Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND PLANT FOR PRODUCING SHEATHED CONTINUOUSLY CAST PRODUCTS
BACKGRODND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing thin
metal strip, wherein the liquid molten metal is contacted with
and welded to a flat metal product, preferably a hot strip of
steel. The present invention also relates to a plant for
carrying out the method by means of an oscillating mold, i.e.,
stationary mold, or a travelling mold, preferably a two-roller
casting unit, in which the flat metal product of hot strip is
continuously introduced in such a way that it surrounds the
entering molten metal on all sides and is discharged through a
delivery s~~stem.
s;. Description of the Related Art
Flat products having thicknesses of less than 2mm are to
date primarily produced by rolling hot strip which can then also
be cold rolled down to thicknesses of 0.2mm. The requirements
with respects to sectional dimensions and planarity of the cold
strip must already be adjusted by percentage during the hot
rolling process because a percentage correction of the accuracy
to dimension is no longer possible during the cold rolling
process. ~~tarting at a width/thickness ratio of about 100/1, the
CA 02229750 1998-02-18
resistance to flow in the roller gap becomes practically
infinitely large in the width direction, so that a percentage
correction of sectional dimension and planarity is no longer
possible. For this reason, developments of casting technologies
which attempt to realize, for example, sizes with widths of
greater than 1,OOOmm and thicknesses of less than lOmm have up
to now essentially not gone past the status of a semi-production
unit.
When casting thin sizes with an oscillating mold with the
formation of a slag film thickness inherent in the system and
with the prerequisite of the same production as it is achieved in
a conventional slab unit, the main problem is that, corresponding
to the lowering of the casting thickness and the increase of the
casting speed, the surface area produced per unit of time
increases x>y a multiple and the slag film thickness decreases
simultaneously. Also increasing at the same time are the
solidification heat released per unit of time and, thus, the heat
flux in the mold.
Fig. 1. of the drawing illustrates the resulting heat flux in
dependence on the casting thickness and the casting speed. For
example, if' the critical heat flux of a conventionally produced
slab having a thickness of 200mm and being cast with lm/min is
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about 1MW/m2, the lowering of the casting thickness to, for
example, 50mm while simultaneously increasing the casting speed
to 6m/min results in an increase of the heat flux to 2.8MW/m2.
Corresponding to the shrinkage index, the danger of the
occurrence of longitudinal cracks increases to 2.8 from the
standardized value 1 in the case of a standard slab, i.e., the
susceptibility to longitudinal cracks is 2.8 times greater than
in a standard slab with the width being equal.
The limit analysis of the partial illustrations of Fig. 1
exhibits a minimum relative slag film thickness of 0.001 or
0.02mm and essentially reflects the conditions when casting
without casting powder. The integral heat flux reaches a value
of about 5MW/m2 and can also be found in billet plants in which
casting is carried without casting powder or casting slag.
Similar conditions with respect to the heat flux also exist
when casting strip with travelling molds. The lack of slag also
results in these processes in heat fluxes on the level of the
limit analysis.
When transferring these results to strip plants, the
following becomes clear:
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1. The slag-free casting results in a significantly higher
heat flux in the mold during the solidification and, thus,
2. z-esults in a significantly higher shrinkage during the
casting process, as well as
3. causes a correspondingly higher thermal load of the
mold which leads to the danger of longitudinal cracks in the
strip surface and leads to lower service lives of the mold.
At the present time, the problems described above lead to
uncontrollable surface defects when casting thin strands having a
thickness of less than 30mm in plants with a travelling mold, for
example, in accordance with the two-roller method (Bessemer
principle), the Hazelett method or in belt units and the one-
roller method.
The best results have so far been achieved using twin
rollers. In this method, liquid steel is cast symmetrically
between twa rollers which rotate in opposite directions, the
steel solidifies in the narrowest gap between the rollers, i.e.,
the kissing point, and is discharged through a delivery unit.
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The surface quality which is achieved by this method and
particularly the accuracy to dimension in thickness direction and
width direction, i.e., planarity and sectional size, are not
reproducible and cannot be realized with the percentage
tolerances for cold strip demanded by the market. This is
particularly true for the production of C-steel.
