TechnicalField
[0001] The present invention relates to a method for measuring a thickness of slag
floating on a surface of molten metal, and particularly, to a method for measuring a thickness
of slag, capable of continuously measuring a thickness of slag with high accuracy even when
the slag is thin.
Background Art
[0002] Continuous casting of steel is a technique of manufacturing a slab by injecting
molten steel into a water-cooled mold and continuously extracting the slab downward of the
mold while making contact with the mold for cooling to form a solidified shell.
[0003] In such continuous casting, a tundish is employed as an intermediate container
between a ladle and a mold in order to temporarily store molten steel in the event of
replacement of the ladle used to supply the molten steel and distribute the molten steel to a
plurality of molds.
[0004] At the end of the work for supplying the molten metal of the ladle to the tundish, a
small amount of slag floating on a surface of the molten steel inside the ladle may be
discharged from the ladle to the tundish along with the molten steel. The slag flowing from
the ladle to the tundish is dispersed into the molten steel inside the tundish, and most of the
slag rises up on the molten steel and is separated from the molten steel so as to form a layer on
a surface of the molten steel inside the tundish and float as slag inside the tundish. In
addition to the slag flowing from the ladle along with the molten steel, flux is further added
into the tundish for coating the molten steel inside the tundish. The flux is molten by the heat
of the molten steel and becomes slag.
[0005] A part of the slag that does not rise up the molten steel is carried to a water-cooled
mold along with the molten steel by supplying hot water through a submerged nozzle of the
tundish and remains in the slab as a nonmetallic inclusion after solidification of the molten
steel. This causes a surface defect of a product or the like. As the amount of the slag
increases inside the tundish, involvement of slag may occur in the event of replacement of the
ladle. In addition, as the slag that does not rise up the molten steel increases, a qualify
problem such as an internal defect in the resulting slab may occur.
[0006] If the molten steel inside the tundish is reduced in the event of replacement of the
ladle, and a proportion of the slag increases, slag involvement easily occurs. For this reason,
if the thickness of slag inside the tundish exceeds a predetermined value, the slag is discharged
to the outside through a discharge hole provided in an upper part of the tundish. This
discharge may cause a secondary trouble such as a fire when it is scattered. Furthermore, a
part of the molten steel is discharged, which may also cause a product yield to be degraded.
[0007] In view of preventing a breakout caused by a discharge of slag from the inside of
the mold and improving work safety and productivity, the thickness of slag inside the tundish
is an important control factor.
[0008] In this manner, in terms of quality management of the slab and improvement of an
yield and safety, it is important during a continuous casting work to know the amount of slag
inside the tundish including both the slag flowing from the ladle and the slag originated from
the flux intentionally added. Naturally, it is a major premise to minimize the slag discharged
from the ladle to the tundish.
[0009] ln Patent Literature l, there is proposed the method of suppressing a discharge of
slag caused by supplying molten steel as small as possible. In this method, an open/close
operation of the sliding nozzle used to inject molten steel of the tundish to the mold is
controlled on the basis of an opening level signal of the sliding nozzle and a meniscus level
signal of molten steel inside the mold.
[0010] However, in the method proposed in Patent Literature 1, the amount of slag inside
the tundish is not measured. For this reason, if this method is applied to supply the molten
steel from the ladle to the tundish, and an operation for closing the sliding nozzle is performed
earlier during a continuous casting work in view of reliably preventing a discharge of the slag
from the ladle, such a problem arises that much molten steel remains inside the ladle, and an
yield of the molten steel is significantly reduced.
[0011] In the current continuous casting works, the amount of slag inside the tundish is
controlled through the thickness of the slag. As a technique therefor, typically, an operator
opens a part of the lid on the tundish and inserts a metal measurement probe into the molten
steel inside the tundish to measure the thickness of the slag attached to the measurement probe.
[0012] Although the slag inside the tundish seldom abruptly increases during the casting,
the amount of slag inside the tundish gradually increases as the number of ladles, that is, the
ladle replacement frequency in the continuous casting increases. For this reason, it is
indispensable to measure the thickness of slag whenever the ladle is replaced. Howeveq
since the measurement of the slag thickness using the measurement probe is a transient action
manually performed by an operator, there is a problem that it increases a work load and
generates a large difference of the measurement value depending on operators, and in addition,
continuous and stable measurement cannot be performed.
[0013] In addition, it is necessary to open a part of the lid on the tundish when the
measurement probe is inserted into molten steel. As a result, shut-offof the ambient air by an
inert gas atmosphere (such as an Ar gas) inside the tundish becomes insuffìcient, so that such a
problem arises that an inclusion may be generated in the molten steel due to secondary
oxidation caused by intrusion of the ambient air, which degrades internal quality of the slab.
