Method for producing thin slabs in a continuous casting plant

A method 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. In a first vertically extending first segment 0 of a strand guide, exclusive strand reduction, also called casting and rolling, is carried out. The segment 1 arranged immediately underneath the first segment 0 carries out bending of the strand through several bending points into the inner circular arc. Prior to final solidification, the strand is bent back through several return bending points into the horizontal.

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. 4, 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 
G1 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).times.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.degree. C. 
1 cp = 10 exp3 Ns/m exp2 
Olive oil 
at 20.degree. C. 
80 cp = 
Honey at 20.degree. C. 
10000 cp 
Nivea at 20.degree. C. 
40000 cp 
Margarine 
at 20.degree. C. 
100000 cp 
Bitumen at 20.degree. C. 
1000000 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 G1. 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 G1 for the arrangement of the lowest liquidus point in 
dependence on the casting speed and the straight line G2 for the lowest 
solidus point or the final solidification in dependence on the casting 
speed, begins in the case of the maximum casting speed of 10 m/min at the 
end of segment 0 which carries out the strand thickness reduction. 
In FIG. 3 of the drawing, partial illustration 3a, i.e., the left half of 
FIG. 3, also shows as an example the pattern of the different phases of a 
strand having a thickness of 100 mm from the meniscus in the mold with a 
subsequent strand thickness reduction in the segment 0 having a length of 
2 m from 100 mm to 80 mm solidification thickness to the final 
solidification in the last segment number 14 for the maximum casting speed 
of 10 m/min. Partial illustration 3a once again makes it very clear that 
segment 0 imparts into the strand the highest possible deformation caused 
by the strand thickness reduction and the bending process from the 
vertical into the inner circular arc through five bending points as well 
as poor conditions for oxides rising into the meniscus, and, thus, into 
the casting slag. 
Partial illustration 3a also illustrates that the reduction speed which 
acts on the shell of the strand for reducing the thickness from 100 mm to 
80 mm, i.e., by 20%, is 0.833 mm/s at a casting speed of 5 m/min and is 
1.66 mm/s at a casting speed of 10 m/min. This reduction speed of the 
strand thickness represents a direct measure of the deformation of the 
strand shell which at the entry into the segment 0 has a thickness of 
about 10.3 mm at a casting speed of 5 m/min and about 7.3 mm at a casting 
speed of 10 m/min. This strand deformation caused by casting and rolling 
is high and is not only doubled from 0.83 to 1.66 mm/s by the speed 
increase from 5 to 10/min, as expressed by the simplified variable 1.66 
mm/s, but the speed increase enters the deformation with a quadratic 
function. 
These high deformations, additionally superimposed by the bending processes 
in segment 0, lead to the danger of cracks of the inner strand shell as 
well as of the outer strand shell, particularly in the case of steel 
qualities which are sensitive to cracks. 
SUMMARY OF THE INVENTION 
Therefore, in view of the findings and relationships described above, it is 
the primary object of the present invention, based on devices for the 
strand thickness reduction immediately below the mold, to propose a method 
and a plant concept for a high-speed continuous casting plant for slabs 
which ensure an optimum surface quality and internal quality of the steel 
strand. 
In accordance with the present invention, in a first vertically extending 
first segment 0 of the strand guiding means, exclusive strand reduction, 
also called casting and rolling, is carried out. The segment 1 arranged 
immediately underneath the first segment 0 carries out bending of the 
strand through several bending points into the inner circular arc. Prior 
to final solidification, the strand is bent back through several return 
bending points into the horizontal. 
The continuous casting plant according to the present invention for 
carrying out the above-described methods includes a vertically extending 
segment 0 for a strand thickness reduction of between 40 and 10 mm. The 
following segment 1 has at least three bending points and the radius of 
the inner circular arc of this segment is between and 6 and 3 m. For 
bending the strand back from the inner circular arc into the horizontal, 
at least three straightening points are provided and the last return 
bending point at 80% of the maximum casting speed has a distance from the 
sump tip of at least 2 m. 
The present invention provides an unexpected solution for the various 
complex problems described above, as described below in more detail. 
