Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 022l99l0 l997-l0-30
W09~'3S53~ PCT/SE96/00598
MOULD FOR CONTINUOUS CASTING
The invention relates to a mould for continuous casting
of metals, preferably steel, according to claim 1.
Since the beginning of continuous casting with through-
flow moulds, professional circles have dealt with problems
relating to the creation of air gaps between the strand shell
and the mould wall below the surface of the molten metal.Of
course, this gap formation deteriorates the heat transfer
between the mould and the strand shell considerably and causes
an uneven cooling of the strand, which results in deficiencies
in the billet, such as rhombic shape, cracks, structural
defects, etc. In order to generally shape as good a contact as
possible between strand and mould wall along the whole length of
the mould, and thereby the best possible conditions for the
deviation of heat, a number of suggestions have been made, such
as pressing in a coolant into the airgap and mould cavities of
different conicities, etc.
US-A-4 207 941 describes a mould for the continuous
casting of steel strands with polygonal, in particular square
cross-sections. The cross-section of the mould cavity, that is
open at both sides, is at the inlet side a square with corner
fillets and at the outlet side an irregular dodecagon. However,
when moulding with this mould a jamming of the strand may easily
arise, which may lead to a tearing up of the strand and a break-
through. Further, a dodecagon is moulded instead of a square. It
is particularly difficult to dimension such moulds for different
moulding speeds during a continuous moulding, which is necessary
at long sequential mouldings with many changes of ladles.
In DE-A-3 907 351 a mould for continuous casting of a
bloom is described. The two longitudinal sides are provided with
CONFIRMATION
COPY
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bulges at the inlet side of the mould, which are continuously
retrograded along a part height of the mould. At the outlet side
of the mould, the cross-section of the mould cavity is right-
angled and provided with the desired bloom cross-section. The
only purpose of the two bulges facing each other is to provide
sufficient place for a moulding pipe. There are no bulges on the
two narrow sides and also no shaping of the strand shell by
means of the mould walls.
AT-B-379 093 describes a mould for the continuous
lo casting of a bloom. The circumference line of the cross-section
of the mould cavity at the inlet side can be divided into four
circumference sections. At two circumference sections at the
inlet side, which simultaneously form the longitudinal sides of
the bloom section, are provided cross-sectional enlargements
with the shape of protruding bulges in relation with the same
cross-section at the outlet side of the strand. The measure of
the bulge, which in this case corresponds to the arch height,
diminishes continuously in the moving direction of the strand
and is nil at the exit of the mould. At the two other
circumference sections, i.e., the short sides of the bloom, run
the narrow-side walls, which contrary to the two longitudinal or
broad sides, diverge in the direction of the strand. These
short- or narrow-sides diverging in the direction of the strand,
are necessary for solving the posed problem, namely to prevent a
jamming and a formation of creases on the broad sides. An
improvement and an even dlstribution, respectively, of the
cooling over the whole circumference of the mould, and thereby
an improvement of the strand quality relative to outer surface
and structure, is not possible to obtain with this mould, since
the narrow and broad sides are cooled differently strongly. An
increase of the moulding speed is limited by the undefined
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cooling conditions on the narrow sides. The strong cooling of
the broad sides and the poor cooling of the narrow sides further
increase the risk for a break-through both at strongly varying,
and in particular at high moulding speeds.
In EP-A-498 296 a mould for continuous casting is
disclosed according to which the circumference line of the inlet
side has circumference sections between the corners with cross-
sectional enlargements of the mould cavity. These enlargements
have the shape of bulgings, the chordal heights of of the
lo bulgings decreasing in all circumference sections in the moving
direction of the strand along at least a partial length of the
mould cavity. According to this publication, a measurable
cooling of the strand shell should thereby be obtained along the
whole circumference in order to on one hand improve the strand
quality and on the other hand increase the moulding rate.
Moreover, the construction shall also make possible differences
in the moulding rate during the process of casting.
The construction according to EP-A-498 296 has only
partly managed to fulfil the posed problems. The reduction of
the cross-sectional area has sometimes turned out to be too big,
which has caused problems of jamming of the cast strand and of
an increased mould wear. In existing plants, the chosen casting
rate has turned out to be too high for this mould. It has been
constructed in first hand for high rates, not for freedom from
cracks and a generally faultless casting. However, it does not
fully compensate for corner shrinkage, wherefore the corner
design leaves more to be desired. Furthermore, the strand binds
or "pinches" at lower casting rates.
A primary object of the present invention is to
compensate for the corner shrinkage of the strand, both at
higher and lower casting rates.
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A further object of the present invention is to
simplify the design of the mould as much as possible.
Still another object of the present invention is to
obtain an optimal contact between the strand and the mould wall
along the whole length of the mould.
A further object of the present invention is to
compensate for the shrinkage gradient that arises between the
inlet and outlet of the mould, thereby minimizing pull and
pressure stresses.
lo These and further objects have been solved in a
surprising way by shaping the mould in the way as defined in the
characterizing clause of claim 1.
