U.S. patent application number 12/298537 was filed with the patent office on 2009-10-15 for stirrer.
This patent application is currently assigned to ABB AB. Invention is credited to Jan-Erik Eriksson, Sten Kollberg.
Application Number | 20090255642 12/298537 |
Document ID | / |
Family ID | 38625289 |
Filed Date | 2009-10-15 |
United States Patent
Application |
20090255642 |
Kind Code |
A1 |
Kollberg; Sten ; et
al. |
October 15, 2009 |
STIRRER
Abstract
A device for casting of metal. The device includes a mold, a
casting tube via which molten metal is supplied to molten metal in
the mold in a region at a distance below a meniscus of the molten
metal, and at least one stirrer including an iron core and a coil
arranged around the iron core. The iron core is arranged to be
elongated along the broad side of the mold and adapted to apply a
magnetic field to the molten metal for stirring the melted metal.
An upper part of the iron core is positioned at a distance from the
meniscus of from 50 mm above the surface of the meniscus to 195 mm
below the surface. The length of the iron core in relation to the
length of the broad side of the mold is between at least 50% and
80% of the length of the broad side.
Inventors: |
Kollberg; Sten; (Vasteras,
SE) ; Eriksson; Jan-Erik; (Vasteras, SE) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20043-9998
US
|
Assignee: |
ABB AB
Vasteras
SE
|
Family ID: |
38625289 |
Appl. No.: |
12/298537 |
Filed: |
April 25, 2007 |
PCT Filed: |
April 25, 2007 |
PCT NO: |
PCT/SE07/50269 |
371 Date: |
October 27, 2008 |
Current U.S.
Class: |
164/146 |
Current CPC
Class: |
B22D 11/115
20130101 |
Class at
Publication: |
164/146 |
International
Class: |
B22D 11/115 20060101
B22D011/115 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 25, 2006 |
SE |
0600919-5 |
Claims
1. A device for continuous or semicontinuous casting of metal,
comprising: a mold comprising two broad sides and two narrow sides,
wherein a ratio of the broad sides to the narrow sides amounts to
2:1, through which a molten metal passes during a casting process,
a casting tube via which a molten metal is supplied to a molten
metal already present in the mold in a region at a distance below a
meniscus of the molten metal, at least one stirrer comprising an
iron core and a coil applied around, the iron core, wherein the
iron core is arranged to be elongated along a broad side of the
mold and a length of the iron core in relation to a length of the
broad side of the mold is between 50% and 80% of the length of the
broad side and is adapted to apply a magnetic field to the molten
metal to stir said melt, and wherein the iron core is arranged such
that an upper part of the iron core is positioned at a distance
from the meniscus of from 50 mm above a surface of the meniscus to
195 mm below said surface of the meniscus.
2. The device according to claim 1, wherein the iron core is
arranged such that the upper part is positioned at a distance from
the meniscus of from 50 mm above the surface of the meniscus to 150
mm below said surface of the meniscus.
3. The device according to claim 1, wherein the iron core is
arranged such that the upper part is positioned at a distance from
the meniscus of from 50 mm above the surface of the meniscus to 100
mm below said surface of the meniscus.
4. The device according to claim 1, wherein two stirrers are
located symmetrically around a center line of the broad sides of
the mold and on both sides of said broad sides.
5. The device according claim 1, wherein two stirrers are located
asymmetrically, on respective sides of the broad sides of the mold.
Description
TECHNICAL FIELD
[0001] The present invention relates to a device for continuous or
semicontinuous casting of metals, comprising a stirrer according to
the preamble to claim 1.
BACKGROUND OF THE INVENTION AND PRIOR ART
[0002] During continuous or semicontinuous casting, a molten metal
is supplied to a casting mould, hereinafter designated mould, in
which it is cooled and formed into an elongated strand. Depending
on its cross-sectional dimensions, the strand is designated BILLET,
BLOOM or SLAB. During the casting, a primary flow of hot, molten
metal is supplied to the cooled mould, in which the metal is cooled
and at least partially solidifies into an elongated strand. The
cooled and partially solidified strand then continuously leaves the
mould. At the point where the strand leaves the mould, it has at
least a mechanically self-supporting, solidified casing that
surrounds a non-solidified centre. The cooled mould is open at two
opposite ends in the casting direction and preferably connected to
means for supporting the mould and means for supplying coolant to
the mould and supporting means. The mould is preferably made of a
copper-based alloy with good thermal conductivity.