[0014] Knowing the thickness of slag floating on a surface of the molten steel inside the
ladle is also important to suppress a discharge of the slag into the tundish. Similar to the
thickness of the slag inside the tundish, the thickness of the slag flowing from a converter
furnace to the ladle is measured using the measurement probe manually inserted. This also
increases a work load of an operator.
Citation List
Patent Literatures
[0015] Patent Literature 1 : JP H05-000359 A
Summary of lnvention
Technical Problem
[0016] As described above, since the thickness of slag inside the ladle or the tundish
during a continuous casting work is measured manually by an operator, there are problems of
generation of a work load, and measurement accuracy. In addition, since the atmospheres
inside the ladle and the tundish have high temperatures, the manual measurement is not
preferable in view of safety. Furthermore, since it is necessary to open the lid to measure the
thickness of slag inside the tundish, such a problem arises that the ambient air intrudes into the
inside of the tundish, which degrades internal quality of the slab.
[0017] The present invention was made in view of the aforementioned problems. It is
therefore an object of the present invention to provide a method capable of measuring a
thickness of slag with high accuracy independently from an operator.
Solution to Problem
[0018] The inventor studied automatically measuring a thickness of slag using a
microwave range flrnder. As the microwave range finder, a frequency modulation continuous
wave (FMCW) type widely employed to measure a distance with high accuracy was used.
[0019] In the FMCW technique, a microwave frequency is continuously modulated at a
predetermined amplitude and a predetermined period with respect to a predetermined center
frequency. A microwave is transmitted from an antenna of the microwave range finder to a
measurement target, and a reflected microwave (reflection wave) reflected on the
measurement target is received by the same antenna. Since the microwave frequency is
modulated, a frequency of the reflection wave of the measurement target (the reflection wave
reflected on the measurement targef, hereinafter this similarly applies to the following
description) received by the microwave range finder is different from a frequency of the
microwave transmitted from the microwave range finder at the timing when this reflection
wave is reaeived. For this reason, a time elapsing from transmission of the microwave to
receiving of the reflection wave of the measurement target can be calculated on the basis of a
difference between a frequency of the received reflection wave and a frequency of the
microwave transmitted at the receiving timing. In the FMCW technique, a value obtained by
multiplying the time resulting from this calculation by a velocity of the microwave in the
ambient air and dividing the multiplication result by two is set as a distance from the antenna
to the measurement target. That is, a value L (mm) obtained from the following equation (1)
is set as a distance from the antenna to the measurement target.
L: c't/2 (l),
where "c" denotes a velocity (mm/s) of the microwave in the ambient air, and "Í"
denotes a time (s) calculated from a difference between a frequency of the received reflection
wave and a frequency of the microwave transmitted from the microwave range frnder at the
receiving timing.
t0020] Currently, the microwave range finder is used to measure a meniscus level of
molten steel inside a converter fumace. Using the microwave range finder, a distance to the
measurement target can be continuously measured independently from an operator.
Therefore, by applying the microwave range finder, it is conceived that the thickness of slag
can also be continuously measured. In addition, by placing the antenna inside the tundish,
there is no need to open the lid of the tundish during measurement of the thickness of slag.
Note that, if "4" is calculated (or analyzed) on the basis of the measurement result of the
microwave range finder, this may be referred to as "A is measured" herein. In addition, in
the following description, a "reflection wave of B" refers to a microwave reflected on an
object B (reflection wave).
[0021] When the thickness of slag is measured using the microwave range finder, a
microwave is transmitted to molten steel and slag floating on a surface of the molten steel
from an antenna, and a reflection wave of the microwave on a surface of the molten steel and a
reflection wave on the slag surface are received by the same antenna. A first period of time
elapsing from transmission of the microwave to receiving of the reflection wave on the surface
of the molten steel and a second period of time elapsing from transmission of the microwave
to receiving of the reflection wave on a surface of slag are calculated on the basis of a
difference between a frequency of the received reflection wave and a frequency of the
microwave transmitted from the microwave range finder at the receiving timing. First, if a
difference between a microwave velocity in the ambient air and a microwave velocity in the
slag is neglected, a distance L0 from the antenna to the surface of molten steel and a distance
Ll from the antenna to the surface of slag can be measured using the microwave range finder
on the basis of the equation (1) described above and first and second periods of time f\ andt2.
In addition, it is conceived that a value AL obtained by subtracting the distance L1 from the
distance L0 corresponds to the thickness of slag. The difference ÂL can be expressed as the
following equation (2).
