U.S. patent application number 10/155040 was filed with the patent office on 2002-12-05 for casting, vertical casting method and vertical casting apparatus.
This patent application is currently assigned to DAIDO TOKUSHUKO KABUSHIKI KAISHA. Invention is credited to Amano, Hajime, Kumura, Soichiro, Takahashi, Hajime.
Application Number | 20020179279 10/155040 |
Document ID | / |
Family ID | 26616149 |
Filed Date | 2002-12-05 |
United States Patent
Application |
20020179279 |
Kind Code |
A1 |
Takahashi, Hajime ; et
al. |
December 5, 2002 |
Casting, vertical casting method and vertical casting apparatus
Abstract
A molten steel of a special steel or the like including a
high-alloy steel is poured in a mold whose bottom portion is closed
with a dummy head of a lift table. As the lift table is vertically
moved down at a given casting speed, a casting whose lower end is
supported by the dummy head is continuously pulled out from the
bottom portion of the mold. As a pair of movable molds that
constitute the mold are relatively moved away from each other in
synchronism with the downward movement of the lift table, both
widthwise side surfaces of the casting are provided with required
tapering.
Inventors: |
Takahashi, Hajime; (Tokai,
JP) ; Amano, Hajime; (Tokai, JP) ; Kumura,
Soichiro; (Tokai, JP) |
Correspondence
Address: |
KODA & ANDROLIA
Suite 3850
2029 Century Park East
Los Angeles
CA
90067-3024
US
|
Assignee: |
DAIDO TOKUSHUKO KABUSHIKI
KAISHA
|
Family ID: |
26616149 |
Appl. No.: |
10/155040 |
Filed: |
May 24, 2002 |
Current U.S.
Class: |
164/491 ;
164/436 |
Current CPC
Class: |
B22D 7/00 20130101; B22D
11/1213 20130101; B22D 11/1206 20130101 |
Class at
Publication: |
164/491 ;
164/436 |
International
Class: |
B22D 011/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2001 |
JP |
2001-165557 |
Jun 1, 2001 |
JP |
2001-167345 |
Claims
What is claimed is:
1. A casting of a predetermined length cast by pouring a molten
special steel including a high-alloy steel and tool steel in a mold
open to the top and bottom thereof and vertically pulling out the
casting having a required cross-sectional shape and having a shell
formed on a surface from a bottom portion of said mold, at least a
pair of opposing sides being tapered in such a way that an opposite
side size of both sides becomes smaller toward said bottom portion
from a top portion.
2. A vertical casting method for casting a casting of a
predetermined length by pouring a molten special steel including a
high-alloy steel and tool steel in a mold open to the top and
bottom thereof and vertically pulling out the casting having a
required cross-sectional shape and having a shell formed on a
surface from a bottom portion of said mold; wherein at a time of
casting said casting, tapering at least a pair of opposing sides of
said casting in such a way that an opposite side size of both sides
becomes smaller toward said bottom portion from a top portion by
relatively moving at least a pair of opposing movable molds of said
mold away from each other while pulling out said casting from said
bottom portion of said mold.
3. The vertical casting method according to claim 2, wherein a
casting speed of pulling out said casting from said mold is set
equal to or less than 0.2 m/min to thereby suppress occurrence of a
center porosity and segregation in said casting to be cast.
4. A vertical casting apparatus for casting a casting of a special
steel including a high-alloy steel and tool steel, comprising: a
mold which is open to the top and bottom thereof and has at least a
pair of movable molds that are relatively moved close to and away
from each other by movable means and where a molten special steel
including said highalloy steel and tool steel is poured; a lift
table, provided below said mold in a vertically movable manner, for
supporting a lower end of the casting having a shell formed on a
surface thereof and pulling out said lower end of said casting from
a bottom portion of said mold; and lifting means for moving said
lift table up and down.
5. A vertical casting method for casting a casting of a
predetermined length by pouring a molten special steel including a
high-alloy steel and tool steel in a mold open to the top and
bottom thereof and vertically pulling out the casting having a
required cross-sectional shape and having a shell formed on a
surface from a bottom portion of said mold; wherein at a time of
casting said casting, after completion of pouring, covering a top
portion of said mold with a lid member and heating said molten
steel in said mold with a plasma or arc with an internally defined
heating chamber set in an atmosphere of an inert gas and under such
a heating condition as to be able to keep a temperature of a
surface of said molten steel at a solidus casting temperature or
higher, thereby suppressing occurrence of an internal defect in
said casting to be cast.
6. The vertical casting method according to claim 5, wherein said
lid member is preheated to a predetermined temperature before
covering said top portion of said mold with said lid member.
