DESCRIPTION
TITLE OF INVENTION
METHOD OF CONTINUOUSLY CASTING A STRAND
TECHNICAL FIELD
[OOO 11
The present invention relates to a method of continuously casting a strand
designed to reduce the formation of centerline cavities, porosity, and shrinkage cavities,
which are internal defects, by reduction-rolling the strand using a pair of rolls, and in
particular, it relates to a continuous casting method using movable rolls.
BACKGROUND ART
[0002]
Currently, a typical process of manufacturing steel is carried out in such a manner
that a strand is cast by a continuous casting process and the strand is subjected to
processes such as slabbing, blooming, or billeting, and rolling to be formed into a final
product. However, large size steel products having a large cross section such as steel
products for a boiler tank and a large size die as a final product are manufactured in
small lots, and they must be made from a cast steel having a large cross section.
Therefore, at the present time, cast steels which are used for large size steel products are
not produced by continuous casting, but are cast as a large size ingot by pouring molten
steel into a mold and causing it to solidifL therein. Hereinafter, this technique is
referred to as "ingot process."
[0003]
Casting large size ingots by an ingot process is much less efficient than by a
continuous casting process even though they are manufactured in small lots. Moreover,
in view of the need for feed from a feeder in an upper portion of the ingot, retention of
molten steel in a sprue and a stalk, or the like, the production yield is very low. The
term "feed from a feeder" as used herein refers to feeding of molten steel by an amount
corresponding to the solidification shrinkage in order to prevent the formation of
shrinkage cavities and shrinkage cracks due to solidification shrinkage of molten steel
in ingot casting.
[0004]
Furthermore, when a strand having a large cross section is cast by a continuous
casting process, centerline cavities, porosity, which is a bubble defect, and segregation
that occur in a central region of a strand tend to become larger. The term "centerline
cavities" as used herein refers to cavity defects that occur in a central region of an alloy
slab when the slab is cast. Moreover, when the casting comes to an end, after the
supply of molten steel into the mold is discontinued, large shrinkage cavities such as
those seen in a typical ingot process occur due to solidification shrinkage over an area
from the meniscus (molten steel surface) of the strand toward the downstream side
along the casting direction. These internal defects or the like not only decrease the
production yield, but, in some cases, remain in the final product and can be a major
cause of product defects.
[OOOS]
As a method of producing a strand having a large cross section with good internal
quality, Patent Literature 1 proposes using an inverted tapered mold in a
semi-continuous casting process for producing large size ingots such as extra thick flat
ingots that are difficult to cast in conventional continuous casting machines because of
their thickness. In addition, Patent Literature 1 discloses that the ingot quality can
further be enhanced by heating the meniscus in the top (upper portion) of the ingot by
electrical means.
[0006]
Patent Literature 2 discloses that, in continuous casting of a strand, the formation
of internal defects such as centerline cavities and porosity can be reduced by employing,
for the strand shape, a tapered shape in which the distance between the side surfaces of
the strand gradually increases toward an upper portion.
[0007]
In the meantime, it is generally known that continuous casting of a strand may
include a process of performing reduction rolling on the strand surface during the last
stage of solidification for the purpose of reducing internal defects such as porosity and
segregation. For example, Patent Literature 3 discloses a method of reduction-rolling
a strand having a liquid core.
3
[OOOS]
Using a tapered mold or employing a tapered shape for the strand shape as in the
techniques disclosed in Patent Literatures 1 and 2 can substitute, to some extent, for the
function of feed from a feeder as conventionally practiced. However, these methods
require complicated casting processes and high equipment cost, but nevertheless the
effect of inhibiting centerline cavities and porosity is limited, and the effect is reduced
with the increase in the strand cross section. Also, the technique of heating the
meniscus in the upper portion of a strand does not provide the advantage of improving
the internal quality of the strand up to its central region when the strand has a long
length, and further, the technique requires expensive equipment and also is
uneconomical in terms of energy. Therefore, it is not considered to be a very effective
technique.
