Note: Descriptions are shown in the official language in which they were submitted.
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BASIC OXYGEN FURNACE ARGON REFINING BLOW
The present invention relates to the manufacture
of steel and more particularly to an improvement in pneumatic
steel making processes wherein oxygen is introduced through
a lance and blown downwardly onto the surface of a bath to
effect refinement of the melt.
In the`basic oxygen process, substantially
pure oxygen is introduced from above the surface of the bath
in a basic-lined vessel. After the furnace is charged with
controlled amounts of scrap and molten iron, the furnace is
brought into an upright position and the oxygen lance is
lowered to a predetermined position above the surface of the
bath. Oxygen gas issues from the jet nozzle at high
velocity with the action of the oxygen jet being partially
chemical and partially physical. On striking the surface of
the liquid bath, the oxygen immediately starts reacting
leading to the formation of iron oxides with carbon monoxide
being evolved which gives rise to a vigorous boiling action
and accelerates refining metallurgical reactions.
The slag forming fluxes,which are chiefly bu~ned
lime, flu~rspar and millscale, are added in controlled amounts
after the oxygen introduction begins. These materials serve
to produce a slag of the desired basicity and fluidity.
There are several advantages of the basic oxygen
process in its flexibility in handling raw materials of many
types and compositions. The scraP, which is employed, mav be either heavy
or li`ght an~ the oxide charae, if used, mav ~e drv~eres, sinter, p~llets
or millscale. The process may be operated on any kind of
hot metal that can be used in the basic open hearth furnace.
3 ~n the manufacture of steel in the basic oxygen
furnace, a very large percentage of the heats are required to
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be finished at YerX low carbon values. The final car~ons are
frequently as low as 0.03% in order to meet the Q.Q5% ~aximum car-
bon aim in low alloy high strength steel, enamelling s-teels, deep
drawing steels and electrïcal steels~ In many grades of steel it
is advantageous to have a res-i`dual carbon content as low as 0.02%
to obtain certaïn desïred strength and other metallurgical pro-
perties.
A standard basic oxygen furnace charge is approx-
imately 7Q% hot metal and 30~ steel scrap and has an average
carbon content in the range of 2.5 to 3.0%. It is customary to
remove the excess carbon from the melt by oxidation employing
high purity oxygen introduced through a lance as the oxidizing
agent. ~nfortunately, as the carbon is oxidized there is a
simultaneous oxidation of iron. The lower the carbonis in the
melt, the higher is the percentage of iron oxidized to FeO. This
oxidized iron is permanently lost to the formed slag. In melts
of very low carbon i.e. 0.03%, the FeO in the slag may be as high
as 35.0%. As it is normal practice to carry between 250 and 300
lbs of slag per ton of metal, the iron losses are thus consider-
able. In addition, high FeO slags are very fluid and chemically active and
may result in severe attack on the furnace refractory lining.
Melts which are at low carbon levels and contain high slaglevels may have as high as 1000 ppm oxygen dissolved in the steel. This oxygen
is c~nx~ly removed from the steel by addition of aluminum. Apart fram the
high cost of this deoxidation practice, not all of the alumina is rem~ved
from the steel and may be a source of defects in the fmal product.
The above mentioned problems which are an in-
herent feature of the basic oxygen furnace steel making process
have to date been philosophically accepted by the steel industry
and it is~ an object of the present invention to reduce these
problems to a ~inimu~.
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109282B
It is a further object of the present invention to reduce the iron
loss to the slag during the refining process.
It is a further object of the present inven~ion to increase the viscosity
of the slag.
It is yet a further object of the present invention to provide an improved
process for the refining of steel.
Thus, in accordance with the present teachings, an improvement is provided
in the process for the manufacture of steel in a basic oxygen furnace wherein a
bath of metal is refined by the introduction of oxygen through a lance onto the
surface of the bath. The improvement which is provided comprises blowing the
bath with oxygen until the carbon content of the bath is about 0.10% above the
desired finished carbon content and discontinuing the oxygen introduction and
introducing argon through the lance until the desired carbon content of the
bath is achieved.
