Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CLEANING MOLTEN METAL
Background of the Invention
Field of the Invention
The present invention relates to a method for cleaning
molten metal, and more particularly to a method for
obtaining clean molten metal by making nonmetallic
inclusions rise to the surface of the molten metal and
removing the nonmetallic inclusions from the molten metal.
Description of the Related Arts
Since inclusions ( for example, alumina inclusions in
molten steel ) being suspended in molten metal are a cause
of producing defects of quality of products, various methods
for decreasing or removing the inclusions in molten steel
have been proposed.
Out of methods having been very often used as
comparatively effective methods, there is pointed out a
method for removing inclusions from molten metal, wherein
molten metal is bubbled by blowing inert gas from the bottom
of a vessel into the molten metal under atmospheric
pressure, gas bubbles thus produced trap the inclusions in
the molten metal and the inclusions are removed after the
inclusions have risen to the surface of the molten metal.
In the case of manufacturing a high quality steel,
there is a need for limiting the total amount of oxygen in
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molten steel to 15 ppm or less. There, however, occurs a
problem such that molten metal cannot be cleaned to more
than ordinary norms of cleanness by the use of the
above mentioned method. Therefore, a new method for
cleaning molten metal has been expected to be developed.
That is, there have been difficulties in the prior art
bubbling method in that a bubbling zone was limited to a
zone which widened from a gas blow opening of the bottom of
a vessel upwardly in the form of " V " and it was difficult
to bubble molten steel from the entire vessel due to a limit
of a gas blowing method. Moreover, there have been
problems such that, since sizes of produced bubbles were
large, molten metal flows as if it went around the bubbles
during rising of the bubbles, fine inclusions move together
with a flow of the molten metal, avoiding those bubbles,
and, in consequence, the fine inclusions were hard to be
trapped by the bubbles.
To overcome the above-mentioned difficulties, the
present inventors propose the following method:
In this method, molten metal under elevated pressure is
bubbled by a gas soluble in the molten metal, inclusions
being suspended in the molten metal are trapped by gas
bubbles produced by bubbling and by fine gas bubbles
produced by reducing the pressure on pressurized molten
metal. After the inclusions have risen to the surface of
the molten metal, said inclusions are removed. Ordinary
inclusions are trapped by first bubbling. Said trapped
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inclusions rise to the surface of the molten steel. On the
other hand, since pressurized molten metal is bubbled, a
large amount of bubbling gas dissolves in the molten metal.
Thereafter, gas having dissolved in the molten metal appears
as fine gas bubbles from the entire zone of the molten
metal by rapidly reducing a pressure inside a vessel.
During the bubbling, the fine inclusions are trapped by the
bubbles and rise to the surface of the molten metal with the
bubbles.
The above-mentioned method is very effective in removal
of the inclusions in the molten metal. However, since the
molten metal is pressurized at the initial stage of a
processing step, some portion of the bubbling gas having
once dissolved in the molten metal appears as fine gas
bubbles during the reduction of the pressure on the molten
metal, but the rest of the gas remains dissolved in the
molten metal. Accordingly, one more processing step for
degassing the molten metal is required after the
above-mentioned processing of the gas has been carried out.
Therefore, there have been problems in that degassing
capacity and the number of processing steps had to be
increased. In addition, a cost for equipment increased
because of the use of a pressure vessel.
Summary of the Invention
It is an object of the present invention to provide a
method for cleaning molten metal under reduced pressure
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wherein the above-mentioned bubbling method is improved, a
pressure vessel is not used, and degassing of the molten
metal is carried out in the same vessel as that of bubbling
method.
To accomplish the said object, the present invention
provides a method for cleaning molten metal comprising the
steps of:
keeping a pressure inside a vessel having molten metal
therein at a pressure of Ps of atmospheric pressure or
less;
bubbling the molten metal in the vessel by gas soluble
in the molten metal, a portion of said gas dissolving in the
molten metal and the rest of said gas converting to gas
bubbles; and
reducing rapidly the pressure in the vessel to pressure
PE, fine gas bubbles being produced in the molten metal in
the vessel, nonmetallic inclusions being trapped by said
fine gas bubbles and by gas bubbles produced by bubbling and
rising to the surface of the molten metal, and gas
dissolved in the molten metal being removed.
