Metal founding – Combined – Including continuous casting apparatus
Reexamination Certificate
2000-02-18
2003-03-25
Dunn, Tom (Department: 1725)
Metal founding
Combined
Including continuous casting apparatus
C164S424000, C164S441000
Reexamination Certificate
active
06536505
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for producing slabs in a continuous casting plant preferably equipped with a vertical mold, preferably for thin slab plants for casting preferably steel having, for example, a solidification thickness of 60 mm-120 mm, for example, 80 mm, and casting speeds of up to 10 m/min. and a maximum casting output of about 3 million tons per year.
2. Description of the Related Art
The thin slab plants known in the art for producing a slab thickness reduction, realized in a casting and rolling device, reduce the strand thickness immediately underneath the continuous casting mold, which is equipped with one or two pairs of foot rollers, predominantly in the so-called “segment
0
”. In that segment, the thickness of the strand is reduced, for example, from 65 mm to 40 mm over a metallurgical length of about 2 m, i.e., over the entire length of the segment or stand
0
, which is not arranged vertically, wherein the casting speed is at most 6 m/min. A plant having these characteristics results in a strand thickness reduction of at most 38% and a deformation speed in the strand thickness of at most 1.25 mm/s.
During this holding time of the strand with liquid core, the strand shell having a thickness of about 8-12 mm is substantially deformed when entering the segment
0
due to bulging of the strand shell between the rollers of the continuous casting plant. This internal deformation increases with increasing casting speed and height of the plant or also the ferrostatic pressure, and decreases with decreasing spacing between the rollers. It is to be noted in this connection that the roller diameter cannot be less than, for example, 120 to 140 mm because of mechanical construction criteria, i.e., mechanical load, structural limits particularly in the case of intermediately arranged rollers. A possible mechanical solution could be a sliding plate, also called “grid”, which, however, is not suitable for carrying out a reduction of the strand thickness.
In normal continuous casting, the internal deformation is essentially determined by
bulging of the strand between rollers;
bending of the strand from the vertical into the inner circular arc;
straightening of the strand into the horizontal;
deviation of the rollers from the ideal strand guiding line due to
roller jumps;
roller impacts; and
tensile stress.
Added to these internal deformations and also the surface deformations must be the deformations which are produced by the strand thickness reduction or also the casting and rolling process in the segment
0
. This specific internal deformation is superimposed on the deformation already produced in the segment
0
caused essentially by the strand bulging and the bending process from the vertical into the internal circular arc. This cumulation of the individual specific deformations may lead to a total deformation which becomes critical and leads to rupture not only of the inner strand shell but also the outer strand shell.
This type of additional load acting on the strand shell due to casting and rolling or the thickness reduction during the solidification in the segment
0
having a length of, for example, 2 m immediately underneath the mold is described in German patents 44 03 048 and 44 03 049 and is illustrated in detail as an example in the diagram of
FIG. 1
of the drawing.
As shown in
FIG. 1
, a vertical mold having a length of 1 m and provided with one or two pairs of foot rollers is followed by a segment
0
having a length of 2 m in which the strand is bent over several stages into the inner circular arc and is also reduced in its thickness. These two processes or deformations taking place simultaneously lead to a superimposed cumulated total deformation composed of the bending deformation D-B and the casting and rolling deformation D-Gw. The total deformation D-Ge which acts on the strand shell may become greater than the critical limit deformation D-Kr and may lead to ruptures of the inner strand shell as well as of the outer strand shell. This danger increases with increasing casting speed due to a roller spacing or roller diameter in segment
0
which may not become smaller than a certain limit because of mechanical reasons.
In addition, when describing this problem, it must be taken into consideration that the limit deformation D-Kr has a specific behavior in each steel quality. For example, a deep drawing quality is less critical with respect to the absorption of deformations without the consequences of ruptures than, for example, a microalloyed steel quality API X 80.
Moreover, the configuration and extension of the overheated melt or also of the pure molten steel phase in the strand, indicated by the straight line G
1
in dependence on the casting speed, has a significant influence of the internal quality of the strand. In the example illustrated in
FIG. 1
, the pure molten steel phase or also the geometrically lowest liquidus temperature in the middle of the strand extends up to about 1.5 m below the meniscus or casting level at a casting speed VG of 5 m/min and to about 3.0 m underneath the casting level at a casting speed VG of 10 m/min. Underneath this point, the two phase area composed of melt and crystal is present over the entire strand thickness, wherein the two phase area looses melt portion in favor of crystal portion proportionally with increasing distance in the direction toward the sump tip or the final solidification.
When the crystal portion is 50%, i.e., at half the distance between the lowest liquidus point of 1.5 m at, for example, VG5 m/min and the final solidification which takes place at about 15 m, i.e., at 8.25 m (1.5 m+ (15 m-1.5 m)×0.5=8.25 m) (percent by weight), the melt/crystal phase has a viscosity of 10,000 cP. When the crystal portion is 80%, the two phase area has a viscosity of 40,000 cP, while the pure molten steel phase, depending on the steel quality, has to the lowest liquidus point a viscosity of only about 1-5 cP and, moreover, its partial viscosity between the crystals (crystal network or dendrites) is practically not increased, i.e., is constant, up to the final solidification.
To provide a reference of the viscosities in the two phase area mentioned above to known substances of everyday life, the following substances shall be mentioned:
Water
at 20° C.
1
cp = 10 exp3 Ns/m exp2
Olive oil
at 20° C.
80
cp =
Honey
at 20° C.
10 000
cp
Nivea
at 20° C.
40 000
cp
Margarine
at 20° C.
100 000
cp
Bitumen
at 20° C.
1 000 000
cp
These viscosities illustrate that for a good forced convection and, thus, a good destruction of crystals by a strand thickness reduction, a crystal/melt structure should be present in the core of the strand, i.e., at maximum casting speed the strand should have in its core already a two phase area in the region of the segment
0
or the pure molten steel phase or also the overheated area or the penetration zone for the rising of oxides should no longer be present. These conditions in connection with the oxidic degree of purity have led to the finding that, on the one hand, the segment
0
should be vertical and, on the other hand, the segment
0
should only serve for the strand thickness reduction and not also additionally for bending the strand.
In
FIG. 1
, which illustrates the poor conditions described above, the overheated zone or the lowest liquidus points extends to the end of the segment
0
and, thus, already into the inner circular arc of the continuous casting plant in the case of a maximum casting speed of 10 m/min, as indicated by point 1.1 on straight line G
1
. These casting conditions are extremely unfavorable for the strand shell deformation as well as for the oxidic degree of purity.
The two phase area, extending between two straight lines, i.e., the straight line G
1
for the arrangement of the lowest liquidus point in dependence on the casting speed and the straight line G
2
for the lowest solidus point or the final solidification in dependence on the casting
Kueffner Friedrich
Lin I.-H.
SMS Schloemann--Siemag Aktiengesellschaft
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