[0009]
In contrast, the technique of squashing internal porosity during its formation
stage by reduction-rolling the strand at its surface using rolls or the like (in-line
reduction rolling technique), as practiced in continuous casting by a typical continuous
casting machine, is a decisive and very effective technique. However, when the in-line
reduction rolling technique is employed in continuous casting of a strand having a large
cross section, there are two problems as follows.
[OO 1 01
The first problem is that, in the in-line reduction rolling technique, the collapsing
of the porosity formed in a strand should not be carried out at a randomly selected stage
of casting, but must be carried out at an optimal time for reduction rolling. For
example, when the reduction rolling of the strand is to be carried out during the
formation stage of the porosity, the optimal time is considered to be the last stage of
solidification between the time in which the solid fraction at the central region is about
0.5 and the time of complete solidification. When the reduction rolling is to be carried
out after complete solidification, the optimal time is considered to be a time
immediately after solidification at which the temperature of the central region of the
strand is still sufficiently high. Therefore, in a typical continuous casting process,
reduction rolling devices such as reduction rolls are usually disposed at a specified
location such as in the vicinity of the exit of the continuous casting machine.
[OOl 11
However, when a strand having a large cross section is cast, in order to
reduction-roll the strand under optimal conditions for collapsing centerline cavities and
porosity by reduction rolling devices installed in the vicinity of the exit of the
continuous casting machine, it is necessary to extend the length of the continuous
casting machine to ensure time before the strand completely solidifies. It is to be
noted that the length from the meniscus in the mold to the position of the crater end of
the strand is considered to be proportional to the square of the thickness of the strand.
Thus, when a case of casting a strand having a thickness of 300 mm is used as a
reference, for example, casting of a strand having a thickness of 900 mm would require
a continuous casting machine nine times longer, and therefore would result in requiring
considerable cost for construction.
[OO 121
On the other hand, if it is impossible to extend the length of the continuous
casting machine, one approach that may be considered to ensure the time before the
strand completely solidifies is to reduce the casting speed. In general, the casting
speed (the speed of the strand) at the position of the crater end is considered to be
inversely proportional to the square of the thickness of the strand. Thus, when a case
of casting a strand having a thickness of 300 mm at a rate of 1 dmin is used as a
reference, for example, casting of a strand having a thickness of 900 mm would need to
be performed at a rate of 0.11 dmin, which is a very slow speed. Such very slow
speed casting causes insufficient heating at the meniscus in the mold, and leads to
, skinning of the meniscus, formation of a rippled casting surface due to shrinkage of the
solidified shell at the meniscus, and the like, and therefore results in significant
deterioration of the surface quality of the strand. In order to prevent the deterioration
of the surface quality, using both plasma heating and Joule heat for meniscus heating
may be considered, but it results in a high equipment cost and is uneconomical in terms
of energy as described above.
[00 131
The second problem is that, in the case of a strand having a large cross section,
the reduction force does not sufficiently penetrate into the depth of the strand and thus it
may not be possible to filly collapse centerline cavities and porosity.
CITATION LIST
PATENT LITERATURE
[00 141
Patent Literature 1 : Japanese Patent Application Publication No. S62- 16 1445
Patent Literature 2: Japanese Patent Application Publication No. 2004-243352
Patent Literature 3: Japanese Patent Application Publication No. 2000-288705
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[00 151
As described above, there are problems with the techniques of conventional
continuous casting methods for reducing centerline cavities and porosity in a central
region of a strand having a large cross section and reducing shrinkage cavities and
centerline cavities in an upper portion of the strand, in terms of equipment cost and
energy and in terms of surface quality.
[00 161
The present invention has been made in view of such problems with conventional
techniques. Accordingly, it is an object of the present invention to provide a method
which, in continuous casting, reduces centerline cavities and porosity in a central region
of a strand and shrinkage cavities and centerline cavities in an upper portion of the
strand regardless of the size of its cross section at a low equipment cost and without
causing deterioration of the surface quality.
SOLUTION TO PROBLEM
[00 1 71
In order to solve the above problems, the present inventors studied methods of
reduction-rolling a strand in continuous casting. Consequently, they have found that,
by using movable rolls to reduction-roll a strand, it is possible to perform reduction
rolling at an optimal location for collapsing centerline cavities, porosity, and shrinkage
cavities regardless of the size of the strand cross section. In this case, there is no need
for adjusting the length of a continuous casting machine or the casting speed, which
would be required in the case of using rolls fixed at a specified location, and therefore
the equipment cost is very low.
