxygen steel as disclosed~
in U.S. application Serial No. 880,562, filed February 23,
1978, now U.S. Patent No. 4,149,878.
The inert gas must be introduced in such manner as
to cause intensive interaction between the slag and the melt.
Preferably, the inert gas is introduced through the oxygen ~ `
lance by metering it into the oxygen line. Alternatively,
the inert gas may be introduced through a separate lance ,
directed to impinge oxygen-free fluid against the surface of
the melt. However, there is no reason for incurring the extra
expense of adding a second lance to a basic oxygen vessel~
Since the sulfur content of the melt, at the end
of the oxygen blow is one of the most difficult variables to ~;
control in the basic oxygen process, occasionally, even when
the present invention is practiced the sulfur content of the
melt at ~he end of a blow will be higher than desired. How-
ever, the sulfur content of the melt may be lowered in accor-
, ,
dance with the present invention by adding at least onesulfur-removing compound, such as high-calcium lime, to the
slag in the vessel and reblowing the melt with inert gas
alone in such a manner as to cause intensive interaction
between the slag and the melt until the sulfur content is
reduced to the desired level.
.;:
If the slag already contains sufficient sulfur-
removing compounds, then merely reblowing the melt with inert
gas alone in the above described manner may be used to lower
the sulfur content to the desired level.
~ ~ .
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EX~YPLES
The following examples will serve to illustrate
the advantage o practicing the invention. A series of heats
was performed in a BOF (~asic Oxygen Furance) system having
the following characteristics:
nominal vessel volume: 5,000 cubic feet
tap weight of heat: 235 tons
inert gas used: argon
The normal dolomitic lime charge for this vessel
was 16,000 lbs. The normal slag-forming ingredients were
comprised of 16,000 lbs. of dolomitic lime and 24,000 lbs.
of high-calcium lime, and 2,000 lbs. of flurospar.
The dolomitic lime charge was increased to 35,000
lbs. and the high-calcium lime charge was reduced to 10,000
lbs. and argon was injected into the vessel in accordance
with the method of add:ition disclosed in Thokar et al
mentioned previously. The argon was injected at a constant
rate of 3,500 standard cubic feet per minute during the latter
portion of the oxygen blow. As a result of practicing the
invention, the life of the lining of this vessel was extended
from a previous life of about 780 heats to about 1100 heats.
In spite of the very large amounts of dolomitic lime added
as slag forming ingredients, no difficulty was encountered
making steel having sulfur content as low as desired. Further-
more, during this test, flurospar, which is known to aid
sulfur removal, was not used. In spite of the omission of
the normal amount of flurospar, the process was still able to
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produce steel meeting low-sulfur specifications. Hence,
another advantage of the invention is a saving in the amount of
flurospar charged to the vessel. The typical maximum sulfur
content allowed by the specifications for steel made by this ' ~-
vessel was 0.025% sulfur.
It can be seen that the practice of the invention
yields significant results. The life of the vessel lining :
was increased to more than 40% beyond that which had been
previously obtained for this vessel.
