Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR CONTINUOUS CASTING OF
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A SLIGHTLY DEOXIDIZED STEEL SLAB
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The present invention relates to a process for
producing by continuous casting a so-called slightly
~; deoxidized steel, which is very simllar to rimmed or
semikilled steels
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~ 5 Attempts have been made for many years to produce
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steels corresponding to rimmed and semikilled steels by con-
tinuous casting. However, such continuously cast steels
have not been practically produced to date because of
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problems in the continuous casting operation and the steel
- 10 quality, especially the defect of blow holes an the surface
of the slab. Such phenomenon as the rimming action
i,
occurring in the conventional ingot casting presents in the
powder-casting operationj which is adopted in most of the
I ~modern continuous casting processes, difficulties in
^~ 15 production, such as breakout and the like. Prior to
continuous casting deoxidation is, therefore, controlled so
~}~ as not to cause the rimming aciton. However, when the
amount of free oxygen in the so deoxidation adjusted molten
steel is more than from approximately 50 to 70 ppm at a
solidication temperature of from 1520 to 155o2C, blow holes
are caused on the surface of a strand. These blow holes are
exposed to the ambLent air prior to the rolling and are left
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as surface flaws on the rolled products, because the inner
surfaces of the blow holes are oxidized by the ambient air.
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The oxygen concentration mentioned above can be measured by
an oxygen concentration cell using a zirconium dioxide
(ZrO2) stabilized by a calcium oxide (CaO) as a solid
electrolyte, a mixture of chromium and a chromium oxide
(Cr2O3) as a standard electrode and iron (Fe) as a counter
electrode.
In the present state of the art of continuous casting
of steels corresponding to the rimmed or semikilled steels,
the steels are excessively deoxidized by means of the
deoxidant or vacuum degassing so as to prevent the rimming
~ action from occurring. As a result, the high production
; rate of the continuous casting is completely utilized. On
, the other hand, in several reports, attention is directed to
i~ the fact that the surface defects of blow holes on the
strand are caused by insufficient rimming action for
~ removing the blow holes. In these reports processes are
i proposed for assisting the rimming action of the slightly
i ~ deoxidized steel or the deoxidation unfinished steel. In
the proposed processes, the molten steel is subjected to an
electromagnetic stirring force within the mold. These
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processes are divided into a process wherein the molten
steel within the mold is s*irred in a horizontal or vertical
direction by means of an electromagnetic stirring device
installed in the interior of the mold, so that a circulating
flow or convection is created, and a process wherein the
molten steel within the mold is stirred by means of an
electromagnetic stirring device provided below the mold, so
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process of installing the electromagnetic stirring device in
the mold interior includes Japanese Laid Open Patent
Application (hereinafter referred to as JA-OS) No. 51-2621
and Japanese Published Patent Application (hereinafter
referred to as JA-AS) No. 53-34164, while the prior art of
the process of installing the electromagnetic device below
the mold includes JA-OS No. 49-126523 and JA-OS No.
50-68915. When the molten steel within the mold is
subjected to the electromagnetic stirring force so as to
assist the rimming action, the following inconveniencies in
a practical operation are caused. Since generated bubbles
are moved upwards and removed by the effects of the rimming
action in the processes mentioned above, the flow speed of
molten steel required for floating the bubbles is relatively
high. It should be noted here that the flow speed is
dependent on the oxygen concentration in the molten steel.
Since the oxygen level of the practically castable
deoxidation unfinished or slightly deoxidized steels is
lower than that réquired for an appreciable rimming action,
a flow speed of approximately 3.0 m/sec may be required.
However, when the bubbles are moved upwards or removed by a
high flow speed, a disturbance of the molten steel surface
" i
l~- is caused by the vigorous stirring movement of the molten
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steel within the mold. The continuous casting powder, which
. . .
uhould be present on the molten steel surface, has the
purposes of: lubrication between the mold and the strand;
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j~ ~ prevention of a decrease in the temperature of molten steel;
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i prevention of reoxidation of the molten steel, and;
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absorption of inclusions contained in the molten steel. The
disturbance of the molten steel surface causes in its turn
the disturbance of the continuous casting powder on the
molten steel sur~ace, with the result that the essential
functions of the powder are not exhibited, and also, such
problems as entrapment of the powder and breakout may be
caused. Since the continuous casting powder on the molten
; steel surface in the mold is indispensable for the currently
conducted continuous casting, it is essential to prevent the
disturbance of the molten steel surface. Accordingly, the
method of floating removal of bubbles due to rimming action
is impractical in the currently conducted continuous
casting, which is based on powder castingj because the
disturbance of the molten steel surface unavoidably occurs.
- 15 Regarding the floating removal method, it is to be noted
that, although appreciable disturbance of the molten steel
~; surface might not be caused in the case of a horizontal
rotating flow, the stirring movement of the molten steel for
floating removal of bubbles, which are generating and
growing, must be performed at an extremely high speed of the
flow and the stirring flow rotates the powder on the molten
steel surface. As a result, the continuous casting powder
is gradually accumulated at the central portion of the mold,
;~ and eventually no continuous casting powder exists at the
.,,
interface between the molten steel and the mold walls.
Consequently since the powder cannot be flown between the
mold and the solidified shell of steel so as to perform the
necessary lubri~ation, a breakout may finally be caused.
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The concept involved in the process disclosed in
JA-OS No. 51-2621, mentioned above, is to impart the
rotational flow to the entire molten steel in the mold and,
; therefore, the danger of powder entrapment is great. Since
the currently conducted continuous casting is based on
- powder casting as explained above, the casting operation
itself becomes difficul-t due to the imparting o~ a stirring
force equivalent to the rimming action to the molten steel
in the mold. Such stirring processes cannot, therefore, be
adopted practically.
In a process disclosed in the Belgian Patent
No. 864218, a linear motor is installed at the both long
sides of a slab mold, in such a manner that the propulsion
forces by the linear motor are led into directions opposite
to one another, thereby creating a horizontally rotating
flow even in the central portion of the mold. The object of
the disclosed process is to separate and remove inclusions
from the molten steel by a centrifugal force and not to
prevent disturbance of liquid surface from occurring. The
process disclosed in this Belgian patent, in which even
molten steel in the central portion of the mold undergoes
the flowing movement, disadvantageously causes the
disturbance of continuous casting powder on the molten steel
surface. As a result, normal powder Gasting, on which the
presently conducted continuous casting is based, is not
performed in this disclosed process.
The prior art of continuously casting a steel with a
high oxygen content by utilizing an electromagnetic stirring
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force includes JA-OS No. 51-122625, in addition to the prior
art mentioned above. However, measures for preventing the
disturbance of powder are not mentioned in JA-OS No.
- 51-122625.
The prior arts of an electromagnetic stirring device
disposed in various type molds includes JA-AS No. 51-9858,
JA-AS No. 33-2768, JA-AS No. 47-32468, JA-AS No. 49-27487,
JA-AS No. 53-8535, JA-AS No. 54-4325, JA-OS No. 50-150640,
JA-OS No. 52-5625, JA-OS No. 52-60233, JA-OS No. 52-56015,
JA-OS No. 52-88541, JA-OS No. 52-97327, JA-OS No. 53-25235,
JA-OS No. 53-26731, JA-OS No. 53-28033, JA-OS No. 53-28034,
JA-OS No. 53-88631, JA-OS No. 53-113225, JA-OS No.
