U.S. patent application number 10/490459 was filed with the patent office on 2004-12-09 for method for refining molten iron containing chromium.
Invention is credited to Igarashi, Masao, Nakao, Ryuji, Sumi, Makoto, Tanaka, Tomoaki, Yamashita, Kosuke, Yoshino, Koichiro.
Application Number | 20040245682 10/490459 |
Document ID | / |
Family ID | 27532002 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040245682 |
Kind Code |
A1 |
Yamashita, Kosuke ; et
al. |
December 9, 2004 |
Method for refining molten iron containing chromium
Abstract
A refining method and refining apparatus, able to shorten the
time required for refining and reduce the refining costs in
decarburization refining of a chromium-contained molten steel,
which refining method for chromium-contained molten steel etc.
performing decarburization refining by blowing a gas containing
oxygen gas into a chromium-contained molten steel under a vacuum or
atmospheric pressure and vacuum, said refining method for a
chromium-contained molten steel etc. characterized by having a
first step for blowing oxygen gas while making the inside of the
vessel a pressure of a range of 400 Torr (53 kPa) to atmospheric
pressure, a second step for blowing oxygen gas while evacuating the
inside of the vessel to 250 to 400 Torr (33 to 53 kPa), and third
step for blowing gas while evacuating the inside of the vessel to
not more than 250 Torr (33 kPa). Further, a refining method and
refining apparatus for an ultra-low carbon chrome melt
characterized by performing a first vacuum refining until the third
step, then restoring the pressure in the vessel to at least 400
Torr (53 kPa), then performing second vacuum refining while making
the bottom blowing gas blow rate at least 0.4 Nm.sup.3/min per ton
steel.
Inventors: |
Yamashita, Kosuke;
(Futtsu-shi, Chiba, JP) ; Nakao, Ryuji;
(Hikari-shi, Yamaguchi, JP) ; Tanaka, Tomoaki;
(Hikari-shi, Yamaguchi, JP) ; Igarashi, Masao;
(Hikari-shi, Yamaguchi, JP) ; Yoshino, Koichiro;
(Hikari-shi, Yamaguchi, JP) ; Sumi, Makoto;
(Kitakyushu-shi, Fukuoka, JP) |
Correspondence
Address: |
KENYON & KENYON
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
27532002 |
Appl. No.: |
10/490459 |
Filed: |
March 22, 2004 |
PCT Filed: |
September 20, 2002 |
PCT NO: |
PCT/JP02/09701 |
Current U.S.
Class: |
266/208 ;
75/512 |
Current CPC
Class: |
F27D 2003/164 20130101;
C21C 5/30 20130101; F27D 3/0032 20130101; C21C 5/35 20130101; F27D
2003/168 20130101; F27D 17/001 20130101; F27D 3/0025 20130101; F27D
17/004 20130101; F27D 2003/166 20130101; C21C 7/10 20130101; C21C
7/0685 20130101; F27D 3/16 20130101; C21C 5/005 20130101 |
Class at
Publication: |
266/208 ;
075/512 |
International
Class: |
C21C 007/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 20, 2001 |
JP |
2001-286694 |
Sep 20, 2001 |
JP |
2001-286695 |
Nov 5, 2001 |
JP |
2001-339046 |
Dec 25, 2001 |
JP |
2001-391274 |
Aug 13, 2002 |
JP |
2002-235726 |
Claims
1. A refining method refining by blowing a mixed gas including
oxygen gas into a chromium-contained molten steel in a refining
vessel, said refining method for a chromium-contained molten steel
characterized by having a first step of blowing in said mixed gas
while making the inside of the vessel a pressure of a range of 400
Torr (53 kPa) to atmospheric pressure, a second step of blowing
said mixed gas while evacuating said vessel to 250 to 400 Torr (33
to 53 kPa), and a third step of blowing said mixed gas while
further evacuating the inside of the vessel to not more than 250
Torr (33 kPa) and by refining step by step while switching from the
first step to the second step at a concentration of carbon in the
melt of 0.8 to 0.3% and switching from the second step to the third
step at a concentration of carbon in the melt of 0.4 to 0.1%.
2. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by refining while making the mixed
gas blow rate at said second step at least 0.4 Nm.sup.3/min per ton
melt.
3. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by, in said first step, performing
refining comprising refining the entire amount under atmospheric
pressure, refining the entire amount under a vacuum, or refining
first at atmospheric pressure, then under a vacuum.
4. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by, when refining under atmospheric
pressure of said first step, refining using both top blowing and
bottom blowing as the blowing of said mixed gas.
5. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by, when refining under atmospheric
pressure of said first step, refining using only oxygen for the
blowing of said mixed gas.
6. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by, in said third step, refining by
further evacuating step by step the inside of the vessel along with
the decrease in concentration of carbon in the melt.
7. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by, in said third step, refining by
any means of supplying only inert gas for the blowing of said mixed
gas, gradually reducing the ratio of supply of oxygen gas in said
mixed gas along with the decrease in concentration of carbon in the
melt, or supplying inert gas after the ratio of oxygen gas in said
mixed gas decreases.
8. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by starting evacuating the inside
of said refining vessel, then blowing inert gas, nitrogen, or
another non-oxidizing gas or a mixed gas of the same to reduce the
concentration of oxygen in the exhaust gas to not more than 7 vol
%, then blowing said mixed gas into said evacuated refining vessel
and starting refining.
9. A refining method for a chromium-contained molten steel as set
forth in claim 1, characterized by, in said third step, reducing
the concentration of carbon in the melt to not more than 0.08%,
then restoring the pressure in the vessel to at least 400 Torr (53
kPa), then bottom blowing mixed gas and vacuum refining at a mixed
gas blow rate of at least 0.4 Nm.sup.3/min per ton melt so as to
reduce the carbon to an ultra-low level.
10. A refining method for a chromium-contained molten steel as set
forth in claim 9, characterized, after said third step, by
restoring the pressure inside the vessel to at least 400 Torr (53
kPa), then bottom blowing mixed gas, reducing the ratio of the
oxygen gas in the blown mixed gas to not more than 30%, reducing
the pressure inside the vessel to not more than 100 Torr (13 kPa),
and continuing refining.
11. A refining apparatus for a chromium-contained molten steel,
said refining apparatus for a chromium-contained molten steel
characterized by comprising a vacuum refining vessel, an alloy and
sub-material addition unit provided above the vacuum refining
vessel, exhaust gas cooler, vacuum valve, one-stage or
multiple-stage ejector type vacuum exhaust unit, and water-sealed
type vacuum pump arranged successively and by having a pressure
control valve under vacuum for returning part of the exhaust gas
from down stream side of said water-sealed type vacuum pump to the
upstream side of said water-sealed type vacuum pump.
12. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by being provided with a means
for adjusting the opening degree of said vacuum control use
pressure adjusting valve to control the degree of vacuum inside
said vacuum refining vessel to return part of the exhaust gas
exhausted from said water-sealed type vacuum pump to the upstream
side of the exhaust gas passage of said water-sealed type vacuum
pump.
13. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by providing a means arranging
a vacuum valve between an exhaust side of said one-stage or
multiple-stage ejector type vacuum exhaust unit and said
water-sealed type vacuum pump and said vacuum refining vessel side
of said exhaust gas cooler, closing said vacuum valve before the
start of vacuum refining to place said ejector type vacuum exhaust
unit and said water-sealed type vacuum pump in a vacuum state in
advance, and opening said vacuum valve simultaneously with the
start of vacuum refining to raise the degree of vacuum of the
vacuum refining vessel.
14. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by providing a means for
adjusting the opening degree of said vacuum control use pressure
control valve under a vacuum in advance to restore up to 10% of the
flow of exhaust gas to the upstream side of said water-sealed type
vacuum pump and then immediately adjusting the degree of vacuum in
said vacuum refining vessel when adding alloy and sub-material
during refining under a vacuum in the vacuum refining vessel.
15. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by providing a seal unit
having a seal valve for sealing an addition port at the bottom of
said alloy and secondary material adding unit and setting a dummy
lance integrally with said seal unit at the bottom of said seal
valve or setting it elevatably linked with said seal unit.
16. A refining apparatus for a chromium-contained molten steel as
set forth in claim 15, characterized by providing a seal port for
blowing seal gas to a clearance between inside walls of the
addition port of said alloy and secondary material unit and said
dummy lance.
17. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by providing a center cover
having a cooling function at the bottom of said alloy and secondary
material addition unit.
18. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by providing at the back of
said exhaust gas cooler inside the refining apparatus system a
water leakage detection unit able to detect water leakage by
measuring at least one of a steam temperature or steam pressure in
the exhaust gas.
19. A refining apparatus for a chromium-contained molten steel as
set forth in claim 11, characterized by arranging at the back of
said one-stage or multiple-stage ejector type vacuum exhaust unit
and said water-sealed type vacuum pump a return water storage tank
linked with these and attached to a gas ventilation unit.
20. A refining apparatus for a chromium-contained molten steel as
set forth in claim 19, characterized by providing a water-sealed
cover having a partition cover provided, without being fixed, at
the top of said return water storage tank.
21. A refining apparatus for a chromium-contained molten steel as
set forth in claim 20, characterized in that the weight of said
water-sealed cover satisfies the following formula
(1):(W1+W2).times.9.8>P.times.S (1)where, W1: weight of
partition cover (kg) W2: weight of weight placed on partition cover
(kg) P: maximum gas pressure acting inside return water storage
tank (Pa) S: maximum area of projection of inside surface of
movable partition cover on horizontal plane (m.sup.2).
22. A refining apparatus for a chromium-contained molten steel as
set forth in claim 20, characterized in that the water-sealing
height of said water-sealed cover satisfies the following
formula:H-L>9.8.times.10.su- p.3.times.P (2)where, H: height of
outside outer tube of partition cover side walls of water-sealed
cover (m) P: maximum gas pressure acting at inside of return water
storage tank (Pa) L: height of sealing water passage between inner
tube and outer tube in water-sealed cover (m).
Description
TECHNICAL FIELD
[0001] The present invention relates to a refining method and
refining apparatus for chromium-contained molten steel which refine
chromium-contained molten steel in a refining vessel while blowing
a gas containing oxygen gas.
BACKGROUND ART
[0002] When refining chromium steel, in particular stainless steel
and other chromium steel including at least 9% of chrome, the
method of decarburization refining by the AOD method of blowing
oxygen gas or a mixed gas of oxygen gas and an inert gas into a
melt contained in a refining vessel has been extensively used. In
the AOD method, when the decarburization proceeds and the
concentration of carbon in the melt drops, the chromium becomes
oxidized more easily, so the method has been adopted of raising the
ratio of the argon gas or other inert gas in the blown gas along
with the drop in the concentration of carbon to suppress the
oxidation of chromium. However, in the region of low concentration
of carbon, the decarburization rate falls, so a long time is
required until reaching the desired concentration of carbon.
Further, to raise the ratio of the inert gas in the blown gas, the
amount of consumption of the expensive inert gas greatly increases.
This is also not advantageous economically.
[0003] As a method for promoting the decarburization in the region
of low concentration of carbon, utilization of the vacuum refining
method may be mentioned. Japanese Unexamined Patent Publication
(Kokai) No. 6-287629 discloses the method of supplying oxygen gas
or a mixed gas of oxygen gas and inert gas as the blown gas,
decarburizing the melt until the concentration of carbon in the
melt falls to 0.5 wt %, evacuating in the vessel to not more than
200 Torr (26 kPa), and continuing to decarburize the melt after the
concentration of carbon falls below this value. Since performing
this treatment under a vacuum from a relative high concentration of
carbon and performing the decarburization by a mixed gas with
oxygen gas under a vacuum, the oxygen efficiency for
decarburization is improved, so the decarburization rate is
improved with the same amount of supply of oxygen, the reduction
silicon prime units and expensive inert gas prime units can be
reduced, and the refining time can be shortened. The pressure
inside the vessel in the vacuum treatment is made not more than 200
Torr (26 kPa) because it is considered that the oxygen efficiency
for decarburization falls at a pressure higher than that.
[0004] Japanese Unexamined Patent Publication (Kokai) No. 9-71809
as well discloses a refining method comprising decarburizing a melt
by blowing a gas containing oxygen gas in the atmosphere, then
switching from atmospheric treatment to vacuum treatment at the
stage when the concentration of carbon drops to 0.7 to 0.05 wt %
and blowing a gas containing oxygen gas under a vacuum of 200 (26
kPa) to 15 Torr (2 kPa). The vacuum condition is made not more than
200 Torr (26 kPa) because it is considered the vacuum treatment
cannot be effectively performed under a pressure higher than
this.
[0005] By performing the vacuum treatment in a carbon concentration
region of a concentration of carbon of not more than 0.5 wt % or a
concentration of carbon of not more than 0.7 wt % and blowing gas
containing oxygen gas in the vacuum treatment, it is possible to
realize an improvement of the decarburization rate and a reduction
of use of the expensive insert gas, but if it were possible to
achieve a much shorter refining time or reduced amount of use of
inert gas, this would contribute greatly to the reduction of the
production costs and improvement of the productivity.
[0006] On the other hand, it is extremely difficult to refine
ultra-low carbon chromium steel with a concentration of carbon of
not more than 0.01% by the AOD method. As the method for promoting
decarburization in such a region of low concentration of carbon,
utilization of the vacuum refining method may be mentioned. As the
utilization of the vacuum refining method, the VOD method of vacuum
refining by decarburization in a converter until a suitable
concentration of carbon, then shifting the melt to a vacuum
refining vessel and the method of using a vacuum AOD furnace for
vacuum refining while placing an exhaust hood over the AOD furnace
are general.