In the method of the present invention, the molten
metal is not processed under elevated pressure as in the
prior art method. The molten metal is processed under
reduced pressure after the molten metal has been bubbled by
blowing gas soluble in the molten metal at atmospheric
pressure or less. Particularly, in this processing under
reduced pressure, not only the fine gas bubbles are simply
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produced, but also said bubbling gas remaining dissolved in
the molten metal is removed together with the fine gas
bubbles.
The above objects and other objects and advantages of
the present invention will become apparaent from the
detailed description to follow, taken in connection with the
appended drawings.
Brief Description of the Drawings
Fig.1 is a graphical representation showing the
relation between a pressure inside a vessel during bubbling
of molten metal by blowing nitrogen gas into the molten
metal and a pressure inside the vessel during degassing of
the molten metal according to the present invention;
Fig.2 is a vertical sectional view illustrating a
refining apparatus for a ladle which carries out VOD
processing according to the present invention;
Fig.3 is a vertical sectional view illustrating a
refining apparatus for a ladle which carries out VAD
processing according to the present invention;
Figs. 4 and 5 are explanatory view of a gas tight
vessel having a horizontal rotation axis capable of rotating
according to the present invention;
Fig.6 is a graphical representation showing a
change of the total amount of oxygen in the molten steel
with the lapse of time according to the present invention;
Fig.7 is an explanatory view designating another
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example of the present invention wherein the same apparatus
as in Fig.4 is used.
Fig.8 is a graphical representation showing the
total amount of oxygen in the molten steel in another
example different from Fig.6; and
Figs.9 and 10 are explanatory views designating a
vessel of rectangular parallelepiped having a gate therein.
Description of the Preferred Embodiment
Preferred Embodiment-1
Fig.1 shows a result of having studied the case when
molten steel was used as molten metal and nitrogen as
gas soluble in the molten steel. Namely, Fig.1 is a
graphical representation designating preferable zones out of
coordinates of pressure Ps of atmosphere inside a vessel,
under which the molten metal is bubbled by blowing said gas
into the molten metal, and of reduced pressure PE of
atmosphere inside the vessel. Units of Ps and PE are Torr.
Zone B shown with oblique lines is a preferable zone. Zone A
shown with crossing lines is more preferable zone. The zone
B is a zone where the inclusions decrease. However, since
the content of nitrogen ~N ~ in the molten steel poses
problems depending on steel species, a denitrification step
is sometimes required after processing of the molten steel
under reduced pressure.
In Fig.1, a boundary ~ between the zones A and B,
namely, PE = 40 Torr is determined in such a manner as
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described below. The relation between the pressure PN
inside the gas tight vessel and the content of nitrogen (N)
in the molten steel is determined by an equilibrium relation
of the folIwing equation (1) in the case of ordinary steel:
~ N~ = 450 (PN)1/2 .... - -- ( 1 )
Units of ~N ~ and PN are ppm and atm, respectively.
The allowable iargest value of ~N ) is estimated at 100 ppm.
~N ~ ~ 100 is obtained. ( N ) ~ 100 is substituted for
the equation (1) and the pressure unit is converted from atm
to Torr. PN ~ 38 Torr is obtained. PE = 40 Torr is
obtained by rounding PN ~ 38 Torr.