: 53-142923, JA-OS No. 53-142924, JA-OS No. 54-4241, USP No. 3153820, USP No. 3995678, USP No. 4042007, Belgian
Patent No. 27898 and Belgian Patent No. 27899. The prior
arts mentioned above basically disclose measures for
preventing entrapment of inclusions or slags into a
: solidifying shell of a strand, but not concrete measures for
preventing the disturbance of the continuous casting powder.
It is, therefore, an object of the present invention
to remove the disadvantages of the known processes for
producing deoxidation unfinished or slightly deoxidized
;: steels by continuous casting, and to make it possible to
: continuously cast these steels with neither rimming action
:~ ~ 25 occurring in the molten steel within the mold nor surface
;~ defects on the strand, thereby achieving the merits of
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producing the steels mentioned above by continuous casting
and, at the same time, decreasing the amount of deoxydant
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used for the production of a unit of steel.
It is another object of the present invention to make
it possible to continuously cast deoxidation unfinished or
slightly deoxidized steels, without eliminating the merits
achieved by the normally conducted powder casting at
present.
It is an object of an emhodiment of the present
invention not to create a disturbance at the solidification
interface, explained hereinbelow, by an ejected stream from
outlet ports of a regularly adopted immersion nozzle.
In accordance with the present invention there is
provided a process for continuous casting of a slab of
slightly deoxidized steel, wherein a continuous casting
powder and a nozzle immersed into molten steel within a mold
are used, characterized by the combination of:
casting into the mold molten steel having a
concentration of free oxygen in the molten steel in the
range of from 50 to 200 ppm;
providing the inner surface of both short sides
of said mold with a concave shape seen in the horizontal
'~ cross section of the short sides;
locating a device for generating an electro-
j magnetic force at each of both long sides of the mold and
, above an outlet port of the immersion nozzle;
orienting the propulsion force of the device for
generating the electromagnetic force in directions along
, I
said long sides opposite to one another;
i energizing the device for generating a flow
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having an essentially constant flow speed, said flow
horizontally rotating entirely around a solidification
interface and in the proximity thereof, and being formed
from the poisition of a molten steel surface within the mold
to the proximity of a predetermined vertical position of
said solidification interface, and;
providing said horizontal flow with a flow speed
'in the range of from 0.1 to 1.0 m/sec.
T,he present invention will be explained in detail
- 10 with refere~ce to the drawings, in which;
' Fig. 1 is a graph for illustrating a formation
','~ mechanism of bubbles;
Fig. 2 is a graph of the distribution of the
concentration of the elements in steel;
, 15 Fig. 3 is a schematic illustration of the strand
being cast and illustrates a principle of the present
, invention;
Fig. 4 is a graph similar to Fig. 2;
Fig. 5 is a graph of flow speed distribution;
, 20 Fig. 6 is a plan view of a continuous casting mold;
, Fig. 7 is a cross sectional view along the
line VII-VII in Fig. 7;
~, Fig. 8 is a plan view of a slab mold with four
', corners and illustrates a rotational flow of molten steel;
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~, 25 Fig. g is a plan view of a slab mold with a curved
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, Fig. 10 is a graph of flow speed distribution along a
,~ distance (x) from one of the short sides of the mold;
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Figs. 11 A, B, and C are partial plan views of the
molds which can be used in the present invention;
Figs. 12 A and B are a plan view and a vertical cross
sectional view of a mold, respectively, and illustrate a
flow of molten steel moving toward the center of mold;
Figs. 13 A and B are drawings similar to Figs. 12A
and B, respectively, and illustrate a horizontal rotational
flow;
Fig. 14 is a graph of the thickness of a
solidification layer;
Fig. 15 is a partial cross sectional view of a mold;
Figs~ 16 and 17 are partial cross sectional views of
a mold and an immersion nozzle with horizontal outlet ports;
Fig. 18 is a partial cross sectional view of a mold
and an immersion nozzle with downward outlet ports;
Fig. 19 is a plan view of a mold;
Fig. 20 is a similar drawing to Fig. 19, and;
Fig. 21 is a schematic illustration of the
macroscopic structure of a strand.
The present Inventors firstly investigated in detail
the factors involved in generation of bubble on the strand
surface during the solidification of a molten steel having a
low deoxidation degree. Referring to Fig. 1, the formation
procedure of the bubbles during solidification is divided
into a stage denoted as NUCLEATION, in which the nuclei of
` the bubbles are generated, and a stage denoted as GROWTH, in
which the nuclei are grown to bubbles. As seen in Fig. 1, a
partial pressure Pco in the bubbles already generated of
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approximately 1 atm or more (Pco ~ 1 atm) is sufficient for
the further growth of the bubbles, although a partial
pressure Pco of from approximately 2 to 3 atm (Pco ~ 2 3
atm) is necessary for the nucleation at the solidification
interface. This fact means that, although ~he nuclei of
bubbles are difficult to generate, the already generated
nuclei can easily grow into bubbles. The growth of the
bubbles is terminated when the ferrostatic pressure PFe f
the molten steel exerted on the bubbles exceeds the partial
pressure Pco. The generation of nuclei of bubbles, which
later grow into bubbles, is mainly influenced by the
concentration of carbon and oxygen in the molten steel. As
wil1 be understood from Fig. 2, the component elements in
the molten steel are concentrated at the solidification
interface, i.e. the interface between the solid and liquid,
while the solidification proceeds. In Fig. 2, Ci indicates
the element concentration at the solidification interface, ~-
C indicates the element concentration in the solid phase
~i 5
~ and Ce indicates the element concentration in the liquid
; 20 phase.
The critical concentration required for the
generation of nuclei of bubbles is denoted in Fig. 2 by Cx.
Even in a case where the element concentration Ce in the
liquid does not arrive at the concentration Cx ~ the
j~ 25 concentration Ci may exceed Cx ~ due to the concentration
phenomenon mentioned above, and the so generated nuclei will
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~¦ later grow to bubbles~ which are exposed on the surface
portion of the strand. This fact implies that: the
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generation of nuclei, which later grow to bubbles, commences
even at beginning of the solidification of molten steel,
namely at the molten steel surface within the mold, and; in
order to suppress the generation of nuclei of bubbles, the
element concentration at the solidification-interface at the
molten steel surface in the mold must be controlled so that
it is below the critical concentration Cx of the nuclei
generation of bubbles. The results of research by the
present Inventors are as follows.
A. The nuclei of bubbles are difficult to generate
as compared with the growth thereof and the generation of
the nuclei of bubbles requires more than a certain
concentration of the elements.
B. The nuclei of bubbles are already generated at
the solidification beginning point, namely at the
solidification interface at the molten steel surface within
the mold.
C. The concentration of elements, such as carbon
and oxygen, is relatively very high at the solidification
interface.
The facts A, B and C, above, were taken note of when
creating the present invention, which provides measures to
reduce the concentration of elements at the solidification
interface in the neighborhood of the molten steel surface
within the mold to a level less than the critical
I concentration of the generation of nuclei of bubblas, while
;~ causing no disturbance of the molten steel sur~ace.