[0007] As an example of the VOD method, Japanese Unexamined Patent
Publication (Kokai) No. 51-142410 discloses the method of oxygen
refining in a converter, then decarburizing the melt in a vacuum
decarburization ladle to make the concentration of carbon after
vacuum treatment 0.008%.
[0008] As a method using a vacuum AOD furnace, Japanese Examined
Patent Publication (Kokoku) No. 60-10087 discloses the method of
refining chromium steel by first refining by oxygen gas at the
initial ordinary temperature until the carbon falls to about 0.2 to
0.4 wt %, then stopping the supply of oxygen gas while continuing
to agitate the melt by the inert gas in the same vessel,
continuously lowering the pressure inside the vessel to about 10
Torr (1.3 kPa), and lowering the concentration of carbon after
vacuum treatment to 0.13 wt %.
[0009] With the above method, the carburization under vacuum uses
only inert gas, so the oxidation of chromium is suppressed, but the
oxygen source of the decarburization becomes the oxygen in the melt
or the oxygen in the slag and the rate of supply of oxygen becomes
slow, so a drop in the decarburization rate is invited. Therefore
this cannot be said to be an efficient decarburization refining
method. As opposed to this, Japanese Unexamined Patent Publication
(Kokai) No. 6-287629 discloses a decarburization refining method
for chromium-contained molten steel comprising supply a mixed gas
of oxygen gas and inert gas as the blown gas, performing
decarburization refining under atmospheric pressure until the
concentration of carbon in the melt falls to 0.5 wt %, then, after
the concentration of carbon falls below this value, evacuating the
inside of the vessel to not more than 200 Torr (26 kPa) and
continuing to decarburize the melt. In this method, gas including
oxygen gas is supplied even in the vacuum refining. Due to this,
the oxygen efficiency for decarburization is improved, so an
improvement in the decarburization rate is achieved and the
refining time can be shortened, so it is possible to achieve a
large reduction in the refining costs and improvement in the
productivity and refining down to the ultra-low carbon region of a
concentration of carbon of not more than 0.01 wt % becomes easy. In
this invention, the total amount of the blown gas during the vacuum
annealing is made 0.3 Nm.sup.3/min.multidot.T.
[0010] In decarburization refining of ultra-low carbon
chromium-contained molten steel, by applying vacuum refining to the
decarburization in the low carbon concentration region and using a
gas containing oxygen gas as the bottom blown gas used at the time
of vacuum refining, refining of the ultra-low carbon area of a
concentration of carbon of not more than 0.01 wt % becomes
possible, but the decarburization rate gradually falls along with
the fall in the concentration of carbon, so to decarburize the melt
until this ultra-low carbon region, an extremely long refining time
is required compared with decarburization refining down to the
ordinary low carbon region. Therefore, compared with usual refining
of low carbon chromium steel, a drop in productivity of the
decarburization refining is invited and an increase in the refining
costs is caused.
[0011] Further, regarding the refining apparatus for a
chromium-contained molten steel, vacuum refining furnaces comes in
various types such as VOD, AOD, RH, and REDA, but vacuum exhaust
equipment is required for evacuating the inside of the furnace. The
vacuum exhaust equipment for industrially evacuating the inside of
a vacuum refining furnace generally achieves a predetermined degree
of vacuum inside the furnace by combining a large number of
ejectors. The degree of vacuum is controlled in accordance with the
progress in refining in the vacuum refining furnace, but normally
one or more ejectors with capacities commensurate with the targeted
degree of vacuum are operated among a large number of ejectors to
secure the predetermined degree of vacuum.
[0012] On the other hand, one type of vacuum exhaust unit used
industrially is a water-sealed vacuum pump. When using this alone,
due to the problem of cavitation, the attainable degree of vacuum
is about 61 Torr (8 kPa). To obtain a higher degree of vacuum, it
is necessary to jointly use the above-mentioned ejectors.
[0013] When controlling the degree of vacuum using only ejectors,
nitrogen, air, etc. is blown in before the ejectors and the blow
rate is controlled so as to control the degree of vacuum in the
furnace or the ducts.
[0014] When refining a melt using gaseous oxygen under vacuum, the
CO gas produced by the decarburization reaction causes the metal
and slag to splash from the surface of the melt toward the top of
the vacuum refining furnace. The amount of this generated increases
sharply when the degree of vacuum rises (when a high vacuum is
reached) and deposits on the alloy addition port, furnace cover,
ducts, etc. at the top of the refining vessel to block the same or
cause trouble in various equipment and operations and obstruct
productivity. If raising the degree of vacuum and increasing the
oxygen blow rate, a rapid decarburization reaction will proceed and
the phenomenon will arise of the CO gas generated causing a large
amount of metal to be blown upward all at once from near the
surface of the melt, that is, boiling will be caused. This will
also become major trouble in the equipment and worsen the
productivity.
[0015] In this way, vacuum oxygen decarburization of a carbon melt
is an operation which requires extreme care. The point is to
control the degree of vacuum and the oxygen blow rate in accordance
with the concentration of carbon in the melt. Among these, the
oxygen blow rate can be controlled to a certain extent by the flow
adjustment valve of the oxygen gas, but no sufficient control
method has been established for the degree of vacuum.
[0016] In the above prior art, when using ejectors, the method of
successively starting and stopping a large number of ejectors does
not allow extremely fine control of the degree of vacuum since the
ranges of capacity of the ejectors themselves are broad. Further,
as seen in Japanese Unexamined Patent Publication (Kokai) No.
10-1716, the method of allowing gas to leak in from the outside
while operating the exhaust unit (for example, using nitrogen)
enables control of the degree of vacuum to a certain extent, but
has the defect that the gas costs rise. As a means for slashing the
gas costs, there is the method of using air as an alternative to
nitrogen. However, while control of the degree of vacuum itself is
possible, the exhaust gas sucked in contains a high concentration
of CO gas, so when mixing in air containing a combustion-assisting
gas constituted by oxygen, there is the danger of combustion and
explosion. Employment for actual machinery is extremely dangerous.
Further, if allowing gas to leak in from the outside, the load on
the exhaust unit increases. For example, the power used by the
vacuum pump increases. Therefore, this is not preferable from the
viewpoint of energy conservation. Further, the method of
controlling the amount of supply of steam to an ejector used in
this patent relies on the fact that the optimum steam flow rate of
an ejector is distinctive, so changing this remarkably reduces the
exhaust performance of the ejector itself. Further, at the same
time, a slight fluctuation in the amount of steam is overly
sensitively reflected in the ejector performance, so extremely fine
control of the pressure inside the refining vessel becomes
difficult.
[0017] On the other hand, the method of using a water-sealed type
vacuum pump is currently employed for control of the degree of
vacuum by pump units, but this is not used together with ejectors,
the capacity is insufficient for realizing a high vacuum by this
alone, and extremely fine control of the degree of vacuum is
impossible.
[0018] Further, in a vacuum refining vessel, in most cases, for
efficient refining or for final adjustment of the ingredients of
the melt, alloy or secondary materials are added to the melt in the
middle of refining or at the end stage of refining. Normally, these
are charged into the vessel and added to the melt by allowing them
to naturally drop from an alloy hopper provided at the top of the
refining vessel through a chute.
[0019] However, due to the argon blown into the refining vessel for
agitating the melt or the oxygen blown for promoting
decarburization, splash of the metal and slag, generation of dust,
etc. occur inside the refining vessel. Therefore, the metal
deposits at the alloy and secondary material addition port linked
with the inside of the vessel and accordingly the addition port
becomes blocked or other trouble easily occurs. Therefore, to
suppress the occurrence of such trouble, the means has been adopted
of providing the alloy and secondary material addition port with
side walls resistant to the effects of the metal and slag or, in
the case of a refining vessel with a high tank height, providing a
top cover. Further, the means has also been adopted of using the
alloy and secondary material addition port jointly as the insertion
port of the top blowing lance. If considering continuous long term
operation of a vacuum refining vessel, however, neither means is
sufficient in practice.
[0020] Further, in treatment of the exhaust gas of a metallurgical
furnace, including atmospheric and vacuum refining vessels, it is
necessary to cool the high temperature exhaust gas produced.
Therefore, sometimes a water-cooled type gas scrubber is provided
in the middle of the ducts or the ducts are water cooled in the
middle. In this case, heat is exchanged between the high
temperature exhaust gas and the large amount of cooling water. Due
to abrasion and reduced thickness of the piping and ducts, cracking
due to thermal stress, etc., sometimes the cooling water leaks from
the piping and ducts to the inside of the exhaust gas passage.
Exhaust gas treatment equipment is generally closed, however, so it
is impossible to obtain a grasp of the state of water leakage
inside. Therefore, sometimes operation is continued while not being
able to confirm internal water leakage and the water leakage
becomes serious and leads to a remarkable drop in the degree of
vacuum or the inability to remove dust from the system due to the
water leakage or other trouble in equipment or operation.
[0021] Therefore, operation has been stopped on a scheduled basis
at a certain frequency and the inside of the ducts checked and the
gas cooler checked. Further, the practice has been to install an
electrostatic capacity type detection rod at the dust collector at
the bottom of the gas cooler and utilize the fact that dust changes
in electrostatic capacity when wet by water leakage so as to detect
water leakage.
[0022] If stopping operation and conducting checks on a scheduled
basis, however, the operating efficiency of the facilities will be
reduced and the productivity blocked. On the other hand, with the
above-mentioned electrostatic capacity type detection rod, it is
difficult to adjust the electrostatic capacity of the detection rod
according to the state of wetness of the dust. For example, with a
small amount of water leakage, if the temperature is high or under
a vacuum, the water will easily turn into steam, so detection of
water leakage will not be possible. The detection rod is predicated
on detection of a large amount of water leakage. Therefore, it is
extremely difficult to detect water leakage in advance while still
slight.
[0023] Further, vacuum exhaust equipment for industrially
evacuating a vacuum refining vessel generally achieves a
predetermined degree of vacuum in the furnace by combining a large
number of ejectors or using a vacuum pump. Vacuum ejectors utilize
the so-called "mist-blowing principle" and suck in and exhaust the
exhaust gas in the vacuum refining vessel and the ducts and other
parts of the vacuum path by the ejected media. For the ejected
medium, usually steam is used industrially. Steam is condensed by
the cooling water at a condenser after the ejectors to become water
again and therefore only the exhaust gas is exhausted to the next
stage. The cooling water of the condenser and the condensed water
of the steam are temporarily collected and stored at a water
storage tank near the ground and are pumped to the cooling tower by
a pump. On the other hand, as the vacuum pump, industrially a
water-sealed pump is used and a large amount of water is used. The
water used by the vacuum pump is collected and stored in a water
storage tank in the same way as the condenser water.
[0024] Exhaust gas contains a large amount of CO gas. The condenser
water is accompanied by large numbers of bubbles of exhaust gas
containing CO which flow into the water storage tank along with it.
Therefore, the inside of the water storage tank becomes an
atmospheric gas containing CO gas in composition. In the sense of
preventing the gas inside the tank from leaking outside the tank,
closeability and sealability are very important as functions
required for a water storage tank.
[0025] Water storage tanks come in generally two types: steel seal
pots and concrete (the top cover part made of steel) hot wells.
Steel seal pots have a good closeability, but suffer from the
problems of corrosion and swelling capital costs. On the other
hand, concrete hot wells are free from corrosion and relatively
inexpensive in terms of capital costs as well, but suffer from
problems in the sealability with the top steel covers. In the
following description, the invention will be explained taking as an
example mainly the latter concrete hot wells, but the invention may
similarly be applied to steel seal pots.
[0026] There are two issues with hot wells. The first is that there
is leakage of CO-containing gas from a hot well. The second is the
suppression of damage to the equipment when the cooling water
inside a hot well overflows.
[0027] As means for dealing with this, the method of forcibly
evacuating the inside of the hot well by a suction fan is widely
employed. Due to this, the inside of the hot well becomes a
constantly negative pressure and the danger of leakage of the
inside gas is remarkably reduced. However, the inside of a hot well
being made negative pressure due to suction of gas means suction of
air from the seal parts. The clearance of the seal parts therefore
gradually expands. If the suction fan were to stop in this state
for some reason or another, a large amount of CO-containing gas
would leak from the expanded clearance of the seal parts.
[0028] Further, even if the power of the system of the return pump
of the hot well is cut off for some reason and the return pump
stops, the supply pump of the large-sized cooling tower will
continue to operate. This being so, the cooling water in the hot
well will continue to increase and will overflow. As a measure
against this, it may be considered to attach a switch valve from
another power source system to the supply pipe to the condenser and
water-sealed pump, but tremendous expense would become required for
the long distance pipeline and the large switching valve.
DISCLOSURE OF INVENTION
[0029] The present invention has as its object the provision of a
refining method for a chromium-contained molten steel comprising
refining by blowing a gas containing oxygen gas into a
chromium-contained molten steel in a refining vessel and enabling a
reduction of the amount of use of inert gas or oxygen gas and
shortening of the refining time.
[0030] Further, the present invention has as its object the
provision of a refining method able to shorten the time required
for refining and reduce the refining cost in decarburization
refining of an ultra-low carbon melt.
[0031] Further, the present invention provides a vacuum control
method and apparatus in vacuum exhaust equipment able to control
the degree of vacuum in a vessel or ducts at the time of refining a
melt by oxygen decarburization in a vacuum refining vessel.
[0032] Further, the present invention has as its object the
provision of a seal unit and seal method able to avoid blocking of
an alloy and secondary material addition port even under refining
conditions where the metal and slag are remarkably violently
splashed.