Line ~ passing the lower ends of the zone B is
determined for the following reason:
The amount of nitrogen removed from the molten steel
which was required for trapping the inclusions and making
the inclusions rise to the surface of the molten steel
needed to be 50 ppm or more by experience of the present
inventors. The amount of removed nitrogen is the difference
between ~N ~ s and ~N ~ E . (N ~ s is the initial content
of nitrogen increased by bubbling the molten steel by
blowing nitrogen into the molten steel. ~N ~ E is the final
content of nitrogen decreased by degassing the molten steel
under reduced pressure. That is to say, the line ~ is
determined by the use of the following equation obtained by
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putting said equation (1) into ~N ~ s - ~N ~ E 2 50 and
representing pressures in Torr:
tPS) (PE)1/2 = 3.06 (2)
The intersection point where the line ~ crossed the line ~
was ( 40, 88 ) in coordinates ( PE. PS ), PE = 40 being
substituted for the equation (2). Further, in the case PN
is less than 75 Torr in the equation (1), it takes much time
to make an equilibrium state between the pressure PN and the
content ~ N ~ and this is ineffective. Ps of less than 75
Torr is considered difficult to apply. As a result, the
line ~ passing the lower ends of the zone A was
determined by connecting the point ( 0, 75 ) with the point
( 40, ~8 ). More preferable zone A is two-dimensional
rectangular coordinates, whose ordinate is Ps Torr and whose
abscissa is PE Torr. The zone A is enclosed with Ps=760,
PE=0 and point (40, 88) and (0, 75) represented with
(PE, PS). The preferable zone B is two-dimensional
rectangular coordinates, whose ordinate is Ps Torr and whose
abscissa is PE. The zone B. is enclosed with Ps=760, PE=40
and (PS)1/2 - (PE)1/2= 3.06 . Out of the zone B, a zone
enclosed with Ps=760, PE=40, PE=400 and (Ps) 1/2 - (PE)1 2
= 3.06 is more preferable in the two-dimensional rectangular
coordinates, whose ordinate is Ps Torr and whose abscissa is
PE Torr. Further, a zone enclosed with Ps=760, PE=40,
PE=200 and (PS)1/2 - (PE)1/2 = 3.06 is most desired.
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Example-1
An example of the present invention will be described
with specific reference to the appended drawings. Fig.2 is
a vertical sectional view illustrating a refining apparatus
for a ladle having been used for the example of the present
invention. In the drawings, referential numeral 30 denotes
a gas tight vessel. Ladle 32 of 50 ton capacity, into which
molten steel 31 is charged, is put into the vessel. Cover 33
of the gas tight vessel 30 is arranged in a removable state.
Lance 3~ is fixed to the cover to be used for VOD ( Vacuum
Oxygen Decarbonization ). Referential numeral 35 denotes
an exhaust opening for making the gas tight vessel vacuum.
Referential numeral 36 denotes a porous plug for blowing gas
into the molten steel, which is arranged at the bottom of
the ladle.
50 t of molten steel charged into the ladle 32 was kept
at 1660 ~C and at 300 Torr in said gas tight vessel. 6 Nm3
of N2 was blown into the molten steel from the bottom of the
ladle 32 for 10 min. Then, cover 33 was changed for another
cover 38 having three electrodes as shown in Fig.3 to carry
out VAD ( vacuum arc degassing ). Pressure in the vessel
was rapidly reduced to 1 Torr,a heat compensation being made
for the molten steel by means of said electrodes 37, and the
pressure of 1 Torr was kept for 20 min. At the same time,
the molten steel was bubbled by Ar gas blown into the molten
steel from the bottom of the ladle at 150 N Q /min. A black
point shown in Fig.1 corresponds to the Example-1.
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The content of the molten steel obtained by processing
of the molten steel in said Example-1 will be shown in
Table 1 (all values are by weight). Example-2 shown in
this Table will be described later. Results obtained in
the case of using the prior art pressure elevation and
reduction method (Control-1) wherein molten steel was
degassed under reduced pressure after the molten steel had
been bubbled by N2 gas under elevated pressure of more than
atmospheric pressure and an Ar gas bubbling method
(Control-2) are shown as controls in Table 1. In the
pressure elevation and reduction method, the pressure in
the vessel was increased to 3 atm during blowing of N2 gas
and then reduced to 100 Torr during reduction of the
pressure. As for other conditions, the molten steel was
processed under the same conditions as those of the example
of the present invention. Ar gas, however, was not blown
into the molten steel during pressure reduction.
In the Ar gas bubbling method, 50 t of molten steel
was kept at 0.5 to 1 Torr in a vacuous ladle and bubbled by
Ar gas blown into the molten steel from the bottom of the
ladle at a rate of 150 NQ/min for 20 min.