In the present invention, a rotational flow of molten
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~lSZ723
- 12 -
steel in the form of a film is formed as shown by the
hatched sides of the solidification interface defined by the
frame-like bold lines in Fig. 3. The rotational flow,
hereinafter denoted by the reference numeral 3, is: formed
-~ S around essentially the entire perimeter of a solidification
interface 2; limited to only around the perimeter of the
solidification interface, namely not extended toward the
interior of the molten steel, and; formed in the
~- neighborhood of a molten steel surface l in the mold. Due
- 10 to the filmy rotational flow of molten steel, a strand 4
which is produced is provided at the surface portion thereof
with a non-defective solidification layer 5, in which the
element concentration is less than the critical limit for
generating nuclei of the bubbles. It is to be noted that:
since the molten steel at the circumference of the
solidification interface in the neighborhood of the molten
steel surface is rotated during the casting, the
concentration phenomenon of the component elements in the
"I
molten steel can ~e suppressed, and; since the imparted flow
of the molten steel is in the form of a film formed only at
~' ~ the circumference of the solidification interface, no
disturbance of the molten steel surface and the continuous
casting powder on the molten steel surface is caused by the
, ,
i~ flow of molten steel. Referring to Fig. 4, although the
; ~ 25 concentration C. of the molten steel at the rest state
., 1
i ~ ~ (dotted line) is more than the critical concentration Cx for
the generation of nuclei of bubbles, the concentration is
redu~ed to value Ci , which is less than Cx ~ by
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- 13 -
subjecting the solidification interface to the filmy flow.
In accordance with the present invention, the de-
oxidation unfinished and slightly deoxidized steels provided
with a non-defective solidi~ication layer without blow holes
on a strand surface, are steels falling within the range of
the following points of deoxidation degree. Below the
minimum point of oxygen concentration, blow holes including
pin holes are generated on the surface of strand, which is
cast without imparting any flow to the molten steel. This
minimum point is dependent upon the operational conditions,
such as the comFonents other than oxygen, the temperature of
the molten steel and casting speed, and corresponds, in most
operating conditions, to an oxygen concentration of approxi-
mately from 50 to 60 ppm at a solidification temperature
ranging from 1520 to 1530C. Above the maximum point of
oxygen concentration, the casting operation can be
inconveniently regulated. When the oxygen concentration is
'~ too high, the rimming action is caused to occur in the mold,
so that not only does the so generated serious disturbance
of the molten steel surface impede the normal powder casting
but, also, the casting operation itself is impossible in the
worst case. The minimum oxygen concentration, for
generating the rimming action corresponds to approximately
200 ppm of oxygen. The deoxidation unfinished and slightly
deoxidized steels mentioned herein refer to steels having an
oxygen concentration, in terms of free oxygen concentration,
~i of not less than 50 ppm and not more than 200 ppm. The
oxygen concentrations mentioned above are measured by an
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- 14 -
oxygen concentration cell using a zirconium dioxide (ZrO2)
stabilized by calcium oxide (CaO) as a solid electrolyte, a
mixture of chromium (Cr) and chromium oxide (CrO2) as a
standard electrode and iron (Fe) as a counter electrode.
When the steel to be cast has an oxygen content of
more than 200 ppm, i.e. the maximum value, the steel should
be subjected to carbon deoxidation in a vacuum degassing or
deoxidation by the deoxidants of Al, Si, Ca and the like,
and after adjusting the oxygen concentration to a level less
--
than the maximum point, the steel is subjected to the
casting process of the present invention.
, .
In the present invention, the entire circumference of
the solidification interface in the neighborhood of the
molten steel surface in the mold is subjected to the
15 rotational flow of molten steel having a flow speed as
" explained hereinafter. The flow speed of molten steel
required for the suppression of the nuclei of bubbles is
only such that the concentration of elements at the
solidificaiton interface is decreased to a level less than
20 the concentration of elements required for the nuclei
generation. Accordingly, the flow speed in the present
invention may be considerably slower than the flow speed for
'I
~ removing the bubbles in the conventional processes. The
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'~ maximum flaw speed is approximately 1.0 mtsecond. In other
words, even by a slow speed of less than approximately 1.0
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5~ m/second the concentration of elements at the solidification
lntérface can be decreased to a level less than that
reguired for the nucleis generation. When the flow speed
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llSZ7Z3
- 15 -
exceeds approximately l.0 m/second, the flowing movement,
which may be a rotational movement around the entire
circumference of the solidification interface in the mold,
tends to cause the disturbance of the molten steel surface
or of the continuous casting powder on the molten steel
surface. The maximum flow speed is, therefore, regulated so
as not to induce disturbance of the molten steel surface and
the continuous casting powder. The minimum flow speeds are
in the range of from 0.1 to 0.4 m/second. Below the minimum
flow speeds, the desired effects of the concentration
decrease cannot be achieved. The flow speed according to
the present invention is slow and is in the range of from
0.1 to l.0 m/second, preferably in the range of from 0.5 to
0.8 m/second. The flow speed within these ranges should be
a constant value over the entire solidification interface.
Although the flow speed of the present invention may partly
overlap the conventional flow speeds for removing bubbles,
~) the present invention is directed toward a slow flow speed,
,;,
because there is a difference in the mechanism of
suppressing the bubbles between the present invention and
the conventional processes for flowing molten steel. The
,,
concept of the present invention resides in the fact that
the generation of nuclei is suppressed in a stage prior to
growth of the nuclei into bubbles, while, in the concept of
the conventional processes the already grown bubbles are
moved upwards and then removed.
The depth of the rotational flow will now be
explained. This depth has a relationship to the thickness
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- 16 -
of the non-defective surface solidification layer of a
strand without blow holes. ~heoretically, when the minimum
non-defective solidification layer is present on the surface
layer of a strand, any blow holes within the surface layer
are pressed tightly during the subsequent rolling steps and
present no problems in the practical use of the rolled
product. However, practically speaking, since a
considerable amount of scale off occurs until the rolling.
for example, during the casting and in the heating furnaces,
the blow holes may be exposed to the strand surface unless
the scale off is taken into consideration. Since the amount
of scale off is approximately from 0.7 to 5 mm, the
rotational flow realized at the entire circumference of the
solidification interface extends from the molten steel
surface where the solidification initiates, to a depth where
the solidification surface equal to the scale off amount is
formed. The so imparted rotational flow is formed on the
entire circumference of the solidification interface in the
upper portion of liquid pool in the mold, and such
rotational flow has a strip form wherein its width is in the
vertical direction in the liquid pool. The position of the
solidification layer having a thickness in the range of from
0.7 to 5 mm is dependent upon the casting speed, but is from
approximately 50 to 200 mm below the molten steel surface
when the casting condition is such as that usually adopted.
Regarding the thickness of the rotational flow
mentioned above, such thickness is preferably as small as
possible, so as to save energy and to decrease effects of
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1527Z3
- 17 -
the rotational flow on the molten steel surface to the
lowest degree. Hereinafter, the thickness of the rotational
flow will be explained.
Referring to Fig. 5, the flow within the mold has a
distribution of speed depending upon distance from the mold
wall. This distribution is dependent upon the propulsion
force imparted by, for example, the flow-imparting device
explained hereinbelow, and the thickness of a copper plate
of the mold. When these conditions are properly adjsuted, a
flow speed of as high as 1.0 m/sec at the surface of the
mold wall can be decreased to a value less than one half of
1.0 m/sec at a portion in the mold from 10 to 20 mm distant
from the surface of mold wall, as exemplified in Fig. 5.