AL: LO-LI = (c.tl-c.t2)12: c.(t1-t2)12 (2),
where "c" denotes a velocity (mm/s) of the microwave in the ambient air, "tl" denotes a
first period of tirne (s), and "t2" denotes a second period of tirne (s).
100221 The thickness of slag floating on a surface of molten steel inside the tundish is
typically within a range of 10 to 20 mm. As a result of the inventor's study, if a microwave
having a center frequency of 20 GHz and a frequency modulation amplitude (hereinafter,
referred to as a 'lnodulation amplitude") of 4 GHz, that is, a so-called general-purpose
microwave is employed, the thickness of slag inside the tundish is too thin, so that the
reflection wave on the surface of molten steel and the reflection wave on the surface of slag
are not separated from each other, and it is difficult to clearly recognize the reflection wave on
the surface of slag. As a result, it is found that the thickness of slag cannot be measured. In
contrast, the inventor found out that even the thin thickness of slag can be measured by using a
microwave having a center frequency of 24 to 32 GHz and a modulation amplitude of 8 to 10
GHz.
[0023] In addition, the inventor found out that the thickness of slag can be measured with
high accuracy through correction by multiplying the value ÀL calculated from the first and
second periods of time measured using this microwave by a constant. This study will be
described below in more detail. lì
100241 The present invention was made in view of the aforementioned findings. The
summary of the present invention is to provide a method for measuring a thickness of slag
floating on a surface of molten steel as described below.
[0025] According to an aspect of the invention, there is provided a method for measuring
a thickness of slag floating on a surface of molten metal using a microwave range finder that
transmits and receives a frequency modulated microwave having a center frequency of 24 to
32 GHz and having a frequency modulation amplitude of 8 to 10 GHz through an antenna, the
method comprising: using the microwave range finder, transmitting the microwave from the
antenna to the molten metal and the slag and receiving a reflection wave of the transmitted
microwave on a surface of the molten metal and a reflection wave on a surface of the slag
through the antenna; calculating a first period of time elapsing from transmission of the
microwave to receiving of the reflection wave on the surface of the molten metal on the basis
of adifference between afrequency of the reflection wave on the surface of the molten metal
received through the antenna and a frequency of the microwave transmitted from the
microwave range finder at the timing when the reflection wave on the surface of the molten
metal is received; calculating a second period of time elapsing from transmission of the
microwave to receiving of the reflection wave on the surface of the slag on the basis of a
difference between a frequency of the reflection wave on the surface of the slag received
through the antenna and a frequency of the microwave transmitted from the microwave range
finder at the timing when the reflection wave on the surface of the slag is received; and
calculating a calculation value AL of the above described equation (2) using the first and
second periods of time, wherein the calculation value is calculated by measuring a thickness of
slag, the thickness being known, using the microwave range finder, and a correction function
for corecting the calculation value to the thickness of the slag, the thickness being known, is
obtained in advance, and a value obtained by correcting the calculation value, which is
continuously measured using the microwave range finder during a work, on the basis of the
correction function is set as a thickness ofthe slag.
10026) In the method described above, the molten metal may be molten steel stored in a
continuous casting tundish.
100271 In the method described above, the correction function may be a multiplication of
the calculation value by a constant, and the constant may be a relative dielectric constant of the
slag to the power (-0.5).
[0028] Herein, "flux" refers to powder added to a surface of molten metal, and "slag"
refers to a molten material of the flux.
Advantageous Effects of lnvention
100291 Using the method for measuring a thickness of slag floating on a surface of
molten metal according to the present invention, it is possible to measure the thickness with
high accuracy independently from an operator even when the thickness is equal to or smaller
than 150 mm. Since the slag inside the tundish can be measured without opening the lid of
the tundish, it is possible to suppress generation of an inclusion in molten steel caused by
secondary oxidation caused by intrusion of the ambient air and obtain slab having high internal
quality. In addition, by controlling a continuous casting work using the slab thickness
measurement value, it is possible to suppress involvement of slag occurring in the event of
replacement of the ladle during a continuous casting and obtain high-quality slab with a high
yield.
