Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND
This invention relates, in general, to refining of
steel, and more particularly, to an improvement in the basic
oxygen process, i.e. a process wherein molten steel contained
in a vessel is refined by top blowing oxygen into the melt.
More specifically, this invention is directed to a method for
increasing the nitrogen content of steels made by the basic
oxygen process.
The manufacture of steel by the basic oxygen pro-
cess, also referred to as the BOP or BOF process, is wellknown in the art. When low-carbon steel is made by this
process, its diss01ved nitrogen content is subject to wide
variations. eer~ain grades of steels have specifications
requiring a low nitrogen content, and methods have been
devised to achieve this, for example, as disclosed by
Glassman's U.S. Patent No. 3,769,000 and Pihlb]ad's U.S.
Patent No. 3,307,937.
On the other hand, some steels have specifications
calling for high nitrogen contents, hence methods of increas-
ing the nitrogen content have also been devised. Many ofthese methods require a separate nitrogen addition step
after completion of the conventional decarburization step.
Examples of such methods are shown in U.S. Patent No.
2,865,736, U.S. Patent No. 3,402,756, and U.S. Patent Nos.
3,356,493; 3,322,530; and 3,230,075.
U.S. Patent No. 3,180,726 disclcses blowing the
~elt with pure nitrogen or nitrogen together with an inert
2.
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gas and adding a stabilizing or ixing element after the
blow. This method, however, does not allow the steel maker
to adjust the nitrogen content of the steel independently,
i.e. without altering the composition of the melt by adding
other alloying elements. All of the above methods have the
disadvantage o~ requiring an additional step after oxygen
refining, thereby increasing the time required to make each
heat of steel. Furthermore, some require the addition of
other elements in order to fix the nitrogen in the melt,
while others require complex teeming apparatus.
Still another approach used by the prior art has
been to increase the nitrogen content of the melt during
decarburization. U.S. Patent No. 3,754,894 shows how the
nitrogen content of steels may be increased during decar-
burization provided, however, that the decarburizing gases
and nitrogen are injected from beneath the surface of the
bath. This method is not easily combined with the BOF pro-
cess wherein all gases are injected rom above the surface
of the melt. If nitrogen gas is merely blown into a basic
oxygen vessel from above the bath during conventional decar-
burization practice, the results will not be reproducible,
and the aim nitrogen content will be achieved onLy fortuitously
It has not been possible, prior to the present
invention, to make steels having high nitrogen contents by
the basic oxygen process without performing a separate step
after decarburization and/or adding elements to the melt in
addition to nitrogen.
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OBJECTS ^
Accordingly, it is an object of the present inven-
tion to make basic-oxygen steel having a nitrogen content
that is reproducible and greater than that made by conven-
tional BOP techniques.
It is another object of the present invention to
make basic-oxygen steel having a high nitrogen content with-
out requiring a nitrogen addition step after decarburization.
It is a further object of the present invention to
increase the nitrogen content of basic oxygen steel without
adding other alloying or stabilizing elements either ~ogether
with or in addition to the nitrogen.
SUMMARY OF THE INVENTION
The above and other objects, which will readily
be apparent to those skilled in the art, are achieved by
the present invention, which comprises: in a process for
the production of steel by decarburizing a ferrous melt con-
tained in a vessel by blowing oxygen into the melt from above
the surface thereof, the improvement comprising producing
steel having a high nitrogen content, within a preselected
range, by:
(a) introducing a nitrogen-rich gas into
the melt, simultaneously with said oxygen during the
latter portion of the decarburization step, in an
amount equal to at least 100 NCF ~f nitrogen gas per ton
of molten metal (3 NM3 of nitrogen gas per metric ton) and
in such a manner as to promote intensive interaction ~f the
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nitrogen-rich gas with the molten metal,
(b) refining the mel~ with the oxygen and
nitrogen-rich gas by blowing the melt to a manganese content
at least as low as 0.10 percent, and
(c) maintaining the partial pressure of nitro-
gen in the vessel head-space a~ least equal to and preferably
higher than that calculated to be in equilibrium with the
aim dissolved nitrogen content of the melt at 1600C (2912F).
The term "steels having a high nitrogen content"
or "high-nitrogen steels" is used to mean steels having
nitrogen contents of at least about 0.01 percent, or 100 ppm.
The term "aim nitrogen content" is intended to mean
the final nitrogen content t~e steel maker is attempting to
achieve.
The term "nitrogen-rich gas" as used in the present
specification and claims is intended to mean a gas containing
sufficient nitrogen to satisfy the equilibrium requirement
of step (c)~ above. The preferred nitrogen-rich gases are
industrially pure nitrogen or air. Gaseous nitrogen compounds
that liberate sufficient nitrogen upon reacting in a BOF
vessel, e.g. ammonia, may also be used.
The abbreviation "NCF" is used to mean normal cubic
foot of gas measured at 70F and 1 atmosphere pressure.
The abbreviation "NM3" is used to mean normal cubic
meter of gas measured at 0C and one atmosphere pressure.