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Table 1
Time,(min) Sol A~ T. T.
after C % Si % Mn % P % S % (0) (N~
Pressure % ppm ppm
Reduction
0 0.13 0.32 1.12 0.023 0.006 0.048 42 147
Example-1
0.13 0.31 1.12 0.023 0.006 0.047 8 38
0.13 0.31 1.12 0.023 0.006 0.047 7 31
0 0.14 0.33 1.14 0.021 0.005 0.049 42 147
Example-2
0.14 0.32 1.13 0.021 0.005 0.047 7 60
0.14 0.32 1.13 0.021 0.005 0.047 5 35
Control-1
0 0.13 0.33 1.13 0.020 0.005 0.050 41 632
Pressure
Elevation 10 0.13 0.32 1.13 0.020 0.005 0.048 9 279and
Reduction
Method 20 0.13 0.32 1.13 0.020 0.005 0.047 9 162
Control-2
0 0.12 0.33 1.15 0.018 0.005 0.054 42 20
Ar Gas
Bubbling 10 0.12 0.32 1.14 0.018 0.005 0.054 24 19
Method
0.12 0.31 1.15 0.018 0.005 0.052 18 18
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In Table 1, when Example-1 is compared with Control-1
or Control-2 relative to the total amount of oxygen T.[0)
and the total amount of nitrogen T.(N) in the molten steel
20 min later after the pressure reduction,T.(0) in Example-1
decreased and T.(N) greatly decreased in comparison with the
pressure elevation and reduction method. In comparison with
the Ar gas bubbling method, it is recognized that T.(0)
decreased. In this case, the amount of nitrogen increased
slightly by blowing nitrogen gas, but such T.(N) does not
pose any specific problem except for specific cases.
Preferred Embodiment-2
~ hen the molten steel is stirred by blowing inert gas
into the molten steel under reduced pressure as in the
Example-1, soluble gas is remarkably removed from the
molten steel and the amount of nitrogen decreased to the
extent enough to be able to be put to practical use inspite
of bubbling of the soluble gas in comparison with Control-1
and Control-2 in Table 1. The soluble gas, however, is
removed during the pressure reduction and, at the same
time, the occurrence of fine gas bubbles is also decreased.
Accordingly, it is thought that the effect of rising and
separation of nonmetallic inclusions decreases with the
lapse of time. Therefore, it is intended in Example-2 to
keep the effect of the rising and separation of the
nonmetallic inclusions by blowing the soluble gas together
with the inert gas during the pressure reduction.
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Example-2
An example of the present invention will be described
specifically. As in Example-1, a refining apparatus for
a ladle as shown in Figs.2 and 3 was used. Soluble gas was
blown into molten steel before a pressure reduction, 50 t of
molten steel being kept at 1660~C and at 300 Torr. 6 Nm3
of N2 gas was blown from the bottom of the ladle as shown in
Fig.2 into the molten steel by means of a porous plug for
10 min. Then, cover 33 was changed for cover 38. Heat
compensation was made for the molten steel by the use of arc
heat of electrodes. Pressure in the ladle was rapidly
reduced to 1 Torr and the pressure of 1 Torr was kept for 20
min. The molten steel was bubbled by blowing Ar gas as
inert gas together with N2 gas from the bottom of the ladle
at a rate of 150 N~/min. Flow of N2 gas was decreased to
zero in the last five minutes as shown in Table 2 so as to
make the amount of soluble gas as small as possible.
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Table 2
Time after Pressure Reduction
0 ~ 10 10 ~ 15 15 ~ 20
Amount of Gas
Blown into Ar 100 30 150
Molten Steel
N~/min
N2 50 120 0
The components of molten steel which were obtained as
a result of having processed the molten steel in Example-2
will be shown in Table 1. When Example-2 is compared with
Example-1 in this Table, it is recognized that T.(0) was
decreased by blowing nitrogen into the molten steel under
reduced pressure while T.(N) was increased slightly.