Brief Description of Drawings
[0030] FIG. I is a diagram illustrating a configuration of an experimental device used to
measure a thickness of flux;
FIG. 2 is a diagram illustrating a relationship between a thickness X of the flux in a
container, a distance La from an antenna to a bottom of the container measured using
general-purpose microwaves, and a distance Lb from the antenna to a surface of the flux;
FIG. 3 is a diagram illustrating a relationship between the thickness X of the flux in the
container, a distance La from the antenna to the bottom of the container measured using
microwaves having a center frequency of 32 GHz and a modulation amplitude of 8 GHz, and a
distance Lb from the antenna to the surface of the flux;
FIG. 4 illustrates exemplary measurement data obtained by using a microwave having a
center frequency of 32GHz and a modulation amplitude of 8 GHz;
FIG. 5 is a schematic diagram illustrating a measurement state of the thickness of slag
using the microwave range finder;
FIG. 6 illustrates exemplary measurement data for showing states of the flux and the
slag floating on a surface of the molten steel;
FIG. 7 is a diagram illustrating a configuration of a test device used to measure the
thickness of slag;
FIG. 8 is a diagram illustrating a relationship between flux adding frequency and
thicknesses of slag and flux obtained through manual measurement; and
FIG. 9 is a diagram illustrating a relationship between flux adding frequency, a thickness
of slag obtained through manual measurement, and a thickness of slag obtained through a slag
thickness measurement method according to the present invention.
Description of Embodiments
[0031] A study for completing the present invention and embodiments of the present
invention will now be described.
[0032] 1. Details of Study
1-1. Check for Measurable Flux Thickness (First Preliminary Experiment)
A thickness of slag floating on a surface of molten steel inside a tundish is typically
within a range of 10 to 20 mm. In this regard, it was confirmed through an experiment (first
preliminary experiment) in a laboratory whether or not it was possible to measure a distance
from an antenna of a microwave range finder to a surface of slag and a distance from the
antenna to a surface of molten steel to a degree that the thickness of slag could be calculated,
in a case where a so-called general-purpose microwave was employed.
[0033] In the first preliminary experiment, flux was used instead of slag. That is, in the
first preliminary experiment, flux was regarded as slag, and a bottom of the container that
10
contains flux was regarded as a surface of molten steel.
[0034] FIG. 1 is a diagram illustrating a conflrguration of an experimental device used to
measure a thickness of flux. The experimental device includes a microwave range finder 1
and a container 10 that contains flux 13. The microwave range finder 1 is a FMCW type
microwave range finder. The microwave range finder 1 includes an antenna 2 that transmits
and emits a microwave onto an object as a target whose distance is to be measured (hereinafter,
referred to as a "measurement target") and receives a reflection wave of the transmitted
microwave reflected by the measurement target, an amplifier 3 that amplifies a signal intensity
of the received reflection wave, and a personal computer 4 thaÍ. controls transmission of the
microwave and collects and analyzes data regarding the received reflection wave,
[0035] The microwave range finder 1 is a FMCW type system in which a microwave
frequency is continuously modulated at a predetermined amplitude and a predetermined period
with respect to a predetermined center frequency. The microwave range finder 1 calculates a
time elapsing from transmission of the microwave to receiving of the reflection wave from the
measurement target on the basis of a difference between a frequency of the received reflection
wave of the measurement target and a frequency of the microwave transmitted from the
microwave range finder I at the receiving timing as described above. In addition, a value
obtained by substituting this calculated time into the aforementioned equation (1) is set as a
distance from the antenna to the measurement target.
[0036] The following Table I shows characteristics of flux used in the first preliminary
experiment. The Table 1 shows compositions, basicities, and viscosities of main components,
and the remaining components other than main components of flux are impurities.
[0037]
[Tâble l]
Chemical Composition of Flux (mass%) Basicity
CaO/SiO2
Viscosity
CaO sio2 AlzO¡ Na"O F* T.C (Pa's)
28.6t38.1 33.3147.3 L418.4 5.61lt.2 2.01s.7 015.6 0.6/1.1 0.11/0.96
li
Numerical values in the table refer to a (minimum/maximum) value.
"T'C" refers to a total carbon amount.
"Viscosity" refers to a value when slag with molten flux has a temperature of 1400oC.
[0038] In the first preliminary experiment, a so-called general-purpose microwave having
a center frequency of 20 GHz and a modulation amplitude of 4 GHz was employed. If the
microwaves are emitted to the inside of the container 10, a part of the microwaves is reflected
on a surface of the flux 13, and remaining microwaves transmit through the flux l3 and are
reflected on a bottom 10a of the container 10. In this preliminary experiment, the amount of
the flux 13 with which a distance from the antenna 2tothe surface of the flux 13 could be
measured (thickness X of the flux 13 in the container l0) was examined by changing the
amount of the flux 13 inside the container 10 while a distance from the antenna 2 to the bottom
10a of the container 10 was constant. In addition, a relationship between a distance La from
the antenna 2 to the bottom 10a of the container 10 measured using the microwave range
finder 1 and a thickness X of the flux 13 was investigated.