[00 1 81
The present invention has been achieved based on the above-mentioned findings,
and the summaries thereof are set forth in the following (1) to (3) relating to methods of
continuously casting a strand.
[00 191
(1) A method of continuously casting a strand, characterized in that the method
comprises: using a pair of rolls configured to interchangeably perform guiding and
supporting of a strand and reduction rolling of the strand, and to be movable in a
vertical direction along the strand below a mold, wherein, while the strand is being
withdrawn, the pair of rolls is held in a stopped condition and guides and supports the
strand, and after the withdrawing of the strand is completed, the pair of rolls is moved in
a vertical direction and accordingly reduction-rolls the stopped strand.
[0020]
(2) The method of continuously casting a strand according to the above (I),
characterized in that the move of the rolls while reduction-rolling the strand is in a
vertically upward direction.
[002 11
(3) The method of continuously casting a strand according to the above (1) or (2),
characterized in that the strand has a circular transverse cross section.
ADVANTAGEOUS EFFECTS OF INVENTION
[0022]
With the method of continuously casting a strand according to the present
invention, it is possible to significantly reduce centerline cavities, porosity, and
shrinkage cavities regardless of the size of the strand cross section as well as to cast a
strand with high production yields, using a continuous casting machine for which the
equipment cost is low, without causing deterioration of the surface quality.
BRIEF DESCRIPTION OF DRAWINGS
[0023]
[FIG. 11 FIG. 1 is a configuration diagram of a continuous casting machine to
which the method of the present invention is applicable, wherein FIG l(a) is a front
view thereof and FIG. l(b) is a side view thereof.
[FIG 21 FIG. 2 is a diagram illustrating a casting process in accordance with the
continuous casting method of the present invention, wherein FIG 2(a) shows a state at
the start of casting; FIG. 2(b) shows a state during withdrawing of the strand; FIG. 2(c)
shows a state in which the movable rolls have been moved to the lower end of the
movable range after completion of the withdrawing; FIG. 2(d) shows a state in which
the movable rolls are raised while they are reduction-rolling the strand; and FIG. 2(e)
shows a state in which the reduction rolling has been completed.
[FIG 31 FIG 3 is a graph illustrating the relationship between the ratio of the
reduction amount to the liquid core diameter of the strand (reduction amount/liquid core
diameter) and the area fraction of defects, wherein FIG 3(a) shows the results obtained
in a constant region and FIG. 3(b) shows the results obtained in an upper portion of the
strand.
DESCRIPTION OF EMBODIMENTS
[0024]
FIG. 1 is a configuration diagram of a continuous casting machine to which the
method of the present invention is applicable, wherein FIG. l(a) is a front view thereof
and FIG. l(b) is a side view thereof. The continuous casting machine shown in FIG. 1
is of the vertical type and configured to cast a strand in a vertically downward direction.
This continuous casting machine includes: a ladle 1 that contains molten steel; a mold 2
to which the molten steel is supplied from the ladle 1 via a submerged entry nozzle (not
shown); and a movable reduction roll unit 4 that reduction-rolls a strand 3 that has been
withdrawn downward from the mold 2. The mold 2 is composed of a set of half molds.
The movable reduction roll unit 4 includes a pair of rolls 5 and a frame 6 that supports
the rolls 5. The frame 6 is integral with the rolls 5 and vertically movable upward and
downward along the strand 3 below the mold 2.
[0025]
Immediately below the mold 2 are arranged support rolls 7 as shown in
later-described FIG. 2 (not shown in FIG. I), which form a support region for a
8
solidified shell 3a of the strand 3. In the continuous casting machine, it is preferred
that the solidified shell 3a be supported, at least immediately below the mold 2, at a
region extending a length about one fourth to about the same as that of the mold 2.
FIG. 2, which will be later described, shows an embodiment in which the support region
has a length about the same as that of the mold 2.
