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| United States Patent |
4,042,378 |
| Ramelot |
August 16, 1977 |
Controlling pig iron refining
Abstract
Industrially pure oxygen is blown into molten pig iron in a converter by
means of at least one tuyere located below the upper surface of the molten
metal. A quantity (such as amplitude, frequency, acceleration, or speed)
characterizing movement of the converter is measured continuously while the
oxygen is blown in. An instant at which this quantity undergoes a sudden and
considerable decrease is detected. From that instant onwards, the amount of
oxygen necessary and sufficient to provide the steel with a desired quality is
blown in. The required amount of oxygen is determined from an empirical
relationship between the amount of oxygen blown in after the instant and either
the phosphorus content of the steel or the iron content of the slag.
| Inventors: |
Ramelot; Daniel Leon (Saint-Nicolas, BE)
|
| Assignee: |
Centre de Recherches Metallurgiques-Centrum
voor Research in de (Brussels, BE) |
| Appl. No.: |
682668 |
| Filed: |
May 3, 1976 |
Foreign Application Priority Data
| Current U.S. Class: |
75/375 |
| Intern'l Class: |
C21C 005/30 |
| Field of Search: |
75/60 |
References Cited [Referenced
By]
U.S. Patent Documents
Primary
Examiner: Rosenberg; P. D.
Claims
I claim:
1. In a pig iron refining process in which industrially
pure oxygen is blown into molten pig iron surmounted by a layer of slag in a
converter by means of at least one tuyere located below the upper surface of the
molten metal, in order to obtain steel of a desired quality, the improvement
comprising the steps of: continuously measuring movement of the converter in the
direction of an axis common to a pair of pivot pins on which the converter is
pivotably mounted while the oxygen is blown in; detecting an instant at which
the movement measurement undergoes a sudden and considerable decrease; and, from
that instant onwards, blowing in only that amount of oxygen, which is necessary
and sufficient to provide the steel with the desired quality at the end of the
refining operation, the amount of oxygen being determined from an empirical
relationship, determined from previous refining operations in the converter,
between the amount of oxygen blown in after the instant and at least one of the
phosphorus content of the steel and the iron content of the slag.
2. The
process of claim 1, in which the measurement is the amplitude of the movement.
3. The process of claim 1, in which the measurement is the frequency of
the movement.
4. The process of claim 1, in which the measurement is the
acceleration of the movement.
5. The process of claim 1, in which the
measurement is the speed of the movement.
6. The process of claim 1, in
which the movement is in a frequency range of up to 50 Hertz.
7. The
process of claim 6, in which the frequency range is between 10 and 25 Hertz.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a
pig iron refining process in which industrially pure oxygen is blown in by means
of at least one tuyere located below the surface of the molten metal in a
converter.
In known installations the tuyere is generally in the bottom
of the converter and comprises two co-axial ducts, the inner duct being arranged
to convey oxygen whereas the space between the inner duct and the outer duct is
designed to convey a fluid, preferably an endothermic-decomposition fluid such
as a hydrocarbon liquid or gas, to protect the tuyere and the refractory
materials against the action of the oxygen.
Input and output parameters
are monitored and input parameters are controlled in order to obtain steel of a
desired quality. Fixed input parameters include the temperature and composition
of the pig iron; variable input parameters include oxygen flow rate and the
amount and timing of additions to the molten metal; output parameters include
the temperature and composition of the waste gases. The desired quality may be,
for example, the content of at least one component of the steel.
A
number of methods of controlling the refining operation when blowing oxygen
through the bottom of the converter have already been suggested, for example
methods based on material and heat balances and a mathematical model for
calculating charges.
Theoretically, such methods permit the desired
composition and temperature for the refined metal to be regularly obtained upon
turn-down of the converter. However, a certain scatter of the results thus
obtained occurs in practice. The dispersion may be due to a lack of information
on the charged materials, for example on the weight or the precise composition
of the pig iron, the scrap, or the lime. To remedy such a situation, premature
turn-down of the converter is generally provided, the iron content and the
temperature of the slag are rapidly measured, and blowing is re-started with or
without additions and is continued for the time necessary to obtain the desired
composition and temperature of the steel.
The results thus obtained have
been found to be satisfactory. Scatter, which generally occurs when no premature
turn-down is practiced, is substantially reduced. However, premature turn-down
of the converter has the disadvantage of prolonging the duration of the refining
operation.
SUMMARY OF THE INVENTION
An object of the present
invention is to provide a process which allows this disadvantage to be
eliminated and refining to be terminated with no need for the converter to be
prematurely turned down.
It has unexpectedly been found that, by
observing the movement of the converter, it is possible to detect two instants
which are significant for the refining operation, viz. first of all the instant
at which the slag melts, and a little later the instant at which
dephosphorization terminates. The movement of the converter may be characterized
by its amplitude, speed, or acceleration. It has also been possible to
experimentally determine the existence of a relationship between (a) the amount
of oxygen blown in starting from either of these significant instants and (b)
the phosphorus content of the steel or the iron content of the slag.
Accordingly, measurement of the movement of the converter may be used for
determining the precise moment at which the refining operation is terminated.