Accordingly, a flow having a thickness of from 10 to 20 mm
and being adjacent to the mold wall, is an essential portion
of the flow which participates in the suppression of the
nuclei of bubbles, and the flow distant from the mold wall
by more than 20 mm exerts almost no influence on the
behaviour of the molten steel surface. From the point of
view of the function of suppressing the nuclei of bubbles,
only a flow of molten steel having a thickness of from 10 to
20 mm participates in such function.
In the present invention, there is provided a
concrete means for imparting a rotational flow to the molten
steel which is present around the entire circumference of
the solidification interface in the upper portion of liquid
pool within the mold. Such means is preferably a device for
generating an electromagnetic force, especially a linear
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- 18 -
motor, which forms the rotational filmy flow, from the point
of view of economy and stability. In the embodiment of the
rotational flow imparting means illustrated in Figs. 6 and
7, the linear motors 8 and 8' are located in the cooling
boxes of both long sides 9 of the mold 7. The propulsion
forces of the linear motors 8 and 8' are directed in the
directions a and b which are opposite to one another,
thereby providing the rotational f low 3. The installing
position of the linear motors 8 and 8' in the vertical
direction is illustrated in Fig. 7. The linear motors 8 and
8' installed at the position shown in Fig. 7 subject the
solidification interface 2 to the strip-shaped rotational
f low 3. The region of the solidification interface 2
subjected to the rotational flow in the form of a stup
extends from the initiating point of solidification at the
molten steel surface l to the position of the solidification
interface 2 where the thickness of the solidification layer
is more than the amount of scale off, for example,
approximately 0.7 to 2.0 mm. The thickness of the
rotational f low 3 having the predetermined f low speed
explained hereinabove, is from approximately lO to 20 mm.
Theoretically, it is possible to subject the entire
solidification interface of the region as explained above to
the electromagnetic flow, but this is actually rather
difficult, especially when the product to be cast is a slab
having a rectangular shape in lateral cross section. In the
case of a bloom or billet having a round or square cross
sectional shape, a smooth electromagnetic flow can be
11527Z3
-- 19 --
relatively easily obtained, because of the un-form distance
of the center from the wall of the mold. On the other hand,
in the case of a slab, since there is a significant
difference between the lengths of the long sides and short
sides of the mold, and further, since the distance from the
wall to the center of the mold is not uniform, the
electromagnetic flow is not uniform. Furthermore, since the
flow speed itself imparted to the molten steel according to
the present invention is slow, the flow of molten steel may
be interrupted at the four corners of the slab mold due to
the stagnation of electromagnetic flow, with the result that
the intended objects cannot be achieved as explained
hereinàfter with reference to Fig. 8. In Fig. 8, the linear
motors 8 and 8' are installed along the direction of the
long sides 9 of the slab mold 7, and qenerate propulsion
forces having directions a and b opposite to one another.
The stagnation may be caused at the four corners Cl through
C4 of the slab mold 7.
In the present invention, a predetermined position of
the solidification layer is stably subjected to the
electromagnetic flow having a predetermined speed, without
causing a disturbance of the molten steel surface and
stagnation in the slab mold. These conditions of the
electromagnetic flow were investigated and established as
follows.
First, the present Inventors conducted detailed
research on the configuration of the short sides of a slab
~ mold having such a shape that an electromagnetic field is
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- 20 -
created without stagnation.
In Fig. 9, the short sides 10 of the mold 7 are
provided with an outwardly concave or inwardly convex shape
having a radius of curvature corresponding to one half the
radius of curvature (R) of the effective le~gth of the short
side 10 of the mold 7. In the case of using the mold
illustrated in Fig. 9, the flow of molten steel ln the mold
- has a pattern 5 as schematically illustrated in Fig. 9.
When the short sides 10 have a concave shape as illustrated
10 in Fig. 9, the flow generated at one of the long sides 3 can -
be satisfactorily transmitted to~one of the short sides 10
with the radius of curvature (R) as shown in Fig. 9 and even
to the opposite long side 9, with the consequence that a
continuous horlzontal rotational flow 3 without stagnation
can be formed within the mold 7. The formation of the inner
; surface of the mold short sides in a concave form is known
by itself in the continuous casting of steel from JA-OS
Wo. 52-117234. However, JA-OS No. 52-117234 has the object
of providing a solidiication shell of the short sides of a
slab with an arch structure and, thus, preventing the
,~, :
bulging of the short sides of the slab during the casting
procedure. Therefore, JA-OS No. 52-117234 does not suggest
the continuous horizontal rotational flow of the molten
, ,~ . .
steel according to the present invention. The concave shape
~` 25 of the inner surface of the continuous casting mold short
- sides is also known from USP No. 2781562, although this
~ patent is related to a nonferrous metal. However, the
:: ~
~ object of this patent is to decrease a solidification gap
`
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between the ingot and the mold, and thus, also does not
suggest the rotational flow mentioned above.
Second, the present Inventors conducted research on
the flow patterns obtained by using short sides of the mold
having various radiuses of curvature (R). It was proven, as
a result of the research, that the flow pattern equivalent
to the pattern illustrated in Fig. 9 can be obtained at a
radius of curvature (R) of from one half to two times the
effective thickness (d) of the short sides of the strand.
Referring to Fig. lO, the relationships between the flow
speed and the distance x are illustrated in the cases of
using various shapes of the short sides. The distance x
designates the distance from one of the short sides to the
center of the long sides in the direction of the long sides
at the middle portion of the mold thickness or one half of
the thickness (d) of the strand or the mold short sides. As
seen in Fig. lO, when the radius of curvature (R) of the
mold is from one half to two times the strand thickness (the
solid and dotted lines), a relatively rapid flow is formed
at the short sides of the mold and the flow speed becomes
abruptly slow when the flow leaves the short sides.
Contrary to this, when the radius of curvature (R) is three
times or more the strand thickness ~d) (the chain line), the
flow speed is relatively decreased to less than one half of
that of the two curves mentioned above, and further, there
is no appreciable change in the distribution of speed in the
direction toward the center of the mold long sides. These
facts mean that the electromagnetic flow is effectively
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llSZ723
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obtained at the solidification interface in the former two
radiuses of curvature, while in the latter radius of
curvature, the horizontal rotational flow exerts no
appreciable influence upon the solidification interface at
the short sides and is dispersed toward the interior of the
mold. The dispersion of the flow in the latter case may
cause dlsturbance of the molten steel surface or stagnation
portions, with the result that the entire circumference of
the solidification interface cannot be subjected to the
desired electromagnetic flow. Such tendency of the
horizontal rotational flow in the case of R < 3d is common
in the conventional molds with linear short sides. The
disturbance of the molten steel surface mentioned above may
bring about in its turn a non uniform distribution of the
continuous casting powder on the molten steel surface and
entrapment of such powder into the molten steel, while the
interruption of the electromagnetic flow due to the
stagnation, may bring about the formation of bubbles. As
will be understood from the above explanation, in order to
subject the predetermined region of the solidification
interface to the electromagnetic flow, the shape or
: : configuration of the short sides must be selected so that
: the radius of curvature (R) of the short sides is in the
: range of from one half to two times, preferably from one
25 half of and equal to the strand thickness (d).