7. A vertical casting apparatus for casting a casting of a
predetermined length by pouring a molten special steel including a
high-alloy steel and tool steel in a mold open to the top and
bottom thereof, supporting a lower end of the casting having a
shell formed on a surface thereof and pulling out said lower end of
said casting from a bottom portion of said mold with a lift table,
provided below said mold and movable up and down vertically by
lifting means, comprising: a lid member capable of covering a top
portion of said mold and set an internally defined heating chamber
in an atmosphere of an inert gas; and heating means for heating
said molten steel in said mold covered by said lid member with a
plasma or arc.
8. The vertical casting apparatus according to claim 7, further
comprising preheating means for preheating said lid member.
9. The vertical casting apparatus according to claim 7 or 8,
wherein said lid member is constructed by covering an outer side of
an inside lining member of a refractory material with an outside
lining member of a heat insulating material.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a casting of a
predetermined length, which is acquired by vertically pulling out a
casting that has been cooled down in a mold so as to have only its
surface portion coagulated (or a shell formed on the surface), and
a vertical casting method and a vertical casting apparatus which
cast the casting.
DESCRIPTION OF THE RELATED ART
[0002] In the field of nonferrous metals, such as aluminum, a
vertical casting method is known that casts a casting of a
predetermined length by pouring molten steel in a mold, which is
open to the top and bottom, supporting the lower end of a casting,
which has been cooled down in the mold so as to have a shell formed
on the surface, on a dummy head of a lift table provided below the
mold in a movable manner and vertically pulling out the casting
from the bottom portion of the mold by vertically moving the lift
table downward at a given speed.
[0003] Because the vertical casting method is advantageous over the
ingot-making method in various factors, such as energy saving,
power dissipation, attempts have been made to cast particularly
castings with large cross sections with general kinds of special
steels including high-alloy steel and tool steel and ensure still
standing coagulation of the castings. However, castings that are
obtained from the steels by the vertical casting method suffer the
occurrence of multiple internal defects, such as the center
porosity and center segregation or V-shaped segregation, thus
degrading the quality of the castings and lowering the yield. In
addition, the conventional vertical casting method may have an
internal defect of a casting head cavity occurring in a casting. In
other words, the conventional vertical casting method could not
produce castings that could sufficiently meet such a present strict
demand on the quality of castings as required of special steels
including high-alloy steel and tool steel.
SUMMARY OF THE INVENTION
[0004] The present invention has been proposed to solve the problem
of the prior art, and aims at providing a casting which has fewer
internal defects, such as the center porosity, casting head cavity
and center segregation or V-shaped segregation, and has improved
quality and yield, and a vertical casting method and a vertical
casting apparatus which can cast such casting.
[0005] To solve the problem and achieve the above object
preferably, according to one aspect of the invention, there is
provided a casting of a predetermined length that is cast by
pouring a molten special steel including a high-alloy steel and
tool steel in a mold open to the top and bottom thereof and
vertically pulling out a casting having a required cross-sectional
shape and having a shell formed on a surface from a bottom portion
of the mold, and characterized in that at least a pair of opposing
sides being tapered in such a way that an opposite side size of
both sides becomes smaller toward a bottom portion from a top
portion.
[0006] To solve the problem and achieve the above object
preferably, according to another aspect of the invention, there is
provided a vertical casting method for casting a casting of a
predetermined length by pouring a molten special steel including a
high-alloy steel and tool steel in a mold open to the top and
bottom thereof and vertically pulling out a casting having a
required cross-sectional shape and having a shell formed on a
surface from a bottom portion of the mold;
[0007] wherein at a time of casting the casting, tapering at least
a pair of opposing sides of the casting in such a way that an
opposite side size of both sides becomes smaller toward a bottom
portion from a top portion by relatively moving at least a pair of
opposing movable molds of the mold away from each other while
pulling out the casting from the bottom portion of the mold.
[0008] To solve the problem and achieve the above object
preferably, according to a different aspect of the invention, there
is provided a vertical casting apparatus for casting a casting of a
special steel including a high-alloy steel and tool steel, the
apparatus comprising:
[0009] a mold which is open to the top and bottom thereof and has
at least a pair of movable molds that are relatively moved close to
or away from each other by movable means, and where a molten
special steel including a high-alloy steel and tool steel is
poured;
[0010] a lift table, provided below the mold in a vertically
movable manner, for supporting a lower end of the casting having a
shell formed on a surface thereof and pulling out the lower end of
the casting from a bottom portion of the mold; and
[0011] lifting means for moving the lifting table up and down.