The soluble gas is removed by pressure reduction in
Preferred Embodiment-1 and Preferred Embodiment-2. However,
in case a depth of a molten metal bath is large, a static
pressure in a bottom portion of the molten metal bath
becomes large. In consequence, it becomes difficult to
degass the molten steel by the use of only processing of
molten steel under reduced pressure. Specifically, when
the depth of the molten steel is 1.5 m or more, the above-
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mentioned tendency became remarkable. To degass the molten
metal in the bottom portion of the molten metal bath of
large depth, it is thought to turn a gas tight vessel
having the molten metal therein upside down and to greatly
stir said molten metal under reduced pressure. This example
will be described according to Example-3 and Example-4 shown
below with specific reference to Figs.4 to 8.
Example-3
Fig.4 shows an example of a gas tight vessel 1 being
able to be turned upside down with horizontal rotaton axis
in a central portion thereof. Said gas tight vessel 1 is
made cylindrical by tightly jointing vessel la and vessel lb
, each of which has a form of a ladle of 2 m in diameter and
3 m in height. Gas blow opening 11 is positioned on an end
face of the vessel lb. Exhaust opening for exhausting an
atmospheric gas from vessel 1 is arranged in joint portion
13 of the vessels la and lb.
In this example, molten steel was cleaned by the use of
the gas tight vessel 1 constituted with the vessels la and
lb as described above. 50 t of molten steel 31 was charged
into the vessel lb. Another vessel la was tightly jointed
to the vessel lb from above to form the vessel 1. Then, N~
gas was blown into the vessel 1 through the gas blow opening
11 at a rate of 100 N ~/min to bubble the molten steel 31.
Gas was exhausted from the vessel 1 through said gas exhaust
opening 12 so that the pressure inside the gas tight vessel
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1 could not exceed a predetermined pressure. 20 minutes
later, bubbling was stopped. In the above-mentioned state,
the inside of the gas tight vessel 1 was made vacuous
through said exhaust opening 12 with the use of a vacuum
pump ( not shown ) so that the pressure inside the vessel 1
could be reduced to lo-2 Torr. When this degree of vacuum
was kept,N2 having dissolved in the molten steel 31 appeared
as fine gas bubbles which trapped fine inclusions in the
molten steel 31 and made the fine inclusions rise to the
surface of the molten steel.
minutes later after this, the gas tight vessel was
turned counterclockwise at 180 ~ as shown in Fig.5 and was
made to be in the upsidedown state as shown in Fig.4. Since
a portion of the molten steel bath which had been in a deep
position and had been under large static pressure was
changed for a portion of molten steel bath of small depth,
a static pressure decreased rapidly and a large amount of
fine gas bubbles were produced from the portion of the
molten steel bath of small depth. After the molten steel
had been left in this state for 5 minutes, the gas tight
vessel was turned clockwise at 180~ to be again in the state
of Fig.4. In this case, the molten steel 31 was again
stirred. Moreover, since the portion of the molten steel
bath of large depth was changed for the portion of the
molten steel bath of small depth, pressure on the portion of
the molten steel bath having been under large static
pressure was rapidly reduced, N2 gas having remained in the
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molten steel 31 appeared as fine gas bubbles. The molten
steel was left in this state for 5 minutes. A vacuum pump
was driven to keep the degree of vacuum at 10 -2 Torr during
the pressure reduction.
Fig.6 shows a change of the total amount of oxygen in
the molten steel 31 with the lapse of time in the example.
According to Fig.6, the total amount of oxygen in the molten
steel could be decreased from initial ~0 ppm to final 12
ppm.
Example-4
An apparatus used for this method was the apparatus
having been used in Example-3 as shown in Fig.4. As in
Example-3, after molten steel had been kept in the gas tight
vessel 1 and bubbled by blowing N2 gas into the molten
steel through gas blow opening 11 positioned in the bottom
of the vessel 1, pressure inside the gas tight vessel was
reduced to 10-2 Torr by evacuating the gas tight vessel and
the vacuum was kept. After the degree of vacuum had been
kept for 5 min,the gas tight vessel 1 was turned at 90~ with
central horizontal rotation axis 10 in a central portion
thereof. The gas tight vessel 1 came to be in the state
such that a longitudinal direction of the gas tight vessel
of cylindrical shape was kept hori~ontally. Therefore, an
area of bath of the molten steel 31 became large and a depth
of the entire molten steel bath became small. As a
result, a static pressure on a portion of the molten steel
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31 which had been in a deep portion was reduced and fine gas
bubbles began to be actively produced.