[0039] FIG 2 is a diagram illustrating a relationship between the thickness X of flux in
the container, the distance La from the antenna to the bottom of the container measured using a
general-purpose microwave, and the distance Lb from the antenna to the surface of the flux.
The distances of FIG. 2 are obtained by substituting a calculated value of a time elapsing from
transmission of the microwave to receiving of the ref'lection wave from the measurement
target into the aforementioned equation (1). In addition, the dotted line in FIG. 2 is formed
by simply connecting a point measurable using the microwave and a point where the thickness
of flux is zero.
[0040] When the thickness X of the flux 13 in the container 10 was smaller than 150 mm
as a result of the experiment, the reflection wave from the bottom 10a of the container 10 and
the reflection wave from the surface of the flux 13 were not separated from each other.
Therefore, it was impossible to clearly recognize the reflection wave from the surface of the
flux 13. As a result, it was impossible to measure the distance from the antenna 2 to the
surface of the flux 13. However, as illustrated in FIG. 2, when the thickness X of the flux 13
t2
in the container 10 was equal to or larger than 150 mm, both the reflection wave from the
bottom 10a of the container 10 and the reflection wave from the surface of the flux 13 could be
clearly recognized. Therefore, it was possible to measure the distance from the antenna 2 to
the surface ofthe flux 13.
[0041] As illustrated in FIG. 2, asthe thickness X of the flux 13 increased, the distance
La from the antenna 2tothe bottom lOa of the container 10 measured using the microwave
range fìnder 1 increased. This was because a dielectric constant is different between the flux
and the air, and a velocþ of the microwave transmitting through the flux is affected by the
flux. As a result, it was recognizedthat a value obtained by subtracting the distance Lb from
the antenna 2 to the surface of the flux 13 measured using the microwave range finder 1, from
the distance La from the antenna 2 to the bottom l0a of the container 10 measured using the
microwave range finder 1 ("La - Lb:' hereinafter, referred to as a "difference value") was
larger than the actual thickness X of the flux 13.
[0042] Here, the difference value (La - Lb) will be described, A time elapsing from
transmission of the microwave to receiving of the reflection wave from the bottom 10a of the
container 10 was calculated on the basis of a difference between a frequency of the reflection
wave on the bottom 10aof the container l0 received by the antenna 2 anda frequency of the
microwave transmitted from the microwave range finder 1 at the timing that this reflection
wave was received, and this time was set as a first period of time. In addition, a time
elapsing from transmission of the microwave to receiving of the reflection wave from the
surface of the flux 13 was calculated on the basis of a difference between the frequency of the
reflection wave on the surface ofthe flux 13 received by the antenna 2 and the frequency of
the microwave transmitted from the microwave range finder 1 at the timing that the reflection
wave was received, and this time was set as a second period of time.
[0043] The aforementioned equation (2) expresses a difference (L0 - Ll) between the
distance L0 from the antenna to the surface of molten steel and the distance Ll from the
antenna to the surface of slag measured for the slag on a surface of molten steel using the
microwave range finder. If this equation (2) is applied to the distance La from the antenna2
13
to the bottom 10a of the container 10 measured using the microwave range finder 1 and the
distance Lb from the antenna 2 to the surface of the flux 13 measured using the microwave
range finder 1, the difference value (La - Lb) can be expressed as the following equation (3):
La-Lb : c'(t1-t2)12 (3),
where "c" denotes avelocify (mm/s) of the microwave in the ambient air, "tl" denotes
the first period of time (s), and "12" denotes the second period of time (s).
[0044] 1-2. Optimum Selection of Microwave and Correction of Thickness of Flux
Measured using Microwave Range Finder (Second Preliminary Experiment)
The inventor studied the result of the first preliminary experiment and contemplated that
the thickness of flux could be continuously measured with high accuracy using the microwave
range finder if a correction function for correcting the difference value to the actual thickness
X of flux was established in advance on the basis of a relationship between the actual
thickness X of flux and the difference value (La - Lb) between the distances La and Lb
measured using the microwave range finder, and the difference value based on the values
continuously measured using the microwave range finder was corrected using this correction
function. This correction function is a multiplication of the diflerence value by a constant as
described below. The constant is a relative dielectric constant of flux to the power (-0.5).
[0045] After the first preliminary experiment was perfomed, it was found that it was
difficult to stably detect the reflection wave on the surface of flux using a general-purpose
microwave. The inventor thought that this problem was because the measurement accuracy
was low due to a long wavelength of the general-purpose microwave of ten-odd millimeters.