[0003] From a casting box, also designated tundish, the molten
metal is supplied to the mould via a casting tube that extends down
in to the mould. The casting tube preferably extends so far down
into the mould that it projects into the molten metal that exists
therein. When the melt from the tube flows into the melt that
already exists in the mould, it generates a so-called primary flow
and a so-called secondary flow. The primary flow leads downwards in
the casting direction, whereas the secondary flow leads from the
region of the walls of the mould upwards towards the surface of the
metal bath located therein, designated the meniscus, and downwards.
In different metal bath, designated the meniscus, and then
downwards again. The meniscus is covered by a layer consisting of
casting powder intended to act as protection against the
surrounding atmosphere and to minimize heat losses.
[0004] In different parts of the metal bath that is present in the
mould, periodic velocity fluctuations occur during the casting
process. Thus, upper and lower loops arise, in which the melt flows
around in a manner known per se. Due to resonance phenomena, which
are associated with the periodic oscillations of such loops, large
bubbles, for example argon gas bubbles, oxide inclusions from the
casting tube, and slag of various kinds from the surface of the
metal bath will be transported far down in the casting direction,
that is, far down into the cast strand that is initially formed in
the mould. This results in inclusions and irregularities in the
finished, solidified cast strand.
[0005] If the hot metal flow is allowed to enter into the mould in
an uncontrolled manner, the flow will penetrate deep into the cast
strand, which probably will have a negative influence on the
quality and productivity. An uncontrolled hot metal flow in the
cast strand may result in encapsulation of non-metallic particles
and/or gas occlusions in the solidified strand, or cause casting
defects in the inner structure of the cast strand. A deep
penetration of hot metal flow may also cause a partial remelting of
the solidified surface structure so that the melt penetrates the
surface layer below the mould, which causes severe disturbances in
production and a long downtime for repair.
[0006] Velocity variations caused by oscillating flow in the mould
give rise to pressure variations at the meniscus, and, in addition,
variations in height arise at the meniscus. When the velocity of
flow and hence the turbulence at the meniscus become too high, this
leads to slag being drawn down from the casting powder and further
down into the solidified strand, and results in an increased risk
of cracking due to uneven shell growth.
[0007] On the other hand, when the velocity becomes too low at the
meniscus, there is a risk of temperature differences arising, which
may lead to local solidification at the meniscus with ensuing risks
of cracking and of slag particles adhering under the shell that is
solidifying at the meniscus. It is thus important, especially at
low casting speeds, to maintain a flow at the meniscus that is
optimal with respect to the speed in order to supply heat for
melting of casting powder while at the same time endeavouring to
obtain a low turbulence. In the region around the casting tube,
there is a considerable risk of a local, unfavourable flow or
stagnation of the melt arising, which leads to cracks being formed
in the cast strand. Further, the oscillating flow provides an
unsymmetrical velocity downwards in the mould. In certain
situations, the velocity on one narrow side of the mould may become
considerably higher than on the opposite narrow side, which results
in a heavy downward transport of inclusions and gas bubbles with an
ensuing deterioration of the quality of the cast object.
[0008] From Japanese patent publication JP-57017355, it is known to
arrange electromagnetic stirrers, wherein the stirrer in the
vertical direction is placed such that the distance from its upper
edge to the meniscus is larger than or equal to 200 mm at the long
side of the mould for the purpose of preventing casting powder from
being drawn down from the meniscus of the melt and further down
into the cast strand. The dimension of the stirrer in relation to
the broad side of the mould amounts to 0.4-0.7 times the dimension
of the broad side of the mould (0.4-0.7)*b. This solution, however,
is only intended to create stirring a certain distance down in the
melt and does not completely solve the previously mentioned
problems relating to velocity variations.
SUMMARY OF THE INVENTION
[0009] The object of the present invention is to provide a device
for continuous or semicontinuous casting of metals, especially
intended for casting of slabs, which contributes to reduce or
eliminate the disadvantages mentioned above. In particular, a
device is aimed at which creates an even flow at the meniscus for
different speeds of the inflowing melt.
[0010] This object is achieved by means of the device described in
the introductory part of the specification, which is characterized
in that the iron core is arranged such that its upper part is
positioned at a distance from the meniscus that lies from 50 mm
above the surface of the meniscus to 195 mm below said surface.
