Note: Descriptions are shown in the official language in which they were submitted.
CA 02403081 2002-09-13
OXYGEN-BLOWING LANCE IN VACUUM REFINING APPARATUS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oxygen-blowing lance in an RH vacuum
refining apparatus for refining molten steel in an additional refining process
of a steel
making process for manufacturing very low carbon steel. More particularly,
this
invention relates to the oxygen-blowing lance for the vacuum refining
apparatus which
is installed in the side wall of a vacuum chamber of the RH vacuum refining
apparatus
with a predetermined inclination angle for blowing oxygen toward molten steel
to
decarburize molten steel within the RH vacuum refining apparatus, and which
has
cooling means for preventing fracture or erosion due to the temperature of
molten steel
without deteriorating the degree of vacuum in the vacuum chamber.
2. Description of the Related Art
In general, an RH vacuum refining apparatus 10 used for the manufacture of
very low carbon steel containing carbon of 70ppm or less, as shown in Fig. 1,
comprised a ladle 12 for storing molten steel tapped from a converter and was
not
deoxidized, a vacuum chamber 14 arranged above the ladle and submerged
conduits 16
for defining a circulation path for the flow of molten steel between the ladle
and the
vacuum chamber. The submerged conduits 16 were constituted of an ascending
conduit 16a through which molten steel ascends to the vacuum chamber 14 from
the
ladle 12 and a descending conduit 16b through which molten steel descends to
the ladle
12 from the vacuum chamber 14 where molten steel was decarburized. A vacuum
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pump 18 was connected to the vacuum chamber 14.
When backflow gas was provided from backflow gas feeding means 22 to the
ascending conduit 16a and the internal pressure of the vacuum chamber 14 was
lowered
to several or tens of torn by the operation of the vacuum pump 18, molten
steel stored in
the ladle ascended to the vacuum chamber 14 through the ascending conduit 16a
while
carrying out decarburizing reaction that carbon in molten steel reacts with
oxygen, and
then flown into the ladle 12 through the descending pipe 16b. As a result, the
carbon
content in molten steel was lowered.
As well known in the art, an oxygen feeding apparatus forcibly feeds oxygen to
molten steel introduced into the vacuum chamber 14 in order to accelerate the
above
decarburizing reaction. As shown in Fig. 2, the oxygen feeding apparatus had
an
oxygen-blowing lance 24 exposed within the vacuum chamber 14 for blowing
oxygen.
Further, the oxygen feeding apparatus was provided with cooling means for
preventing
the fracture or erosion of the oxygen-blowing lance 24 due to the high
temperature
within the vacuum chamber 14. According to cooling means, the oxygen-blowing
lance 24 was assorted into a water-cooled blowing lance and an air-cooled
blowing
lance, of which each outer surface was provided with coolant and cooling air,
respectively.
This had a problem containing a risk that the explosion of the vacuum chamber
was resulted from the leakage of cooling water to molten steel when the water-
cooled
blowing lance was locally fractured or eroded due to the internal heat of the
vacuum
chamber.
In the meantime, as shown in Fig. 3, the air-cooled blowing lance 30 was
constituted of an inner pipe 32 for blowing oxygen and an outer pipe 34 for
surrounding
the outer surface of the inner pipe 32, and the inner and outer pipes 32 and
34 were
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coaxial. In this case, cooling gas composed of inert gas was sprayed into the
vacuum
chamber through a space between the outer surface of the inner pipe and the
inner
surface of the outer pipe, thereby the end of the blowing lance 30 exposed
within the
vacuum chamber was cooled. Then, the gas of Ar is sprayed through the inner
pipe 32
while continuously blowing cooling gas through the space in order to prevent
molten
drops dispersed from molten steel within the vacuum chamber 14 from contacting
with
the end of the spray lance 30 after stopping oxygen-feeding through the inner
pipe 32.