Particularly, the present invention ensures and combines the following 
features: 
a minimum ferrostatic pressure or also a minimum plant height between the 
meniscus in an oscillating vertical mold, advantageously driven 
hydraulically, and the final solidification in the horizontally extending 
portion of the strand guiding means; 
minimized deformation density distribution of the total deformation 
composed of casting and rolling deformation and the bending deformation in 
a vertical bending unit with concavely constructed wide sides of the mold, 
predetermined roll diameters in the strand guiding means and up to maximum 
casting speeds of, advantageously 10 m/min; 
a complete elimination of the overheating phase or penetration zone for 
rising oxides in the vertical portion of the continuous casting plant, 
i.e., in segment 0 which is the machine element for carrying out the 
strand thickness reduction at a maximum casting speed of, for example, 10 
m/min, for ensuring a strand symmetry in the range of overheating or pure 
molten steel phase; 
a casting and rolling process at maximum casting speed of, for example, 10 
m/min in segment 0 in which the two phase area melt/crystal is present in 
the middle of the strand at the latest at the end of the segment 0 which 
carries out strand thickness reduction or casting and rolling; 
a deformation speed of the strand shell in segment 0 of at most 1.2 mm/s; 
a minimized bending deformation density in segment 1 from the vertical 
through several bending points into the inner circular arc independently 
of the casting and rolling deformation in the segment 0 which is arranged 
directly in front of segment 1; and 
a minimized straightening deformation density from the inner plant radius 
through several straightening or return bending points into the 
horizontal, preferably at least 12 s or at least 2 m in front of the final 
solidification in relation to an average casting speed of 80% of the 
maximum casting speed. 
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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 and FIG. 3a have already been described above. 
FIG. 2 and FIG. 3b illustrate the method and the apparatus according to the 
present invention. 
FIG. 2 of the drawing shows the distribution of the internal strand 
deformation according to the present invention over the strand guiding 
length with an indication of the plant configuration for the casting 
speeds 5 and 10 m/min. and the extension of the pure molten steel phase, 
the final solidification in dependence on the casting speed and the limit 
deformation. 
In accordance with the present invention, the continuous casting method is 
set up in such a way that the strand deformation density is minimized over 
the strand guidance and each type of deformation takes places successively 
independently of the other type of deformation. The deformation curves D-5 
and D-10 extend underneath the critical and, thus, limit deformation D-Kr. 
Moreover, the deformation curves show that a cumulation of the deformation 
caused by casting and rolling and by bending is avoided because, in the 
illustrated embodiment, the strand thickness reduction D-Gw is carried out 
in a vertical segment 0 having a length of 3 m and bending D-B of the 
strand is carried out in the subsequent segment 1 through, for example, 5 
bending points. 
FIG. 2 further shows that the lowest liquidus point 1.1 or the overheating 
zone or the penetration zone in the interior of the strand which 
constitutes about 10% of the solidification time with overheating of the 
steel of 25.degree. C. in the distributor extends at the maximum casting 
speed of 10 m/min up to 3 m underneath the meniscus or 2 m deep into the 
segment 0. This ensures that the oxides can rise freely and symmetrically 
until strand solidification in the vertically arranged pure molten steel 
phase and that, simultaneously, underneath the lowest liquidus point from 
which the two phase area of the strand interior completely fills out the 
strand metal, the destruction of the crystals and the suppression of the 
macro segregation and middle segregation due to the casting and rolling 
process can take place over the remaining length of lm in the segment 0. 
The two phase area is located between the straight line G1 which represents 
the lowest position of the liquidus point and the straight line G2 which 
represents the position of the sump tip in dependence on the casting 
speed. In the case of VG 5 m/min, the two phase area crystal/melt begins 
at about 1.5 m (liquidus point 1.2) underneath the meniscus or 0.5 m after 
the strand enters the segment 0 and ends at about 15.1 m (2.2) in FIG. 2 
with the sump tip; in the case of a casting speed of 10 m/min, the two 
phase area begins at about 3 m (1.1) and ends with the sump tip at about 
30.2 m (2.1), as seen in FIG. 2. 
The strand reduction or the casting and rolling process with the full two 
phase area between the strand shells extends in the case of VG 5 m/min 
casting speed over 2.5 m of the remaining length of the segment 0 and in 
the case of VG 10 m/min. over 1 m of the residual length of the segment 0. 
In both cases, a forced convection of the two phase area and, thus, an 
improvement of the interior quality of the strand are ensured. 
In accordance with FIGS. 3a and 3b , bending back of the strand from the 
inner radius of, for example, 4 m through several return bending points, 
for example, 5 straightening points, into the horizontal, is carried out, 
for example, in segment 4 having a length of 2 m in order to ensure a 
smooth return deformation D-R and simultaneously prevent a negative 
influence of the strand deformation on the final solidification and, thus, 
the internal quality of the strand. 
Moreover, FIG. 3 must be discussed. Particularly as compared to FIG. 3a, it 
is apparent that the casting and rolling deformation D-Gw from 100 to 80 
mm takes place over a segment 0 having a length of 3 m and, thus, with a 
deformation speed of only 1.11 mm/s in the case of a casting speed of 10 
m/min. and with a deformation speed of 0.55 mm/s in the case of a casting 
speed of 5 m/min. This deformation speed is significantly reduced as 
compared to that of 1.66 mm/s in the case of a segment 0 having a length 
of 2 m and a casting speed of 10 m/min. Consequently, the deformation 
speed is below the value of 1.25 mm/s which is known to be critical. 
The advantages provided by the present invention result from ensuring a 
continuous casting method for thin slabs from a solidification thickness 
of preferably between 60 and 120 mm with a casting and rolling stage 
immediately underneath the vertical mold in a vertically arranged segment 
0. 