[0011] By this device, the metal flow at the meniscus is directed
away from the narrow sides of the mould inwards towards the casting
tube and uniformly across the whole width of the melt, and, in
addition, a homogeneous flow configuration is obtained at the
meniscus which provides the lowest turbulence when the flow is
uniform across the whole mould width. With a stirrer placed as
previously described, a sufficiently large counter-directed
meniscus flow is obtained uniformly over the whole width of the
casting mould while at the same time the turbulence is restricted.
The location of the stirrer also contributes to obtain a good
rotation of the melt around the casting tube and the installation
of the stirrer is considerably simpler compared with prior art
solutions. By arranging the stirrer as described above, the
secondary flow is utilized in an optimum way while at the same
time, with the help of the stirrer, it is modified so as to obtain
a good symmetrical flow of the melt in the mould including a good
horizontal flow of the melt around the casting tube, which promotes
an even shell growth while at the same time the amount of
inclusions in the finished strand is reduced. By an optimum flow is
meant that the velocity of the melt at the meniscus (the secondary
flow) is maintained at a constant level without varying in time
while at the same time the velocity of the metal flow (the primary
flow) directed downwards from the casting tube is to be kept at as
low a level as possible to minimize the risk of inclusions
accompanying the melt far down into the solidified strand. The
dimension of the iron cores of the stirrer in the vertical
direction is usually 240-280 mm.
[0012] According to an alternative embodiment, the iron core is
arranged such that its upper part is positioned at a distance away
from the meniscus that lies from 50 mm above the surface of the
meniscus to 150 mm below said surface.
[0013] According to an alternative embodiment, the iron core is
arranged such that its upper part is positioned at a distance away
from the meniscus that lies from 50 mm above the surface of the
meniscus to 100 mm below said surface.
[0014] According to a preferred embodiment of the invention, two
stirrers are arranged symmetrically around the centre line of the
broad sides of the mould and on both sides of said broad sides.
Since the iron cores of the stirrers only need to cover part of the
width of the cast strand, such a device provides a cost-effective
solution since a good rotation of the melt around the casting tube
as well as an even velocity profile over the thickness of the width
of the cast strand are obtained.
[0015] According to a further embodiment of the invention, two
stirrers are placed asymmetrically, on respective sides of the long
sides of the mould. This embodiment provides advantages such as
lower weight, lower power consumption and reduced influence of
magnetic fields on the surroundings. In addition, the pole pitch is
large, which results in a maximally effective stirrer.
[0016] Additional advantages and advantageous features of the
invention will become clear from the following description and the
other dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present invention will now be explained in greater
detail by means of various embodiments and with reference to the
accompanying drawings.
[0018] FIG. 1 is an explanatory sketch of the device according to
the invention.
[0019] FIG. 2 is a top view according to one embodiment of the
device according to the invention.
[0020] FIG. 3 is an exploded view of a continuous casting device
according to the invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0021] The invention will now be described by means of various
embodiments.
[0022] FIG. 1 shows an explanatory sketch of the invention,
comprising a mould 1 enclosing a melt 2 which is supplied to the
mould 1 by means of a casting tube 3 lowered into the melt. The
melt 2 is cooled and a partially solidified strand is formed. The
strand is then moved continuously out of the mould 1. According to
the invention, at least one stirrer 4 is arranged which has an iron
core and a coil applied around it and, with the iron cores arranged
so as not to cover the whole length of the broad sides of the mould
but instead at least 50% of the broad sides of the mould and at
most 80% of the broad sides of the mould, symmetrically about the
centre line 5 of the mould 1 on both sides of the broad sides of
the mould. The iron cores are arranged such that their upper parts
are positioned at a distance from the meniscus that lies from 50 mm
above the surface 7 of the meniscus to 195 mm below said surface 7,
in order to create a rotating stirring of the melt below the
meniscus 7 by means of a period low-frequency travelling field. By
arranging the stirrers 4 as described above, a good rotating
stirring of the melt in the mould, including a good stirring of the
melt around the casting tube 3, are obtained. Furthermore, the fact
that the stirrers 4 do not cover the whole mould width means that
the normal flow pattern that arises, when the melt is supplied to
the mould via the casting tube 3, is not adversely affected.
[0023] FIG. 2 shows an alternative embodiment of the invention,
wherein the stirrers 8 are located asymmetrically on respective
sides of the broad sides 10 of the mould 9 and arranged such that
the upper parts of the iron cores are positioned at a distance from
the meniscus that lies from 50 mm above the surface of the meniscus
to 195 mm below said surface.
[0024] The invention is not limited to the embodiments shown, but
may be varied and modified within the scope of the following
claims.
* * * * *