However, when the air-cooled spray lance 30 is used, cooling gas is
continually fed into
the vacuum chamber 14 through the space even after stopping the oxygen-feeding
process through the inner pipe 32 as described above.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made to solve the foregoing
problems and it is an object of the present invention to provide an oxygen-
blowing
lance for forcibly feeding oxygen in an RH vacuum refining apparatus, in which
the
oxygen-blowing lance has cooling means capable of effectively cooling the
oxygen-blowing lance without worsening the degree of vacuum in the vacuum
chamber
and having no leakage of cooling water in order to improve decarburizing
efficient from
molten steel in the RH vacuum refining apparatus.
In order to obtain the above object, the invention provides an oxygen-blowing
lance for feeding oxygen installed in an RH vacuum refining apparatus which
includes a
ladle for storing molten steel, a vacuum chamber arranged over the ladle and a
submerged conduits defining circulation channel for the flow of molten steel
between
the ladle and the vacuum chamber, and the oxygen-blowing lance penetrates
through the
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side wall of the vacuum chamber with a certain inclination angle; it is
characterized in
that the oxygen-blowing lance comprises an oxygen feeding pipe for blowing
oxygen
into the vacuum chamber; a tubular body which surrounds the outer surface of
the
oxygen feeding pipe and extends from a leading end of the oxygen feeding pipe
exposed
within the vacuum chamber toward the side wall of the vacuum chamber for a
certain
length, the tubular body being closed at both ends; and volatile liquid
substance stored
in a closed storage space defined between an inner surface of the tubular body
and the
outer surface of the oxygen feeding pipe, wherein the volatile liquid
substance is
evaporated due to the internal temperature of the vacuum chamber, and the
evaporated
gas substance is condensed due to the temperature outside the vacuum chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present
invention will be more clearly understood from the following detailed
description taken
in conjunction with the accompanying drawings, in which:
Fig. 1 is a conceptual view illustrating a conventional RH vacuum refining
apparatus:
Fig. 2 is a conceptual view illustrating the vacuum refining apparatus with an
oxygen-blowing lance installed in a vacuum chamber according to an embodiment
of
the prior art:
Fig. 3 is a sectional view illustrating the leading end of a typical
oxygen-blowing lance:
Fig. 4 is a conceptual view illustrating an RH vacuum refining apparatus with
an oxygen-blowing lance installed in a vacuum chamber according to an
embodiment of
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the invention:
Fig. 5 is a front elevation view illustrating the vacuum chamber having the
oxygen-blowing lance installed in the sidewall according to the embodiment of
the
invention:
Fig. 6 is a sectional view of the oxygen-blowing lance according to the
embodiment of the invention:
Fig. 7 is a sectional view taken along a line A-A in Fig. 6:
Fig. 8 is an enlarged view illustrating regions of the oxygen-blowing lance in
which a bellows-type coupling is installed according to the embodiment of the
invention: and
Fig. 9 is a graph illustrating temperature variations measured from the
oxygen-blowing lance installed in the sidewall of the vacuum chamber according
to the
embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The following detailed description will disclose a preferred embodiment of the
present invention with reference to Figs. 4 to 9, in which the same reference
numerals as
in the conventional art is used to designate the same or similar components.
An RH vacuum refining apparatus 10 for manufacturing very low carbon steel
has a ladle 12 for storing molten steel, a vacuum chamber 14 arranged over the
ladle 12
and a circulation conduits 16 constituted of an ascending conduit 16a through
which
molten steel within the ladle 12 ascends to the vacuum chamber 14 and a
descending
conduit 16b through which molten steel descends to the ladle 12 after being
decarburized in the vacuum chamber 14. The vacuum chamber 14, as shown in Fig.
5,
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is divided into an upper vessel 14a maintained in vacuum by a vacuum pump 18
and a
lower vessel 14b into which molten steel flows through the ascending pipe 16a
from the
ladle 12. The outside wall of the vacuum chamber 14 is made of a steel sheath,
and
refractory are constructed inside the steel sheath.
According to the present invention, an oxygen-blowing lance 40 for forcibly
feeding oxygen into the vacuum chamber 14 is installed in a lower section of
the upper
vessel 14a and penetrates the refractory of the vacuum chamber 14 with a
predetermined inclination angle 8.