[0033] Further, the present invention has as its object to detect
with a high precision water leakage in an exhaust gas treatment
apparatus in a metallurgical furnace or vessel of an atmospheric
refining or vacuum refining apparatus, in particular a water-cooled
duct, exhaust gas cooling unit, or other unit using cooling water
and provides a detection unit able to detect even a slight amount
of water leakage during treatment, easily to manage and maintain,
and superior in durability.
[0034] Further, the present invention has as its object the
provision of an apparatus for simply solving the problems in the
hot well, that is, suppressing leakage of CO-containing gas from
the hot well and damage to equipment at the time of overflow of the
cooling water in the hot well.
[0035] The present invention was made to solve the above problems
and has as its gist the following:
[0036] (1) A refining method refining by blowing a mixed gas
including oxygen gas into a chromium-contained molten steel in a
refining vessel, said refining method for a chromium-contained
molten steel characterized by having a first step of blowing in
said mixed gas while making the inside of the vessel a pressure of
a range of 400 Torr (53 kPa) to atmospheric pressure, a second step
of blowing said mixed gas while evacuating said vessel to 250 to
400 Torr (33 to 53 kPa), and a third step of blowing said mixed gas
while further evacuating the inside of the vessel to not more than
250 Torr (33 kPa) and by refining step by step while switching from
the first step to the second step at a concentration of carbon in
the melt of 0.8 to 0.3% and switching from the second step to the
third step at a concentration of carbon in the melt of 0.4 to
0.1%.
[0037] (2) A refining method for a chromium-contained molten steel
as set forth in (1), characterized by refining while making the
mixed gas blow rate at said second step at least 0.4 Nm.sup.3/min
per ton melt.
[0038] (3) A refining method for a chromium-contained molten steel
as set forth in (1) or (2), characterized by, in said first step,
performing refining comprising refining the entire amount under
atmospheric pressure, refining the entire amount under a vacuum, or
refining first at atmospheric pressure, then under a vacuum.
[0039] (4) A refining method for a chromium-contained molten steel
as set forth in (1) or (3), characterized by, when refining under
atmospheric pressure of said first step, refining using both top
blowing and bottom blowing as the blowing of said mixed gas.
[0040] (5) A refining method for a chromium-contained molten steel
as set forth in any one of (1) to (4), characterized by, when
refining under atmospheric pressure of said first step, refining
using only oxygen for the blowing of said mixed gas.
[0041] (6) A refining method for a chromium-contained molten steel
as set forth in (1), characterized by, in said third step, refining
by further evacuating step by step the inside of the vessel along
with the decrease in concentration of carbon in the melt.
[0042] (7) A refining method for a chromium-contained molten steel
as set forth in (1), characterized by, in said third step, refining
by any means of supplying only inert gas for the blowing of said
mixed gas, gradually reducing the ratio of supply of oxygen gas in
said mixed gas along with the decrease in concentration of carbon
in the melt, or supplying inert gas after the ratio of oxygen gas
in said mixed gas decreases.
[0043] (8) A refining method for a chromium-contained molten steel
as set forth in (1), characterized by starting evacuating the
inside of said refining vessel, then blowing inert gas, nitrogen,
or another non-oxidizing gas or a mixed gas of the same to reduce
the concentration of oxygen in the exhaust gas to not more than 7
vol %, then blowing said mixed gas into said evacuated refining
vessel and starting refining.
[0044] (9) A refining method for a chromium-contained molten steel
as set forth in (1), characterized by, in said third step, reducing
the concentration of carbon in the melt to not more than 0.08%,
then restoring the pressure in the vessel to at least 400 Torr (53
kPa), then bottom blowing mixed gas and vacuum refining at a mixed
gas blow rate of at least 0.4 Nm.sup.3/min per ton melt so as to
reduce the carbon to an ultra-low level.
[0045] (10) A refining method for a chromium-contained molten steel
as set forth in (9), characterized, after said third step, by
restoring the pressure inside the vessel to at least 400 Torr (53
kPa), then bottom blowing mixed gas, reducing the ratio of the
oxygen gas in the blown mixed gas to not more than 30%, reducing
the pressure inside the vessel to not more than 100 Torr (13 kPa),
and continuing refining.
[0046] (11) A refining apparatus for a chromium-contained molten
steel, said refining apparatus for a chromium-contained molten
steel characterized by comprising a vacuum refining vessel, an
alloy and sub-material addition unit provided above the vacuum
refining vessel, exhaust gas cooler, vacuum valve, one-stage or
multiple-stage ejector type vacuum exhaust unit, and water-sealed
type vacuum pump arranged successively and by having a pressure
control valve under a vacuum for returning part of the exhaust gas
from down stream side of said water-sealed type vacuum pump to the
upstream side of said water-sealed type vacuum pump.
[0047] (12) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by being provided with a
means for adjusting the opening degree of said vacuum control use
pressure adjusting valve to control the degree of vacuum inside
said vacuum refining vessel to return part of the exhaust gas
exhausted from said water-sealed type vacuum pump to the upstream
side of the exhaust gas passage of said water-sealed type vacuum
pump.
[0048] (13) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by providing a means
arranging a vacuum valve between an exhaust side of said one-stage
or multiple-stage ejector type vacuum exhaust unit and said
water-sealed type vacuum pump and said vacuum refining vessel side
of said exhaust gas cooler, closing said vacuum valve before the
start of vacuum refining to place said ejector type vacuum exhaust
unit and said water-sealed type vacuum pump in a vacuum state in
advance, and opening said vacuum valve simultaneously with the
start of vacuum refining to raise the degree of vacuum of the
vacuum refining vessel.
[0049] (14) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by providing a means for
adjusting the opening degree of said vacuum control use pressure
control valve under a vacuum in advance to restore up to 10% of the
flow of exhaust gas to the upstream side of said water-sealed type
vacuum pump and then immediately adjusting the degree of vacuum in
said vacuum refining vessel when adding alloy and sub-material
during refining under a vacuum in the vacuum refining vessel.
[0050] (15) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by providing a seal unit
having a seal valve for sealing an addition port at the bottom of
said alloy and secondary material adding unit and setting a dummy
lance integrally with said seal unit at the bottom of said seal
valve or setting it elevatably linked with said seal unit.
[0051] (16) A refining apparatus for a chromium-contained molten
steel as set forth in (15), characterized by providing a seal port
for blowing seal gas to a clearance between inside walls of the
addition port of said alloy and secondary material unit and said
dummy lance.
[0052] (17) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by providing a center
cover having a cooling function at the bottom of said alloy and
secondary material addition unit.
[0053] (18) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by providing at the back
of said exhaust gas cooler inside the refining apparatus system a
water leakage detection unit able to detect water leakage by
measuring at least one of a steam temperature or steam pressure in
the exhaust gas.
[0054] (19) A refining apparatus for a chromium-contained molten
steel as set forth in (11), characterized by arranging at the back
of said one-stage or multiple-stage ejector type vacuum exhaust
unit and said water-sealed type vacuum pump a return water storage
tank linked with these and attached to a gas ventilation unit.
[0055] (20) A refining apparatus for a chromium-contained molten
steel as set forth in (19), characterized by providing a
water-sealed cover having a partition cover provided, without being
fixed, at the top of said return water storage tank.
[0056] (21) A refining apparatus for a chromium-contained molten
steel as set forth in (20) or (21), characterized in that the
weight of said water-sealed cover satisfies the following formula
(1):
(W1+W2).times.9.8>P.times.S (1)
[0057] where,
[0058] W1: weight of partition cover (kg)
[0059] W2: weight of weight placed on partition cover (kg)
[0060] P: maximum gas pressure acting inside return water storage
tank (Pa)
[0061] S: maximum area of projection of inside surface of movable
partition cover on horizontal plane (m.sup.2)
[0062] (22) A refining apparatus for a chromium-contained molten
steel as set forth in (20) or (21), characterized in that the
water-sealing height of said water-sealed cover satisfies the
following formula:
H-L>9.8.times.10.sup.3.times.P (2)
[0063] where,
[0064] H: height of outside outer tube of partition cover side
walls of water-sealed cover (m)
[0065] P: maximum gas pressure acting at inside of return water
storage tank (Pa)
[0066] L: height of sealing water passage between inner tube and
outer tube in water-sealed cover (m)
BRIEF DESCRIPTION OF DRAWINGS
[0067] FIG. 1 are views of a refining vessel of the present
invention, wherein (a) shows the state at the time of vacuum
refining and (b) shows the state at the time of atmospheric
pressure refining.
[0068] FIG. 2 is a view of the relationship between the pressure
inside a refining vessel and oxygen efficiency for
decarburization.
[0069] FIG. 3 is a view of the relationship between the pressure
inside a refining vessel and a dust generation index.
[0070] FIG. 4 is a view schematically showing an exhaust gas
treatment unit of a vacuum refining facility.
[0071] FIG. 5 is a view of the trends in the vacuum treatment time
and the change in degree of vacuum in a vacuum refining furnace and
a vacuum exhaust unit.
[0072] FIG. 6 is a view schematically showing a seal unit in a
conventional vacuum refining unit.
[0073] FIG. 7 is a view of an embodiment of a seal unit according
to the present invention.
[0074] FIG. 8 is a view schematically showing the area around a hot
well.
[0075] FIG. 9 is a view of a side view of a hot well water-sealed
cover.
BEST MODE FOR CARRYING OUT THE INVENTION
[0076] In the present invention, at the time of vacuum refining,
for example, when the refining vessel 1 shown in FIG. 1(a) performs
atmospheric pressure refining, for example, a refining vessel 1
shown in FIG. 1(b) is used. Refining gas is blown into the
chromium-contained molten steel in the refining vessel through a
bottom blowing tuyere 2. Further, the refining vessel 1 has a
detachable exhaust hood 3. At the time of vacuum refining, as shown
in FIG. 1(a), an exhaust hood 3 is attached to the refining vessel
1 and gas is sucked out to evacuate the refining vessel. At the
time of atmospheric pressure refining, as shown in FIG. 1(b), the
exhaust hood 3 is not attached, so as the blown gas, it is also
possible to blow gas while using not only the bottom blowing tuyere
2, but also a top blowing lance 12.
[0077] The present invention, as explained in the above (1), has as
its biggest feature having a step of blowing a gas containing
oxygen gas while evacuating the inside of the vessel to 250 to 400
Torr (33 to 53 kPa) in the refining process. This step is called
the "second step". By arranging this step (hereinafter generally
referred to as the "second step") in the medium carbon region
around a concentration of carbon of 0.4 wt % and vigorously
stirring the melt simultaneously, it is possible to maintain the
oxygen efficiency for decarburization in the medium carbon region
at a high value and further possible to suppress the generation of
dust.
[0078] FIG. 2 shows the relationship between the pressure inside
the refining vessel and the oxygen efficiency for decarburization
when making the bottom blowing gas blow rate 0.4 to 0.9
Nm.sup.3/min per ton melt. It is learned that up until the region
above a pressure inside the vessel of 400 Torr (53 kPa), a high
oxygen efficiency for decarburization can be maintained. Note that
at under 100 Torr (13 kPa), the amount of generation of dust is
large and operation not possible.
[0079] FIG. 3 is a view of the relationship between the pressure
inside the refining vessel and dust generation index when making
the bottom blowing gas blow rate 0.4 to 0.9 Nm.sup.3/min per ton
melt. The dust generation index is a value indexed to the average
value of the dust generation at a pressure inside the vessel of 400
Torr (53 kPa). It is learned that by making the pressure inside the
refining vessel at least 250 Torr (33 kPa), it is possible to
greatly reduce the dust generation.
[0080] By making the pressure the range of 250 to 400 Torr (33 to
53 kPa) at the second step, it is possible to achieve an increase
of the bottom blowing gas blow rate and as a result possible to
achieve a shorter refining time. The bottom blowing gas blow rate
is preferably made at least 0.4 Nm.sup.3/min per ton melt. Due to
this, it is possible to realize strong agitation for obtaining a
high oxygen efficiency for decarburization by a pressure of at
least 250 Torr (33 kPa) and shorten the refining time and possible
to keep the dust generation to a low level even if the blow rate of
the bottom blowing gas is at least 0.4 Nm.sup.3/min per ton melt if
the pressure is at least 250 Torr (33 kPa). The bottom blowing gas
blow rate can give even more preferable results if over 0.5
Nm.sup.3/min per ton melt.
[0081] As the timing for shifting from the first step where the
pressure inside the refining vessel is at least 400 Torr (53 kPa)
to the second step of 250 to 400 Torr (33 to 53 kPa), it is
preferable to shift when the concentration of carbon in the melt is
0.8 to 0.3%. This is because in the carbon region where the
concentration of carbon is higher than 0.8%, even if refining under
a vacuum, setting the pressure to a pressure higher than 400 Torr
(53 kPa) and increasing the oxygen gas blow rate enables more
efficient refining or refining under atmospheric pressure and
jointly using blowing of top blown oxygen gas secures a high oxygen
gas blow rate and enables efficient refining. Of course, even if
starting the second step from the region where the concentration of
carbon is at least 0.8%, for example, a concentration of carbon of
1.0%, it is possible to obtain the effect of the present invention.
On the other hand, if continuing the refining at a pressure over
400 Torr (53 kPa) up to the carbon region of a concentration of
carbon lower than 0.3%, a reduction in the oxygen efficiency for
decarburization is caused and prolongation of the refining time is
led to, so this is not preferable. Of course, even if starting the
second step from the area where the concentration of carbon is not
more than 0.3%, for example, a concentration of carbon of 0.2%, it
is possible to obtain the effect of the present invention. Most
preferably, it is sufficient to shift to the second step when the
concentration of carbon in the melt is 0.5 to 0.4%.