[0012] To solve the problem and achieve the above object
preferably, according to a different aspect of the invention, there
is provided a vertical casting method for casting a casting of a
predetermined length by pouring a molten special steel including a
high-alloy steel and tool steel in a mold open to the top and
bottom thereof and vertically pulling out the casting having a
required cross-sectional shape and having a shell formed on a
surface from a bottom portion of the mold;
[0013] wherein at a time of casting the casting, after completion
of pouring, covering a top portion of the mold with a lid member
and heating the molten steel in the mold with a plasma or arc with
an internally defined heating chamber set in an atmosphere of an
inert gas and under such a heating condition as to be able to keep
a temperature of a surface of the molten steel at a solidus casting
temperature or higher, thereby suppressing occurrence of an
internal defect in the casting to be cast.
[0014] To solve the problem and achieve the above object
preferably, according to a further aspect of the invention, there
is provided a vertical casting apparatus for casting a casting of a
predetermined length by pouring a molten special steel including a
high-alloy steel and tool steel in a mold open to the top and
bottom thereof, supporting a lower end of the casting having a
shell formed on a surface thereof and pulling out the lower end of
the casting from a bottom portion of the mold with a lift table,
provided below the mold and movable up and down vertically by
lifting means, characterized by comprising:
[0015] a lid member capable of covering a top portion of the mold
and setting an internally defined heating chamber in an atmosphere
of an inert gas; and
[0016] heating means for heating the molten steel in the mold
covered by the lid member with a plasma or arc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic structural diagram illustrating a
vertical casting apparatus according to a preferable first
embodiment of the present invention;
[0018] FIG. 2 is a schematic plan view showing a transverse section
of the vertical casting apparatus according to the first
embodiment;
[0019] FIG. 3 is a schematic perspective view of a casting cast by
the vertical casting apparatus according to the first
embodiment;
[0020] FIG. 4 is an explanatory diagram showing the size of
tapering given to the casting according to the first
embodiment;
[0021] FIG. 5 is an explanatory diagram showing coagulation front
side angles of castings according to the prior art and the first
embodiment;
[0022] FIG. 6 is a graph showing the results of measuring the
center segregation of C and the coagulation front side angle;
[0023] FIG. 7 is a schematic structural diagram illustrating a
vertical casting apparatus according to a preferable second
embodiment of the invention;
[0024] FIG. 8 is a schematic cross-sectional view of a lid member
of the vertical casting apparatus according to the second
embodiment;
[0025] FIG. 9 is an explanatory diagram showing heating means of
the vertical casting apparatus according to the second
embodiment;
[0026] FIG. 10 is an explanatory diagram showing a heating step
performed by the vertical casting apparatus according to the second
embodiment;
[0027] FIG. 11 is an explanatory diagram showing coagulation front
side angles of castings according to the prior art and the present
invention (second embodiment);
[0028] FIG. 12 is a graph showing the results of measuring the
cut-off length of an upper end of a casting;
[0029] FIG. 13 is a graph showing the results of measuring the
segregation of C; and
[0030] FIG. 14 is a graph showing the results of measuring the
coagulation front side angle.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0031] A description will be given below of castings according to
the present invention, vertical casting methods and vertical
casting apparatuses capable of casting said castings according to
preferred embodiments of the present invention with reference to
the accompanying drawings.
[0032] (First Embodiment)
[0033] FIG. 1 schematically illustrates a vertical casting
apparatus 10 that executes a vertical casting method according to a
first embodiment of the present invention. The vertical casting
apparatus 10 has a mold 12 open to the top and bottom thereof
placed on a casting bed 14. The vertical casting apparatus 10 is
designed in such a way that molten steel of a special steel
including a high-alloy steel and tool steel is poured in the mold
12 via a ladle 16 and a tundish 18 both provided above the mold 12.
The mold 12 is designed in such a way as to be oscillated up and
down by an unillustrated oscillation unit to reduce friction
between a casting 26 to be pulled out from the bottom portion of
the mold 12 and the mold 12, thereby preventing burning.
[0034] As shown in FIG. 2, the mold 12 comprises a pair of fixed
molds 28, 28 set apart in the thickness direction of the casting 26
and a pair of movable molds 30, 30 provided slidable between both
fixed molds 28, 28 and set apart in the widthwise direction of the
casting 26. Molten steel is poured in the space that is defined by
those four molds 28, 28 and 30, 30. Each movable mold 30 is
connected with a rod 32a of a hydraulic cylinder (moving means) 32
which is placed on the casting bed 14 and uses hydraulic pressure
or the like. As both hydraulic cylinders 32, 32 are urged in the
forward and backward directions synchronously, the pair of movable
molds 30, 30 relatively move close to or away from each other. As
the pair of movable molds 30, 30 are relatively moved away from
each other while pulling out the casting 26 from the bottom portion
of the mold 12 as will be described later, both widthwise sides (a
pair of opposing sides) of the casting 26 pulled out from the mold
12 are tapered in such a way that the widthwise size of both sides
(opposite side size) becomes smaller toward the bottom portion from
the top portion (see FIG. 3). Actually, the casting 26 is tapered
in a range of 4 to 15 mm per 1 m (see FIG. 4).