In addition, the inventor thought that the reflection wave from the surface of flux as well as the
reflection wave from the bottom of the container could be stably detected, and the accuracy of
the thickness of flux measured using the microwave range finder could be improved by using a
microwave having a wavelength shorter than that of the general-purpose microwave and
increasing the modulation amplitude. In the FMCW technique, the frequency modulation
amplitude and the center frequency are important to improve the measurement accuracy.
[0046] In this regard, the inventor performed the second preliminary experiment using a
14
i:li.ii:"-:'ì-j-l:i::1-:.i:-l.i:i: :.. -: ì::: r. ..;,
microwave having a center frequency of 32 GHz and a modulation amplitude of 8 GHz and
the experimental device of FIG. 1.
l004ll FIG 3 is a diagram illustrating a relationship between the thickness X of flux in
the container, and the distance La from the antenna to the bottom of the container and the
distance Lb from the antenna to the surface of flux measured using a microwave having a
center frequency of 32GHz and amodulation amplitude of 8 GHz. As illustrated in FIG.3,
if this microwave was used and the thickness X of flux in the container was equal to or larger
than 15 mm, it was possible to measure the distance from the antenna to the surface of flux
even when the thickness X was smaller than 150 mm. In addition, the inventor confìrmed
that the same measurement could be performed using a microwave having a center frequency
of 24 to 32 GHz and a modulation amplitude of 8 to l0 GHz.
[0048] FIG. 4 illustrates exemplary measurement data obtained when a microwave
having a center frequency of 32 GHz and a modulation amplitude of 8 GHz is used. FIG. 4
shows measurement data obtained by setting the thickness X of the flux 13 in the container 10
in 40 mm. From FIG. 4, it is recognized that the reflection wave on the surface of flux and
the reflection wave on the bottom of the container are clearly separated and can be stably
detected. In addition, the distance La from the antenna to the bottom of the container
measured using the microwave range finder was 545 mm, and the distance Lb from the
antenna to the surface of flux was 484 mm, A difference value of this distance (La - Lb) was
61 mm, which was larger than an actual thickness of flux (X :40 mm).
[0049] As apparently recognized from the experimental results of FIGS. 3 and 4, there is
a relationship between the actual thickness of flux and the difference value calculated from
each distance measured using the microwave range fìnder. In this regard, the inventor
obtained the following equation (4) as a correction function for correcting the calculated
difference value to the actual thickness of flux. In the equation (4), the difference value (La -
Lb) calculated from each distance measured using the microwave range finder is multiplied by
a constant. According to the inventor's study, this constant corresponds to a relative
dielectric constant e¡ of flux to the power (-0.5). ln the case of the measurement data of FIG.
15
4, the relative dielectric constant e¡ of flux is set to 2,33. The inventor confirmed that it was
possible to calculate the thickness of flux with high accuracy by correcting the difference
value obtained from the distances La and Lb continuously measured using the microwave
range flrnder on the basis ofthe equation (4).
x : (La-Lb).€L-o s (4),
where "X" denotes a thickness (mm) of flux, "La" denotes a distance (mm) from the
antenna to the bottom of the container measured using the microwave range finder, "Lb"
denotes a distance (mm) from the antenna to the surface of flux measured using the microwave
range finder, and"ey" denotes a relative dielectric constant of flux.
[0050] 2. Measurement Test for Thickness of Slag using Microwave Range Finder
FIG. 5 is a schematic diagram illustrating a measurement state of the thickness of slag
using the microwave range fìnder. As illustrated in FIG. 5, for the flux 13 floating on a
surface of the molten steel 11 and the slag 12 where the flux l3 is molten, a test of measuring a
distance L0 from the antenna 2 of the microwave range finder to the surface of the molten
steel 11, a distance Ll from the antenna 2to an interface between the slag 12 and the flux 13,
and a distance L2 from the antenna 2 to a surface of the flux 13 was performed using a
microwave having a center frequency of 24 to 32 GHz and a modulation amplitude of 8 to l0
GHz with the FMCW microwave range finder.
[0051] FIG. 6 illustrates exemplary measurement data concerning a state where flux and
slag float on a surface of molten steel. As a result of the measurement test, as illustrated in
FIG, 6, it is recognized that a reflection wave on a surface of the molten steel, a reflection
wave on an interface between slag and flux, and a reflection wave on a surface of the flux
were clearly separated from each other and could be stably detected.