[0082] As the timing for shifting from the second step where the
pressure inside the refining vessel is 250 to 400 Torr (33 to 53
kPa) to the third step where the pressure is not more than 250 Torr
(33 kPa), it is preferable to shift when the concentration of
carbon in the melt is 0.4 to 0.1%. This is because by making the
carbon region where the concentration of carbon is higher than 0.4%
a pressure of 250 to 400 Torr (33 to 53 kPa), it is possible to
sufficiently obtain the effect of the present invention of
improving the refining efficiency and reducing the dust generation.
Of course, even if shifting to the third step from the
concentration of carbon of 0.5%, it is possible to obtain the
effect of the present invention. On the other hand, if continuing
refining by a pressure over 250 Torr (33 kPa) up to the carbon
region with a concentration of carbon lower than 0.1%, a reduction
in the oxygen efficiency for decarburization is caused and
prolongation of the refining time is caused, so this is not
preferable. Of course, even if starting the third step from the
region where the concentration of carbon is not more than 0.1%, for
example, the concentration of carbon is 0.05%, the effect of the
present invention can be obtain. Most preferably, it is sufficient
to shift to the third step at a concentration of carbon in the melt
of 0.3 to 0.2%.
[0083] As to the type of the blown gas of the bottom blown gas at
the second step, it may be made a mixed gas of oxygen and an inert
gas from the start of the second step, but it is also possible to
use a pattern of first blowing oxygen gas alone and then
successively increasing the ratio of the inert gas in the second
step.
[0084] The pressure in the refining vessel at the second step can
be held at a certain pressure in the range of 250 to 400 Torr (33
to 53 kPa), but if adopting a pattern of successively changing from
a high pressure to a low pressure, it is possible to decarburize
the melt while maintaining a substantially constant high oxygen
efficiency for decarburization without mixing in inert gas, so more
preferable results can be obtained.
[0085] Regarding the stage before the second step, that is, the
first step, it is sufficient to employ either of the case of
refining the entire amount under atmospheric pressure, the case of
refining the entire amount under a vacuum, and the case of refining
first under atmospheric pressure and then under a vacuum.
[0086] When refining under atmospheric pressure at the first step,
since no exhaust hood 3 is provided for vacuum refining above the
refining vessel, it is possible to jointly use top blowing and
bottom blowing as the gas blowing. Further, since the exhaust gas
is treated under atmospheric pressure, the exhaust gas suction
capability can be increased compared with vacuum refining. Under
such conditions, by top blowing in addition to bottom blowing, it
is possible to increase the overall amount of blown gas and promote
the progress in the decarburization refining. The lower the
concentration of carbon, the lower the carbon monoxide partial
pressure P.sub.CO in the gas at equilibrium with the chromium in
the melt. Therefore, in refining under atmospheric pressure, to
prevent oxidation loss of chromium, it is necessary to mix argon or
another inert gas in the blown gas, reduce the concentration of
carbon, increase the ratio of inert gas, and reduce the P.sub.CO in
the atmosphere.
[0087] When refining under atmospheric pressure in the first step,
it is possible to use only oxygen as the blown gas. This is because
with a range of carbon in the first step of 0.8 to 0.3% or more,
the P.sub.CO at equilibrium with the chromium in the melt is at
least 0.7 atm. Even if using only oxygen as the blown gas, the
extent of decline of the oxygen efficiency for decarburization is
small and a high decarburization rate is obtained. Further, it is
possible to suppress use of expensive inert gas. Note that if
making the range of carbon in the first step at least 0.5%, the
P.sub.CO at equilibrium with the chromium in the melt becomes at
least 0.9 atm, so a higher effect is obtained.
[0088] It is possible to perform the reduction of the first step
under atmospheric pressure at first and then perform it under a
vacuum of a pressure of at least 400 Torr (53 kPa). If adopting
vacuum refining in the latter half of the first step, compared with
the case of refining the same region under atmospheric pressure, it
is possible to hold the P.sub.CO low even when reducing the ratio
of mixture of inert gas or blowing only oxygen gas not using inert
gas at all and perform refining preventing oxidation of chromium.
As the timing for shifting from atmospheric pressure to a vacuum,
it is preferable to shift in the region of the concentration of
carbon of 0.8 to 0.5%. This is because below the concentration of
carbon, addition of a means for reducing the P.sub.CO so that the
P.sub.CO at equilibrium with the chromium in the melt becomes not
more than 1 atm enables more efficient decarburization. The reason
for making the pressure at least 400 Torr (53 kPa) is that if in
the region of concentration of carbon of the first step, the
content of carbon becomes high, so it is possible to obtain a
sufficiently excellent oxygen efficiency for decarburization even
under high pressure. Further, in the carbon region, it is important
to secure the amount of blown gas and secure a high refining
efficiency, but if using the same vacuum suction unit, the higher
the pressure, the greater the exhaust gas suction capacity and the
greater the amount of blown gas that can be obtained. Together with
this, a high pressure enables generation of dust and splashing of
the fine particles of metal produced from the melt surface in the
vacuum refining vessel to be suppressed even with the same gas blow
rate.
[0089] Regarding the degree of vacuum in each step, vacuum oxygen
decarburization is possible while controlling the vacuum to the
target degree of vacuum by the later explained control. Further,
there may be a plurality of target degrees of vacuum controlled in
each step.
[0090] While the extent of the effect becomes smaller compared with
the second step, in the first step as well, the higher the gas blow
rate from the bottom blowing, the greater the agitation force of
the melt and the higher the level the oxygen efficiency for
decarburization can be held at, so it is preferable to make the
rate at least 0.4 Nm.sup.3/min per ton melt. Further, the higher
the blow rate, the higher the oxygen supply rate obtained and the
shorter the refining time can be made.
[0091] It is also possible to perform the vacuum refining from the
start of the first step. For example, when there is extra leeway in
the production capacity and the refining time can be extended,
vacuum refining is performed from the start of the first step. Due
to this, the supply rate of the oxygen falls and refining time
becomes longer, but it becomes possible to hold the oxygen
efficiency for decarburization at a high level in the refining as a
whole. For example, it becomes possible to secure an oxygen
efficiency for decarburization of the refining as a whole of at
least 90%. Along with this, it becomes possible to keep use of
expensive dilution gas to a minimum.
[0092] Regarding the step after the second step, that is, the third
step, the inside of the vessel is evacuated to 250 Torr (33 kPa)
and gas blown in. The more the concentration of carbon in the melt
falls, the lower the optimal pressure in the vessel for obtaining a
high oxygen efficiency for decarburization, so in the third step
where decarburization proceeds, it is preferable to employ a
pressure lower than the second step. Along with this, the lower the
concentration of carbon, the greater the effect of melt agitation
on the decarburization reaction. With the same gas blow rate, the
lower the pressure inside the vessel, the larger the expansion of
the gas and the greater the melt agitation force, so the pressure
is preferably made lower than the second step.
[0093] In the third step, it is preferable to successively evacuate
in the vessel step by step along with the decline in the
concentration of carbon in the melt. It is further preferable to
successively evacuate the inside the vessel to a pressure inside
the vessel at the final stage of the decarburization refining of
not more than 50 Torr (7 kPa). In the region of low concentration
of carbon, along with the drop in the concentration of carbon, the
P.sub.CO at equilibrium with the chromium in the melt rapidly
falls. For example, at a carbon of 0.2%, the equilibrium P.sub.CO
is about 0.3 atm, but at a carbon of 0.1%, it becomes not more than
0.1 atm. If evacuating the vessel step by step corresponding to
this, it is possible to stably hold the oxygen efficiency for
decarburization at a high level.
[0094] In the third step, the concentration of carbon sufficiently
falls, so the blown gas may be made a mixed gas not containing
oxygen gas or only an inert gas. Further, when supplying a mixed
gas of oxygen gas and an inert gas as the blown gas, it is
preferable to gradually reduce the ratio of the oxygen gas in the
mixed gas along with the decline in concentration of carbon in the
melt. Compared with when the blown gas is just an inert gas, when
suitably mixing in oxygen gas, efficient decarburization can be
performed after securing the rate of supply of oxygen, so it is
possible to shorten the refining time. Further, along with the drop
in the concentration of carbon, the P.sub.CO at equilibrium with
the chromium in the melt rapidly falls, so if reducing the ratio of
oxygen gas of the blown gas, efficient decarburization becomes
possible. Further, there are cases where the refining is performed
while making the blown gas only inert gas in the final stage of the
third stage. Further, it is possible to charge ferrosilicon
immediately before or after making the blown gas an inert gas so as
to reduce the chromic acid in the slag on the melt and improve the
yield of chromium (chromium) or other valuable metals.
[0095] As explained above, the lower the concentration of carbon,
the greater the effect of the melt agitation on the decarburization
reaction. The third step evacuates the vessel more than the second
step, but the rate of the blown gas is preferably made at least 0.4
Nm.sup.3/min per ton melt as well. Note that if the rate of blown
gas becomes too large, a large amount of splash will be generated
and will hinder operation, so it is preferable to make the rate not
more than 1.0 Nm.sup.3/min per ton melt.
[0096] Note that when supplying bottom blown gas inside the
refining vessel, generally a double tuyere is used. With a double
tuyere, the refining gas is passed through an inner tube and the
cooling gas through an outer tube. Even when blowing in oxygen gas
alone in the present invention, the outer tube is supplied with a
small amount of a cooling gas such as nitrogen or argon or propane
or another hydrocarbon gas or a mixed gas of the same. Further, the
gas mixed with the oxygen (O.sub.2) may be argon or another inert
gas, N.sub.2, CO, or CO.sub.2 alone or in a mixture.
[0097] In the vacuum refining method of the present invention,
compared with the conventional vacuum refining method, the amount
of blown gas is increased, so it becomes necessary to consider a
vacuum exhaust unit for evacuating the inside the refining vessel.
An increase in the amount of heat generation due to the increase in
the amount of exhaust gas can be dealt with by increasing number of
the gas coolers 8 installed in the exhaust pipe 7 between the
exhaust hood 3 and the vacuum exhaust unit (steam ejector 10 or
water pump 11) shown in FIG. 1(a) or the cooling capacity per unit.
Further, an increase in the amount of dust generation due to the
increase in the amount of exhaust gas can be dealt with by
increasing number of the bag filters 9 installed in the exhaust
pipe between the exhaust hood 3 and the vacuum exhaust unit or the
dust treatment capacity per unit. In the present invention, as a
result of making the pressure inside the refining vessel in the
second step higher than in the past, the amount of dust generation
is reduced, so even when increasing the bag filters, the minimum
increase is enough.
[0098] Further, in the present invention, when refining an
ultra-low carbon chrome melt, the pressure in the vessel is
restored to at least 400 Torr (53 kPa) after the first vacuum
refining up to the third step. By restoring the pressure in this
way and then performing the second vacuum refining and making the
gas blow rate of the second vacuum refining at least 0.4
Nm.sup.3/min per ton melt, it is possible to greatly improve the
oxygen efficiency of decarburization in the ultra-low carbon
region. If producing ultra-low carbon chromium steel with a
concentration of carbon of not more than 0.01% in the first-stage
vacuum refining as in the past, it is necessary to continue the
vacuum refining for at least 20 minutes, while if restoring the
pressure in the middle of the vacuum refining for two-stage
evacuation as in the present invention, it becomes possible to
shorten the total time of the vacuum refining by about 10 minutes
and produce similar ultra-low carbon steel.
[0099] When the concentration of carbon falls to a predetermined
concentration, the refining under atmospheric pressure is
suspended, the exhaust hood 3 is attached to the refining vessel 1,
and the vacuum refining is started. In the process of reduction of
the degree of vacuum at the time of start of the vacuum refining
from atmospheric pressure, a rapid decarburization reaction
proceeds even without the supply of oxygen gas. An amount of oxygen
at equilibrium with the CO gas partial pressure of the atmosphere
dissolves in the melt. By evacuating the vessel, the CO gas partial
pressure of the atmosphere falls, so the oxygen which cannot
dissolve bonds with the carbon in the melt resulting in the
reaction. This is called "natural decarburization". The inventors
conducted various experiments and found quantitatively that amount
of natural decarburization does not greatly depend on the melt
composition, melt temperature, evacuation, or other conditions and
is about 0.05%.
[0100] The reason why decarburization in the ultra-low carbon
region is promoted by restoring the pressure in the middle of the
first vacuum refining and making the gas blow rate of the second
vacuum refining at least 0.4 Nm.sup.3/min per ton melt is not
necessarily clear, but it is believed that under strong agitation
by the bottom blown gas, the above-mentioned effect of natural
decarburization is obtained even in the region where the
concentration of carbon falls. That is, it is believed that by
restoring pressure in the middle of the vacuum refining, the
concentration of oxygen dissolving in the melt increases and that
by again evacuating the vessel, a decarburization reaction easily
arises in the process of decline in the concentration of
dissolvable oxygen.
[0101] As the timing of restoration of pressure, if restoring
pressure when the concentration of carbon falls to 0.05 to 0.12 wt
%, the effect of the present invention can be obtained. As
explained above, the amount of natural decarburization occurring
when evacuating the vessel is about 0.05%. It is sufficient to
decarburize the melt to the concentration of carbon at the time of
restoration of pressure minus this amount in the second vacuum
refining. If the concentration of carbon at the time of restoration
of pressure exceeds 0.12%, the amount of decarburization at the
second vacuum refining increases and a sufficient effect can no
longer be obtained. As set forth in the above (9) of the present
invention, it is possible to obtain the most preferable effect if
restoring the pressure after decarburizing the melt to a
concentration of carbon in the melt of not more than 0.08 wt % in
the first vacuum refining.