At the completion of the argon blow, the amount of oxygen remaining
dissolved in the molten steel is considerably below the level encountered when
the melt is blown in the usual normal manner with 100% oxygen only. The use
of the argon blow results in a lower level of non metallic deoxidation products
and thus a cleaner steel product is obtained. Ladle deoxidant recoveries, when
argon is employed following the oxygen blow, have also been found to be much
higher, as for example, the aluminum deoxidation efficiences are in the range
of 60 to 75%. This compares very favorable with 25 to 50% efficiences which is
experienced with heats which have been blown with oxygen only in melts containing
similar carbon, manganese and temperature levels.
The use of argon in completing the refining of the melt has also been
found to result in a significant increase in the viscosity of the slag.
During tapping of the melt, the much stiffer obtained slag is more readily
retained in the furnace and the exclusion of large volumes of iron oxide
bearing slag from the ladle is a contribut-
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lO~Z828
ing factor in improyed deoxidant effici.encx. It should als,o be
noted that increased s:lag vi.~cosity also acts to reduce the attack
on the furnace and ladle lini`ngs whi:ch results: in a significant
increase i`n the life of the refractory employed for such li.nings.
Although applicant does not wish to be confined
to any theoretical discuss:ion it is belîeved that the intimate
mixing of the slag containing FeO which.is produced during the
oxygen blow and the ~etal containing carbon would provide the
chemical reac~ion
FeO + C ~ Fe + CO.
The carbon monoxide escapes to atmosphere and the reduced metallic
iron reverts back to the metai bath. During the argon blow, the
free volume of the vessel is filled with argon gas and the partial
pressure of CO in contact with the bath would be very low. This
further accelerates the FeO + C reaction. In view of the turbul-
ence of the bath with ~he introduction of the argon through the
lance, the dissolved oxygen in the metal also reacts with the
carbon
C + 1/2 2 ~ CO.
In accordance with the present process, it is
desired to produce a steel having a finished carbon content of
0.04%. The steel is blown with oxygen in the basic oxygen furnace
in the normal manner until the carbon content of the bath is
approximately 0.14% or about 0.10% above the required finishing
carbon content. In place of oxygen the blow is now continued
employing 100% argon gas. To create the emulsion between the
slag and the metal, the flow rate of the argon gas is substantially
the same as that which is employed for oxygen in conventional
basic oxygen furnace processes and typically a 120 ton capacity
vessel would employ a flow rate in the order of about 7,3Q0 cu.
ft/min.
28Z8
The following Example and data is being provided
to illustrate the present invention and is not intended to limit
the scope of the concept.
EXAMPLEI
Oxygen was; b.lown downwardly onto the 12Q ton heats.unti.l th.e
resi.dual carbon co~tent of the melt reacfied approximately Q.2 to
0.25% (or as indicated in ~able 1~. The oxygen flow was stopped
and a sample of each. melt was taken (Sample I~. Following this,
pure argon was substi.tuted for oxygen and blown onto the melt for
two periods of 2 mins. each at a flow rate of between 3000 and
6000 cu.ft./min. Samples were taken after each period of treat-
ment with.the argon (Samples II and III). The resulting reduction
in the amounts- of carbon and dissolved oxygen remaining in the
melt is shown in Table 1.
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lO~Z828
As may readily be observed from the above data
the carbon content is effectively reduced to the desired levels
with no significant reduction in manganese. In addition, through
the use of the present teachings, some of the oxidized iron which
has been permanently lost to the slag is through the use of the
argon blow reduced to metallic iron and reverts back to the metal
bath--~urthermDre, the finishing blow employing argon also acts
to remove dissolved oxygen thereby reducing the oxidation practice
normally effected.
It will be obvious to those skilled in the art
that various modifications may be resorted to without departing
from the spirit of the invention.
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