[0035] As shown in FIG. 1, a plurality of guide rolls 44, 44 that
hold the casting 26, which has been cooled down (first cooling) in
the mold 12 to have an outer shell (shell) formed on the surface,
from both sides in the widthwise direction are arranged directly
below the mold 12 in a freely rotatably manner. As the casting 26
immediately after being pulled out from the bottom portion of the
mold 12 is held by the guide rolls 44, 44 from both sides, bulging
is prevented. A plurality of nozzles 46 set apart in the up and
down direction are provided below the laid-out locations of the
guide rolls 44, 44 at both widthwise direction of holding the
casting 26 in such a way as to face one another. As cooling water
(water) is directly sprayed toward the casting 26 from each nozzle
46, secondary cooling is carried out to accelerate coagulation of
the casting 26. It is to be noted that the guide rolls 44 and the
nozzles 46 are designed in such a way as to be retractable to the
positions where they do not interfere with a lift table 50 to be
discussed later, so that the lift table 50 is allowed to move up
and down between the guide rolls 44, 44 and the nozzles 46, 46
opposing in the widthwise direction.
[0036] The lift table 50, which has a dummy head 48 for supporting
the lower end of the casting 26, is provided below the mold 12 in
such a way as to be vertically movable. Pulleys 52, 52 are
rotatably laid on both sides from which the casting 26 is held. A
wire 54 whose one end is connected to an adequate fixed portion is
put around both pulleys 52, 52, with its other end connected to a
winch 56 whose speed is variable. The lift table 50 is suspended by
the wire 54 stretched between both pulleys 52, 52 and is moved up
and down by lifting means 58 that comprises the wire 54 and the
winch 56. In other words, as the winch 56 is rotated in the
direction of winding up the wire 54, the lift table 50 moves upward
via the wire 54, and as the winch 56 is rotated in the direction of
letting the wire 54 out, on the other hand, the lift table 50 moves
downward via the wire 54. The lift-down speed (casting speed) of
the lift table 50 in this case is set to a very low speed of 0.2
m/min or less to thereby suppress the occurrence of internal
defects, such as the center porosity and center segregation or
V-shaped segregation, in the cast casting 26.
[0037] (Operation of First Embodiment)
[0038] A description will now be given of the operation of the
vertical casting method that is executed by the vertical casting
apparatus according to the first embodiment. The winch 56 is
rotated in a predetermined direction to move up the lift table 50.
With the bottom portion of the mold 12 closed with the dummy head
48, molten steel of a special steel including a high-alloy steel
and tool steel is poured in the mold 12 via the ladle 16 and the
tundish 18. The molten steel poured in the mold 12 is subjected to
first cooling in the mold 12, thus forming a shell on the surface
of the molten steel. As the winch 56 is rotated in the reverse
direction to vertically move down the lift table 50 at a given
casting speed, the casting 26 whose bottom end is supported by the
dummy head 48 is pulled out from the bottom portion of the mold
12.
[0039] The casting 26 immediately after being pulled out from the
mold 12 is held by the guide rolls 44, 44 from both sides in the
widthwise direction, as shown in FIG. 1, thus preventing the
occurrence of bulging. As cooling water is sprayed on the casting
26 from the nozzles 46, 46 the secondary cooling of the casting 26
is carried out.
[0040] In synchronism with the downward movement of the lift table
50, the pair of movable molds 30, 30 relatively move away from each
other under the urging force of the hydraulic cylinders 32, 32,
thereby giving both widthwise sides of the casting 26 with required
tapering in such a way that the widthwise size of both sides
becomes smaller toward the bottom portion from the top portion (see
FIG. 3). Tapering the casting 26 this way suppresses the occurrence
of internal defects, such as the center porosity and center
segregation or V-shaped segregation, in the casting 26.
