Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROVND OF THE INVENTION
One type of metallurgical vessel known as the argon-oxygen
or AOD converter consists of a pivotable, generally pear-shaped
vessel having an upper opening. A plurality of tuyeres, gener-
ally two or three in number, extend radially through the side
wall of the vessel and they are spaced 40 to 60 apart, usually
in a horizontal position, 17 to 25 centimeters above the flat
vessel bottom. This places the inner ends of the tuyeres below
bath depth which is typically one meter.
In a typical conversion process, the AOD vessel is charged
with electric furnace hot metal containing between .8 and 1.5%
carbon. A mixture of oxygen and argon having a ratio of about
3 parts oxygen to 1 part argon is blown through the center
tuyere while argon or air is delivered through the outer tuyere
pipe as a coolant or shielding fluid. During the process
cycle, the ratio of argon and oxygen delivered to the center
tuyere pipe is reversed in programmed steps.
Argon-oxygen vessels are commonly supported on a trunnion
ring having radially extending trunnion pins supported on
bearings. One trunnion pin is also coupled to a drive mechanism
whereby the vessel may be tilted for pouring, deslagging and
sampling.
The argon-oxygen process suffers a disadvantage of rela-
tively high cost as a result of refractory and equipment wear
and high argon consumption. ~or example, the refractory sur-
rounding the tuyeres in AOD vessels erodes relatively rapidly
despite the use of expensive argon as a cooling fluid. Also,
as a result of the blowing pattern, AOD vessels have a tendency
to oscillate causing wear in the support bearings and drive
assembly.
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SUMMARY OF THE INVENTION
It is an object of the invention to provide a new and
improved metallurgical method.
A further object of the invention is to provide an argon-
oxygen metallurgical conversion method wherein there is a
significant cost saving over conventional argon-oxygen processes.
A further object of the invention is to provide an argon-
oxygen process wherein refractory erosion and drive system wear
is minimized.
These and other objects and advantages of the present
invention will become more apparent from the detailed descrip-
tion thereof taken with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The single drawing figure schematically illustrates a
metallurgical apparatus in which the method according to the
present invention may be practiced.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The system in which the method according to the invention
may be practiced consists of a bottom-blown metallurgical
vessel 10 and a system 11 for supplying process gases and other
materials.
The vessel 10 includes a metallic shell 12 and a refractory
lining 13. A conventional trunnion ring 14 is provided for
supporting the vessel 10 and has a trunnion pin 15 extending from
each of its opposite sides. The trunnion pins 15 are suitably
supported in a well-known manner on bearing structures tnot
shown) and are coupled to a suitable drive mechanism (not
shown) for tilting vessel 10 to each of a plurality of positions
as may be required during a process cycle. A smoke hood 16 may
be disposed above the opén, upper end of vessel 10 when the
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latter is in its vertical position as illustrated in the drawing
to prevent discharge of pollutants during operation of the
vessel.
The vessel 10 may have a removable refractory bottom 17
having a bottom plate 19 for supporting a plurality of tuyeres
21 which extend through openings 22 in refractory bottom 17.
The tuyeres 21 are preferably arranged in a symmetrical pattern
and each includes an inner tuyere pipe 21a and an outer tuyere
pipe 21b both of which are adapted to be connected to the gas
supply system 11. Inner tuyere pipe 21a defines a first tuyere
passage and pipe 21b is larger than and spaced from pipe 21a to
provide a second tuyere passage. Also affixed to the bottom
plate 19 is a lime distributor 22 connected to an oxygen input
pipe 23 and by manifold 24 to the inner tuyere pipes 21a. Each
of the outer tuyere pipes 21b are similarly connected by manifold
25 to a shielding fluid inlet pipe 26.
The gas and material supply system 11 includes a pair of
vessels 27 and 28 in which materials such as burnt lime, lime-
stone, iron oxide, carbon, fluorspar and other desulfurizing
agents may be stored. While only two vessels 27 and 28 are
shown, it will be understood that there may be as many pressure
vessels as there are types of powdered materials which are to
be injected into the molten metal within the vessel 10.