DETAILED DESCRIPTION OF THE INVENTION
When decarburizing steel by top blowing, i.e. by
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blowing oxygen into a melt from above the melt's surface,
it is known that the nitrogen content of the melt first
decreases as bubbles of CO gas formed during decarburizing
sparge nitrogen from the melt. During the la~ter stages of
decarburization by the conventional BOF process, the CO
bubble generation rate decreases. This decrease is believed
to have at least three important effects. First, the de-
crease in CO generation rate allows more atmospheric nitro-
gen to infiltrate the vessel because of reduced off-gas
velocity at the mouth of the vessel. Second, some of this
atmospheric nitrogen becomes entrained in the oxygen being
blown into the melt and is ultimately absorbed. Third, the
decreased CO generation also results in a decreased nitrogen
sparging rate which further contributes to an increased final
nitrogen level. The relationship between these factors is
essentially uncontrollable, and therefore the final nitrogen
content is not reproducible and tends to vary from heat to
heat despite apparently identical decarburization conditions.
Furthermore, the final nitrogen content of steels refined by
the conventional basic oxygen process is usually lower than
that required by the specifications for "high nitrogen" grades
of steel. When this occurs renitrogenation is required.
The following describes the preferred practice of the present
invention for renitrogenation of basic-oxygen steel during
decarburization.
Nitrogen-rich gas must be introduced into the melt
simultaneously with the oxygen during the latter portion of
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the oxygen decarburization step. The preferred method of
accomplishing this is by introducing the nitrogen-rich gas
into the oxygen stream. This may be accomplished most simply
by installing an extra connection into the oxygen line that
feeds the oxygen lance, and piping a source of nitrogen-rich
gas to the extra connection. Of course other, more expensive,
methods may be used, as for example, a separate lance for the
nitrogen-rich gas, or the use of lances having separate
parallel passages for the oxygen and nitrogen-rich gas streams.
Such passages may be either concentric or adjacent to each
other within the same lance. An in-line mixer could also be
included in the lance. However, these more complex methods
offer no apparent advantage over the preferred method of
practicing the invention.
The flow rate of the nitrogen gas must be sufficient
to maintain a partial pressure of nitrogen in the head space
above the melt that is at least equal to, and preferably
greater than that which would be in equilibrium with the aim
dissolved nitrogen content in the molten metal.
The amount of nitrogen-rich gas introduced must be
at least equal to 100 NCF of nitrogen gas per ton of molten
metal (3 NM3 of nitrogen gas per metric ton) to achieve repro-
ducible results. The amount of nitrogen absorbed by the melt
increases with the amount of nitrogen introduced. However,
the amount of nitrogen absorbed will vary from BOP system to
BOP system. Once the relationship between amount of nitrogen
introduced and final nitrogen content is experimentally deter-
mined for a particular BOF system, and provided other variables
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are held constant, reproducible results can consistently be
attained by practice of the present invention, as long as the
prescribed minimum amount of nitrogen is injected into the melt
The oxygen and nitrogen-rich gas mixture must be
blown in to the melt in a manner that promotes intensive inter-
action between the nitrogen-rich gas and the bath. If this is
not implemented, then consistent results will not be obtained.
One means of accomplishing the intensive interaction
is to employ lance pressures significantly greater than those
normally used. Each BOP system has a normal oxygen blowing
pressure used during conventional decarburization. It is
believed that the normal oxygen lance blowing pressure in
most BOF shops is insufficient to accomplish the interaction
necessary to practice this invention. Substa~tially
increasing the lance pressure during nitrogen-rich gas addi-
tion will accomplish the desired result. For example, it
was found that in a 235 ton (213 metric ton) BOF vessel
equipped with a lance having four 1.75 inch (~.45 cm) diameter
ports, an increase in lance pressure from about 115 psig
(8.1 kg/cm2 gage) to about 150 psig (10.6 kg/cm2), i.e.
about a 30 percent increase in gage pressure, was sufficient
to generate the requisite interaction of the gas with the
melt. Note, however, that penetration of the gas je~ and
the resultant stirring action is not completely predictable
from vessel to vessel and can only be empirically determined.
The blowing pressures used ln some BOF s~ ps may not require
an increase in order to achieve the gas-melt interaction
required by the present invention.
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Another method of obtaining the required inten-
sive interaction is to blow the mixture of nitrogen-rich
gas and oxygen with the lance in lower positions than
normal. As with lance pressure, all BOP shops have normal
Lance positions for various stages of conventional o~ygen
decarburization. Typically the lance is gradually lowered
as decarburization proceeds. Conventional lance positions
may not produce sufficient interaction of the gas with the
melt to reproducibly renitrogenate the melt. This problem
can be corrected by moving the lance to a lower position
than normal during the latter stages of decarburization,
while introducing the nitrogen-rich gas.
Still another method of accomplishing the requisite
gas-melt interaction is to inject the nitrogen-rich gases
with nozzle velocities that are higher than normally used
in conventional BOF practice. Hence, in order to practice
the present invention, some BOF shops may have to increase
their lance gas velocities by using lances with smaller
diameter gas discharge nozzles.