The shapes of the mold short sides as illustrated in
Figs. 11 A, B and C, are e~bodiments of a continuous casting
mold.~ These concave polygon shapes can be practically
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~152723
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employed for providing a relatively effective magnetic flow,
but are not as ideal as the short sides having a radius of
curvature in the range of from one half to two times the
strand thickness. In short, as will be understood from
Fig. 8, the stagnation is generated by the flow disturbance,
which results from the flow advancing in the direction from
the long to short sides and colliding against the short
sides. Consequently, the entire solidification interface
can be subjected to the electromagnetic flow, when the short
sides of the mold are provided with a concave shape,
including a concave polygon snape, so that the flow is
smoothly guided from the long sides toward the short sides.
Third, the present Inventors conducted experiments
regarding the installation position of the linear motors
provided at both long sides of the mold, so as to stably
obtain a thickness of a non-defective solidification layer
which was more than the predetermined thickness. This is
because, by means of only the measures explained above,
which make it possible to provide the electromagnetic flow
20 without stagnation, it is difficult to obtain the thickness
of the solidification layer mentioned above regarding a slab
and, therefore, the installation position of the linear
motors is important. In the experiments conducted by the
Inventors, the linear motors were installed at various
25 depths fram the position corresponding to the malten steel
surface in the mold. In one experiment, the linear motors 8
and 8' were installed considerably below the molten steel
surface, so that, as seen in Figs. 12 A and B, the
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predetermined position of the molten steel in the mold,
namely the molten steel surface, would be subjected to the
electromagnetic flow. In addition, a portion of the
solidification interface, which portion extends from the
liquid surface down to the position where t~e solidification
layer having a thickness of scale off is formed, was also
subjected to the electromagnetic flow. Furthermore, the
mold short sides had a radius of curvature (R) of from one
half to two times the strand thickness (d), which radius is
optimum for obtaining the electromagnetic flow without
stagnation. The flow speed was slow because it was from 0.1
to 1.0 m/sec. Although the shape of the mold short sides
was ideal and, further, the flow speed was slow, the
obtained force of electromagnetic flow 13 was directed not
lS in the horizontal rotating direction but in the directions
from both of the mold short sides toward the mold center.
Due to the directions of the electromagnetic flow mentioned
above, the continuous casting powder 14 on the molten steel
surface is gathered up toward the center of the molten steel
surface, with the consequence that no powder is present on
the molten steel surface at both short sides. Normal powder
casting cannot, therefore, be carried out and such accidents
as breakout are caused by the electromagnetic flow as
explained above. The phenomenon of the gathering up of the
continuous casting powder is caused by the upward components
15, as explained with reference to Fig. 12B, which
illustrates a vertical cross sectional view of the mold. In
Fig. 12B, the linear motors 8 and 8' are installed at such a
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depth from the molten steel surface l that the components of
the electromagnetic flow in the laminar form along the long
sides are not transmitted satisfactorily to the molten steel
surface. ~en these components of the electromagnetic flow
collide against the short sides, these components are
divided into upward components 15 and downward
components 16. In a case where the upward components 15 are
stronger than the horizontal electromagnetic flow 3, the
phenomenon mentioned above appears.
As will be understood from this explanation, if the
linear motors 8 and 8' are installed below a certain
position, the horizontal electromagnetic flow 3 tends to be
weak, while the upward components 15 of the flow, which
later advance from the short sides to the center of the
mold, tend to be strong and the most influential.
Therefore, in the present invention the linear motors are
installed as close as possible to the molten steel surface,
provided that the installing position of the linear motors
is within the range in which the non-defective
solidification layer having a thickness corresponding to the
thickness of scale off mentioned above is provided.
According to research conducted by the present
Inventors, the phenomenon illustrated in Figs. 12A and B is
conspicuous when the installing position of the linear
motors is 150 mm or more from the molten steel surface and
is particularly pronounced at 20~ mm or more from the molten
steel surface. Therefore, the linear motors are preferably
installed at a distance from the molten steel surface of
.. , . , .. . ~ .. . .
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,
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~15~,723
,
- 26 -
less than 200 mm, more preferably less than 150 mm. These
distance values can be applied for the linear motor
installation in almost all molds and are not dependent on
the size of the mold.
In the arrangement as illustrated in Fig. 13A, the
short sides of the mold are provided with a radius of
curvature (RJ which is from one half to two ti~es the strand
thickness (d) as mentioned hereinabove, and in the
arrangement illustrated in Fig. 13B the linear motors 8, 8'
are installed in the range of from 150 to 200 mm below the
molten steel surface, e.g., 150 mm. ~he flow pattern
obtained by the mold and the linear motors illustrated in
Figs. 13A and 13B is illustrated in Fig. 13A. The
electromagnetic flow without stagnation, which flow causes
no disturbance at predetermined positions of the molten
steel surface as can be understood from Fig. 13A, is not
obtained unless both an ideal shape of the short sides and
an optimum installing position of the linear motors are
provided. The pattern of electromagnetic flow in Fig. 13A
20 causes no disturbance of the continuous casting powder on
the molten steel surface, because the horizontal
electromagnetic flow 3, which will later collide against the
short sides 10, is sufficiently strong for: not causing
disturbance of the powder on the molten steel surface, and;
25 obtaining an electromagnetic flow 3 which is continuously
distributed only on the solidification interface.
Furthermore, the downward components 16 of the
electromagnetic flow are at a position below a portion of
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`` 1152~7Z3
- 27 -
the solidification interface, which portion is to be
subjected to the electromagnetic flow. These downward
components do not, therefore, exert an undesirable influence
on the formation of the non-defective solidification layer.
When the linear motors are installed at a position
approximately 100 mm below the molten steel surface, such
installing position does not always lead to serious
accidents, such as breakout, in the case of a conventional
flat shape of the short sides. However, it cannot be said
that definitely no danger of these accidents exists, because
the liquid pool slightly swells at the short sides, due to
the flat shape thereof, when the electromagnetic flow
collides against the flat short sides, and; as a result, the
supplied amount of the continuous casting powder may be too
lS low at the swelled portion of the liquid pool.
As explained in detail above, in order that neither
disturbance of the molten steel surface nor stagnation are
caused, and further, that the predetermined region of the
solidification interface is subjected to the continuous
electromagnetic flow, both the shape of the short sides of
the mold and the installing position of the linear motors
should satisfy the conditions in accordance with the present
invention.
In an ideal embodiment of the present invention, the
linear motors are installed in a cooling box of the mold, in
such a manner that the centers of the cores of the linear
motors are located at the level of the molten steel surface.
In addition, the continuous casting operation is carried out
.. . , . . . ~ . ~
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l~S27Z3
- 28 -
in such a manner that: the region of the liquid pool
influenced by the laminar flow, which is caused by the
linear motors, extends as deep as 200 mm ~rom the molten
steel surface, and; the flow speed of the electromagnetic
flow 3 is in the range of from 0.1 to 1.0 mmlsec in this
section.
Actually, the linear motors at the ideal position
mentioned above may pose an installation difficulty.