Further, the oxygen-blowing lance 40 of the invention, as shown in Fig. 6, has
an oxygen feeding pipe 42 for guiding the flow of oxygen so that oxygen fed
from an
oxygen feeding tank (not shown) is blown into the vacuum chamber 14. The
oxygen-blowing lance 40 is provided with a cylindrical tubular body 44 which
is sealed
at both ends by an upper plate 44a fixed to the outer surface of the oxygen
feeding pipe
42 placed in a refractory layer of the vacuum chamber 14 and a lower plate
44b.
Between the outer surface of the oxygen feeding pipe and the inner surface of
the tubular body, a storage space of a proper size is formed, in which
volatile liquid
substance 50 is provided. In the meantime, the upper plate 44a is provided
with a vent
pipe 46 connected to a vacuum pump (not shown) for ventilating air within the
storage
space so as to maintain the storage space at the atmospheric pressure or
below, in
particular, in the vacuum state. The vent pipe 46 is provided with a first
valve 46a.
Further, the upper plate 44a is provided with a volatile material-feeding pipe
48 for
feeding the volatile liquid substance 50 into the storage space. The volatile
material-feeding pipe 48 is provided with a second valve 48a.
As described above, in the oxygen-feeding pipe 42 with the tubular body 44
being fixed by the upper plate 44a and the lower plate 44b, a portion of the
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oxygen-feeding pipe 42 located in the storage space maintains a separated
structure in
order to prevent any damage due to thermal stress that will be described
later. The
first oxygen feeding pipe 42a and the second oxygen feeding pipe 42b resulted
from the
separated structure are connected to each other by installing a bellows-type
coupling 52
in separated region of the first and second oxygen feeding pipes 42a and 42b.
In order
to prevent the leakage of oxygen through a clearance C between the opposed
separated
regions of the first and second oxygen feeding pipes 42a and 42b, a pipe 54 is
provided
in the separated regions inside the bellows-type coupling 52 to close the
clearance C.
Referring to Fig. 8, both ends of the bellows-type coupling 52 are supported
by
the first stoppers 56a and 56b each of which is provided on the first and
second oxygen
feeding pipe 42a and 42b, respectively in order to help the smooth expansion
and
contraction of the oxygen feeding pipe 42 due to the thermal stress and the
like and to
prevent the release of the first oxygen feeding pipe 42a from the second
oxygen feeding
pipe 42b. Further, a lower portion of the pipe 54 is supported by the second
stopper
56c provided on the outer surface of the first oxygen feeding pipe 42a within
the
bellows-type coupling 52 in order to effectively prevent the leakage of oxygen
through
the cleavage C.
As set forth above, the oxygen-feeding pipe 42 is installed with the
predetermined inclination angle in the refractory layer of the vacuum chamber
14 so
that the volatile liquid substance 50 stored in the storage space between the
outer
surface of the oxygen feeding pipe 42 and the inner surface of the tubular
body 44 is
gravitationally placed within the lower portion of the oxygen feeding pipe 42,
i.e. on the
lower plate 44b which is exposed within the vacuum chamber 14. In this case,
the
volatile liquid substance 50 is vaporized by the absorption of heat within the
vacuum
chamber 14, in particular, around the lower end of the oxygen-feeding pipe 42,
and
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resultant gas moves toward an upper portion of the storage space, i.e. the
upper plate
44a. In this case, since the temperature of the upper plate 44a become lower
due to its
placement adjacent to the steel sheath outside the vacuum chamber 14 having a
relatively lower temperature, the vaporized gas moved toward the upper plate
44a is
condensed. Resultant liquid material moves toward the lower plate 44b along
the
outer surface of the oxygen feeding pipe 42 and/or the inner surface of the
tubular body
44.
A volatile liquid substance, which volatilizes due to the absorption of
external
heat, is selected by considering physical properties such as specific gravity,
surface
tension, viscosity and latent heat of vaporization; price; available
temperature; material
of the tubular body and the oxygen feeding pipe and so on.
The following Table 1 indicates melting point, boiling point and available
temperature range for volatile liquid substances.