~nother requirement for obtaining reproducible
results is that the manganese content of the melt be blown
to less than 0.10 percent during decarburization. The
manganese is merely an "indicator" reflecting ~he conditions
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within the melt necessary for reproducibility of ni~rogen
pick-up, and is not intended to infer a causal relationship
between manganese in the steel and nitrogen absorption. In
normal BOP practice, the manganese level is adjusted to final
specification subsequent to the decarburization with the
addition of various ferromanganese alloys. Hence, the pro-
cess is only minimally affected by consistently blowing to
less than 0.10 percent manganese during decarburization.
The following examples illustrate the preferred
practice of the invention.
EXAMPLES
Six 235 ton (213 metric ton) heats were refined by
top blowing with pure 2 in a BOP refining system in accord-
ance with standard BOP operating practices. Table I below
shows the values of the variables that were experimentally
manipulated and the results obtained. In each case, indus-
trially pure nitrogen was introduced admixed with oxygen via
the oxygen lance, beginning "t" minutes before the estimated
completion of the decarburization step. The value of "t"
varied from heat to heat as shown in the Tables I and II
below.
TAB~E I
Heat No. 1 2 _ _ 3
Oxygen Blow rate,
NCFM (NM3/min) 20,000 (530)18,000 (470) 20,000 (530
10 .
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TABLE I (Continued)
Heat No. 1 2 __ 3
Nitrogen Blow rate,
NCEM (NM3/min)3,400 (90)5,600 (150)3,000 (80)
t 9 approximate
duratiOn of N2
Blow, minutes 7 1/2 7 1/4 10
Amount of N2 per
ton NCF/ton
~NM~/metric ton) 109 (3.2) 173 (5.0) 128 (3.7)
Lance pressure
during N~ Blow,
PSig (Kg7Cm2) 150 (10.6) 152(10.7) 155 (10.9)
Melt Temperature at
Turndown F (C)2,975 (16J5) 2,840(1560) 2 ,950 (1621)
Melt Analysis at
Turndown, (%)
C 0.03 0.03 0.03
Mn 0.08 0.07 0.08
FeO (in slag)36.18 33.90 30.82
N 0.0153 0.017 0.0161
Aim Nitrogen
Content,(%) 0.014 0.016 0.016
Nitrogen Specification
Range, (%) 0.012-0.016 0.014-0.018 0.014-0.018
The first three heats shown in Table I, illustrate
the correct practice of this invention, adhering to all its
requirements, the main requirements being that:
(1) The requisite mixing intensity be obtained,
here by employing a lance pressure higher than normal during
the nitrogen introduction. The normal lance pressure for
the vessel is approximately 115 psig (8.1 kg/cm2)i
(2) At least 100 NCF (3 NM3) of nitrogen be
introduced per ton (metric ton) of steel, and
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(3) The manganese content be blown to 0.10~/o
or less.
It is to be noted that for each of these three
heats, the final nitrogen content was easily wqthin 10% of
the aim nitrogen content, and all were within ~he range of
acceptable nitrogen specification for the intended grade.
This kind of reproducibility has not been possible prior to
the present invention.
TABLE II
10 Heat No. 4 5 6
Oxygen Flow Rate,
NCFM (NM3/min)20,000 (530)14,000 (370)20,000 (530)
Nitrogen Blow Rate,
NCFM (NM3/min)5,600 ~150)5,000 (130) 4,000 (110)
t, approximate
duration of N2
Blow, minutes 4 1/2 5 4 1/2
Amount of N2 per
ton NCF/ton
(NM~/metric ton)109 (3.2) 105 (3.1) 78 (2.3)
Lance pressure during
N2 Blow, psig
(kg/cm2) 165 (11.6) 118 (8.3) 150 (10.6)
Melt Temperature at
Turndown, F (C)3,020 (1660)2,895 (1591)2,880 (1582)
Melt Analysis at
Turndown, (%)
C 0.03 0.05 0~03
Mn 0.12 0.09 0.09
FeO (in slag) 24.12 n.a. n.a.
N 0.0105 0.0110 0.0111
Aim Nitrogen
Conten~, (%) 0.014 0.016 0.014
Nitrogen Specification
Range, (%) 0.012-0.016 0.014-0.018 0.012-0.016
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Heats 4, 5, and 6, shown in Table II, illustra~e
the unsatisfactory results obtained when one of the three
requirements of the invention is not followed. The turndown
nitrogen contents of Heats 4, 5, and 6 fall significantly
short of the aim, i.e. by 40 to 50 percent, and lay outside
of the acceptable nitrogen specifications for the intended
grades. Heat 4 had proper gas-melt interaction, obtained
by increased lance pressure and the proper amount of nitrogen,
but the manganese content was not blown below 0.10%. For
Heat 5, all requirements of the invention were fulfilled,
except that the low lance pressure employed produced insuf-
ficient interaction between the melt and the nitrogen. An
insufficient amount of nitrogen was the only requirement
vioLated in Heat 6.
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