The explanation for this is the solidification behavior of
the steel. Under the casting conditions present in the twin
rollers, the steel solidifies on a "cold" roller surface in a
very short time. The heat flux of about 5MW/m2 flows radially
into the roller surface and is decreased by the roller cooling
means.
The fallowing effects occur in this connection:
- The roller-type mold which is subjected to a high
thermal load has, for example, a cylindrical cold dimension and
expands in the contact area due to the thermal increase with the
solidifying strand and imparts a negative crown on the strip.
However, a correction of the sectional dimensions during further
processing in the rolling mill is practically no longer possible
because of the lacking transverse flux in the roller gap in the
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case of the width/thickness ratios of greater than or equal to
100/1.
- The strand surface shrinks during the solidification
parallel to the roller surface in the casting direction as well
as in the width direction, so that a tensional stress exists in
the strand shell and a compressive stress exists in the mold
surface. Simultaneously, the mold surface expands during a
rotation cycle due to the heat flux in radial direction as well
as in axial direction and produces a tensile stress in the strand
surface in both directions.
The superposition of these stresses with the stresses caused
by the shrinkage of the strand shell leads to a shrinkage and
adherence of the strand shell to the mold surface. This, in
turn, leads to the formation of longitudinal cracks due to the
axially acting stresses, i.e., transversely of the casting
direction, and to transverse cracks due to the radially acting
stresses, i.e., longitudinally of the casting direction. With
increasing width of the cast sizes, the absolute value and, thus,
the influence of the axial shrinkage on the danger of the
longitudinal crack formation and on the adjustment of a
percentage acceptable planarity and freedom from cracks also
increase.
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Various patent documents, for example, DE 34 40 234, DE 34
40 235, DE 34 40 237, report about travelling molds in the form
of strips and/or rollers. In these instances, it is attempted to
improve the surface quality by minimizing the friction between
the solidifying surface of the molten metal and the mold
material, in order to avoid cracks and achieve a uniform surface
quality. A connection or welding between the cold surfaces and
the crystallized material are intentionally avoided.
In accordance with another proposal disclosed in DE 34 06
730, a metal foil is continuously supplied into the mold of a
horizontal continuous casting plant for the purpose of
lubricating the mold wall.
Not salved in the prior art until today are the problems of
the high heat flux and the profile and planarity tolerances in
the casting of strips, preferably of steel, having widths of
between 400mm and 1600mm and thicknesses of less than lOmm.
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SU~iARY OF THE INVENTION
Therefore, it is the primary object of the present invention
to propose a continuous casting method and a plant for carrying
out the method on the basis of an oscillating or travelling mold
in which the heat flux from the solidifying melt is controlled in
such a way that the shrinkage constitutes a subcritical value, a
welding connection between the solidifying melt and the flat
product, for example, a hot strip, takes place simultaneously,
and an optimum strip geometry with respect to sectional size and
planarity is ensured.
In accordance with the present invention, the solidus
temperature of the metal sheathing is always smaller than or
equal to the solidus temperature of the molten metal.
In the mold of the plant according to the present invention,
the sheathing material is guided along at least one of the wall
surfaces of the mold.
The unexpected solution according to the present invention
resides in the fact that the critical shrinkage of the
solidifying strand surface which is subjected to a tensile stress
is compensated by the superposition of an oppositely acting
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compressive stress caused by the expansion of the sheathing which
is located between the mold and the liquid metal.
The sheathing itself initially absorbs the heat flux
capacitively and, consequently, remains relatively cold at the
side facing the mold wall and is capable of absorbing greater
stress on this side without the formation of cracks.
The various features of novelty which characterize the
invention are pointed out with particularity in the claims
annexed to and forming a part of the disclosure. For a better
understanding of the invention, its operating advantages,
specific objects attained by its use, reference should be had to
the drawing and descriptive matter in which there are illustrated
and described preferred embodiments of the invention.
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BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
Fig. 1 is a diagram showing the heat fluxes occurring in
dependence on casting thickness and casting speed;
Fig. 2 is a schematic illustration of a two-roller casting
machine according to the present invention;
Fig. 3 is a diagram explaining the basic requirements for a
composite material connection between the solidifying melt and
the sheathing; and
Fig. 4 is a perspective view, on a larger scale, showing the
casting procedure.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 of the drawing is discussed above.