Furthermore, a disadvantage is involved in the flow speed in
the range of from 0.1 to 1.0 m/sec at a depth of 200 mm from
the molten steel surface. It is, therefore, practically
advisable to install the linear motors in such a manner that
the centers of the cores of the linear motors are positioned
approximately 200 mm below the molten steel surface, thereby
utili~ing upper and lower laminar flows for realizing the
influence mentioned above. In this installation, the
interface between the liguid pool and the solidification
layer having a thickness of up to 5 mm is effectively
subjected to the continuous electromagnetic flow having a
speed in the range of from 0.1 to 1.0 m/sec, while neither
disturbance of the molten steel surface nor stagnation are
caused. Since the main object of the present invention is
to obtain a non-defective solidification layer at the
required minimum region of the solidification interface, as
will be understood from the explanation above, the number of
linear motors arranged in a vertical direction may be one or
more.
One important aspect of the present invention will
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1~52723
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now be explained. Referring to Fig. 14, the progress of
solidification at the solidification interface, which is to
be subjected to the electromagnetic flow of molten steel, is
delayed as (indicated by the broken line) as compared with
5 the progress of solidification of the conventional process
without electromagnetic flow (the solid line). In order to
obtain a non-defective solidification layer having a
required thickness, it is necessary to deepen the position
of the electromagnetic flow as compared with such position
lO in the conventional method. In this regard, the
relationship between the electromagnetic flow and the
streams of molten s~eel ejected from an immersion nozzle
which is used for pouring the molten steel into the mold, is
important from a practical point of view. Since the process
15 of present invention is based on powder casting, exposed
casting stream cannot be employed for pouring in the present
invention. If so called open pouring is used, disturbance
of the continuous casting powder is caused during the
pouring of the molten steel into the mold. Therefore, the
20 immersion nozzle, namely the immersion type pouring nozzle,
which is immersed into the molten steel in the mold, is
indispensable in the process of the present invention.
Referring to Fig. 9, the linear motors 8 and 8' are
installed in the cooling boxes of the mold 7 for a slab
25 along the long sides thereof. The solidification interface,
where the thickness of solidification layer is equal to or
less than the thickness of scale off, is subjected to the
electromagnetic flow, which is generated by the propulsion
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~15Z723
- 30 -
force of the linear motors in directions a and b opposite to
each other. In accordance with the present invention the
electromagnetic ~low is generated in such a manner that the
streams ejected from the immersion nozzle 11, for pouring
the molten steel into the mold, do not impede the
electromagnetic flow, to which the solidification interface
mentioned above is subjected. Regarding a conventional
immersion nozzle, the outlet ports of such a nozzle coincide
in most cases with the position of forming the
electromagnetic flow. If this conventional immersion nozzle
is used in the present invention, the electromagnetic flow,
which is slow so as not cause the disturbance of the molten
steel surface within the mold, is impeded by the streams
ejected from the immersion nozzle. Namely, the
electromagnetic flow is formed only partially due to the
influence of the ejected stream. It is to be noted in this
regard that, at the region of the solidification interface,
where the thickness of the solidification layer is equal to
or less than the thickness of scale off, the progress of
solidification is delayed due to the electromagnetic flow,
to which this region of solidification interface is
subjected, as seen in Fig. 14, wherein the solid and dotted
lines indicate casting without and with electromagnetic
flow, respectively. Accordingly, if the influence of the
25 ejected streams exists as mentioned above, the suppresion of
the nuclei of bubbles and the formation of a psuedo-rimmed
layer are only partially performed. This fact is the result
of the generation of a stagnation portion of the
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l~SZ723
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electromagnetic flow. ~ue to such stagnation portion, it is
impossible to form the filmy, electromagnetic flow, which
must be continuous along the solidification interface so as
to form a non-defective solidification layer around the
entire surface of the strand. The electromagnetic flow,
which is disadvantgeously discontinuous, is formed during
the casting whether the ejected streams of molten steel are
directed to the short or long sides of the mold, although
the degree of discontinuousness varies according to the
direction of the streams. However, if the flow speed of the
electromagnetic flow is high, the elimination of the
disadvantage mentioned above might seem to be possible.
When the flow speed of the electromagnetic flow is, however,
adjusted to exclude the influence by ejected streams
mentioned above, the stagnation disappears but dlsturbance
of molten steel surface i5 caused. This is because the
collision force of the electromagnetic flow against the
walls is large, since the flow speed of the entire flow is
high. As a result, the upward components of the flow are so
strong that they are stronger than the horizontal
electromagnetic flow. When the disturbance of molten steel
surface is caused, due to the upward components, the merits
of the powder casting are lost. In accordance with the
present invention, the ejecting position of the immersion
nozzle is set deeply within the liquid pool by means of:
providing the immerssion nozzle with a relatively large
length; orienting the ejecting direction of the immersion
nozzle downwardly or; a combination of a relatively long
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li52723
- 32 -
nozzle and a downward ejecting direction. As a result of
such adjusting methods, the electromagnetic flow mentioned
above is formed between the molten steel surface and the
streams ejected from the immersion nozzle, thereby not
allowing the stream ejected from the immersion nozzle to
impede the electromagnetic flow, which is required for
forming the non-defective solidification layer having a
predetermined thickness.
~ continuous casting apparatus according to an
embodiment of the present invention is illustrated in
Fig. 15 in the form of a cross sectional view along line B-B
of Fig. 9. ~s seen in Fig. 15, the linear motors 8 and 8'
are installed at a portion of the mold corresponding to a
level between the molten steel surface 1 in the mold 7 and
the ejected streams 17. The output of the linear motors 8
and 8' is adjusted so as to: subject a region of the
solidification interface to a continuous electromagnetic
flow; form the solidification layer free from blowholes,
and; extend the region of the solidification interface
mentioned above from the molten steel surface to a portion
of the solidification interface where the thickness of the
solidification layer is equal to the thickness of scale off.
The position of the streams 17 ejected from the immersion
nozzle 11 is below the electromagnetic flow.
In addition to the installing position of the linear
motors 8 and 8' as illustrated in Fig. 15, the linear motors
may be installed at such positions as illustrated in
Figs. 16 and 17. In Fig. 16, the linear motors 8 and 8' are
.
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- 33 -
installed at the level of the molten steel surface 1, and in
Fig. 17 the linear motors 8 and 8' are installed over the
mold region extending from the molten steel surface 1 to the
position of the streams ejected from the immersion
nozzle 11. In both Figs. 16 and 17, the installing position
of linear motors 8 and 8' extends from the molten steel
surface 1 to the ejected streams 17, so as to thereby obtain
a predetermined thickness of the non-defective
solidification layer at a predetermined region of the
solidification interface. When the position of the ejected
streams 17 is 300 mm below the molten steel surface 1, the
installing position of the linear motors as illustrated in
Fig. 15 may be from 10 to 20 mm below the molten steel
surface. In this installation of the linear motors, a
lS 2-4 mm thick non-defective solidification layer having the -
thickness more than the thickness of scale off is secured by
the effects of the electromagnetic flow, while the ejected
streams 17 exert no influence on the growth of the
solidification layer.
In practicing the process of the present invention,
the direction of streams 17 ejected from the immersion
nozzle is important. In this regard, it is possible to use
the following measures employed in the art of continuous
casting. According to these measure the upward flow is
generated as low as possible from the stream which is
ejected from the immersion nozzle and, then, collides a-
gainst the walls of mold, and the outlet ports of the
immersion nozzle are provided with such an angle that the
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115Z7Z3
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horizontal rotational flow which is formed above the ejected
streams is not disturbed. As a result, it is possible to
bring streams 17 ejected from the immersion nozzle close to
the molten steel surface. The ejected streams mentioned
above may be horizontal, as illustrated in ~igs. 15, 16 and
17, or may be oriented in a downward direction, as
illustrated in Fig. 18. The ejected streams 17 of Fig. 18
are particularly preferable.