[0102] The gas blow rate in the second vacuum refining is made at
least 0.4 Nm.sup.3/min per ton melt. For example, even if restoring
the pressure in the middle of the vacuum refining, with a gas blow
rate in the second vacuum refining of about 0.3 Nm.sup.3/min per
ton melt of a level like the past, the vacuum refining time for
producing the ultra-low carbon steel can only be shortened by about
1 to 3 minutes compared with the conventional one-stage vacuum
refining. Further, even if making the gas blow rate in the
first-stage vacuum refining at least 0.4 Nm.sup.3/min per ton melt
in the same way as in the present invention, it is only possible to
obtain a very slight shortening of the vacuum refining time. More
preferable results can be obtained if making the gas blow rate in
the second vacuum refining at least 0.5 Nm.sup.3/min per ton melt.
The concentration of carbon after natural decarburization in the
second vacuum refining is not more than 0.05%. The decarburization
reaction becomes completely regulated by the diffusion of carbon.
In promoting progress in decarburization, the gas blow rate becomes
an important factor. In the present invention, the inventors
discovered that the rate is at least 0.4 Nm.sup.3/min per ton
melt.
[0103] At the time of the start of the second vacuum refining, the
concentration of carbon is reduced to not more than 0.1% or so, so
the pressure inside the vessel is made a pressure of not more than
200 Torr (25 kPa) to suppress the oxidation of chromium and secure
a high oxygen efficiency for decarburization. Further, as set forth
in (10) of the present invention, the pressure inside the vessel at
the second vacuum refining is preferably made not more than 100
Torr (13 kPA). This is because the lower the pressure in the
vessel, the lower the concentration of oxygen dissolving in the
melt and because with the same gas supply rate, the agitation force
due to expansion of the gas becomes larger and therefore the
decarburization rate becomes higher. To enjoy these effects, it is
effective to make the pressure not more than 100 Torr (13 kPa).
More preferably the pressure inside the vessel in the second vacuum
refining is made not more than 50 Torr (7 kPa).
[0104] The gas blown in the second vacuum refining may be made a
mixed gas of oxygen gas and an inert gas. In the second vacuum
refining, the concentration of carbon falls, so to suppress
oxidation of chromium and obtain a high oxygen efficiency for
decarburization, it is not possible to make the ratio of the oxygen
gas that high. As set forth in the above (10) of the present
invention, the ratio of the oxygen gas in gas blown in the second
vacuum refining is preferably made not more than 30%. If the ratio
of the oxygen gas exceeds 30%, the amount of oxygen used for the
oxidation of the chromium in the melt rapidly increases. Over half
of the oxygen gas blown in is used for oxidation of the chromium,
so the ratio is preferably made not more than 30%. More preferably,
the ratio of the oxygen gas may be made about 10%.
[0105] Next, the refining apparatus according to the present
invention will be explained by the drawings.
[0106] A conceptual view of the exhaust gas treatment equipment of
the present invention is shown in FIG. 4. The exhaust gas 15
produced in the vacuum refining furnace 1 passes through the
water-cooled duct 13 and is cooled by an exhaust gas cooler 16
connected there. Next, it passes through the duct 14, is cleaned of
dust by the dust collector 9, passes through the multiple-stage
ejector-type vacuum exhaust unit 10, is further sucked in by the
water-sealed type vacuum pump 11, and is discharged into the
atmosphere.
[0107] Here, the degree of vacuum of any of the vacuum meter 17 in
the furnace, the vacuum meter 18 after the exhaust gas cooler, the
vacuum meter 19 after the dust collector, and the vacuum meter 20
after the multiple-stage ejector type vacuum exhaust unit is
measured and the pressure signal input to the control unit 21. Part
of the exhaust gas is returned to the front of the vacuum pump 11
while adjusting the opening degree of the vacuum control use
pressure adjustment valve 22. Due to this, it becomes possible to
control the inside of the vacuum refining vessel or the inside of
the ducts to a predetermined target degree of vacuum. In
controlling the degree of vacuum, it is possible to freely select
which signal of the vacuum meters to use according to the stage of
refining.
[0108] The level of degree of vacuum controlled to depends on the
amount of splashing of metal from the vacuum refining vessel and
the amount of oxidation of chromium in the melt. In general, if the
degree of vacuum becomes better (the pressure value becomes lower),
the carbon in the melt will be preferentially oxidized and the
amount of oxidation of the chromium will be reduced. However, the
amount of metal and slag splashed from the vacuum refining vessel
will increase. That is, from the region of low chromium oxidation
loss, it is better to increase the degree of vacuum, but from the
region of low metal and slag, it is better to reduce the degree of
vacuum. Therefore, considering the two, there is an optimal range
of the degree of vacuum controlled to. Further, the amount of
oxidation of the chromium in the melt and the amount of splash of
the metal and slag also depend on the amount of carbon in the
melt.
[0109] Next, the method of use of this apparatus will be explained
based on FIG. 4.
[0110] Before starting the vacuum refining, a vacuum valve 23 at
the front of the vacuum exhaust unit is closed and the vacuum
exhaust equipment side, including the ejectors and the water-sealed
type vacuum pump, and the vacuum refining vessel side, including
the exhaust gas cooler or the dust collector, are separated by the
vacuum valve 23. Here, the inside of the vacuum equipment side is
controlled in degree of vacuum to a target 98 Torr (13 kPa) based
on the signal of the vacuum meter 20. (This is called "operation
prevacuum treatment".)
[0111] The vacuum pump 11 controls the degree of vacuum while
setting the above degree of vacuum since when the degree of vacuum
becomes 51 to 61 Torr (7 to 8 kPa), the water rapidly evaporates
and cavitation is caused. In the past, when reaching below 61 Torr
(8 kPa), a cavitation prevention valve was used to relieve the
pressure and adjust the degree of vacuum, but the increase in the
frequency of operation of the prevention valve caused the problem
of leaks of the valve body. However, due to the present invention,
the frequency of operation of the prevention valve is sharply
reduced and there is no longer any leakage from the valve body.
Accordingly, the degree of vacuum is controlled to a range of 61
Torr (8 kPa) or more.
[0112] Further, when then equalizing pressure with the atmospheric
pressure refining vessel side, it is preferable that the degree of
vacuum of the prevacuum treatment be as high a degree of vacuum as
possible to suppress a drop in the degree of vacuum. Accordingly,
the range of control of the degree of vacuum of the prevacuum
treatment was made 61 to 205 Torr (8 to 27 kPa) in consideration of
the controllability of the vacuum control use pressure adjustment
valve 22.
[0113] After the end of preparations for treatment at the refining
vessel side, the inside of the furnace starts to be evacuated.
Simultaneously with the start of treatment, the vacuum valve 14 is
opened, the vacuum exhaust equipment side and the vacuum refining
vessel side are made the same degree of vacuum, then the passage as
a whole is quickly made a high vacuum by the vacuum exhaust
unit.
[0114] When starting the vacuum treatment and evacuating the
passage as a whole, it is desirable to quickly close the vacuum
control use pressure adjustment valve 22 and raise the degree of
vacuum. However, before opening the vacuum valve 23, the pressure
adjustment valve 22 becomes close to fully opened by control of the
degree of vacuum. For example, with control of the degree of vacuum
based on feedback control by the signal of the vacuum meter 17
inside the vessel, it is difficult to quickly close the pressure
control valve in opening degree. Therefore, by forcibly fixing the
opening degree of the pressure adjustment valve to not more than
20%, preferably to fully closed, at the same time as the signal for
starting the vacuum and eliminating the return of the exhaust gas
after the vacuum pump, it becomes possible to quickly increase the
degree of vacuum. The effect of raising the degree of vacuum of (a)
of FIG. 5 is obtained. Here, from the general valve characteristics
of the pressure adjustment valve 22, if the opening degree becomes
not more than 20%, it becomes close to fully closed and has the
characteristic of blocking fluid.
[0115] To shorten the treatment time, it is desirable to start the
oxygen decarburization as quickly as possible after the start of
vacuum. However, a large amount of CO gas is produced
simultaneously with blowing oxygen. If oxygen remains in the vacuum
refining vessel or the vacuum ducts, it will react with the
produced CO gas and give rise to the danger of combustion and
explosion. Therefore, it is necessary to quickly reduce the
concentration of oxygen in the vacuum refining vessel and vacuum
ducts to below the explosion limit. As the method for this, it is
effective to blow into the vacuum refining furnace a large amount
of inert gas, not containing oxygen, or nitrogen or a mixed gas of
the same. However, if not blowing in a dilution gas in the state
after raising the degree of vacuum, a large amount of dilution gas
becomes necessary. The concentration of oxygen in the exhaust gas
becoming the explosion limit of CO was found as a result of
experiments by the inventors to be from over 7 vol % to not more
than 9 vol %. Accordingly, the concentration of oxygen in the
exhaust gas is made not more than 7 vol %.
[0116] When oxygen decarburizing a melt in a vacuum refining
vessel, there is the danger of the CO gas produced in the above way
causing violent splashing of the metal and slag from the melt and
boiling where the metal is splashed rapidly. Therefore, it is
necessary to quickly lower the degree of vacuum after starting to
blow oxygen and control the vacuum to a degree of vacuum able to
avoid trouble in operation. Therefore, the vacuum control use
pressure control valve 22 is opened to return the exhaust gas from
the rear to the front of the vacuum pump to lower the degree of
vacuum, but before the start of blowing oxygen, control of the
degree of vacuum results in the vacuum control use pressure
adjustment valve 22 becoming close to fully closed. With an
automatic mode, it is difficult to rapidly open the vacuum control
use pressure control valve 22 in opening degree. Therefore, by
forcibly fixing the opening degree of the vacuum control use
pressure adjustment valve 22 to at least 80% simultaneously with
the signal for the start of blowing oxygen and increasing the
return of the exhaust gas after the vacuum pump to the upper limit
of the capacity of the adjustment valve, it becomes possible to
quickly lower the degree of vacuum. If making the opening degree at
least 80% from the general valve characteristics of a pressure
adjustment valve, a flow rate of close to the fully open state
flows, so here the opening degree was made at least 80%.
[0117] In the embodiment of FIG. 5, by fixing the opening degree of
the pressure adjustment valve 22 to 100% for 50 seconds after the
start of blowing oxygen to the inside of the refining vessel as
shown in (c), it was possible to quickly return the degree of
vacuum once raised to 152 Torr (20 kPa) to 300 Torr (40 kPa) in
control. The degree of vacuum controlled to differs depending on
the carbon concentration in the melt and the oxygen blow rate.
Research of the inventors found that a range of 60 to 403 Torr (8
to 53 kPa) is suitable. Further, the time for fixing the vacuum
control use pressure adjustment valve 22 to at least 80% after the
start of blowing oxygen is determined by the degree of vacuum to be
controlled to and the internal volume to be made a vacuum from the
vacuum refining vessel to the vacuum exhaust unit. Experience of
the inventors found that 30 seconds to 120 seconds was the optimal
range. Accordingly, by fixing the opening degree of the vacuum
control use pressure adjustment valve 22 to at least 80% for a
predetermined time after the start of blowing oxygen to the inside
of the refining vessel, it is possible to quickly control the
degree of vacuum to a degree of vacuum of 60 to 403 Torr (8 to 53
kPa).
[0118] When vacuum oxygen decarburizing melt in the above way, it
is necessary to lower the degree of vacuum to a certain extent
(raise the pressure) for the oxygen decarburization to avoid
splashing of the metal and slag and boiling. However, there is a
suitable degree of vacuum determined by the carbon concentration in
the melt and the oxygen blow rate. The lower the carbon
concentration or the lower the oxygen blow rate, the more the
danger of splashing or boiling of the metal can be avoided. On the
other hand, the drop in the carbon concentration in the melt causes
the oxidation loss of the iron and chromium to increase, so making
the degree of vacuum rise as much as possible is preferable
metallurgically for suppression of such oxidation loss. Therefore,
the degree of vacuum is controlled so that when the carbon
concentration of the melt is high, the degree of vacuum is lowered,
while when the carbon concentration becomes low, the degree of
vacuum is relatively raised. By this, it is possible to
simultaneously satisfy the requirements of avoidance of upward
boiling and boiling of the metal and reduction of the oxidation
loss of the iron and chromium.
[0119] As embodiments of the present invention, control was
performed by a degree of vacuum of 300 Torr (40 kPa) for a carbon
concentration in the melt, by weight percent, of 0.60 to 0.40%, by
a degree of vacuum of 205 Torr (27 kPa) for a carbon concentration
in the melt of 0.40 to 0.25%, and by a degree of vacuum of 100 Torr
(13 kPa) for a carbon concentration in the melt of 0.25 to 0.20%.
These levels of degree of vacuum differ depending on the type of
the steel being refined, the oxygen blow rate, the type and
condition of the refining vessel, and other operating conditions
and have to be determined so as to meet with local conditions.
Further, successively reducing the oxygen blow rate, like the
degree of vacuum controlled to, in accordance with the reduction in
the carbon concentration in the melt is also effective
operationally and metallurgically. The present invention has
control of the degree of vacuum based on this as its scope. It is
founded on successively controlling the degree of vacuum to the
high vacuum side by the fall in the carbon concentration in the
melt.
[0120] In the control of the degree of vacuum, in the method of
successively switching the degree of vacuum to be controlled to a
high vacuum along with a drop in the carbon concentration in the
melt, it is preferable to switch to the higher vacuum quickly.