Fig.8 is a graphical representation designating
a change of the total amount of oxygen T.(0) in the molten
steel 31 which was processed by the above-mentioned
degassing method wherein the depth of the molten steel bath
was made small. As shown in Fig.8, it is understood that
the degassing method is highly effective in processing of
the molten steel since T.(0) in the molten steel was
decreased from initial 80 ppm to final 15 ppm. In Example-3
wherein the gas tight vessel 1 was turned to 180~ , when
rotation of the gas tight vessel 1 was stopped for a while
at the position where the vessel was turned to 90 ~ , the
effect of turning the gas tight vessel 1 at 180~ and 90 ~
can be obtained.
Example-5
Fig.9 is a schematic illustration showing another
example of a method for promoting degassing of molten steel
by making a depth of molten steel bath small. In this
example, gas tight vessel 2 of parallelepiped of 3 m in
width, 3 m in height and 8 m in length was used. Removable
gate 3 of 3 m in length, 2.3 m in width and 0.5 m in
thickness was arranged inside the gas tight vessel 2. The
inside of the gas tight vessel 2 was divided into two
chambers 2a and 2b. In the drawing, referential numeral 22
denotes an exhaust opening for exhausting inside atmosphere
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which is arranged in a ceiling of the gas tight vessel 2, 21
a gas injection opening arranged in the bottom of the
chamber 2a, 23 an exit port for outflow of the molten steel
which is arranged in the bottom of the chamber 2b and 24 an
inlet for inflow of the molten steel which is arranged in a
ceiling of the chamber 2a. Said gas injection opening 21
and said inlet for inflow of the molten steel are arranged
so that they can be opened or closed if necessary.
A method for cleaning molten steel by the use of the
gas tight vessel 2 constituted in such a manner as described
above will be described. In Fig.9, gate 3 is positioned
2 m away from the left face of the gas tight vessel 2.
Approximately 90 t of molten steel 31 is charged through the
inlet 24 for inflow of the molten steel into one chamber 2a
separated from the other chamber 2b by the gate 3. During
charging of the molten steel, the exit port 23 for outflow
of the molten steel is closed. The volume of the molten
steel 31 comes to be 12 m3 of 3 m in length, 2 m in width
and 2 m in depth. N2 gas is blown through the gas blow
opening 21 at a rate of 100 N Q /min and the molten steel 3
is bubbled by N2 gas. During bubbling, the inside
atmosphere is simultaneously exhausted through said gas
exhaust opening 22 so that there cannot be any excessive
pressure inside the gas tight vessel. The bubbling of the
molten steel is stopped 20 minutes later and the inside of
the gas tight vessel 2 is evacuated through said gas exhaust
opening 22 by the use of a vacuum pump ( not shown ). An
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opening is made between the bottom face and the lower ends
of the gate 3 by lifting the gate 3 upwardly as shown in
Fig.10 when the pressure inside the gas tight vessel 2 is
reduced to 10-2 Torr. Then, the molten steel having been
stemmed by said gate 3 spreads in the whole vessel 2. As
a result, the depth of the molten steel 31 having been 2 m
initially comes to be 0.5 m.
Since the depth of the molten steel becomes rapidly
small and the area of the surface of the molten steel bath
widens, fine gas bubbles are actively produced. ~hen the
molten steel 31 is discharged by opening the exit port 23
for outflow of the molten steel, approximately the total
amount of the molten steel flowed out of the vessel 2 a
quarter of an hour later.
It is clearly understood that the above-mentioned
method is highly effective in cleaning of molten steel since
the total amount of oxygen was decreased from initial 80
ppm to final 12 ppm as a result of having cleaned the molten
steel in such a manner as described above.
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