Table 1
Boiling Point Available
Substance Melting Point
(C) Temperature
Range
Freon I I -11 I 24 -40 ~ 120
Methanol -98 64 10 ~ 130
Water 0 100 30 ~ 200
Potassium (K) 62 774 500 ~ 1000
Sodium (Na) 98 892 600 ~ 1200
Lithium (Li) 179 1340 1000 ~ 1800
Silver (Ag) 960 2212 1800 ~ 2300
In the meantime, relative values for the materials being capable of the heat
transfer can be indicated with Merit Number (M) and expressed as in Equation
1:
M = P~~ ... Equation I ,
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herein, p is specific gravity of volatile liquid substance, Q is surface
tension
of volatile liquid substance, h is latent heat of vaporization, and p is
viscosity of
volatile liquid substance.
In order to select a volatile liquid substance as mentioned above, it should
be
considered about price, available temperature and materials of tubular body
and oxygen
feeding pipe in addition to the physical properties. So, although Li is higher
in Merit
Number than other materials shown in Table l, to use Li requires an expensive
alloy
that can endure Li. On the contrary, to use Na or K as the volatile liquid
substance can
utilize a relatively cheap stainless steel.
Table 2 indicates materials for the volatile liquid substances.
Table 2
Volatile Liquid SubstanceMaterial
Methanol Cu, Stainless Steel
Water Cu, Monel
Potassium (K) Ni, Stainless Steel, Inconel, Ti
Sodium (Na) Stainless Steel, Ni, Inconel 800
Lithium (Li) Nb-1% Zr Alloy, Mo
Silver (Ag) W-26%Rh
Therefore, the material forming the storage space for storing the volatile
liquid
substance as above is selected by considering compatibility to the volatile
liquid
substance, thermal conductivity, convenience of processing, wettability and so
on.
In the meantime, the volatile liquid substance stored in the storage space is
charged to occupy about 15 to 20% of the entire volume of the storage space.
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Hereinafter, it will be described about the operation of the oxygen-blowing
lance 40 of the invention.
First, the oxygen-blowing lance 40 of the invention penetrates the wall of the
vacuum chamber of the RH vacuum refining apparatus 10 to be installed. The
vent
pipe 46 is connected to the vacuum pmnp that is not shown; the volatile
material-feeding pipe 48 is connected to a volatile material storage tank (not
shown).
In the open state of the first valve 46a, the storage space defined between
the
oxygen-feeding pipe 42 and the tubular body 44 is maintained in vacuum as air
contained therein is vented by the operation of the vacuum pump. If the second
valve
46b is opened while the first valve 46a is closed, the volatile liquid
substance 50 is
introduced into the storage space from the volatile material storage tank. The
second
valve 48a is closed when the volatile liquid substance SO is introduced for a
designated
amount.
In this case, water, methanol, Na, K and so on can be utilized as the volatile
liquid substance 50 depending on the available temperature as set forth above.
According to the invention, Na liquid can be used as the volatile liquid
substance 50 by
the consideration of the internal temperature of the vacuum chamber 14 in the
RH
vacuum refining apparatus 10, that is about 800 to 1200°C. In general,
Na has a
melting point of about 98°C and a boiling point of 892°C under
the atmospheric
pressure. Therefore, the boiling point of Na descends in the storage space
maintaining
its pressure at the atmospheric pressure or below as described hereinafter. In
this case,
the Na liquid 50 in use is charged into the storage space as much as to occupy
about 10
to 25% of the entire storage space in a completely condensed state. If the
quantity of
the Na liquid is under 10%, the liquid exists at sides of the storage space by
a large
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amount in a repeating process of evaporation and condensation so that the Na
liquid
may not exist in the bottom. At the quantity exceeding 25%, the Na liquid
evaporates
thereby reducing a reserved space necessary for the Na liquid to evaporate and
move
therein.