Right before switching the degree of vacuum, however, experience
shows that the drop in the flow rate of the exhaust gas causes the
pressure adjustment valve 22 to close to fully close. With an
automatic mode, it is difficult to rapidly close the pressure
control valve in opening degree right after switching to a high
vacuum. Therefore, at the same time as the switching signal to the
higher vacuum, the opening degree of the pressure adjustment valve
22 is forcibly fixed to 0% to 20% and held for 60 seconds. The
results are shown in (d) of FIG. 5. Due to this, exhaust gas no
longer returns after the vacuum pump and the degree of vacuum can
be quickly improved. However, here, "0%" means completely closing
the pressure control valve 22. From the general valve
characteristics of the pressure adjustment valve 22, when the
opening degree becomes less than 20%, the valve becomes close to
fully closed and has the characteristic of shutting off the fluid.
Therefore, the opening degree was made not more than 20%. Further,
when switching the degree of vacuum to the high vacuum side, the
time for fixing the opening degree of the vacuum control use
pressure adjustment valve 22 to not more than 20% is determined by
degree of vacuum to be controlled to and the inside volume etc. to
be made a vacuum from the vacuum refining vessel to the vacuum
exhaust unit. It is learned from experience that 30 seconds to 120
seconds is the optimum range.
[0121] The secondary materials, alloy iron, etc. are sometimes
added to the vacuum refining vessel during control of the degree of
vacuum. In this case, the secondary material, alloy iron, etc. to
be added are stocked in advance in an intermediate hopper and are
added to the vessel after making the intermediate hopper a degree
of vacuum substantially the same as the inside of the furnace.
Therefore, there should be almost effect on the flow rate of the
exhaust gas at the time of addition. If however the secondary
materials to be added include quicklime, gas components are
produced such as the residual CO.sub.2 in the quicklime or a sharp
gas producing reaction is caused in the vessel due to the other
alloys, secondary materials, etc. The gas produced here causes the
flow rate of the exhaust gas to rapidly increase, so the opening
degree of the pressure adjustment valve can no longer keep up and a
rapid deterioration in the degree of vacuum (rise in pressure) is
caused. Therefore, for 40 seconds after addition of the alloy,
secondary materials, etc. inside the vessel, the opening degree of
the pressure adjustment valve is fixed to 0% to positively suck in
the exhaust gas. Due to this, the deterioration in the degree of
vacuum due to the rapid increase in the flow rate of exhaust gas
can be suppressed as shown in (e) of FIG. 5. However, here, "0%"
means completely closing the pressure control valve.
[0122] From the general valve characteristics of the pressure
adjustment valve 22, when the opening degree becomes less than 20%,
the valve becomes close to fully closed and has the characteristic
of shutting off the fluid. Therefore, the pressure adjustment valve
22 is adjusted to return up to 10% of the flow of the exhaust gas
to the upstream side of the water-sealed type vacuum pump 11 so as
to improve the degree of vacuum inside the vacuum refining vessel
quickly. If the flow rate of the returned exhaust gas exceeds 10%,
however, the degree of vacuum will not be quickly improved, so this
is made not more than 10%.
[0123] Further, the time for adjusting the opening degree of the
pressure adjustment valve 22 for control of the degree of vacuum
after addition of the alloy, secondary materials, etc. in the
vessel and returning 10% of the flow rate of the exhaust gas is
determined by the degree of vacuum to be controlled to, the
capacity of the alloy addition hopper, the degree of vacuum inside
the hopper, and the inside volume to be made a vacuum from the
vacuum refining vessel to the vacuum exhaust unit. It is learned
from experience that 30 seconds to 90 seconds is the optimum
range.
[0124] The secondary materials, alloy iron, etc. added to the
vacuum refining vessel normally have a cooling effect on the melt,
so the melt temperature falls. Further, since addition is
intermittent, the amounts of addition become certain considerable
sizes and the melt temperature is temporarily greatly cooled. When
the melt temperature falls, the oxygen efficiency for
decarburization deteriorates metallurgically and the oxidation loss
of the iron, chrome, etc. becomes larger. To suppress this, it is
effective to raise the degree of vacuum and raise the oxygen
efficiency for decarburization at the timing when the temperature
temporarily drops. Therefore, even after the temporary increase in
the flow rate of the exhaust gas quiets down after the addition of
the secondary materials, alloy iron, etc. to the vacuum refining
vessel, the opening degree of the pressure adjustment valve 22
continues to be fixed at 0% for 120 seconds so as to hold the
degree of vacuum at a higher vacuum. Due to this, it becomes
possible to suppress a drop in the decarburization reaction
efficiency due to the drop in the melt temperature caused by the
addition of the secondary materials and alloy. However, here, "0%"
means completely closing the pressure control valve. From the
general valve characteristics of the pressure adjustment valve 22,
when the opening degree becomes less than 20%, the valve becomes
close to fully closed and has the characteristic of shutting off
the fluid. Therefore, the opening degree of the pressure adjustment
valve 22 for control of the degree of vacuum is made 0 to 20%.
Further, the time for making the opening degree of the pressure
adjustment valve 22 for control of the degree of vacuum after
addition of the alloy, secondary materials, etc. to the vessel less
than 20% is determined by the degree of vacuum to be controlled to,
the amount of addition of alloy, the carbon concentration in the
melt, the concentrations of copper, nickel, and other alloy
components in the melt, and the inside volume to be made a vacuum
from the vacuum refining vessel to the vacuum exhaust unit. It is
learned that 90 seconds to 240 seconds is the optimum range.
[0125] FIG. 6 and FIG. 7 schematically show one embodiment of a
seal unit of the present invention. When vacuum decarburizing a
melt in the vacuum refining vessel 1, the top of the furnace 1 is
covered by a vacuum cover 30, while a middle cover 31 is arranged
for preventing splashing of the metal and slag at the top of the
space below the vacuum cover 30. However, the center of the middle
cover 31 is formed with a large opening for adding the alloy and
secondary materials. Normally, the upwardly blown metal directly
reaches the alloy and secondary material addition port provided at
the vacuum cover 30.
[0126] Therefore, in the present invention, a dummy lance 33 is
provided as an integral structure with the valve body at the bottom
of the bottom seal valve 34. Further, in the present invention, the
inner walls of the alloy and secondary material addition port 40
are provided with a seal hole 37 for blowing seal gas (nitrogen) to
the side walls of the dummy lance 33. The narrower the clearance
between the side walls of the dummy lance 33 and the inner walls of
the alloy and secondary material addition port 40, the greater the
seal effect, but it is necessary to set the extent of the clearance
while considering the lateral shaking at the time of elevation or
descent of the bottom seal valve 34 and dummy lance 33 and
unavoidable deposition of some metal. For example, it is preferable
to set a clearance of 10 to 20 mm.
[0127] The bottom seal valve 34 and the dummy lance 33 normally are
connected to an elevator unit arranged at the top (not shown in
FIG. 6 and FIG. 7) and are raised or lowered through pneumatic
pressure, oil pressure, or a winch through a sieve. If it were
possible to keep the lateral shaking at the time of elevation or
descent by the elevator unit smaller, it would be possible to
further narrow the clearance between the side walls of the dummy
lance 33 and the inside walls of the alloy and secondary material
addition port 40 and enhance the seal effect.
[0128] To avoid interference with the alloy and secondary materials
at the time of charging the alloy and secondary materials when
elevating or lowering the bottom seal valve 34 provided with the
dummy lance 33, the elevation stroke has to be made longer. That
is, it is necessary to make it longer than the conventional
elevation stroke by the amount of the height of the dummy lance
33.
[0129] Further, the space above the vacuum refining vessel 1
normally has a conveyor, hopper, or other equipment and apparatuses
for conveying, charging, and storing the alloy, secondary
materials, etc., a vacuum cover or vacuum duct for evacuating the
vacuum refining vessel, and an elevator unit, ancillary units, etc.
for the same arranged in it, so forms an extremely crowded space.
Therefore, it is difficult to arrange an elevator unit with a long
stroke there.
[0130] Therefore, in the present invention, as a means to deal with
this, a pair of elevator units 36 (for example, air cylinders or
hydraulic cylinders) are arranged at the two sides of the alloy and
secondary material charging chute, a rod linked with the bottom
seal valve is connected with the top of the connection bar of the
elevator units, and this is pushed upward by the pair of elevator
units 36 so as to raise or lower the valve body (bottom seal valve
and dummy lance). Due to this means, it becomes possible to
effectively use the crowded space above the vacuum refining vessel
1 and extend the elevation stroke of the bottom seal valve 34 with
the dummy lance 33. In the present invention, the dummy lance 33
will not interfere with the alloy and secondary materials at the
time of charging the alloy and secondary materials. On the other
hand, when there is some leeway in the upper space, it is possible
not to make the bottom seal valve and dummy lance an integral
structure and to arrange the bottom seal valve in the intermediate
vacuum hopper and set the dummy lance alone at the alloy and
secondary material addition port. However, in this case, it is
possible to maintain smooth charging of the alloy and sealability
by raising and lowering the two linked together.
[0131] Further, in the present invention, to further raise the seal
effect, the seal hole 37 for blowing seal gas (mainly nitrogen) to
the dummy lance 33 is provided at the inside walls of the alloy and
secondary material addition port 40.
[0132] The flow rate of the seal gas can be suitably controlled by
a flow adjustment valve (not shown) in accordance with the refining
conditions. In the period from the early to middle phase of the
decarburization where the concentration in the melt is high and the
oxygen blow rate is large, the splashing of the metal and slag is
violent, so the flow rate of the seal gas is made larger. In the
period from the middle to end phase of the decarburization where
the splashing of the metal and slag is small, the flow rate of the
seal gas is reduced. The low flow rate region of the seal gas at
the end phase of the decarburization also contributes to
improvement of the degree of vacuum in the furnace, so this
advantageously promotes the metallurgical reaction and
simultaneously is effective for reduction of the concentration of
nitrogen in the melt.
[0133] Further, at the time of addition of the alloy and secondary
materials, it is preferable to reduce the flow rate of the seal gas
so that the alloy and secondary materials flow smoothly to the
inside of the furnace. At this time, there is a concern that the
metal and slag will enter the alloy and secondary material addition
port 40 and deposit on the inside walls, but the alloy and
secondary materials simultaneously pass through the addition port
40, so the entry of the metal and slag is not a problem at all.
[0134] On the other hand, the method of blowing seal gas includes,
in addition to the above method, the method of introducing the gas
from the outside through a dummy lance and rod of the bottom seal
valve and blowing it out from a plurality of holes provided around
the dummy lance to the inside walls of the alloy addition port 40.
At the top of the space below the vacuum cover, the middle cover 31
is arranged to prevent splashing of the metal and slag, but the
middle cover 31 is cooled by the inert gas (mainly nitrogen).
[0135] In the present invention, it is possible to utilize the
above inert gas as the seal gas to be blown from the seal hole 37
toward the dummy lance 33. Normally, the gas cooling the metal core
of the middle cover 31 is sent in the opposite direction as the
supply route and discharged into the atmosphere, but the gas is
high in temperature and the noise at the time of discharge of gas
becomes a problem, so this has to be handled by complicated
equipment and in the end the capital costs are slashed.
[0136] Further, in the present invention, it is possible to jointly
use a supply source for the gas for cooling the metal core of the
middle cover 31 and the seal gas blown from the seal hole (both
mainly nitrogen), so it is possible to achieve a reduction of the
gas cost.
[0137] Further, the gas (nitrogen) used for cooling the metal core
of the middle cover 31 becomes high in gas temperature, so even if
using the same amount as seal gas, the flow rate of the gas when
discharged from the nozzle of the seal hole and passing through the
clearance between the inside walls of the alloy and secondary
material addition port 40 and the dummy lance 33 will become
larger. As a result, entry of the metal and slag can be prevented
more and the seal effect becomes larger.
[0138] When not using a middle cover 31, the seal gas is blown
directly into the alloy addition port 40, but the method of laying
a pipe to the inside of the high temperature exhaust gas duct for
heat exchange, raising the seal gas temperature, and blowing the
gas to the alloy addition port 40 so as to obtain the effect of a
higher gas temperature and higher flow rate is also included in the
present invention.
[0139] As the seal gas, mainly nitrogen is used, but the gas need
only be inert. In addition to nitrogen, it is possible to use
argon, CO.sub.2, steam etc. alone. Further, it is possible to use a
mixture of these gases.
[0140] The dummy lance is exposed to a high temperature, so it is
preferable to make part of it out of refractories. Further, it may
be cooled by water cooling, air cooling, etc. These methods are
also included in the present invention.
[0141] Next, a water leakage detection unit in the refining
apparatus of the present invention will be explained. The exhaust
gas 15 produced in the vacuum refining furnace 1 passes through the
water cooling duct 13, is sent to the gas cooler 16 connected to
it, and is cooled there. After this, it passes from the gas cooler
16 through the duct 14, is sent to a dry type dust collector 9,
then is further sent through the duct 14 to the vacuum exhaust unit
10, then is discharged to the atmosphere.
[0142] Here, by branching the exhaust gas suction conduit 24 for
the humidity meter and analysis meter from a stage after the dust
collector 9, part of the exhaust gas is branched off and introduced
to the humidity meter 25. As a result, the humidity of the exhaust
gas is measured at the humidity meter 25, but the exhaust gas
analysis meter is also arranged at that position. The exhaust gas
analysis meter is provided after the dust collector 9, but may also
be provided after the gas cooler 16. Further, the analysis meter
provided jointly here may be located at the same location in some
cases, but may also be located separately from the humidity meter
after the vacuum exhaust unit 10 or after the dust collector 9 in
other cases.
[0143] The analysis meter is provided jointly so as to
simultaneously measure at least one of the concentration or partial
pressure of the CO, CO.sub.2, O.sub.2, H.sub.2, or other gas when
measuring the humidity of the exhaust gas. These analysis values
are used to obtain a grasp of the state of progress of the reaction
in the vacuum refining vessel or metallurgical furnace and used as
operation guidance for blowing gas into the metallurgical furnace,
charging the secondary materials and cooling material, etc. or used
as information for judgment of the end of the metallurgical
operation. Further, the measured value of the humidity meter may be
utilized not only as information for judgment of water leakage, but
also as information for judgment of the state of the reaction
inside the vessel or inside the furnace.