It is preferable to adjust the concentration of free
oxygen in the molten steel so that it is within the range of
from 50 to 150 ppm, although the free oxygen concentration
may be in the range of from 50 to 200 ppm, as mentioned
above. ~hen the free oxyg~n concentration is in the range
of from 50 to 150 ppm, the generation of the nuclei of
15 .bubbles can be reliably suppressed by means of an
electromagnetic flow of molten steel having a flow speed in
the range o from 0.1 to 1.0 m/sec, preferably from 0.5 to
0.8 m/sec.
The solidification interface, which should be
subjected to the horizontal electromagnetic flow of molten
steel, extends from the molten steel surface to a portion of
such interface where the thickness of the solidification
layer formed from the molten steel is at least 5 mm. In
order to subject such solidification interface to the
horizontal electromagnetic flow having an almost constant
flow speed in the range of from 0.1 to 1.0 m/sec, preferably
from 0.5 to 0.8 m/sec, the following operating conditions of
the linear motors can be used. The frequency of the
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115~7Z3
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electric current conducted through the coils of the linear
motor can be from 1 to 10 Hz and the product of the electric
current value and number of turns of coils in terms of
amperes.turn is from 1500 to 7000 amperes.turn. It is
assumed in setting this condition that the thickness of the
copper plate of the long sides of the mold is as small as
possible, for example from 5 to 12 mm. It is preferable to
provide the flow of molten steel with such a distribution of
flow speed that the flow speed at the required position in
the mold, i.e., at the solidification interface, falls
within the range of the flow speed mentioned, above, while - --
the flow speed at the inner position within the required
position is extremely slow. Such distribution of the flow
speed can be obtained by adjusting the frequency of the
electric current through the linear motors, so that the
frequency is within the range of from 1 to 10 Hz, for
example, from 3 to 10 Hz. Due to this high frequency the
output of the linear motors tends to be relatively low, so
that it is diffic~lt to obtain the flow speed of from 0.1 to
20 1.0 m/sec, particularly from 0.5 to 1.0 m/sec. Accordingly,
the electric current should be adjusted in such a manner
that a high magnetomotive force in the range mentioned
above, for example, from 4000 to 7000 amperes.turn, makes up
the deficit of the output.
As will be understood from the above explanation, in
a continuous casting process for the steel slab, based on
the employment of continuous casting powder and a nozzle
immersed in fhe molten steel in the mold, the improvement
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11527Z3
- 36 -
according to the present invention comprises: casting a
molten steel containing free oxygen at a concentration in
the range of from 50 to 200 ppm, preferably from 50 to
150 ppm; not allowing an ejected stream of molten steel from
the outlet ports of the immersion nozzle to exert an
undesirable influence on the electromagnetic flow of molten
steel, and; subjecting a predetermined region of the
solidification interface to the electromagnetic flow. Since
there is no undesirable influence on the electromagnetic
flow, the powder casting process conducted by using an
immersion nozzle is not impeded at all and a slab without
blow holes on the surface thereof can be stably cast.
In order to form the non-defective solidification
layer without blow holes on the surface of the strand and to
cause no disturbance of the continuous casting powder on the
molten steel surface, the filmy rotational flow of molten
steel is formed around the entire solidification interface
at the liquid pool in the upper portion of the mold, and is
provided with a strip form, which has a short side in the
vertical direction of the liquid pool. Namely, the filmy
rotational flow of molten steel having the required flow
speed is formed at a limited portion of the liguid pool
adjacent to the mold walls, while in the other portion,
i.e., the center portion of the liquid pool, the molten
steel flows slowly or almost not at all. It is, therefore,
possible to industrially conduct the continuous casting of
the deoxidation unfinished steels or the slightly deoxidized
steels, which correspond to the rimmed and semi-killed
,
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- 37 -
steels.
When practicing the process of the present invention,
attention should be paid to the distance L (Fig. 19) between
the immersion nozzle 11 for pouring the molten steel into
the mold and the walls of the long sides 9 of the mold.
When the distance L is less than 20 mm, the resistance
against the flow, which is provided with the predetermined
flow speed in a portion of the molten steel surface adjacent
to the mold walls, is so high that a smooth flow cannot
always be achieved. The distance L should, therefore, be
20 mm or more. The maximum distance L is ordinarily
determined in accordance with the size of the mold, diameter
of the immersion nozzle and the like.
Since streams ejected from the immersion nozzle have
an influence upon the filmy rotational flow, the ejected
streams 18 are directed below the filmy rotational flow so
as to reduce such influence. It is possible to effectively
: eliminate the remaining influence of the ejected streams
directed below the filmy rotational flow by ejecting the
streams from outlet ports 13 as shown in Fig. 20. The
streams ejected from the immersion nozzle 11 are directed in
almost the same direction as the direction of the rotational
flow.
The present invention will now be explained further
in detail by way of an example and control examples. In
these examples the heats 1 and 2, corresponding to the
rimmed steel, and the heats 3 and 4, corresponding to the
semi-killed steel, as illustrated in the table below, were
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llS2723
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continuously cast. The oxygen concentrations as shown in
the table were obtained by the use of a deoxydant in heats 1
and 2 and by vacuum degassing in heats 3 and 4.
T a b 1 e
Te~nperature
C Si Mn P S So AQ o in Tundish
Heat (~) (%) (%) (%) (%) (%) (ppm) (C)
1 0.05 tr. 0.12 0.013 0.015 tr. 150 1550
2 0.06 tr. 0.12 0.012 0.020 tr. 200 1550
3 0.15 0.15 0.60 0.013 0.012 0 75 1530
4 0.20 0.16 0.57 0.011 0.011 0 70 1530
_
Example
The steel of each heat number was cast under the
following conditions.
Shape of mold: short sides were arched with a radius
of curvature (R) equal to one half of the strand thickness.
Size of mold: 250 mm in thickness and 2100 mm at
maximum length.
Casting speed: 0.7 m/min.
Installing position of the linear motors: the
centers of the linear motors were 200 mm below the molten
steel surface.
Immersion nozzle: the immersion nozzle had an outer
diameter of 100 mm and was disposed at the center of the
mold; the jetting position was 250 mm below the molten steel
115Z7Z3
.
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surface, and; the ejecting direction was toward the short
sides.
Frequency of Electric Current: 5 Hz.
Magnetomotive force: 380A~18 turns
State of rotational flow of molten s~eel: the
effective thickness of flow was from 10 to 20 mm; the
effective depth of flow was from the molten steel surface to
a position 200 mm below the molten steel surface; the
thickness of the solidification layer 200 mm below the
molten steel surface was from 0 to 3 mm, and; the flow speed
was from 0.5 to 0.8 m/sec.
The continuous casting powder:
( 1 ) Cao/Sio2 = 1 . O
A123 = 10%
Na = 3.5%
K = 2.5
F = 4%
C = 4.5%;
(2) viscosity of 2.3 Poise at 1500C, and;
(3) melting point of 1150C.