LO0261
The rolls 5 are configured to interchangeably hction as pinch rolls that guide
and support the strand 3 or as reduction rolls that reduction-roll the strand 3. The rolls
5 are hydraulically pressed from the back side toward the strand 3 to be brought into
contact with the strand 3. Furthermore, the rolls 5 are connected to a large speed
reducer 9 via a universal joint 8, and operate as drive rolls.
[0027]
The frame 6 is supported in a vertically movable manner by jack shafts 10 of
vertically arranged four ball screw jacks, and is also given a driving force to be movable
vertically upward and downward by the jack mechanism of the jack shafts 10.
[0028]
Since the rolls 5 are integral with the frame 6, they are vertically movable upward
and downward along the strand 3, and therefore it is possible to change reduction rolling
positions on the strand 3 and to move the rolls while they are performing reduction
rolling. The move of the rolls 5 can be carried out by rotating the rolls 5 themselves in
a state where they hold the strand 3 therebetween, and the direction of the move can be
changed by changing the rotation direction of the rolls 5. When the rolls 5 are not in
contact with the strand 3, they can be moved by the jack mechanism of the jack shafts
10.
[0029]
FIG. 2 is a diagram illustrating a casting process in accordance with the
continuous casting method of the present invention, wherein FIG. 2(a) shows a state at
the start of casting; FIG. 2(b) shows a state during withdrawing of the strand; FIG. 2(c)
shows a state in which the movable rolls have been moved to the lower end of the
movable range after completion of the withdrawing; FIG. 2(d) shows a state in which
the movable rolls are raised while they are reduction-rolling the strand; and FIG. 2(e)
shows a state in which the reduction rolling has been completed.
[0030]
With reference to FIG. 2, the continuous casting method of the present invention
will be described. Firstly, casting of the strand 3 is started as shown in FIG 2(a), and
the strand 3 is continuously withdrawn as shown in FIG. 2(b). In this process, the rolls
5 are arranged immediately below the mold 2, or actually immediately below the
support rolls 7, and are used as pinch rolls. After the strand 3 has been withdrawn up
to the limitation of the continuous casting machine, the strand 3 is stopped to complete
the withdrawing. Then, the rolls 5 are moved to the lowermost end of the movable
range as shown in FIG. 2(c). Thereafter, they are held waiting until the temperature of
the central region of the strand 3 and the thickness of the solidified shell 3a reach
optimal conditions for reduction rolling.
[003 11
After the state of the strand 3 has reached optimal conditions for reduction rolling,
the rolls 5 are pressed against the strand 3 to the extent that the amount of reduction of
the strand 3 reaches a predetermined amount, and the rolls 5 are rotated in the direction
opposite to the direction at the time of withdrawing, whereby the rolls 5 reduction-roll
the strand 3 while being raised along the axis of the strand 3 as shown in FIG. 2(d).
When the solidified shell 3a contains liquid steel 3b therewithin, the liquid steel 3b is
ejected onto the meniscus in the upper portion as the rolls 5 are raised while
reduction-rolling the strand 3 as shown in FIG. 2(e). The amount of ejected molten
steel is not so large in the case where the strand has a circular transverse cross section
compared to a case where the cross section is of a different shape, although it depends
on the size of the liquid core at the time of reduction-rolling of the strand. That is, the
amount of ejected molten steel is approximately within the capacity of the mold 2. On
the other hand, when raising the rolls 5 while performing reduction rolling after the
strand 3 has been completely solidified up to the core, there is of course no ejection of
liquid steel.
[0032]
By using the movable reduction roll unit 4 to reduction-roll the strand 3 as
described above, it is possible to efficiently reduction-roll the entire strand 3 and
collapse centerline cavities and porosity regardless of the size of the cross section of the
strand 3. The reduction rolling of the strand 3 may be performed continuously or
intermittently only for portions that need reduction-rolling.