[0041] It seems that the occurrence of internal defects, such as
the center porosity and center segregation or V-shaped segregation,
is influenced by the angle of the coagulation front side of molten
steel inside the casting. That is, in case where both widthwise
sides of the casting 26 are straight, the angle (coagulation front
side angle) .theta. with respect to the center line of the
coagulation interface of the molten steel inside the casting is
small as shown in FIG. 5A. This encourages the suction of the
molten steel whose C, S, P or the like has become denser, causing
multiple internal defects. In a case where both widthwise sides of
the casting 26 or both sides in the thickness direction are
tapered, on the other hand, the upper width becomes wider in the
vicinity of the coagulation interface, making the angle .theta.
larger, as shown in FIG. 5B. This reduces the suction of the molten
steel whose C, S, P or the like has become denser, thus making it
possible to adequately suppress the occurrence of the internal
defects. The coagulation front side angle .theta. can be determined
by observing the macro organization of the longitudinal cross
section of the casting.
[0042] The widthwise size of both sides of the casting 26 that has
been cast by the vertical casting apparatus 10 are tapered in such
a way that the widthwise size becomes smaller toward the bottom
portion (BOT portion) from the top portion (TOP portion) as shown
in FIG. 3. This casting 26 has an excellent quality with fewer
internal defects, such as the center porosity and center
segregation or V-shaped segregation, inside. That is, the vertical
casting method according to the first embodiment can sufficiently
cope with steel types, such as special steels including high-alloy
steel and tool steel, that require severe demands on the quality.
In case of casting the casting 26 whose TOP portion has
cross-sectional sizes of 500 mm or greater in thickness and 500 mm
or greater in width, the vertical casting method according to the
first embodiment is particularly effective.
[0043] Although the foregoing description of the first embodiment
has been given of the case where a casting is tapered, a
combination of the tapering scheme and the super slow casting speed
may be employed as well. Setting the casting speed to 0.2 m/min or
slower further increases the coagulation front side angle .theta.,
reducing the suction of the molten steel whose C, S, P or the like
has become denser. This further suppress the occurrence of internal
defects, such as the center porosity and center segregation or
V-shaped segregation, inside the cast casting 26.
[0044] With regard to the tapering of the casting, while only a
pair of sides opposing in the widthwise direction of the casting
are tapered in the first embodiment, only a pair of sides opposing
in the thickness direction may be tapered or all of the four sides
may be tapered. That is, at least one pair of opposing sides should
be tapered.
[0045] Further, the moving means that moves the movable molds of
the mold is not limited to hydraulic cylinders but may take the
form of other various mechanisms, such as a ball screw, a screw and
a rack and pinion, that are actuated by a motor. Although the unit
that is a combination of a wire and a winch is mentioned as the
lifting means for the lift table in the first embodiment, various
other units, such as a hydraulic cylinder or a combination of a
motor and a ball screw, can be used as well.
EXAMPLE 1
[0046] In case of casting a casting whose TOP portion has a
thickness of 600 mm and a width of 750 mm using a special steel
including a high-alloy steel and tool steel as a material, the
results of measuring the center segregation of C and the
coagulation front side angle .theta. for a case where no tapering
is given and the casting speed (Vc) is variable (indicated by
.circle-solid.) and a case where tapering is given and the casting
speed (Vc) is variable (indicated by .tangle-solidup.) are shown in
FIG. 6. The casting was provided with a tapering of 4 mm/m.
[0047] As apparent from FIG. 6, it was confirmed that tapering a
casting could suppress the segregation of C. It was also confirmed
that even in case where no tapering was given, setting the casting
speed slower could suppress the segregation of C in the center of
the casting and could increase the coagulation front side angle
.theta.. It was shown that tapering in addition to the lowering of
the casting speed could further suppress the segregation of C in
the center of the casting and could further increase the
coagulation front side angle .theta..
[0048] (Second Embodiment)
[0049] FIG. 7 schematically illustrates a vertical casting
apparatus 100 that executes a vertical casting method according to
a second embodiment of the present invention. Like or same
reference numerals are given to those members of the vertical
casting apparatus of the second embodiment which are the same as
the corresponding members of the first embodiment to avoid
repeating the detailed description. The vertical casting apparatus
100 has a mold 120 open to the top and bottom thereof placed on a
casting bed 14. The vertical casting apparatus 100 is designed in
such a way that molten steel of a special steel including a
high-alloy steel and tool steel is poured in the mold 120 via a
ladle 16 and a tundish 180 both provided above the mold 120. The
tundish 180 is provided on a carriage 200 that is so constructed as
to be movable along rails (not shown) laid on the casting bed 14,
and is designed in such a way as to move between a casting position
located above the mold 120 and a retracting position set away from
the mold 120. The tundish 180 is provided in such a way as to be
movable up and down with respect to the carriage 200 via an
adequate lifting mechanism. At the time the tundish 180 moves
between the casting position and the retracting position, a duck
nozzle 180a provided on the tundish 180 is moved up to the position
where it does not interfere with the mold 120. The mold 120 is
designed in such a way as to be oscillated up and down by an
unillustrated oscillation unit to reduce friction between a casting
220 to be pulled out from the bottom portion of the mold 120 and
the mold 120, thereby preventing burning.