Thus, the mould has been given a shape that corresponds
to the solidification behaviour of the material. This results in
a better contact and thus a better support for the strand shell,
and thereby an almost tension-free shell. Thereby, the casting
of crack-sensitive sorts is simplified and the risk of corner
cracks in the corner regions, such as cross cracks, corner
cracks and impressions is m; n;m; zed. Further, the risk of a
rhombic strand-shape is decreased, a problem that often arises,
e.g., when casting blooms and billets. The construction
according to the invention exposes the shell to a minimum of
external influence and causes a very small mould wear (contrary
to, e.g., AT-A-379 093), by the fact that the mould has a shape
2~ that substantially follows the free dimension variations of the
strand during casting.
For illustrative but non-limiting purposes, the
invention will now be further described with reference to the
appended drawings. These are herewith briefly presented:
Figure 1 shows a mould according to the invention in a
perspective obliquely from above.
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Figure 2 shows a formalized view straight from above of
two moulds according to the invention.
In fig 1 is generally shown a mould 1 for the
continuous casting of metals, preferably steel, into
substantially rectangular or square cross-sections. It is
usually made of copper or any copper-based alloy. Reference
numeral 2 designates the inlet side of the mould and 3
designates its outlet side. Between these extends the mould
cavity 4. It is suitably made of one sole, straight piece,
lo although bowed moulds and block or plate moulds are also
feasible.
In order to achieve the above mentioned objects, the
mould is shaped with inner walls 5 which bulge inwards, towards
the middle of the mould. At the top, at the inlet side 2 of the
mould, they have a bending radius Rre~, which within given ranges
increases indefinitely towards the outlet side 3 of the mould.
By this shape of the the mould, the m~x;m~l value of the
mechanical tension elongation is reduced in a surprising manner
by at least 25% in the upper parts of the mould and by about 50%
in the lower half of the mould.
In order to generalize the desired dimensioning of the
mould, it is preferred to describe it by ~imension-les5 numbers:
R' = R z' = z
Rre~ L
where R' is the dimensionless radius, R is the radius at the
height z, Rre~ is the radius at the inlet side (see fig 1), z' is
the dimensionless distance from the upper side of the mould, z
is the factual distance from the upper side of the mould and L
is the total height of the mould.
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On basis of the obtained results, the radius R of the
inwardly bulging walls 5 may decrease in the feed direction of
the strand according to the function
..
R(z) = LRref/(L-z), or
R(z) = L2Rref/(L-z)2; or dimensionless:
R' = 1/1-z', or
R' = 1/(1-z') 2
It is easily understood that these functions increase
indefinitely towards the outlet end of the mould.
Referring to the bending radius Rref at the upper edge
of the mould, it should suitably be between 4 and 7 m,
preferably between 5 and 6,5 m.
The dimensionless radius shall suitably be within the
following ranges along the vertical axis of the mould:
Table 1
z' R'uPPer R'lower
0, oO 1 . oo 1 . oo
0.?0 1.56 1.~5
0,4n 2 78 1.67
0,~0 6 ?.5 2.50
0 80 25.00 5.00
o, go 1 oo . 00 10 . 00
1,00 ~ ~
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Preferably, the dimensionless radius lies within the following
ranges:
)
Table 2
Z ~ R ~ ~ppor R lower
0,00 1,00 1.00
0 ~0 1.33 ~.~5
lo 0.40 1.94 1.67
0 60 3.44 . ~ 50
0 70 5.~8 3.33
0,80 lo.oo 5 00
0 90 . 3~.50 10.00
1 00 ~ ~
Preferably, the mould shape goes from bulging sides and
corner angles ~ larger than 90~ at the inlet side, to a
substantially square or rectangular shape at the bottom side of
the mould. According to a preferred embodiment of the present
invention, the corner angles ~ decrease downwards in a
determined way, in order to obtain further advantages with
regard to a m; nimi zed formation of cracks, increased casting
rate and optimized heat transfer between the strand and the
mould. Thus, along the whole length of the mould, there is
advantageously an angle alteration of ~ from between 90 to 98~ at
the top to between 90 to 92~ at the bottom. If the total length
of the mould is divided into twelve equally wide segments
according to fig 1, then up to half the angle alteration should
3c take place in the segments 0 and 1. Up to another 25% of the
angle alteration should occur within the segments 2 to 4,
-
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inclusive. The remaining angle alteration should take place down
to the bottom or outlet side 3 of the mould, i.e., in the
segments 5 to 11, inclusive. As to its amount, the angle
alteration should suitably be between 2 and 8~, preferably
between 2 and 4~.
In a formalized way, fig 2 shows the cross-sectional
shape of the mould cavity 4 at the inlet side 2 and the outlet
end 3, respectively, of the mould. The cross-section of the
inlet side corresponds to line 6 while the cross-section of the
lo outlet end is substantially rectangular and is depicted either
by the rectangle 7 or 8. Thus, as exemplified by the rectangle
7, the corner distance of the mould in one direction may be
lar,ger at the lower edge of the mould than at the upper edge,
i.e. the mould may to some extent obtain an inverted conicity.
Hence, the mould should have some conicity in order to
compensate for the cooling shrinkage of the arising strand
shell, and whose reduction of the corner distance may be added
to the extension of the corner distance that occurs when the
bulging side surface 5 is gradually straightened out.
- According to a preferred embodiment of the present
invention, the bulginess of the mould walls start 5 to 40 mm
from the corners 9, as measured along the substantially planar
wall portion lOi preferably 10 to 30 mm.
