Note: Descriptions are shown in the official language in which they were submitted.
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ThiS inventioll relates to the refining of pig iron, by
~ny of the bottom-blown pneumatic steelmaking processes, eg.
3essemer, SIP, Q-BOP. More particularly, this invention is directed
o a method for achieving more rapid and more e~ficient removal o~ nt
2~ when such processes are employed ~or decreasing the carbon conte
; 15 f a steel melt to a level below 0.2~, and generally below 0.1~. l
In the most widely employed pneumatic steelmaking process,¦
i he Basic Oxygen Process, oxygen is blown from above, through a
allce, so as to pierce through 'che overlying slag layer and pcnetra'~l
nto the iron melt. When it is desired to remove gaseous impurities ,
uch as nitrogen, ~rom a BOP refi~ed steel melt; the steel is teemed
~rom the BOP ~urnace into a ladle and inert gas ~lushing is employed
. ~or periods ranging from about 10 to 25 minutes. By contrast
: lvherewith, in the above noted bottom-blown processes, the oxygen is
. ~lown rrom a point below the top surrace Or the melt) through tuyere i
~ocatcd in the bottom or in the sides o~ the converter. With respec~
lo SIP or Q-BOP, a protective gasj generally a hy~rocarbon, is
, _ .. . . . . , .. , _ _ . _ _ _ . _ _ _ . .
employed to encase or surround the oxygen stream in order to
decrease the inordinately high wear which would occur at
the tuyeres and the converter entry area (i.e. the bottom in
the Q-BOP). One of the significant advantages of the bottom-
blown processes over the BOP, is their adaptability in per-
mitting inert-gas purging to be carried out in the steelmaking
vessel itself. Additionally, the Q-BOP in particular, provides
more effective and efficient degassing in a shorter period
of time, as a result of the comparatively higher gas-flow
la rates which may be employed. With respect to the efficiency
of such purging, a theoretical minimum amount of inert gas
(generally argon) is, of course, required for the removal
of a desired amount of N2 (see, for example, Kollman and
Preusch, Proceedings of the Electric Furnace Conference of
AIME, 1961, pp . 23-42). However, in actual practice many times
more argon is found to be required, because many of the
reactions involved are controlled by mass transfer in the liquid
phase and do not go to completion in a practical time period.
In view, thereof, it is a principal object of this
invention to provide a purging procedure, in which both the
amount of inert gas employed and the time required for effect-
ing the removal of a desired degrée of N2, may significantly
be decreased.
Other objects and advantages of this invention will
become more apparent from the following description when
read in conjunction with the appended claims and the drawing,
in which
The Figure is a graph illustrating the marked
difference in nitrogen removal rate between two steels of
3a differing carbon contents.
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Gaseous impurities, such as N2, are normally re-
moved by purging the steel with argon; such removal being
effected by the lowering of the N2 partial pressure as a
result of the diluting effect of the argon. As noted above,
such purging is`conventionally accomplished only after the
steel melt has been decarburized to the desired extent. It
has now been found that the rate of nitrogen removal, at
high oxygen activities- (for exampLe S00 or even 300 ppm
oxygen) is controlled by a slow chemical reaction on the sur-
face of the molten iron. Thus, at such high oxygen activities,
the iron surface is essentially covered by a layer of adsorbed
oxygen which seriously retards the rate of N2 removal. It
has also been determined that the relative importance of this
retardation effect decreases with oxygen activity, so that
at low oxygen activitiés (for example less than lO0 ppm oxygen)
the overall rate of N2 lS controlled either by liquid phase
mass transfer or by the saturation'of the inert gas to the
equilibrium N2 pressure. In the latter instance (low oxygen
activity) liquid phase mass transfer is the dominant control
n at relatively high nitrogen levels (of the order of 0.01% N2)iwhile saturation control preva~ls at ver~ l~w n~trogen level~ (~0.002% N2),
In view of these findincJs as to the retardation effect o~
adsorbecl oxygen, it may readily b~ understood why the oxygen
blow itself, is not very efficient in effecting the removal
of N2 from the steel melt.
Since oxygen activity is inversely proportional
to carbon activity, it may be seen that the efficiency of inert
gas purging may significantly be enhanced by effecting such
purging at comparatively high carbon contents, i.e. in excess
of 0.3~, and more preferably in excess of 0.5% carbon. The
3a significant benefits resulting from purging at such higher
carbon levels is illustratc~d by the two curves of the figure.