Therefore, the Na liquid 50 introduced into the storage space is positioned
adjacent to the inside of the vacuum chamber 14 due to its weight. The Na
liquid is
evaporated due to heat inside the vacuum chamber 14, and Na gas moves toward
the
upper portion of the storage space, i.e. the upper plate 44a. That is to say,
the Na gas
moves upward due to the difference between the gas pressure at the surface of
the Na
liquid 50 and the gas pressure at the upper portion of the storage space. The
upper
portion of the storage space is maintained at a relatively lower temperature
compared to
the lower portion thereof so that the Na gas is condensed into the Na liquid
50, and the
condensed Na liquid moves toward the lower plate 44b along the side defining
the
storage space. By repeating the evaporation and condensation as mentioned
above, the
Na liquid under the vacuum state absorbs heat during the evaporation to cool
the
surroundings. The evaporated gas moves upward due to the difference of the
internal
pressures, the gas contacts with cold regions in the upper portion to condense
again
while releasing heat, and then the condensed liquid drops downward again due
to the
gravitation. The Na liquid repeats the above cycle to cool hot regions in the
lower
portion.
By blowing oxygen into the vacuum chamber from the oxygen storage tank
through the oxygen-blowing lance 40 cooled as described, erosion of the
oxygen-blowing lance 40 can be prevented.
In the meantime, an unexplained reference numeral 43 in the drawings indicates
a screen made of stainless steel that is fixedly installed on the outer
surface of the
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oxygen-feeding pipe 42. The screen guides the Na liquid 50 condensed in the
upper
portion of the storage space to uniformly flow down across the entire outer
surface of
the oxygen-feeding pipe 42. The screen 43 is preferably sized for 150 mesh or
less
since the installation of the screen 43 having an excessively fine mesh
prevents the Na
gas from contacting with the relatively cold outer surface of the oxygen
feeding pipe 42
through the screen.
Further, an unexplained reference numeral 45 indicates a spring installed
adjacent to the inner side of the tubular body 45. The spring 45 is
manufactured with a
diameter capable of contacting a pipe having about 6mm diameter to the inner
surface
of the tubular body 44. The spring 45 guides the evaporated Na gas to move
toward
the upper portion of the storage space along a spiral route while acts to help
the
condensed Na liquid 50 uniformly exist across the inner face of the tubular
body 44.
Therefore, the evaporated Na gas condenses by contacting with the outer
surface of the
oxygen feeding pipe 42 and the inner surface of the tubular body 44, and the
condensed
Na liquid 50 uniformly exists across the outer surface of the oxygen feeding
pipe 42 and
the inner surface of the tubular body 44 via the screen 43 and the spring 45.
In
particular, the spring 45 causes the upper surface of the Na liquid 50 stored
in the lower
portion of the storage space to slightly move upward across the inner surface
of the
tubular body 44 resultantly preventing the tubular body 44 from the formation
of hot
spots.
The oxygen-feeding pipe 42, the tubular body 44 and the upper and lower
plates 44a and 44b used in the oxygen-blowing lance 40 of the invention are
made of
stainless steel. Although those materials compatible with the Na liquid used
as the
volatile liquid substance include stainless steel, Ni or Inconel 800 as shown
in Table 2,
stainless steel is most adequate considering the workability and price of
material.
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Further, the screen 43 fixedly installed in the outer surface of the oxygen-
feeding pipe
42 is manufactured with stainless steel.
An inert gas of about 60Nm3/hr such as Ar gas is sprayed in order to prevent
clogging of a blowing end of the oxygen feeding pipe due to dispersion of slag
or
molten steel inside the vacuum chamber 14 as Well as to cool the blowing end
while
oxygen-blowing through the oxygen feeding pipe 42 is stopped. When the inert
gas is
blown through the oxygen feeding pipe 42 as above, the temperatures of the
oxygen
feeding pipe 42 and the tubular body 44 are maintained at substantially S00 to
650°C
without substantially temperature difference although they have fluctuation
according to
the conditions inside the vacuum chamber of the RH vacuum refining apparatus
10. In
particular, the temperature of the oxygen-feeding pipe 42 and the tubular body
44 is
substantially uniformly maintained due to the Na liquid in the storage space.
However, in the process of the RH vacuum refining operation, if Al is added to
an upper portion of molten steel while blowing oxygen in order to elevate
temperature,
the heat generated in the oxidation of A1 intermittently elevates the
temperature of
molten steel.