[0144] Regarding the method of use of the apparatus, in treatment
of the exhaust gas of the vacuum refining vessel 1, the high
temperature exhaust gas produced is cooled by providing a gas
cooler 16 in the middle of the ducts or water-cooling the middle
part of the ducts. With the system of this means, the relative
humidity of the exhaust gas is continually measured and monitored
after the dust collector. For example, assume that during vacuum
refining, the water pipes of the gas cooler 16 crack and cooling
water sprays out into the exhaust gas. In this case, the water
leakage is evaporated by the high temperature exhaust gas and the
steam partial pressure of the exhaust gas rises, so the humidity
meter 25 provided after later can detect the rise of the relative
humidity. That is, the case where there is no water leakage inside
the exhaust gas passage and a high humidity continues for a certain
time with respect to the relative humidity of the exhaust gas in
the normal state is judged as meaning the occurrence of water
leakage and action is taken for the equipment and operation. Note
that the invention is not limited to detection of just humidity. It
is also possible to detect the steam partial pressure.
[0145] As a specific example of the measures for the equipment and
operation, the necessary action for the repair work of the water
leakage location, for example, when separating the metallurgical
furnace and exhaust ducts or providing a bypass channel, changing
the path to the bypass side, is taken immediately after detection
of water leakage. Quick repair work for a water leakage location is
important. Early detection of water leakage will enable the repair
locations to be kept minor in most cases and enable the repair to
be finished easily in a short time. Further, in some cases, it is
possible to only issue a warning and suitably stop the operation of
the equipment.
[0146] Normally, when separating part of the exhaust gas and
measuring the humidity in the exhaust gas or analyzing and
measuring the gas, the exhaust gas in the duct is sucked in by the
suction pump and exhaust gas for analysis is directly supplied to
the analysis meter. Accordingly, a single suction pump is enough.
However, when measuring the humidity of the exhaust gas under a
vacuum or analyzing and measuring the gas, two suction pumps have
to be provided. The reasons for this will be explained below. When
sucking in exhaust gas under a vacuum, the gas supplied to the
analysis unit becomes a pressure corresponding to atmospheric
pressure, so the absolute flow rate of the exhaust gas sucked from
the vacuum by the same suction pump (flow rate of gas converted to
standard state) will fluctuate greatly according to the degree of
vacuum. That is, the absolute flow rate of the suction exhaust gas
will become considerably small at the time of a high vacuum
compared with the time of low vacuum. Accordingly, when using the
same suction pump, the flow rate of gas supplied to the humidity
meter or gas analysis and measurement will fluctuate greatly
depending on the degree of vacuum. On the other hand, to maintain
the measurement precision of the humidity measuring unit or gas
analyzer, the fluctuation in the flow rate of gas supplied to these
meters must be avoided. As a means for this, two suction pumps are
provided.
[0147] Note that the steam partial pressure of the exhaust gas
during vacuum refining rises due to reasons other than water
leakage of the equipment in some cases. The vacuum refining vessel
is charged with the alloy iron, cooling material, quicklime, and
other secondary materials during operation. These secondary
materials contain some moisture, so after charging, the steam
partial pressure in the exhaust gas temporarily rises. In
particular, the quicklime and other secondary materials easily
absorb moisture and have large moisture contents, so the amount of
generation of steam after charging remarkably rises. Accordingly,
if hastily judging a rise in relative humidity to mean water
leakage, the result will be erroneous detection. Therefore, the
inventors investigated in detail the behavior of the relative
humidity and as a result found the rise in humidity due to water
leakage becomes continuous. While there is some fluctuation, once
the humidity rises, it continues in a high state until the end of
the treatment. On the other hand, it was learned that the rise in
humidity due to the addition of the alloy, cooling material,
secondary materials, etc. into the refining vessel is short term
and when a certain time elapses after charging, the humidity falls
to the precharging level. Therefore, it is possible to utilize the
difference in behavior of the humidity level to judge if there is
water leakage from the cooling water system.
[0148] Further, as other reasons for the rise in humidity in the
exhaust gas other than water leakage, sometimes the gas fuel, solid
fuel, etc. containing hydrocarbons is burned for the purpose of
providing the heat source at the time of refining in the refining
vessel. For example, if burning LNG, LPG, kerosine, or another
hydrocarbon-based fuel in the vessel, a large amount of steam
enters the exhaust gas. However, the timing of supply and the
amount of supply become clear and the amount of entry of steam into
the exhaust gas can be estimated with relatively good precision.
Therefore, it is sufficiently possible to separate these effects
from the results of measurement of the partial pressure of steam in
the exhaust gas.
[0149] Specifically, to judge water leakage, it is sufficient to
find in advance and similarly set the continuous time of humidity
rise after charging from the advance settings of the rate of change
of humidity and the humidity levels thereof and the types and
amounts of the alloy, cooling material, secondary material, or
other components added to the inside of the vessel at those times,
further set in advance the humidity rise estimated from the time of
supply and amount of supply of the hydrocarbon-containing fuel, and
judge there is water leakage and automatically output a warning
signal or control signal when the settings of the continuous
humidity and time of humidity rise exceed the set humidity level
pattern) and time level.
[0150] Next, the gas ventilation unit and the water-sealed cover of
the return water storage tank in the refining apparatus of the
present invention will be explained.
[0151] The exhaust gas produced in the vacuum refining vessel 1 is
cooled by the exhaust gas cooler 16, cleaned by the dust collector
9, and introduced into the multiple-stage ejector type vacuum
exhaust unit. The multiple-stage vacuum exhaust unit performs first
suction by the No. 1 ejector, condenses the steam at the later No.
1 condenser and repeats the suction and steam condensation at the
No. 2 ejector and No. 2 condenser. Finally, the gas is sucked in by
the water-sealed type vacuum pump 11, then passes through the
separator tank and is discharged into the atmosphere.
[0152] Here, the condenser water from the nos. 1 and 2 condensers,
the sealing water from the water-sealed type vacuum pump, and the
cooling water from the separator tank pass through the pipe 26 and
are collected at the water storage tank constituted by the hot well
27. The cooling water of the hot well 27 is managed in level in the
tank by a water level meter. When rising a certain water level or
more, the return pump 28 is started up and the water is returned
from the hot well 27 to the cooling tower 29 through the return
pipe. The cooling water cooled at the cooling tower passes through
the feed pipe from the feed pump 30 and is sent to the condensers,
water-sealed pump, etc. As explained above, normally the feed pump
belongs to a different power source system than the return pump of
the hot well.
[0153] A detailed example of the area around the hot well 27 will
be shown schematically in FIG. 8. The hot well 27 is a concrete
structure for storing condenser water and sealing water of the
water-sealed pump etc. The top is clad by iron plate 52 at several
locations other than the concrete 50. The condenser water and the
cooling water flowing in from the water-sealed pump sealing water
pipe 26 are temporarily stored in water 53 stored in the hot well.
A supply pump is started up in accordance with the level of the
stored water at the left side of the figure to send the water
through the feed pipe 54 to the cooling tower 29.
[0154] In the prior art, as explained above, the condenser water
and sealing water of the water-sealed pump are accompanied with gas
bubbles containing CO so the CO concentration in the hot well
rises. Further, during the vacuum refining time, the flow rate of
the cooling water greatly changes. Along with this, the inside of
the hot well changes between positive pressure and negative
pressure. When becoming positive pressure, gas containing CO will
leak out from the joints of the top concrete and the iron plate
resulting in an extremely danger state of CO poisoning in the
surroundings.
[0155] Therefore, the practice is to provide an exhaust duct 55 and
ventilate the inside of the hot well by an exhaust blower 56 from
the exhaust outlet port. However, with just exhaust, the inside of
the hot well will become a negative pressure, the above-mentioned
seal parts will break, the clearance will expand, and air will be
sucked in. Normally, this is not a problem, but when the exhaust
blower stops due to a breakdown or blackout, CO will leak out from
the seal parts with the large clearance of the hot well resulting
in a dangerous situation.
[0156] Therefore, the inventors discovered that by evacuating gas
from the exhaust duct connected to the top of the hot well using a
suction means and guiding the ventilation gas from the suction duct
of the ventilation gas connected to the top of the hot well to the
inside of the return water storage tank, it is possible to reduce
the negative pressure inside the hot well and possible to almost
completely eliminate damage to seal parts between the concrete and
iron plate part.
[0157] Specifically, this is achieved by placing the exhaust duct
55 at the top of the hot well, evacuating the inside of the hot
well by an exhaust blower 56 serving as the suction means, placing
an exhaust gas duct 55-1 at the top of the hot well, causing air to
flow from the ventilation gas introduction port 57, and positively
ventilating the inside of the hot well. Here, as the ventilation
gas, it is preferable to use air from the viewpoint of cost and the
viewpoint of safety.
[0158] For example, a ventilating flow occurs in the tank as shown
by the flow 58 of the ventilation gas. The inside of the hot well
becomes an air atmosphere while the Co-containing gas is sucked
out. Further, the negative pressure inside the hot well becomes
smaller than the air flowing in from the duct. It becomes possible
to almost completely eliminate damage to the seals between the rear
concrete and the iron plate part.
[0159] Further, the inventors conducted a detailed survey on the
inner pressure inside a hot well in relation to vacuum refining
operations and as a result found that, as explained above, the
inside of a hot well not only becomes a negative pressure, but also
becomes a positive pressure or a negative pressure. For example, as
an operation before starting the vacuum operation, there is the
method of operation of closing the vacuum valve 23 of FIG. 4,
evacuating the space from the dust collector 9 to the vacuum pump
11 using the water-sealed type vacuum pump 11 in advance
(hereinafter referred to as "prevacuum treatment") and,
simultaneous with the start of operation, opening the vacuum valve
23 and evacuating the vacuum refining vessel side. At this time,
the degree of vacuum of the prevacuum treatment side rapidly
deteriorates (for example, falls from 1.33.times.10.sup.4 Pa to
6.67.times.10.sup.4 Pa), so the condenser water rapidly flows into
the hot well and, while for a short time, the gas inside the hot
well is compressed resulting in a large positive pressure. A survey
by the applicant revealed that 1.96.times.10.sup.3 Pa or more was
reached in many heats. Accordingly, even if sucking out gas by an
exhaust blower, at this timing, the inside of the hot well cannot
be held at a negative pressure. However, with the method of the
present invention, damage to the seal parts is small, so the amount
of leakage of the gas can be kept small. Further, the inside of the
hot well is positively replaced with air, so even if the inside of
the hot well becomes a positive pressure and a small amount of gas
leaks out, the CO gas contained can be kept to a level not causing
any health problems.
[0160] FIG. 9 illustrates the case of providing two water-sealed
covers 51 (side view).
[0161] The water-sealed cover 51 provided at the top of the hot
well is comprised of a double tube shaped cylindrical vessel having
an outer tube 59 and an inner tube 60 on an iron plate 52 of the
top of the hot well and a partition plate 61 able to be inserted in
between the inner and outer tubes. In accordance with need, a
weight 62 is used for increasing the weight of the partition cover.
However, since the weight of the partition cover alone is usually
not enough to withstand the gas pressure in the hot well, the
weight is normally preferably usually used.
[0162] Specifically, the inner tube 59 is lower than the outer tube
60. In the state with the partition cover 61 inserted, the
water-sealed cover sealing water is supplied from the outside of
the outer tube 60. Water is continuously supplied so that the
sealing water enters the inner tube side from the outer tube side
of the partition cover and overflows from the top end of the inner
tube, travels along the inside walls of the inner tube, and flows
into the hot well.
[0163] The sealing water height is designed so that at the time of
a normal vacuum refining operation, due to the sealing water, the
gas inside the hot well will not leak to the outside and the
sealing water will not be cut off even with pressure fluctuations
of positive pressure and negative pressure of the gas in the hot
well. If however the water inside the hot well overflows and is
filled to the inside of the water-sealed cover due to some reason
or another as explained above, the rise in the water level will
cause the partition cover 61 to be lifted up and water to leak to
the outside from the clearance between the inner and outer tubes.
Due to this, it is possible to greatly ease the force acting on the
connecting parts of the iron plate and concrete at the top of the
hot well and damage to the seal parts can be kept extremely
minor.
[0164] The size and number of the water-sealed covers placed in the
hot well may be suitably set in accordance with the total amount of
water of the condenser water supplied, the sealing water for the
water sealed pump, etc. For example, if the total amount of water
is 600 t/h or so, provision of two water-sealed covers of
cylindrical shapes of diameters of 500 mm for allowing the
overflowing water to escape to the outside may be mentioned as a
common sense embodiment.
[0165] Next, a preferable range of settings of the weight of the
partition cover will be explained. The pressure inside the hot
well, as explained above, sometimes reaches more than
1.96.times.10.sup.3 Pa. As pressure, this is small, but if this
pressure acts on an area of a certain size, it becomes a large
pressure. Explaining this using the above-mentioned water-sealed
cover, the cover is a cylindrical shape of a diameter of 500 mm, so
if a pressure of 1.96.times.10.sup.3 Pa acts on it, a force of
about 40 kg will act pushing up the partition cover 61. Therefore,
if the weight of the partition cover is 10 kg, it will be necessary
to adjust the weight by adding a weight of 30 to bring it over 40
kg. Accordingly, the weight of the cover portion of the
water-sealed cover constituted by the partition cover 61 and the
weight 62, if generalized, must satisfy the following formula
(1):
(W1+W2).times.9.8>P.times.S (1)
[0166] where,
[0167] W1: weight of partition cover (kg)
[0168] W2: weight of weight placed on partition cover (kg)
[0169] P: maximum gas pressure acting inside return water storage
tank (Pa)
[0170] S: maximum area of projection of inside surface of movable
partition cover on horizontal plane (m.sup.2)
[0171] In FIG. 9, W1+W2 is the total weight of the movable
partition cover 61 and the weight 62, P is the maximum gas pressure
in the hot well, and S is the horizontal projected area of the
partition cover 61.