In casting the heats 1 through 4, strands having a
non-defective solidification layer could be obtained without
causing the entrapment or disturbance of the continuous
casting powder on the molten steel surface in the mold. The
traversal cross sectional macro-structure of the produced
strands of the heats 1 through 4 was investigated. It was
proven that the 3 mm thick non-defec ive solidification
layer was uniformly formed around the entire surface of the
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strands of all of heats 1 through 4. Blow holes were
present inside the non-defective solidification layer~
The strands in the form of a slab, which were
produced from the heats 1 through 4, were reheated and hot
rolled by a conventional process. Some of the strands were
then cold rolled by a conventional process. No surface
defects were detected in any of the final products so
rolled.
Control Example 1
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same casting
conditions as in the Example, except that the area of
horizontal rotational flow was extended to the center of
mold, and further, that the flow speed at the walls of mold
was approximately 3.0 m/sec. The flow speed of the
horizontal rotational flow was l.0 m/sec.
The molten steel surface was vigorously disturbed and
the continuous casting powder finally gathered in the center
of mold. Since the danger of breakout became high, the
casting had to be terminate. Regarding the cast strands,
the structure thereof was observed after solidification. It
was proven that the continuous casting powder was entrapped
into a considerable number of the strands. In the present
control example, the continuous casting powder was disturbed
due to the rotational flow extending even to the center of
mold.
Control Example 2
Molten steel having the same composition as that of
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the heats 1 through 4 was cast under the same condition as
in the Example, except that the flow speed of the rotational
flow was from 0.1 to l.0 m~sec and, further, the filmy
rotational flow was not imparted to a portion of the molten
steel arrive adjacent to the molten steel surface. Blow
holes or pinholes were detected on the surface of all of the
strands. After the casting, the strands were cnarged into a
heating furnace and then rolled, thereby obtaining the final
products. Numerous surface defects were generated on the
final products and recovery of the final products was,
therefore, too low. It is believed that in the present
control example, wherein the rotational filmy flow for
suppressing the generation of nuclei of bubbles was not
imparted to the molten steel surface, the generation of
lS bubbles could not be suppressed at the outer surface layer
adjacent to the molten steel surface and the bubbles were
thus generated.
Control Example 3
~qolten steel having the same composition as that of
the heats l through 4 was cast under the same condition as
in the Example, except that the short sides of the mold were
flat in shape as in a conventional mold. The continuous
casting powder was finally collected at the center of the
mold and, therefore, the danger of breakout arose. The
casting was then conducted without the propulsion force of
the linear motors, which were stopped in the course of the
casting. As a result, numerous pinholes were generated on
the surface of strands and, therefore, the recovery was
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extremely low.
Control Example 4
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same condition as
in the Example, except that the installing position of the
linear motors was 250 mm and 300 mm from the molten steel
surface. The molten steel surface was vigorously disturbed
and the state of the molten steel surface was similar as
that in Control Example 3.
Control Example 5
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same condition as
in the Example, except that the length of the immersion
nozzle was changed as follows andj further, the flow speed
was more than 1.0 m/sec with regard to the molten steel
having the composition of heats 2 and 4. The length of the
immersion nozæle was changed from that in the Example, so
that the upper surface of the ejected streams was 100 mm
below the molten steel surface. As a result of the casting,
defects of numerous pinholes were generated at portions
denoted by P on the strands 4 as shown in Fig. 21, which
schematically illustrates the strand. It is believed that
such defects result from stagnation and discontinuity of the
electromagnetic flow, due to the influence of the ejected
streams from the immersion nozzle.
The solidification interface could, therefore, not be
subjected to the continuous electromagnetic flow. Regarding
the molten steel having composition of heats 2
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and 4, the disturbance of the molten steel surface was so
serious that the danger of breakout arose. The casting was
then conducted without the propulsion force of the linear
motors, which were stopped in the course of the casting. As
s a result, numerous pinholes were generated on the strand and
the recovery was, therefore, extremely low.
Control Example 6
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same condition as
in the Example, except that the~short sides of the mold
~which was used were flat, as in a conventional mold and,
further, the flow speed of the electromagnetic flow was
0.2 m/sec and 1.3 m/se¢ with respect to the molten steel
: :
having the composition of the heats 2 and 4. When the
molten steel having the composition of the he;ats l and 2 was
cast at a casting speed of 1.3 m/sec, the molten steel
surface was seriously disturbed, and the continuous casting
powder gathered at the center of the mold. Therefore, the
danger of breakout arose.~
i 20 The casting was then conducted without the propulsion
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force of the linear motors, which were stopped in the course
of the~casting,~ As a result, numerous pinholes were
generated on the strands and the recovery was, therefore,
,
extremely low. Regarding molten steel having composition of
the heats 2 and 4 cast at a speed of 0.2 m/sec, numerous
,
pinholes were generated at diagonally opposite portions of
the short sides of strands. It is believed that the
generation of pinholes resulted from the flat shape of the
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1152723
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short sides and low flow speed, which cause stagnation, and
hence, discontinuity of the electromagnetic flow along a
solidification interface.
Control Example 7
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same condition as
in the Example, except that the flow speed of the horizontal
rotational flow at the molten steel surface within the mold
was 0.1 m/sec or less. Large blow holes were generated even
on the surface of slabs, which were obtained from the molten
steel having the composition of the heats 1 and 2.
Therefore, these slabs could not undergo the subsequent
processes. On the other hand, pinholes were detected in all
of the surfaces of the slabs, which were obtained from the
molten steel having the composition of the heats 3 and 4.
Surface flaws were numerously generated on the final
products produced from these slabs, so that the recovery was
extremely low. In the present control example, the flow
speed of the horizontal rotational flow in the mold was not
suficiently high to prevent the generation of nuclei of
bubbles. ~s a result, surface blow holes were generated.
Control Example 8
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same condition as
in the Example, except that the flow speed of the horizontal
rotational flow at the molten steel surface within the mold
was changed to 1.0 m/sec or more. Longitudinal cracks were
--) generated sporadically on the surface of the slabs, which
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were obtained from molten steel having the composition of
the heats 1 throug 4. No matter how small the dimensions of
the longitudinal cracks were, the slabs were subjected to
scarfing, and the scarfed slabs were subsequently subjected
to conventional processes. Surface flaws were generated
sporadically on the surfaces of the final products of these
processes, so that the recovery was low. It is believed
that the generation of the surface flaws resulted from
secondary flaws which were formed due to the scar~ing of
large longitudinal cracks in the slab stage. It was
observed at the casting of the present control example that
the continuous casting powder tended to collect at the
center of the mold due to the strong rotational flow. As a
result, the lubrication effect of the continuous casting
powder was not provided and longitudinal cracks were
generated.
Control Example 9
Molten steel having the same composition as that of
the heats 1 through 4 was cast under the same condition as
in Example, except that the solidification interface where
the solidification thickness was not more than 1.0 mm was
subjected to the electromagnetic flow having a flow speed of
from 0.1 to 0.4 m/sec. The output of the linear motors was
adjusted to provide this electromagnetic flow. In the
present control example, in all the molten steel having the
composition of the heats 1 through 4, the continuous casting
powder was not disturbed, and the thickness of the obtained
-j non-defective solidification layer was 0.5 mm. The strands
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obtained by the casting were subjected to the conventional
processes to produce the final products. Prior to charging
the strands into a heating furnace, the blow holes were
exposed. Due to a conditioning of the strands, the recovery
became extremely low. This is because a non-defective
solidification layer thicker than the amount of scale off at
the processes after the casting was not obtained.
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