The inside B of the leading end of the oxygen feeding pipe 42 has a relatively
narrow diameter so that oxygen can be blown at supersonic speed in a flow rate
condition 1200Nm3/hr of oxygen by the following reason: Oxygen can be easily
dissolved into molten steel when blown at supersonic speed. That is, oxygen
dissolved
into molten steel reacts with A1 to generate heat or reacts with C in molten
steel to raise
decarburizing efficiency.
In general, when gas is blown exceeding supersonic speed through a nozzle
Mach number and the temperature of blown gas can be expressed according to
Equation
2:
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T'oo ---1 + k 2 1 Ma2 .-. Equation 2.
Wherein To indicates the initial temperature of gas, T indicates the gas
temperature at a nozzle exit, Ma indicates Mach number (multiple of sonic
speed), and k
is the ratio of specific heat (1.4).
From Equation 2, it shall be understood that the right term becomes 1 if Mach
number exceeds l and accordingly the gas temperature T at the nozzle exit is
lowered
than the initial temperature of gas T~,. Equation 2 means that the gas
temperature is
abruptly lowered while rapidly cooling around the nozzle.
As set forth above, while oxygen is sprayed with a blowing quantity of about
20
times larger than that of Ar which is used when oxygen is not blown, the
temperature of
the oxygen feeding pipe 42 descends more rapidly than the tubular body 44 due
to a gas
temperature descending effect according to the adiabatic expansion of gas at
the above
supersonic speed nozzle and a cooling effect of a large quantity of oxygen
itself.
Accordingly, internal stress takes place due to the difference in linear
expansion
between the oxygen feeding pipe 42 and the tubular body 44, in which a fragile
region
such as a welded region would break.
Therefore, according to the preferred embodiment of the invention, the
oxygen-feeding pipe 42 is separated at a proper portion, and the separated
region is
connected with the bellows-type coupling 52. The pipe 54 is installed to
prevent a
twisting phenomenon that is resulted from its movement due to the difference
in linear
expansion of the oxygen-feeding pipe 42. This accordingly prevents any
fracture that
may be incurred due to formation of the internal stress based upon the
difference in
linear expansion of the oxygen-feeding pipe 42.
Fig. 9 is a graph illustrating temperature variation measurements of the
oxygen-blowing lance 40 installed in the sidewall of the vacuum chamber 14 of
the
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invention. The oxygen-blowing lance 40 is preheated for two days, and then the
temperature thereof is measured from a terminal time point of preheating. When
about
I hour and 40 minutes have lapsed after the measurement began, RH vacuum
refining
process is started. Numbers marked in Fig. 9 indicate positions of
thermocouples
distanced from the leading end of the oxygen-feeding pipe. That is, 1 em in
the graph
indicates variation of the outer temperature of the tubular body 44 according
to time
lapse at a lcm distanced position from the leading end. This shows that
although the
inside temperature of the vacuum chamber generally indicates about 800 to
1200°C, the
temperature of the inside wall is maintained at about 600°C or below
while the
temperature of the leading end of the RH vacuum refining apparatus
continuously
ascends up to about 1000°C.
Further, the temperature of a 1 cm distanced region from the leading end of
the
oxygen feeding pipe 42 is lowered to 520°C fiom about 570°C at
the terminal point of
preheating process even if Ar gas is blown at 80Nm3/hr through the oxygen
feeding
pipe while oxygen-blowing is stopped as above. However, comparing to the
oxygen-blowing lance of the prior art having a leading end temperature of
1000°C or
higher, the oxygen-blowing lance of the invention maintains the leading end
temperature of 600°C or below thereby effectively preventing oxidation
or erosion of
the lance.
As set forth above, the invention can effectively cool the leading end of the
oxygen-blowing lance without worsening the degree of vacuum in the vacuum
chamber
or having any risk of explosion due to the leakage of cooling water thereby
prolonging
the life time of the blowing lance and enhancing the oxygen-blowing
efficiency.
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Although the preferred embodiment of the present invention has been disclosed
for illustrative purposes, those skilled in the art will appreciate that
various
modifications, additions and substitutions can be made without departing from
the
scope and spirit of the invention as disclosed in the accompanying claims.
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