[0172] Next, the preferable water-sealing height of the partition
cover will be explained. The pressure inside the hot well, as
explained above, sometimes reaches over 1.96.times.10.sup.3 Pa.
Therefore, it is necessary to secure a certain extent of
water-sealing height so that the water seal is not broken and gas
does not leak to the outside.
[0173] For example, in FIG. 9, if assuming that a pressure P of
1.96.times.10.sup.3 Pa acts on the inside, the outside water level
of the side walls of the partition cover 61 would become about 200
mm higher than the inside water wall. Therefore, the height H of
the outer tube 59 at the outside of the partition cover side walls
has to be over (200+L) mm considering the sealing water passage
height Lmm connecting the inside and outside of the partition
cover.
[0174] Accordingly, the water-sealing height of the water-sealed
cover, if generalized, must satisfy the following formula (2):
H-L>9.8.times.10.sup.3.times.P (2)
[0175] where,
[0176] H: height of outside outer tube of partition cover side wall
of water-sealed cover (m)
[0177] P: maximum gas pressure acting at inside of return water
storage tank (Pa)
[0178] L: height of sealing water passage between inner tube and
outer tube in water-sealed cover (m)
EXAMPLES
[0179] The present invention was applied when producing SUS304
stainless steel (8 wt % nickel and 18 wt % chromium) in a 60 ton
melt AOD furnace as shown in FIG. 1. In atmospheric pressure
refining, bottom blowing is performed in the state shown in FIG.
1(b) and, in accordance with need, top blowing is jointly used. In
vacuum refining, bottom blowing is performed after reducing the
pressure inside the refining vessel in the state shown in FIG.
1(a). The concentration of carbon in the melt at the time of start
of production is about 1.6%. Decarburization refining is performed
until a carbon concentration of 0.04%, then the pressure inside the
vessel is returned to atmospheric pressure while adding Fe--Si
alloy iron as a reducing agent for reducing the chromium oxidized
during the decarburization and only argon gas is blown in for
reduction. The steel was taken out to a ladle.
(Example 1)
[0180] The pattern shown in Table 1 was used for refining. The
first step was made atmospheric pressure refining with top and
bottom blowing and use of oxygen gas alone as the bottom blown gas.
A concentration of carbon of 0.5% to 0.15% was made the second
step. The pressure inside the vessel in the second step was made a
two-stage pressure of 350 Torr (46 kPa) and 250 Torr (33 kPa), the
blow rates of the bottom blown gas were made 0.9 and 0.5
Nm.sup.3/min, and the blown gas was made oxygen gas alone. The
third step was made decarburization refining until a concentration
of carbon of 0.04% at a pressure inside the vessel of a two-stage
pressure of 100 Torr (13 kPa) and 40 Torr (5 kPa) and a blow rate
of bottom blown gas held at 0.5 Nm.sup.3/min.
[0181] At the first step, the oxygen gas is blown in alone until
the concentration of carbon reaches 0.5%, so while the oxygen
efficiency for decarburization falls somewhat and the oxidation of
chromium increases, it was possible to slash the amount of use of
the expensive argon gas. Note that in the region of the
concentration of carbon of 0.7 to 0.5% of the first step, if making
the ratio of the bottom blown gas O.sub.2/argon not 1/0, but 4/1,
while the amount of use of the expensive argon gas increases, the
oxygen efficiency for decarburization at the carbon region can be
improved.
[0182] At the second step, the blow rate of the bottom blown gas
was raised to 0.9 to 0.5 Nm.sup.3/min so as to make the pressure
inside the vessel rise to 350 (46 kPa) to 250 Torr (33 kPa) while
maintaining the oxygen efficiency for decarburization. As a result,
it was possible to realize a reduction in the dust generation and a
shorter refining time.
[0183] At the third step as well, the pressure inside the vessel
was made 100 Torr (13 kPa) and the blow rate of bottom blown gas
was maintained at 0.5 Nm.sup.3/min under conditions of 40 Torr (5
kPa), whereby it was possible to maintain the high oxygen
efficiency for decarburization and contribute to a shorter refining
time.
1 TABLE 1 Decarburization phase First step Second step Third step
Step Atmospheric Reduction Class pressure Vacuum phase Pressure 760
350 250 100 40 760 (Torr) (100 kPa) (45 kPa) (33 kPa) (13 kPa) (5
kPa) (100 kPa) Blow rate of 1.4 1.2 0.9 0.5 0.5 0.5 0.5 bottom
blown gas (Nm.sup.3/min/t) O.sub.2/argon 1/0 1/0 1/0 1/0 1/5 0/1
0/1 ratio of bottom blown gas Blow rate of 1.4 1.0 0.0 0.0 0.0 0.0
0.0 top blown gas (Nm.sup.3/min/t) Carbon 1.6 0.7 0.5 0.25 0.15
0.08 0.04 concentration (%)
(Comparative Example 1)
[0184] The pattern shown in Table 2 was employed for refining.
Atmospheric pressure refining was performed for a concentration of
carbon of 1.6 to 0.4% and vacuum refining was performed for a
concentration of carbon of 0.4% and less. The refining conditions
at the atmospheric pressure refining were similar to those of the
first step of Example 1. The blow rate of the bottom blown gas in
the vacuum refining was made 0.3 Nm.sup.3/min like the conventional
level. Since the blow rate of the bottom blown gas was low, from
the viewpoint of preventing a drop in the oxygen efficiency for
decarburization and preventing an increase in the dust generation,
the pressure inside the vessel was made a maximum of 150 Torr (20
kPa).
[0185] Since the blow rate of the bottom blown gas was
overwhelmingly lower than the above example of the present
invention, the refining time was greatly prolonged. Compared with
Example 1, the vacuum refining time was about 2.5 times longer and
the overall refining time required was also about 1.8 times longer.
Therefore, continuous casting for continuously casting charges in a
continuous casting process became impossible.
2 TABLE 2 Decarburization phase First step Second step Third step
Step Atmospheric Reduction Class pressure Vacuum phase Pressure 760
150 150 100 40 760 (Torr) (100 kPa) (20 kPa) (20 kPa) (13 kPa) (5
kPa) (100 kPa) Blow rate of 1.4 1.2 0.3 0.3 0.3 0.3 0.5 bottom
blown gas (Nm.sup.3/min/t) O.sub.2/argon 1/0 1/0 1/0 1/0 1/5 0/1
0/1 ratio of bottom blown gas Blow rate of 1.4 1.0 0.0 0.0 0.0 0.0
0.0 top blown gas (Nm.sup.3/min/t) Carbon 1.6 0.7 0.4 0.25 0.15
0.08 0.04 concentration (%)
(Example 2)
[0186] In the first vacuum refining, the pressure was restored to
atmospheric pressure once when the decarburization progressed to a
concentration of carbon of 0.08%, then the vessel was again
evacuated and decarburization refining was performed until the
target concentration of carbon. The blow rate of the bottom blown
gas in the vacuum refining was made 0.5 Nm.sup.3/min per ton melt.
Table 3 shows the results of the present invention.
[0187] In a comparative example, vacuum refining was performed
continuously until reaching the target concentration of carbon. The
blow rate of the bottom blown gas in the vacuum refining was made
0.5 Nm.sup.3/min per ton melt in the same way as the example of the
present invention until a concentration of carbon of 0.15%. In a
region of concentration of carbon lower than this, it was made 0.3
Nm.sup.3/min per ton melt in the same way as in the past. Table 4
shows the results of the comparative example.
3 TABLE 3 Decarburization phase Atmospheric Restored Reduction
Class pressure Vacuum pressure Vacuum phase Pressure 760 200 150
760 100 50 760 (Torr) (100 kPa) (26 kPa) (20 kPa) (100 kPa) (13
kPa) (7 kPa) (100 kPa) Blow rate of 1.4 1.2 0.5 0.5 0.3 0.5 0.3
bottom blown gas (Nm.sup.3/min/t) O.sub.2 ratio of 100 100 100 100
0 20 0 bottom blown gas Blow rate of 1.4 1.0 0.0 0.0 0.0 0.0 0.0
top blown gas (Nm.sup.3/min/t) Treatment 10.5 11.5 3.0 5.0 5.0 time
(min) Carbon 1.6 0.7 0.5 0.25 0.08 0.01 concentration (%)
[0188]
4 TABLE 4 Decarburization phase Atmospheric Reduction Class
pressure Vacuum phase Pressure 760 200 150 100 40 760 (Torr) (100
kPa) (26 kPa) (20 kPa) (13 kPa) (5 kPa) (100 kPa) Blow rate of 1.4
1.2 0.5 0.5 0.3 0.3 0.3 bottom blown gas (Nm.sup.3/min/t) O.sub.2
ratio of 100 100 100 100 100 0 0 bottom blown gas Blow rate of 1.4
1.0 0.0 0.0 0.0 0.0 0.0 top blown gas (Nm.sup.3/min/t) Treatment
10.5 12.5 21.0 5.0 time (min) Carbon 1.6 0.7 0.5 0.25 0.15 0.08
0.01 concentration (%)
[0189] In the comparative example shown in Table 4, decarburization
refining from a concentration of carbon of 0.08% to 0.01% required
21 minutes of time. On the other hand, in the present invention
shown in Table 3, decarburization refining from a concentration of
carbon of 0.08% to 0.01% was completed in 8 minutes combining the
pressure restoration time and the evacuation time. That is, when
refining ultra-low carbon chromium-contained molten steel of a
concentration of carbon of a target 0.01%, when using the present
invention, it was possible to shorten the refining time by as much
as 13 minutes compared with the past.
[0190] As a result of being able to shorten the decarburization
refining time, it was possible to obtain the effects of slashing
the inert gas prime units, slashing the refractory prime units due
to prolongation of the lifetime of the refining vessel, slashing
the steam prime units used for the vacuum exhaust steam ejectors,
reducing the heat loss due to long refining, etc. Further, with the
method of the present invention, it is possible to produce even
ultra-low carbon steel without greatly prolonging the production
time compared with ordinary concentration of carbon steel and
therefore continuous casting in a continuous casting process became
possible.
Industrial Applicability
[0191] The present invention enables forcible agitation of melt in
the medium carbon region, in particular in the region of a carbon
concentration of 0.2 to 0.5%, in vacuum refining of
chromium-contained molten steel so as to enable vacuum refining of
a high oxygen efficiency for decarburization at a pressure of 250
to 400 Torr (33 to 53 kPa). As a result, generation of dust can be
suppressed and further an increase in the blow rate of the bottom
blown gas can be achieved, so the refining time can be
shortened.
[0192] The present invention further enables selection of a higher
pressure as the atmosphere in the refining vessel even in the
carbon region higher than the carbon region where the vacuum
operation of 250 to 400 Torr (33 to 53 kPa) is performed so as to
enable use of a vacuum operation rather than an atmospheric
pressure operation and thereby enable the amount of use of the
expensive inert gas to be slashed and the productivity to be
improved.
[0193] The present invention further enables adoption of two-stage
vacuum treatment comprising performing decarburization refining of
ultra-low carbon chromium-contained molten steel in an AOD vacuum
refining furnace where the pressure inside the vessel is made to
rise once in a state where the decarburization has progressed to a
certain extent in the refining under a vacuum, then again lowering
the pressure and resuming the refining under a vacuum and a great
increase in the blow rate of the bottom blown gas compared with the
past so as to realize a great improvement in the decarburization
rate in the low carbon region and a great reduction in the overall
decarburization refining time. As a result, it becomes possible to
inexpensively and easily produce ultra-low carbon chromium steel
having a concentration of carbon of not more than 0.01 wt %.
[0194] Further, the present invention establishes a vacuum exhaust
unit and control method enabling control of the degree of vacuum
inside a vacuum refining furnace or its ducts for oxygen
decarburization refining of a melt under a vacuum. The effects in
equipment and operation obtained due to this are as follows:
[0195] First, a shorter overall vacuum treatment time can be
achieved, the productivity can be improved, and the refractory
lifetime of the vacuum refining furnace can be improved.
[0196] Second, splashing of the metal and slag during the vacuum
oxygen refining, boiling of the metal, etc. can be effectively
prevented and prevention of blockage of the alloy addition port,
prevention of deposition of metal on the top cover, prevention of
blockage of the vacuum exhaust ducts, etc. can be achieved. Due to
this, the idling time of equipment is greatly shortened and
slashing of the maintenance costs and improvement of the operating
productivity can be achieved.
[0197] Further, the present invention enables sufficient sealing at
an alloy and secondary material addition port in the refining
process without trouble caused by splashing of the metal and slag,
so it is possible to greatly slash the prime units of the materials
and secondary materials, possible to shorten the operating time,
and possible to greatly reduce the operating costs.
[0198] Further, the present invention can measure and monitor the
humidity of exhaust gas so as to detect a small amount of water
leakage inside the exhaust gas passage and thereby detect water
leakage early and simultaneously strikingly improve the reliability
of detection of water leakage.
[0199] The present invention enables the provision of a method and
apparatus simply dealing with the issues in hot wells, that is, the
leakage of CO-containing gas from the hot well and suppression of
damage to equipment at the time of occurrence of overflow of
cooling water in the hot well.
* * * * *