[0050] The carriage 200 is provided with a lid member 240 which
covers the top portion of the mold 120. The lid member 240 is
designed in such a way as to come to a standby position (FIG. 7)
set away from the mold 120 when the tundish 180 comes to the
casting position and come to a heating position (FIG. 10) at which
the top portion of the mold 120 is covered when the tundish 180
comes to the casting retracting position. The lid member 240 is
provided in such a way as to be movable up and down with respect to
the carriage 200 via an adequate lifting mechanism. At the time the
lid member 240 moves between the standby position and the heating
position, the lid member 240 is moved up to the position where it
does not interfere with the mold 120 or the upper end portion
(which will be discussed later) of the mold 120 that protrudes from
the upper end of the mold 120.
[0051] As shown in FIG. 8, the lid member 240 is formed into a box
with an open bottom and has a three-layer structure comprising an
inside lining member 260 of a refractory material, an outside
lining member 280 of a heat insulating material which covers the
outer side of the inside lining member 260 and an iron shell 300
which covers the outer side of the outside lining member 280. A
heating chamber 240a is defined inside the inside lining member
260. Plural holes (two in the second embodiment) 240b are formed in
the top portion of the lid member 240. Electrodes 340 that
constitute a heating unit (heating means) 320 which heats molten
steel in the mold 120 located on the carriage 200 are inserted in a
retractable manner into the heating chamber 240a through the holes
240b. After completion of pouring to be discussed later, in heating
molten steel at the time of performing still standing coagulation,
a predetermined voltage is applied between the casting 220 in the
mold 120 and the electrodes 340 with the top portion of the mold
120 covered with the lid member 240, so that the molten steel in
the mold 120 is heated by a plasma or arc discharged from the
electrodes 340. At the time of heating the molten steel, an inert
gas, such as Ar or N.sub.2, is supplied into the heating chamber
240a via the holes 240b to set the interior of the chamber 240a in
an atmosphere of the inert gas.
[0052] A refractory material which essentially consists of
Al.sub.2O.sub.3 is preferably used for the inside lining member 260
and a heat insulating material which has SiO.sub.2 doped into
Al.sub.2O.sub.3 for an improved heat insulation is preferably used
for the outside lining member 280. Those materials may however be
replaced with other materials. The use of a heat insulating
material having a low heat conductance for the outside lining
member 280 can reduce the heat release of the lid member 240 at the
time of heating and at the time of preheating which will be
discussed later, thereby ensuring reduction of the applied
power.
[0053] Each electrode 340 of the heating unit 320 is supported to
be movable in the up and down direction by a lifting unit 360
equipped with an electric motor, a pulse generator or the like. The
lid member 240 is provided with a temperature sensor 380 which
detects the temperature of the heating chamber 240a, as shown in
FIG. 9. The temperature detected by the temperature sensor 380 is
input to a temperature control unit 400. The temperature control
unit 400 is designed in such a way as to set the amount of power
applied to the heating unit 320 based on the detected temperature
and set the applied voltage and the supplied current based on the
amount of power in order to hold the temperature of the heating
chamber 240a at a preset target temperature. Based on the applied
voltage, the temperature control unit 400 controls the operation of
the lifting unit 360 to adjust the distance (gap) between the
electrodes 340 and the surface level (meniscus) of the molten
steel, thereby making the heating power variable. The variable
heating power keeps the temperature of the heating chamber 240a at
the target temperature. The target temperature of the heating
chamber 240a is set to the level at which the surface level of the
molten steel in the mold 120 covered by the lid member 240 can be
held at or higher than the solidus casting temperature. This
prevents the surface of the molten steel from being coagulated.
That is, supplying the same amount of heat as the amount of heat
released from the surface of the molten steel keeps the surface
temperature at or higher than the solidus casting temperature.
[0054] Heating of the molten steel by the heating unit 320 is
executed at the time of performing still standing coagulation after
pouring is completed. At this time, upward movement of the lift
table 50 pushes up the upper end portion of the casting 220 having
a shell formed on the surface by a predetermined length from the
upper end of the mold 120 (see FIG. 10A), thereby preventing
electric corrosion or a stray current from occurring on or being
supplied to the mold 120 by the plasma or arc that is discharged
from the electrodes 340. While carbon, tungsten and copper are
available as the material for the electrodes 340, other materials
may be used as well.