[0052] From a knowledge learned from the fìrst and second preliminary experiments, it
can be said that the thicknesses of slag and flux floating on a surface of molten steel can be
calculated using the following equations (5) and (6) on the basis of the distance L0 from the
antenna to a surface of the molten steel measured using the microwave range finder, the
distance Ll from the antenna to the interface between slag and flux measured using the
t6
microwave range finder, the distance L2 from the antenna to the surface of flux measured
using the microwave range finder, a relative dielectric constant es of slag, and a relative
dielectric constant e¡ of flux. Similar to the relative dielectric constant e¡ of flux, the relative
dielectric constant es of slag is obtained in advance on the basis of a relationship befween the
actual thickness of slag and the diflerence value corresponding to the thickness of slag
calculated from each distance measured using the microwave range finder.
11 = (L0_L1).ss-o s
72= (Ll_L2).€L-o s
(5), and
(6),
where "Tl" denotes athickness (mm) of slag, "T2" denotes athickness (mm) of flux,
"L0" denotes a distance (mm) from the antenna to the surface of molten steel measured using
the microwave range finder, "L1" denotes a distance (mm) from the antenna to the interface
between slag and flux measured using the microwave range finder, "L2" denotes a distance
(rnrn) frorn the antenna to the surface of flux measured using the microwave range finder, "rs"
denotes a relative dielectric constant of slag, and "r¡" denotes a relative dielectric constant of
flux.
[0053] As a result of the inventor's study, it was confìrmed that the distance from the
antenna to the interface between slag and flux can be measured if the thickness of slag is equal
to or larger than 2 mm even when the thickness of slag is smaller than a minimum measurable
flux thickness ( l5 mm). This is because the slag in the form of a molten material of flux has
a porosity lower than that of the flux in the form of powder, the relative dielectric constant of
slag is greater than that of flux, and the difference value (L0-L1) corresponding to the
thickness of slag calculated from each distance measured using the microwave range fìnder is
greater than the difference value (Ll-L2) conesponding to the thickness of flux and is output
through amplification.
[0054] In addition, the inventor confirmed that the thickness of slag can be calculated by
setting the distance "Ll" as the "distance from the antenna to the surface of slag measured
using the microwave range finder" in the equation (5), assuming that the flux is perfectly
molten and only the slag floats on a surface of the molten steel.
17
[0055] According to the method for measuring the thickness of slag of the present
invention, it is possible to measure a thickness of slag with high accuracy in a simple and
convenient manner independently from an operator. The thickness of slag can be measured if
it is equal to or larger than 2 mm. Therefore, even a thickness of slag in a tundish having a
relatively thin thickness can be measured if the slag has a thickness by which quality of slab
may be degraded due to involvement of slag or the like. In addition, by placing the antenna
inside the tundish, it is possible to continuously measure the thickness of slag in a non-contact
manner without opening the lid of the tundish.
Examples
[0056] ln order to check the effect of the method for measuring the thickness of slag
according to the present invention, the following test was performed, and the result thereof
was evaluated.
[0057]
FIG. 7 is a diagram illustrating a configuration of a test device used to measure a
thickness of slag. The test device included a microwave range finder 1 and a high-frequency
melting furnace (atmospheric furnace) 15.
[0058] Moltensteel ll inaheatedstatewasstoredinthehigh-frequencyfurnace 15. If
flux was added to the high-frequency furnace 15, the flux was molten by the heat of the molten
steel 11, and alayer of slag 12 (melting layer) and alayer of flux 13 (powder layer) were
separated from each other on a surface of the molten steel 11.
[0059] The microwave range finder I had an antenna 2, awaveguide pipe 5, a reflector 6,
and an amplifier 3. The microwave transmitted from the antenna 2 was reflected on the
reflector 6 and was emitted to the inside of the high-frequency furnace 15, so that it was
reflected on a surface of the molten steel 11, an interface between the slag 12 andthe flux 13,
and a surface of the flux 13. Then, the reflected wave was reflected on the reflector 6 again,
was guided by the waveguide pipe 5, and was received by the antenna 2. In this test, a
distance from a microwave transmilreceive portion of the antenna2 to a microwave reflecting
l8
portion of the reflector 6 was set to 1000 mm. ij
[0060]
Steel of 200 kg was molten in the high-frequency furnace I 5 to form the molten steel I l.
The flux was added to the high-frequency fumace 15 in a dividing manner in six times. The
flux amount in each adding process was set to 1.3 kg. This corresponded to the thickness of
20 mm inside the high-frequency furnace 15 (a value obtained by dividing a volume of flux by
a cross-sectional area ofa cylindrical furnace). The added flux had characteristics shown in
Täble 1.
[0061] Whenever the flux was added, the thicknesses of flux and slag were measured
using the microwave range finder 1, and the thicknesses of flux and slag were measured
through a manual work of an operator (hereinafter, referred to as "manual measurement")
using a metal measurement probe. The microwave range finder I was operated using a
microwave having a center frequency of 32 GHz and a modulation amplitude of 8 GHz.