Fig. 2 of the drawing shows a two-roller casting machine
according to the present invention.
Liquid steel is introduced from a distributor 1 through a
steel supply system 2 into the sheathing 4 which is continuously
supplied at least on one side between the melt 3 and a travelling
mold 5 and is discharged with the solidifying melt. In the
solidifying zone, which simultaneously constitutes the welding
zone 6, the solidifying melt 7 or strand shell is welded to the
sheathing 4. The complete solidification of the metal strip 13
is concluded at the kissing point, the roller apex 8 or in the
area of the strand guide means 10. The strand guide means may be
composed of rollers or plates and can be used for reducing the
strand thickness. This facilitates an increase of the casting
speed, and thus, of the capacity by moving the final
solidification 9 out of the roller apex 8 or kissing point.
A cooling system 14 is arranged immediately following the
travelling mold 5. The area of the strand guiding means is
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provided with a housing 15 and may be operated in a gas-
controlled and/or temperature-controlled manner.
The method according to the present invention takes place as
shown in connection with the example of the twin-roller unit as
follows
The energy released during the solidification of the steel
melt 3 heats the sheathing 4 located between the travelling mold
and the steel melt 3 or the strand shell 7 up to 7So1 sheathing on
the melt side, which leads to welding of the strand shell to the
sheathing 4 in the active area of the mold, i.e., the welding
zone 6. In addition, substances 18 for reducing the melting
point and for reinforcing the welding process can be applied to
the side of the sheathing facing the liquid melt. Due to the
composite material connection of sheathing and solidified melt 7,
i.e., the strand shell, the tensile stress 12 existing in the
strand shell 7 is superimposed and compensated by the compressive
stress 11 existing in the sheathing 4.
This makes it possible to control the planarity and profile
of the strip to be produced and to avoid longitudinal cracks.
The thermal load acting, for example, on roller-type molds, is
substantially reduced by the capacitive heating of the sheathing,
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so that the service life of the mold is increased at the same
time.
By using sheathings of, for example, high-grade steel,
aluminum, copper or other non-iron metals, the invention makes it
possible to produce composite materials, particularly coated
materials, such as stainless coated carbon steel as well as flat
or also long products.
Fig. 3 of the drawing shows the basic requirements for a
composite material connection between solidifying melt and
sheathing.
The solidus temperature of the material serving as sheathing
must always be smaller than or equal to the solidus temperature
of the melt being used, i . e, TS S",, > = TS
When the melt solidifies at the sheathing, energy is still
discharged even when the strand shell cools from Ts s~h to TS ~,
which is helpful to the welding action.
For example, a construction steel melt is used with a
solidus temperature of, for example, 1,520~C which is cast into a
sheathing of high-grade steel having a solidus temperature of,
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for example, 1,460~C. The released solidification heat of the
construction steel heats the sheathing up to 1,460~C and the
steel is welded to the sheathing. Because of the solidus
temperature difference of 60~C., the high-grade steel layer is
partially melted and forms the welding zone during the subsequent
common cooling.
The present invention can be used in Hazelett plants as well
as casting wheels, as shown in Fig. 4.
The present invention, examples of which have been described
above, provides the following advantages:
- Controlled production of a 100 composite material
connection by a certain welding between the melt and the
sheathing of high-grade steel, non-iron metal or other steel
qualities;
- Defined surface in size and planarity;
- Casting of qualities which are sensitive to heat cracks
with high casting speeds;
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- Substantial cost reduction by substituting, for
example, solid stainless steel products by a composite material
having at least one stainless surface;
- Freely selectable final and coating thicknesses;
- New material combinations;
- Connection of the casting process with the rolling
process is possible in-line;
- No scaling because of specified cooling;
- Low thermal load of the mold due to capacitive heating
of the sheathing;
- Possibility of increasing the casting speed (capacity)
by moving the final solidification from the roller apex or
kissing point; and
- Increase of the mold service life.
While specific embodiments of the invention have been shown
and described in detail to illustrate the inventive principles,
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it will be understood that the invention may be embodied
otherwise without departing from such principles.
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