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For example, an argon flush of 2000 ft3/min., performed for
two minutes in a 30-ton heat (whïch had previously been
decarburized to a carbon content of 0.05~) was only capable
of reducing the initial 0.005% nitrogen content to about
0.004~. By contrast, when the same flushing rate was per-
formed on a comparably sized heat prior to the time the
carbon content thereof was reduced to 0.5%, the nitrogen
content was reduced to nearly 0.001~ for the same two-minute
flush.
1~ While it should be understood that the invention is
applicable to all bottom-blown steelmaking processes, the
procedure for carrylng out the teachings thereof will be
described in its specific applicability to the Q-BOP process.
Initial blowing of the pig iron is performed as in conventional
Q-BOP practice. That is, a stream of generally commer~ially
pure oxygen is introduced into the melt through tuyeres located
in or near the converter bottom. The use of oxygen of such
purity, would normally result in extremely rapid wear of
both the tuyeres and the bottom itself. Therefore, each
oxygen stream is surrounded by an encasing or coolant gas
to slow down the violent reaction and thereby achieve sub-
stantially reduced wear. The ratio of oxygen to encasing
gas is desirably held within a critical range so as to per-
mit such wear to proceed in a slow and controlled manner.
Thus, during this initial blowing period there are basically
two different gas throughput rates which are of concern;
Ro the rate at which 2 is introduced, and Rp the rate at
which protective gases (eg. methane) are introduced. As shown
in U.S. patent 3,706,549, the disclosure of which is incor-
porated herein by reference, Ro ~ Rp. In view thereof,
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the average total rate RT may be defined as the sum of Ro
+ Rs, wherein RS iS the rate of introduction of all other
gases. For example, RS may equal Rp plUS RA, wherein RA
is the rate of introduction of argon or other inert
gas. It should be noted that RT is not necessarily constant,
but merely the average total rate of gas introduction. It
is necessary, however, that RT be maintained within a re-
; quisite throughput range of from about 75 NCF/min per ton
of steel to about 160 NCF/min per ton of steel being re-
fined. The minimum rate is dictated by the need
4a.
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to maintain suffi¢lent back pre9sure in the tuyeres, ln order to
revent molten metal ~rom plugging the tuyere openlngs, While rate~
igher than the above noted maxlmu~ would be deslrable ror shortenir g
the length Or the blow (thereby increasing productlon capablllty),
it has been ~ound that rates significantly higher than 160 NCF/min
per ton result in undesirable splashing and spitting above the
onverter,
For the initial portion of the blow, RA will generally be
zero or negligible, while Ro will be greater than o.8 RT Thereaft~ r,
or a steel containing an initial ievel of nitrogen in excess of th~t
~esired in the rinal steel product (l.e ~ 0.002% N, and generally
0.004~ N) the melt will be purged with an inert gas (eg. argon)
~or a time sufflcient to decrease the nitrogen content to the
esired level which generally will be less than 80%, and often
ess than 50~ of said initial level. Although purging may be
initiated at any time before the carbon content of the melt has
een reduced to 0,3%, it is preferable that such purging not be
nitiated until (i) after the silicon portion of the blow (so as to
nsure the achievement of desirablé temperature), but (ii) before
he melt has been decarburized to less than 0.5% carbon (to insure
esirably low oxygen activity). Purglng is preferably conducted by
erminating the oxygen blow and substituting an inert gas therefor;
¦that is Ro will be reduced to zero, while RA ,_ RT. Although less
~esirable, it is not essential, however, that Ro be reduced to zero
hus, the lnert gas may contain a small amount of oxygen, since such
~xygen will rapidly be converted to C0; resultlng in an effective
~as retention time in the bath in which the activity of oxygen will
evertheless be sufficlently low for the purpose hereof. Therefore,
urging within the scope contemplated by this invention may be
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conducted wherein the purge gas rate RA is at least 0.8 RT.
As noted above, purging is conducted for a time sufficient
to decrease the nitrogen content of the bath to the desired
level. Depending both on the amount of nitrogen to be re-
moved and the magnitude of RA, times varying from about one-
half to two minutes will ordinarily be sufficient. Sub-
sequently, decarburization is then resumed by increasing
the rate of oxygen introduction so that Ro is again at
least 0.8 RT; this resumed oxygen blow continuing until
bath ca~bon content is reduced to the desired final level,
generally less than 0.1% carbon.
6.