[0033]
The conditions for reduction rolling of the strand 3 can be varied by changing the
speed at which the rolls 5 are raised. For example, by setting the speed for raising the
rolls 5 to be the same as the speed for withdrawing the strand 3, it is possible to
reduction-roll the entire strand 3 under the same conditions. The reason for this is as
follows. Even after the reduction rolling by raising the rolls 5 is started, solidification
of the liquid steel within the strand 3 proceeds with time to gradually diminish the liquid
core. When the speed for raising the rolls 5 is set to be the same as the speed for
withdrawing the strand 3, the length of time between the casting and the reduction
rolling is constant at any reduction rolling position, and therefore the size of the liquid
core is maintained substantially constant for all reduction rolling positions. It is to be
noted that the speed for raising the rolls 5 may not necessarily be the same as the speed
for withdrawing the strand 3.
[0034]
When the target for inhibition is only the formation of shrinkage cavities and
centerline cavities below the meniscus, the rolls 5 may be raised to a predetermined
position near the lower end of the mold 2 without reduction-rolling the strand 3, and
then, from the position, the rolls 5 may perform reduction rolling on the strand 3 while
being raised to an upper predetermined position. Conversely, the rolls 5 may be raised
to the upper predetermined position above the predetermined position near the lower
end of the mold 2 without reduction-rolling the strand 3, and then, from the position, the
rolls 5 may perform reduction rolling on the strand 3 while being lowered to the
predetermined position near the lower end of the mold 2.
[0035]
With the above steps, an entire process from withdrawing of the strand to
reduction rolling by raising the rolls is completed. Thus, a subsequent casting may be
carried out by repeating the process shown in FIG. 2 again after the strand is taken out.
[0036]
As described above, the use of movable rolls makes it possible to cast a strand
having good internal quality, without replacing the continuous casting machine,
regardless of the size of the strand cross section, at a low equipment cost and without
11
causing deterioration of the surface quality. Furthermore, since the process is a
continuous casting process, higher production yields than by ingot processes are
achieved in casting strands.
[0037]
In the description above, a continuous casting process using a vertical type
continuous casting machine has been described, but the type of continuous casting
machines to which the present invention is applicable is not limited to the vertical type.
The present invention may also be applicable to other forms of continuous casting
machines such as those of the vertical bending type or the arcuate curved type as long as
they have a section where casting can be performed in a vertically downward direction
from immediately below the mold.
[0038]
The type of strand to which this casting is applied is preferably a strand having a
circular transverse cross section. When a strand having a circular transverse cross
section is reduction-rolled by a pair of flat rolls, the solidified shell excluding portions
that are in contact with the pair of rolls is not greatly deformed, and consequently
deformation of only the portions that are in contact with the pair of rolls is sufficient to
inhibit centerline cavities and porosity that are formed in a central region of the strand.
Therefore, it is possible to efficiently collapse centerline cavities and porosity with a
smaller reaction force of the reduction rolling.
[0039]
Furthermore, when a movable reduction roll unit is included in a continuous
casting machine, it is very difficult to install support rolls for a strand and roller aprons
that hold the support rolls, which are installed in conventional continuous casting
machines, because they geometrically interfere with the movable reduction roll unit. If
support rolls are not installed, strand bulging may occur after static pressure of the
liquid steel within the strand is applied to the solidified shell. However, when the
transverse cross section of the strand is circular, the likelihood of the occurrence of
bulging can be reduced even when the solidified shell is subjected to static pressure of
the molten steel in the absence of support rolls to some extent.
[0040]
The reduction rolling of a strand may be carried out in a state where the liquid
core remains within the strand, or in a state where the strand is completely solidified.
Depending on the grade of the steel to which the casting is applied, internal cracking
may occur in the strand when it is reduction-rolled with the liquid core remaining
therein. Thus, in such a case, reduction-rolling may be performed after the strand is
completely solidified. Also, in some grades of steel, centerline cavities and porosity
that will occur are not relatively large, and therefore, in this case, collapsing of
centerline cavities and porosity can be sufficiently achieved even with reduction rolling
after the complete solidification.
EXAMPLES
[004 11
To verify the advantageous effects of the method of continuously casting a strand
of the present invention, the following casting tests (a preliminary test and a final test)
were conducted.