In view of the above, the process according to the present invention
comprises measuring a quantity characterizing movement (or vibration) of the
converter while the oxygen is blown in, detecting the instant (or one of the two
instants) at which the quantity undergoes a sudden and substantial decrease,
and, from the instant onwards, blowing into the converter the amount of oxygen
necessary and sufficient to attain at the end of refining the desired quality of
the steel, the amount of oxygen being determined from an empirical relationship,
previously determined for the installation employed, between the amount of
oxygen blown in after the instant and either the phosphorus content of the steel
or the iron content of the slag depending on the nature of the sudden decrease
in the quantity.
One can conveniently measure the movement of the
converter in the direction of the longitudinal axis passing through the center
of the pivot pins of the converter.
The amplitude and/or the frequency
of the movement of the converter may conveniently be measured. Alternatively,
the movement of the converter may be characterized by the acceleration thereof
or the speed thereof.
The movements of the converter are preferably
measured in the frequency range of up to 50 Hertz, more preferably between 10
and 25 Hertz.
Of course, the above described procedure may be combined
with any method of carrying out the refining operation before determining the
oxygen blowing-in termination. For example, it is possible to proceed as
follows:
a. one performs refining during a first period by blowing in
oxygen a constant flow rate, the flow rate value being determined by means of a
diagram indicating the influence, on this quantity, of parameters such as the
age of the converter, the silicon content of the pig iron, the amount of pig
iron charged into the converter, and the weight of ore charged into the
converter;
b. one measures the carbon monoxide content in the waste
gases leaving the converter, determines the instant at which the CO content
undergoes a sudden and substantial decrease, and, from the appearance of this
decrease onwards, one ends the first refining period by modifying the oxygen
flow rate, which is increased to the highest rate which is technically possible;
c. one performs a second refining period by keeping the oxygen flow rate
at the maximum technically possible value;
d. the refining operation is
terminated on the basis of measurements of the converter movement, in accordance
with the process of the present invention.
BRIEF DESCRIPTION OF THE
DRAWINGS
The invention will be described further, by way of example
only, with reference to the accompanying drawings, in which:
FIG. 1 is a
graph of acceleration (I) of the converter (ordinates) against time (abscissae),
the acceleration being measured in the direction of the longitudinal axis
passing through the pivot pins of the converter;
FIG. 2 is a graph of
the empirical relationship, for the installation used, between the iron content
of the slag (abscissae) and the volume of oxygen blown into the molten metal
(ordinates) after a sudden and considerable decrease of the acceleration of the
converter, this decrease corresponding to melting of the slag; and
FIG.
3 is a graph of the empirical relationship, for the installation used, between
the phosphorus content of the metal (ordinates) and the volume of oxygen blown
into the molten metal (abscissae) after a sudden and considerable decrease in
the acceleration of the converter, this decrease corresponding to the end of the
dephosphorization period.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENT
With reference to FIG. 1, the course of the acceleration of
the converter in the direction of the longitudinal axis passing through the
centers of the pivot pins of the converter is recorded by an accelerometer in
the range of frequencies of from 10 to 25 Hertz. This course may be subdivided
into three periods closely related to the metallurgical phases of the process:
Phase 1: decarburization, taking place during about three quarters of
the total refining time;
Phase 2: dephosphorization, extending up to
about 95% of the total refining time; the duration of this phase depends on the
type of pig iron (phosphoric pig iron or hematite pig iron);
Phase 3:
oxidation of iron, which determines the iron content of the slag, or the
residual phosphorus content of the steel, and terminates the refining operation.
Phase 2 is characterized by two sudden and considerable drops (AB,CD) in
the signal representing the converter acceleration amplitude measured in the
direction of the longitudinal axis passing through the centers of the pivot
pins. The first drop AB corresponds to the melting of the slag and the second
drop CD corresponds to the end of the dephosphorization period. Each drop AB,CD
may be easily observed by the converter operator on the diagram on which the
signal representing the accelerations is recorded.
The drops (AB,CD) are
both sharp and either of them may be used as the reference point from which a
given volume of oxygen will be blown in. The type of pig iron (hematite or
phosphoric) may determine the choice.
Blowing in a predetermined volume
of oxygen, from the selected reference point onwards, allows one to achieve,
with good accuracy, a predetermined iron content of the slag, or a predetermined
phosphorus content of the steel, when the converter is turned down.
FIG.
2 illustrates the relationship between the iron content of the slag and the
volume of oxygen blown in after the drop AB. It will be seen that the iron
content of the slag increases with the volume of oxygen blown in. This
relationship permits ready determination of the volume of oxygen to be blown in,
after the sudden drop, in order to obtain a desired iron content of the slag.
FIG. 3 illustrates the relationship between the phosphorus content of
the steel and the volume of oxygen blown in after the drop CD. It will be seen
that the curve representing the phosphorus content of the steel as a function of
the volume of oxygen blown in after the sudden drop CD has a minimum. This
relationship permits ready determination of the volume of oxygen to be blown in,
after the sudden drop, in order to obtain a desired phosphorus content. Of
course, the phosphorus content desired will usually correspond to the minimum.
Tests on samples of molten material taken during refining have shown
that the above relationship actually correspond to a succession of reproducible
states of the molten material.
* * * * *
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