It will be appreciated that it is necessary to mix the
powdered materials from the vessels 27 and 28 with entraining
gas in a definite proportion. For this purpose, the bottom of
each vessel 27 and 28 is provided with a mixing device 29, the
details of which are not shown but which are well known in the
art. For example, the device 29 may be of a type which withdraws
powdered material from its associated vessel and injects it
into the gas stream. Each mixing device 29 may be operated by
a motive means 30 having a controller 31 responsive to input
signals from a control (not shown) as symbolized by arrows 32.
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The mixing devices 29 are connected to as many sources of
gas as they may be entrained. As, for example, dephosphorizing
agents may be stored in vessel 24 for being entrained in the
oxygen stream, while desulfurizing agents may be disposed in
vessel 26 for being entrained in the argon or nitrogen. The
output of mixing chamber 29 is connected by pipe 34 and valve
36 to the inlet pipe 23 of mixing chamber 22 and the outlet of
pressure vessel 27 is similarly connected thereto by pipe 38
and valve 40. Oxygen may be delivered from a source labeled 2
directly to inlet pipe 23 through pipes 40 and 42, valves 44
and 46, and flow controller 48. Similarly, pipe 50 and valves
52 and 54 connect the hydrocarbon shielding fluid source labeled
HmCn to the inlet pipe 26. Nitrogen from source labeled N2 may
be coupled to the inlet pipe 23 through pipes 56, 58, 60, 62
and 42 and valves 64, 66 and 46. Argon from source labeled Ar
may also be coupled to pipe 23 through pipes 42, 58, 60, 62,
68, flow controller 70 and valves 46, 66 and 72. The argon and
nitrogen sources may also be coupled to tank 27 through pipe 73
and valve 74 and to vessel 28 through valve 75 and pipe 76.
The first flow controller 48 includes any suitable means
for controlling gas flow rate such as a flow meter 78 interposed
in pipe 40 and connected to a flow controller 80 for controlling
a flow control valve 82 also connect~d into pipe 40~ Flow
meter 78 may be any well-]snown type of device which is operative
to produce an electrical output signal functionally related to
the gas flow rate in pipe 40. The controller 80 is electrically
coupled to flow meter 78 and is operative to provide an output
control signal to flow control valve 82 which is functionally
related to its received input signal and valve 82 is operative
to control the flow rate of gas in pipe 40 in relation to its
received signal.
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The argon flow controller assembly 70 similarly includes a
flow meter ~4, a controller 86 and a flow control valve 87
which are interconnected to each other and operative in a
manner similar to that discussed with respect to the flow
control assembly 75 and accordingly, the assembly 70 will not
be discussed in detail. The controllers 80 and 86 are adjustable
in relation to a received input signal so that the proportion
of argon and oxygen which may be delivered to inlet pipe 24 can
be adjusted. Controllers 80 and 86 are also coupled to the
respective flow meters 7~ and 84 for receiving signals func
tionally related to the actual flow rate. Controllers 80 and
86 are then operative to provide corrective signals so that the
desired gas flow ratios can be achieved. A flow controller
which may be employed for this purpose is Model 53-EL-3311BElB
manufactured by Fisher Porter Control corporationO
A gas ratio controller 89 is electrically connected to
controllers 80 and 86 for receiving signals functionally related
to the rate of oxygen and argon delivery and is operative to
provide corrective signals in relation with either a preset
; program or manual adjustments which may be provided by an
operator. In this manner, the ratio of argon to oxygen supplied
to the inlet pipe 26 may be controlled.
In the performance of the method according to the present
invention, vessel 10 initially received a metallic charge. The
specific charge, of course, depends upon the chemical balance
of the desired end product and the availability of materials.