[0042]
1. Preliminary Test
1 - 1. Test Conditions
The strand to be cast was a small-size strand having a diameter of 300 mm and a
length of 1800 mm, and the steel grade selected was a 13% Cr steel, which has a
tendency to have increased centerline cavities and porosity. A continuous casting
machine of the type shown in FIG. 1 was used. However, support rolls for supporting
the solidified shell of the strand were not provided. The movable reduction roll unit
included rolls having a diameter of 450 mrn with a roll force of up to 100 t and a
maximum roll torque of 50 t.m. The speed for raising the movable reduction roll unit
for reduction rolling was set to 0.8 rnlmin and, after the casting of the strand over the
entire length was completed, reduction rolling was performed on the strand over the
entire length. The amount of reduction of the strand was 20 to 70 mm in terms of a
reduction in the strand diameter in the rolling direction. It is to be noted that the cross
section of the strand was flattened as a result of the reduction rolling.
[0043]
The diameter of the liquid core (hereinafter referred to as "liquid core diameter")
at the reduction rolling positions was 70 mm or 110 mm. These values were
determined by defining the solid-liquid interface by an isotherm corresponding to a
solid fraction of 0.8. The position of the interface where the solid fraction becomes
0.8 was determined by one-dimensional unsteady heat transfer and solidification
analysis of the cylindrical cross section. The calculation results were compared
against the results of measurement of the temperature of the strand surface,
measurement of the temperature inside the strand by a thermocouple, and measurement
of the liquid core diameter by addition of a tracer such as S, thereby ~ o ~ r m itnhagt t he
analysis is sufficiently accurate.
[0044]
1-2. Test Results
After completion of the test, each strand was cut so that the longitudinal section
passing through the center of the strand was exposed. The sectioned surface was
ground and polished, and then investigation was made on the formation of centerline
cavities, porosity, and shrinkage cavities. These defects appeared as voids in the
section of the strand, and the extent of the defects was calculated as an area percentage
of.voids (fraction of voids) in the entire area of the, cross section. The fraction of voids
was divided by the fraction of voids of a strand which was cast separately from the
reduction rolled strand without being subjected to reduction rolling by rolls (hereinafter
referred to as "non-reduction-rolled strand"), and the result was defined as the area
fraction of defects and was used as an index showing the extent of the formation of
defects. The area of voids was measured using multipurpose image and photograph
analysis software, but other methods may be used for the measurement.
[0045]
FIG. 3 is a graph illustrating the relationship between the ratio of the reduction
amount to the liquid core diameter of the strand (reduction amount/liquid core diameter)
and the area fraction of defects, wherein FIG. 3(a) shows the results obtained in a
constant region and FIG. 3(b) shows the results obtained in an upper portion of the
strand. The upper portion of the strand refers to a region where centerline cavities and
shrinkage cavities are formed in the case of a non-reduction-rolled strand, and in the
case of a reduction-rolled strand, it refers to a region corresponding to the region in a
non-reduction-rolled strand where centerline cavities and shrinkage cavities are formed.
The constant region refers to a region other than the upper portion of the strand.
[0046]
As shown in FIG. 3(a), it has been found that, when the value of the reduction
amountlliquid core diameter is increased, a significant reduction of centerline cavities
and porosity can be achieved. In addition, fiom FIG. 3(b), it has been observed that an
even greater effect of reducing defects is produced in an upper portion of a strand than
in a constant region.
[0047]
2. Final Test
2- 1. Studies of Casting Conditions
Based on the results of the preliminary test, casting conditions were studied for a
case where the quantity of molten steel is increased as the final test. The strand to be
cast was a strand having a diameter of 800 mm and a length of 10 m, and the steel grade
selected was a 13% Cr steel. The amount of molten steel used in the casting of this
strand was about 40 t. This is equivalent to the amount for four ingots which are cast
by a typical ingot process (the amount of molten steel: 10 t). In typical ingot casting,
feed from a feeder is used to prevent the formation of shrinkage cavities and centerline
cavities in an upper portion of the ingot. For the feed, molten steel in an amount of
10 % of the mass of the ingot is required per ingot, and therefore additional 4 t of
molten steel will be required. After the ingot is cast, the feed portion needs to be cut
off, and therefore a loss is incurred accordingly, but no such loss is incurred in
continuous casting processes.