[0055] As shown in FIG. 7, a power receiving plate 420 of, for
example, carbon is arranged on the casting bed 14 under the lid
member 240 that is at the standby position, and the lid member 240
is placed on the power receiving plate 420. With an inert gas fed
to the heating chamber 240a, as a predetermined voltage is applied
between the power receiving plate 420 and the electrodes 340, a
plasma or arc is discharged from the electrodes 340. The plasma or
arc can preheat the heating chamber 240a or the lid member 240 to a
predetermined temperature. According to the second embodiment, the
heating unit 320 that is used to heat molten steel also serves as
the preheating means that preheats the lid member 240. Instead,
independent separate means may be used or the heating system is not
limited to the heating by a plasma or arc but a burner or the like
can be used as well. While the most preferable preheating
temperature for the lid member 240 is equal to or higher than the
target temperature of the heating chamber 240a, the preheating
temperature may be lower than the target temperature.
[0056] As shown in FIG. 1, the lift table 50 having the guide rolls
44, 44, the nozzles 46 and the dummy head 48 and the lifting means
58 are arranged below the mold 120 as per the first embodiment.
[0057] (Operation of Second Embodiment)
[0058] A description will now be given of the operation of the
vertical casting method that is executed by the vertical casting
apparatus according to the second embodiment. The winch 56 is
rotated in a predetermined direction to move up the lift table 50.
With the bottom portion of the mold 120 closed with the dummy head
48, molten steel of a special steel including a high-alloy steel
and tool steel is poured in the mold 120 via the ladle 16 and the
tundish 180. The molten steel poured in the mold 120 is subjected
to first cooling in the mold 120, thus forming a shell on the
surface of the molten steel. As the winch 56 is rotated in the
reverse direction to vertically move down the lift table 50 at a
given casting speed, the casting 220 whose bottom end is supported
by the dummy head 48 is pulled out from the bottom portion of the
mold 120.
[0059] The casting 220 immediately after being pulled out from the
mold 120 is held by the guide rolls 44, 44 from both sides in the
widthwise direction, as shown in FIG. 7, thus preventing the
occurrence of bulging. As cooling water is sprayed on the casting
220 from the nozzles 46, 46 the secondary cooling of the casting
220 is carried out.
[0060] During pouring of molten steel in the mold 120, the lid
member 240 placed on the power receiving plate 420 is heated at the
standby position by the heating unit 320 with an inert gas supplied
to the heating chamber 240a so that the lid member 240 is preheated
to a temperature near the target temperature. After pouring in the
mold 120 is completed, with the ladle 16 retreated, the tundish 180
moves upward and the carriage 200 moves. As the tundish 180 moves
to the retracting position from the casting position, the lid
member 240 lifted up at the standby position moves to the heating
position (see FIG. 10A). After pouring in the mold 120 is
completed, the lift table 50 moves upward to push the upper end
portion of the casting 220 having a shell formed on the surface by
a predetermined length from the upper end of the mold 120.
[0061] Next, with an inert gas supplied to the heating chamber
240a, the lid member 240 moves downward to cover the top portion of
the mold 120 and the molten steel in the mold 120 is heated by the
heating unit 320 in such a way as to compensate for the heat
released from the surface of the molten steel, as shown in FIG.
10B. This prevents the surface of the molten steel in the mold 120
from being coagulated, thus suppressing the production of a
shrinkage porosity or a casting head cavity. Further, as heating is
applied from the surface of the molten steel to generate large
temperature gradation in the up and down direction of the
uncoagulated portion, the angle .theta. of the coagulation front
side which will be discussed later is increased. This can suitably
suppress the occurrence of internal defects, such as the center
porosity, casting head cavity and center segregation or V-shaped
segregation. Because the top portion of the mold 120 is covered
with the lid member 240 in an approximately sealed manner in this
case, efficient heating of the molten steel by the heating unit 320
is accomplished. What is more, because of the lid member 240
preheated, it is possible to suppress a drop in temperature of the
surface of the molten steel between the point at which the mold 120
is covered with the lid member 240 and the point at which heating
is started. According to the second embodiment, the tundish 180 and
the lid member 240 and the heating unit 320 are laid on the common
carriage 200, making it possible to shorten the time required to
cover the top portion of the casting with the lid member 240 after
pouring is completed. It is also possible to reduce a drop in
temperature of the surface of the molten steel during that
time.