[0062] The relative dielectric constant es of slag was set to "35." For slag having a
predetermined thickness (6.5 mm in manual measurement) and floating on a surface of the
molten steel, this value was obtained in advance from the aforementioned equation (5) on the
basis of the distance from the antenna to a surface of the molten steel measured using the
microwave range finder and the distance from the antenna to an interface between slag and
flux measured using the microwave range finder (difference value : 38.5 mm).
[0063]
FIG. 8 is a diagram illustrating a relationship between the flux adding frequency and the
thicknesses of slag and flux measured through manual measurement. From FIG. 8, it is
recognized that, as the flux adding frequency increases, both the thicknesses ofslag and flux in
the high-frequency furnace increase.
[0064] FIG 9 is a diagram illustrating a relationship between the flux adding frequency,
the thickness of slag measured through manual measurement, and the thickness of slag
measured using the method for measuring the thickness of slag according to the present
invention. Here, the "thickness of slag measured using the method for measuring the
t9
thickness ofslag according to the present invention" refers to a value obtained by substifuting
the distance from the antenna to a surface of molten steel measured using the microwave range
frnder, the distance from the antenna to an interface between slag and flux measured using the
microwave range finder, and the relative dielectric constant of slag (es = 35) to the
aforementioned equation (5).
[0065] From FIG. 9, it is recognized that the thickness of slag obtained through manual
measurement is equal to the thickness of slag obtained through the method for measuring the
thickness of slag according to the present invention. As a result, it is recognized that the
thickness of slag can be continuously measured with high accuracy using the method for
measuring the thickness of slag according to the present invention.
Industrial Applicability
[0066] Using the method for measuring the thickness of slag according to the present
invention, it is possible to measure the thickness of slag in a simple and convenient manner
with high accuracy independently from an operator even when the thickness is thin, that is,
equal to or smaller than 150 mm. Since the slag inside the tundish can be measured without
opening the lid of the tundish, it is possible to suppress generation of an inclusion in molten
steel caused by secondary oxidation caused by intrusion of the ambient air and obtain slab
having high internal quality. In addition, by controlling a continuous casting work using the
slab thickness measurement value, it is possible to suppress involvement of slag occurring in
the event of replacement of the ladle during continuous casting and obtain high-quality slab.
Reference Signs List
[0067] I ... microwave range finder, 2: antenna,3: amplifier, 4: personal computer, 5:
waveguide pipe,6: reflector, 10: container, l0a: bottom, 11: molten steel, 12: slag, 13: flux,
I 5 : high-frequency melting furnace

We claim:
I ' A method for measuring a thickness of slag floating on a surface of molten metal using a
microwave range finder that transmits and receives a frequency modulated microwave having
a center frequency of 24 to 32 GHz and having a frequency modulation amplitude of 8 to 10
GHz through an antenna, tlie method cornprising:
using the microwave range finder,
transmitting the microwave from the antenna to the molten metal and the slag and
receiving a reflection wave of the transmitted microwave on a surface of the molten metal
and a reflection wave on a surface of the slag through the antenna;
calculating a first period of time elapsing from transmission of the microwave to
receiving of the reflection wave on the surface of the molten metal on the basis of a
difference between a frequency of the reflection wave on the surface of the molten metal
received through the antenna and a frequency of the microwave transmitted at the timing
when the reflection wave is received;
calculating a second period of time elapsing from transmission of the microwave to
receiving of the reflection wave on the surface of the slag on the basis of a difference
between a frequency of the reflection wave on the surface of the slag received through the
antenna and a frequency of the microwave transmitted at the timing when the reflection
wave is received; and
calculating a calculation value of the equation (l) using the first and second periods
of time,
wherein the calculation value is calculated by measuring a thickness of slag, the
thickness being known, using the microwave range finder, and a correction function for
correcting the calculation value to the thickness of the slag, the thickness being known, is
obtained in advance, and
a value obtained by correcting the calculation value, which is continuously measured
using the microwave range finder during a work, on the basis of the correction function is set
as athickness of the slag;
2l
LL: c.(t1-t2)12 ...(l)
where "ÂL" denotes the calculated value (mm), ,,c,, denotes a velocity (mm/s) of the
microwave in the ambient air, "tl" denotes the fìrst period of tilne (s), and,,t2', denotes the
second period of time (s).
2. The rnethod according to claim 1, wherein the rnolten metal is molten steel stored in a
continuous casting tundish.
3. The method according to claim 1 or 2, wherein the correction function is a
multiplication of the calculation value by a constant,
the constant is a relative dielectric constant of the slag to the power G0.5).