For example, a solid charge such as scrap iron, scrap steel,
liquid pig iron, iron oxide, or iron bearing materials in other
solid form may be charged into the vessel after which a charge
of liquid pig iron is added. Alternately, solely a liquid
charge may be provided. Prior to the liquid charge, the solid
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charge may be heated by delivering a fuel such as propane,
natural gas or light oil to the outer tuyere pipes 21b and
oxygen to the inner tuyere pipes 21a. A nonoxidizing gas such
as nitrogen or argon may be delivered to the both tuyere pipes
after the preheating step and during the delivery of the molten
metal charge. It will be appreciated that the vessel will
normally be tilted to receive the various metallic charges and
will be returned to its upright position and beneath gas collect-
ing hood 16 before the oxygen is delivered. ~uring the periods
of vessel turn-up and turn-down, nonoxidizing gas such as
nitrogen or argon is delivered to each of the tuyere pipes to
prevent the backflow of molten metal.
After charging has been completed, the liquid metal charge
is blown with fluxes. For dephosphorization, this will normally
consist of lime entrained in the oxygen stream and delivered
through the inner tuyere pipe while hydrocarbon shielding fluid
through the outer pipe. On the other hand, if desulfurization
is required, lime is entrained in either argon or oxygen and
delivered to the center tuyere pipe while the same gas is
delivered through the outer tuyere pipe. After desulfurization
or dephosphorization, the main oxygen blow commences during
which time, oxygen or a mixture of oxygen and argon is delivered
to the center tuyere pipe and a hydrocarbon shielding fluid
delivered through the outer tuyere pipe.
In the production of stainless steel, the charge would
generally include chromium either in the form of chromium
containing scrap or chromium containing hot metal. In order to
avoid the oxidation of chromium, argon will normally be mixed
with the oxygen during the main oxygen blow and the proportion
of argon to oxygen will be increased during the main oxygen
blow. For example, the initial ratio of oxygen to argon will
be about 3 to 1 and this will be increased in increments by the
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flow controller ~9 which operates the controllers 48 and 70
such that the final ratio of argon to oxygen will be about 3 to
1. As those skilled in the art will appreciate, chromium
oxidation is prevented by the reduction of the carbon dioxide
partial pressure within the molten metal by the action of argon
dilution.
In the case of a low carbon steel or electric furnace
steels which are alloyed with silicon, a rapid increase in iron
oxidation noxmally occurs as carbon is oxidized duriny the main
oxygen blow. Iron oxidation is minimized, however, by the
gradual introduction of argon commencing at a point when the
carbon level falls to about .02%. Specifically, the proportion
of argon is gradually increased from about zero to fifty percent
by weight until the end of the main oxygen blow. During this
period, both in the case of stainless steel and carbon steels,
hydrocarbon shielding fluid will be delivered to the outer
tuyere pipe while oxygen and/or oxygen argon mixture is delivered
to the inner tuyere pipe.
After the completion of the main oxygen blow, the bath may
be purged of dissolved gases such as hydrogen and oxygen by an
argon purge during which argon is delivered through both the
inner and outer tuyere passages.
Because hydrocarbon shielding fluid is used as a cooling
medium during the oxygen and argon oxygen blowing periods, the
process is substantially cheaper than the AOD process wherein
argon is employed as a cooling medium. In addition, substan-
tially greater tuyere and refractory life as a result of the
use of hydrocarbon shielding fluid as opposed to argon as a
cooling medium. Further, by injecting the process gases upwardly
and in a symmetrical pattern, the tendency for vessel oscillation
is minimized thereby prolonging the life of the bearings and
drive mechanism.
In a typical example employing a 30-ton vessel with six
1/2" tuyeres, the charge might be electric furnace hot metal
containing about .8-1.5 carbon and about 8% chromium while a
final specification might be a carbon level of .025% and an
18.8% chromium level. Oxygen is delivered at a rate of about
67 normal cubic meters (Nm3) through the center tuyere would be
typical. As the carbon level is reduced, argon is introduced
into the oxygen stream at an increasing rate while the total
flow rata remains substantially the same until a ratio of argon
to oxygen of 3 to 1 by volume exists~
While only a few embodiments of the present invention have
been illustrated and described, it is not intended to be limited
thereby but only by the scope of the appended claims.
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