[0062] The temperature control unit 400 performs feedback control
in such a way as to adjust the gap between the electrodes 340 and
the surface of the molten steel in the heating unit 320 based on
the temperature of the heating chamber 240a detected by the
temperature sensor 380 and keep the temperature of the heating
chamber 240a at the target temperature. Accordingly, a drop in
temperature of the surface of the molten steel in the mold 120 is
kept at or higher than the solidus casting temperature until
coagulation of the casting 220 comes near completion, thereby
adequately preventing the coagulation of the surface of the molten
steel. After a preset time passes, the heating power of the heating
unit 320 is gradually reduced and heating is terminated.
[0063] As mentioned earlier, it seems that the occurrence of
internal defects, such as the center porosity and center
segregation or V-shaped segregation, is influenced by the angle of
the coagulation front side of molten steel inside the casting. That
is, conventionally, the angle of the coagulation front side (the
angle with respect to the center line of the coagulation interface)
.theta. of the molten steel inside the casting 220 would become
small, as shown in FIG. 11A, which would encourage the suction of
the molten steel whose C, S, P or the like has become denser,
causing multiple internal defects. According to the second
embodiment, by way of contrast, the upper width becomes wider in
the vicinity of the coagulation interface, making the angle .theta.
larger, as shown in FIG. 11B. This reduces the suction of the
molten steel whose C, S, P or the like has become denser, thus
making it possible to adequately suppress the occurrence of the
internal defects. The coagulation front side angle .theta. can be
determined by observing the macro organization of the casting
220.
[0064] In other words, even for steel types, such as special steels
including high-alloy steel and tool steel, that require severe
demands on the product quality, the vertical casting method
according to the invention can provide castings which have
excellent quality with fewer internal defects, such as the center
porosity, casting head cavity and center segregation or V-shaped
segregation, inside. In case of casting the casting 220 whose top
portion has cross-sectional sizes of 500 mm or greater in thickness
and 500 mm or greater in width, the vertical casting method and
apparatus according to the second embodiment are particularly
effective.
[0065] Although the description of the second embodiment has been
given of the case where the lid member and the heating unit are
provided on the carriage of the tundish, the lid member and the
heating unit may be provided on another carriage or moving means or
the like so that when the tundish 18 moves to the retracting
position from the casting position, the lid member is moved to the
heating position quickly.
EXAMPLE 2
[0066] In case of casting a casting which has a thickness of 650 mm
and a width of 850 mm using a tool steel as a material, the results
of measuring the cut-off length of the upper end of a casting for a
case where molten steel is not heated at the time of still standing
coagulation after pouring is completed (prior art) and a case where
molten steel is heated (Example 2 of the second embodiment) are
shown in FIG. 12, and the results of measuring the segregation of C
(carbon) are shown in FIG. 13. FIG. 14 shows the coagulation front
side angle .theta..
[0067] As apparent from FIG. 12, according to the prior art that
does not perform heating, multiple internal defects, such as the
casting head cavity, center segregation or V-shaped segregation,
occur, which necessitates discarding of multiple castings, thus
lowering the yield. According to Example 2 (second embodiment) that
performs heating, as seen from the diagram, the occurrence of the
internal defects at the upper end portion of a casting is
suppressed, thus significantly reducing the cut-off length of the
upper end of a casting.
[0068] As apparent from FIG. 13, heating can restrain the
segregation of C as compared with the prior art. As apparent from
FIG. 14, heating increases the coagulation front side angle
.theta.. That is, it is seen that widening the coagulation front
side angle .theta. can preferably suppress the occurrence of
internal defects, such as the center porosity, casting head cavity
and center segregation or V-shaped segregation.
[0069] As explained above, because at least one pair of opposing
sides of each casting according to the present invention are
tapered, even those castings which are cast from steel types, such
as special steels including high-alloy steel and tool steel, that
require severe demands on the product quality, have fewer internal
defects, such as the center porosity and center segregation or
V-shaped segregation, and have a high quality and improved
yield.
[0070] In addition, ultra-slow casting at the casting speed set to
0.2 m/min or slower can further suppress the occurrence of internal
defects, such as the center porosity and center segregation or
V-shaped segregation.
[0071] Even for steel types, such as special steels including
high-alloy steel and tool steel, that require severe demands on the
product quality, the vertical casting method and apparatus
according to another embodiment of the present invention can
provide castings which have an excellent quality with fewer
internal defects, such as the center porosity, casting head cavity
and center segregation or V-shaped segregation, by heating molten
steel in the mold whose top portion is covered with the lid member
at the time of still standing coagulation after pouring of the
molten steel is completed. This results in an improved yield.
Further, preheating the lid member can suppress a drop in
temperature of the surface of the molten steel until heating is
started.
* * * * *