The vertical mold, into which steel is conducted from a distributor V by 
means of a submerged pouring pipe Ta as shown in FIG. 4, should 
advantageously have concave wide side plates and should be hydraulically 
driven in order to ensure 
a precise oscillation and the variation of the moving height, of the 
frequency and the type of oscillation during casting; 
a uniform slag lubrication over the entire strand width; 
a quiet meniscus movement; 
a uniform heat transfer into the mold; 
a concentric strand travel within the mold as well as within the strand 
guiding means; and 
a high casting safety while avoiding ruptures. 
The strand guiding means can also be constructed concavely with a deviation 
from linearity of at most 2.times.12 mm in order to provide a straight and 
secure strand guidance even at high casting speeds. This can be realized, 
for example, with a concavely constructed profile of the strand guiding 
rollers. In addition, the degree of the concave deviation does not have to 
be constant from the mold exit or also from the first strand guiding 
roller to the last roller of the strand guiding means and can decrease 
functionally steadily in the direction toward the strand guiding end to a 
minimum residual concavity or a residual curvature of the strand. 
The segment 0 should be arranged vertically and be used exclusively for the 
strand thickness reduction. The segment 0 should have a minimum length 
which produces at maximum casting speed a reduction speed of the casting 
thickness of less than 1.25 mm/s in the strand and simultaneously, also at 
the maximum possible casting speed, has a minimum length which ensures the 
complete elimination of overheating and as much as possible also a 
destruction of the crystal phase in the two phase area crystal/melt and 
the suppression of the macro segregation and middle segregation. In the 
illustrated example, the segment 0 has a length of 3 m. 
In accordance with the present invention, in segment 1, i.e., immediately 
following the casting process in segment 0, bending of the strand is 
carried out with a two phase mixture between the strand shells through, 
for example, 5 bending points into the inner circular arc of, for example, 
4 m, in order to keep the strand shell deformation density small and not 
to be cumulated with the previously occurring casting and rolling 
deformation. 
In accordance with the geometric relationships and a plant height of, for 
example, about 8 m, a return bending into the horizontal, for example, 
through five straightening points in segment 4 occurs at a distance of 
about 12 m from the meniscus, i.e., a substantial distance in front of 
final solidification which occurs at a distance of about 15 m from the 
meniscus in the case of VG 5 m/min. or at a distance of 30 m from the 
meniscus in the case of VG 10 m/min. Consequently, the time between return 
bending and the resulting deformation of the inner strand shell and the 
final solidification which is extremely sensitive to deformations is 36 s 
or 108 s, so that a harmful influence on the final solidification in the 
area of the sump tip and the resulting defects in the core of the strand 
due to the return bending process are excluded. 
FIG. 4 of the drawing shows an embodiment of the present invention with a 
single-line continuous casting plant for producing a maximum of 3.0 
million tons per year for an average strand thickness of 100 mm at the 
outlet of the vertical mold, wherein the vertical mold has a hydraulic 
drive, the solidification thickness is 80 mm and the maximum casting speed 
is 10 m/min.; the continuous casting plant includes 
a vertical mold having a length of 1.2 m, a width of at most 180 mm in the 
middle of the meniscus and a minimum width of 100 mm in the center and a 
width of 100 mm in the area of the narrow sides at the mold outlet; 
a vertical segment 0 configured as a tong-segment having a length of 3 m 
for reducing the strand thickness to 80 mm; 
a segment 1 with 5 bending points and an inner radius of 4 m; 
segments 2 and 3 in the inner circular arc; 
a segment 4 with 5 straightening points; and 
segments 5-13 in the horizontal portion of the strand guiding means. 
The entire continuous casting plant has a metallurgical length of about 30 
m, wherein about 4 m of the length are arranged vertically (KO), about 8 m 
in the circular arc (segments 1, 2, 3, 4) and about 18 m horizontally 
(segments 5-13). At the casting speed of at most 10 m/min, the lowest 
liquidus point 1.1 extends about 2 m into the segment 0 having a length of 
3 m, so that it is ensured that oxides rise into the casting slag in an 
optimum manner and the oxides remaining in the steel are simultaneously 
symmetrically distributed, while also ensured are a destruction of the 
crystals in the two phase area and a suppression of the core segregation 
in the strand. At a distance of about 16.5 m from the meniscus, a two 
phase mixture of 50% crystal portion (50% by weight) with a viscosity of 
10,000 cP (the same as honey at 20.degree. C.) is present. In addition, 
the final solidification 2.1 takes place in the last segment 13 far away 
from return bending in segment 4. Between the return bending and the final 
solidification in the sump tip area, an undisturbed solidification period 
of about 108 s is available which ensures a good core solidification. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the inventive principles, it will be understood 
that the invention may be embodied otherwise without departing from such 
principles.