[0048]
A continuous casting machine of the type shown in FIG. 1 was used. The mold
used was a water-cooled mold made of copper having a diameter of 800 mm and a
length of 800 mm. Immediately below the mold were provided support rolls with the
length of the support region being 800 mm. The movable reduction roll unit included
rolls having a diameter of 650 mm. Cooling of the strand was carried out by water
spray cooling at a flow rate of 0.2 Ltkg-steel. Withdrawing of the strand was carried
out at a casting speed of 0.25 mlmin, and the withdrawing was discontinued when the
length of the strand became 10 m. The other test conditions than that were the same as
those for the preliminary test as described above.
[0049]
According to heat transfer and solidification analysis that was performed on
continuous casting under the above conditions, it was estimated that the surface
temperature of the strand when the withdrawing was discontinued was about 1220°C at
a location 4 m in the casting direction from the meniscus in the mold and about 980°C
at a location of 10 m. The liquid core diameter at this point of time was estimated to
be about 620 mm at the location of 4 m from the meniscus and 500 mm at the location
of 10 m with the solid fraction of 0.8 being used as a reference. Based on the analysis
results, the amount of reduction of the strand by the movable reduction roll unit was set
to 225 mm, and the speed for raising the movable reduction roll unit was set to 0.25
rnlmin. This speed for raising the rolls is the same as the speed for withdrawing the
strand, and therefore the conditions for reduction rolling (the liquid core diameter and
the surface temperature of the strand at the location to be reduction-rolled) are uniform
over the entire region of the strand.
[0050]
In this case, the liquid core diameter was about 500 mm and the surface
temperature was 980°C at the location to be reduction-rolled at the start of reduction
rolling. When the reduction amount is 225 mm with respect to the liquid core
diameter of 500 mm, the value of the reduction amountlliquid core diameter is 0.45.
Thus, it is estimated from FIG. 3, which shows the results of the preliminary test, that
the area fraction of defects will be significantly reduced both for the constant region and
for the upper portion of the strand, at 20% and 4.8%, respectively. The diameter of the
rolls included in the movable reduction roll unit is 650 mm, and the deformation
resistance of a 13% Cr steel, which is the steel to be cast, is 6 kgf/mm2. Thus,
assuming that the contact angle between the roll and the strand is 32", the necessary roll
force is 650 t.
[005 11
2-2. Test Results
The strands that were cast under the above conditions had less centerline cavities,
porosity, and shrinkage cavities than strands that were cast without the use of a movable
reduction roll unit, and thus exhibited good internal quality and surface quality. In
addition, higher production yields were obtained than in the casting of ingots of a
comparable size by the ingot process.
INDUSTRIAL APPLICABILITY
[0052]
With the method of continuously casting a strand according to the present
invention, it is possible to significantly reduce centerline cavities, porosity, and
shrinkage cavities regardless of the size of the strand cross section as well as to cast a
strand with high production jrields, using a continuous casting machine for which the
equipment cost is low, without causing deterioration of the surface quality.
REFERENCE SIGNS LIST
[0053]
1: ladle, 2: mold, 3: strand, 3a: solidified shell,
3b: liquid steel, 4: movable reduction roll unit, 5: roll pair,
6: frame, 7: support rolls, 8: universal joint,
9: large speed reducer, 10: jack shaft
We claim:
1. A method of continuously casting a strand, characterized in that the method
comprises:
using a pair of rolls configured to interchangeably perform guiding and
supporting of a strand and reduction rolling of the strand, and to be movable in a
vertical direction along the strand below a mold,
wherein, while the strand is being withdrawn, the pair of rolls is held in a stopped
condition and guides and supports the strand, and after the withdrawing of the strand is
completed, the pair of rolls is moved in a vertical direction and accordingly
reduction-rolls the stopped strand.
2. The method of continuously casting a strand according to claim 1, characterized
in that the move of the rolls while reduction-rolling the strand is in a vertically upward
direction.
3. The method of continuously casting a strand according to claim 1 or 2,
characterized in that the strand has a circular transverse cross section.