Note: Descriptions are shown in the official language in which they were submitted.
104673~
The invention relates to a method of continuously
casting steel strands, in particular slabs, in a continuous
casting plant, wherein the strand is withdrawn from a cooled,
oscillating mold containing molten steel covered by a slag
layer, respectively a casting powder layer, and is cooled
further in a secondary cooling zone arranged to follow the
mold.
In methods of this kind the casting slag is delivered
between the mold and the ~orming strand skin. The casting slag
adhering to the strand surface has been found to have
disadvantageous effects in that it remains in the oscillation
marks. Liquid phases of the slag penetrate between the grain
boundaries of the solidified strand skin and lead to
intergranUlar corrosion~ This causes the formation of
transverse cracks which occur on the strand surface transversely
in relation to the lowering direction of the strand. Prior
to a further processing of the strand in the rolling mill
the transverse cracks, which constitute~ surface defects, have
to be eliminated by scarfing, which entails a loss of materialO
Often the voids in the strand sur~ace caused by the corrosion
of the casting slag are so small - some tenths of a mm _
that they are not visible with the naked eye. They do, however,
constitute dangerous points of weakness, which due to
stress concentration may easily develop into severe surface
defects, i.e. to up to 10 mm deep cracksO
For continuous casting various casting powders
are known. Their main constituents in general are CaO, SiO2
and fluxing agents, such as fluor spar, soda and borax. In
the slags resulting from the casting powders the softening
temperatures above which liquid phases occur lie between
q~
1~46731
750 and 1150C.
The surface temperature of the strand in the continuous
casting plant is determined by the amount of cooling water sprayed onto
the strand surface for cooling the strand. In the upper part of ffle
secondary cooling zone, i.e. in the range below the mold, normally
temperatures between 950 and 1250C are run. In the lower part of the
secondary cooling zone a temperature lying between 800 and 1100C is
ma~ntained. Normally the surface temperature of the strand is highest
in the upper part of the secondary cooling zone and diminishes gradually
towards the lower part.
When comparing the softening temperatures of the slags
with the surface temperatures of the strand it becomes understandable
that among many operating conditions occurring in practice it is possible
that liquid slag having a relatively high temperature may act upon the
strand surface for a considerably long period of time, in particular in
the range of the oscillation marks, which then causes the surface defects.
The invention aims at avoiding the above mentioned
disadvantages and difficulties and it is its ob~ect to coordinate the
parameters of the method, in particular the temperature of the strand
surface and the softening temperature of the slag in such a way that a
corrosion attack caused by liquid, hot slag particles is avoided,
respectively that the duration of their influence is kept within limits
that are not dangerous.
Thus this invention seeks to provide in a method of
continuously casting a steel strand, in particular a slab, in a continuous
casting plant, the strand being withdrawn from a cooled, oscillating
mold containing molten steel covered by a layer of casting powder and
casting slag resulting from said casting powder, said casting powder and
casting slag having a certain softening temperature and a certain chemical
composition and said strand having a certain surface temperature being
~046731
further cooled in a secondary cooling zone arranged to follow the mold
by applying a certain amount of cooling water onto said strand, the
improvement comprising co-ordinating the surface temperature of the
steel strand in the secondary cooling zone, said surface temperature
lying between 1250 and 800 C, and the softening temperature of the
casting powder and of the casting slag resulting from said casting
powder, said softening temperature lying between 1150 and 750C, in
such a way that during a period in which the strand passes through the
secondary cooling zone the entire strand surface temperature shall not
exceed a temperature that is 150C above the softening temperature of the
casting slag for longer than 2 minutes.
Suitably the softening temperature of the casting
powder or of the casting slag, respectively, may be adJusted by a
variation of the chemical composition, in particular by a variation
of the content of fluxing agents.
~he surface temperature of the cast steel strand can
be regulated most easily by regulating the amount of cooling water.
Preferably several temperature measuring points are provided along
the secondary cooling zone for currently measuring the surface temper-
ature of the strand and in dependence thereof the amount of cooling
water is regulated while maintaining the temperature limit and the
limit of
104673~
t-hat period during which the cooling water touches the
strand surface.
When following this procedure the danger of corrosion
is eliminated, because the chemical attack with temperatures
o~ the strand surface lying up to 150C above the softening
temperature of the slag can still be considered non-criticalO
No weak points are caused which would lead to the formation
of cracks. At these surface temperatures of the strand the slag
phases are solid or so viscous that the grain boundaries are
not yet attacked. I~ the surface temperature is higher than
the given limit temperature the slag influence duration in the
range of the secondary cooling zone may last maximumly 2
minutes without having to reckon with a dangerous corrosion,
which as a consequence leads to the formation of cracks.
The method of the invention will now be illustrated
in more detail with reference to the accompanying graph, in
which there is plotted on the abscissa the softening temperature
of the slag and on the ordinate the strand surface temperature.
The lines 1 and 2 delimit the range between 750 and 1150C,
within which range there lie the softening temperatures of
slags which result when using various casting powders.
The range of the surface temperatures of the strand in the
casting plant between 800 and 1250C is delimited by the lines
3 and 5.
Field A is separated from field B by line 4. Line 4
corresponds to a temperature lying 150C above the so~tening
temperature of the slag; it indicates that temperature at a
certain point of the strand surface, above which at a slag
influence duration of more than 2 minutes one has to reckon with
cracks in the strand caused by corrosion.
1~46731
The softening temperature of the casting slag is
suitably determined by taking slag samples from the mold,
since the softening temperature of the casting powder and
of the casting slag are normally not equal, because when
the casting powder melts in the mold its chemical composition
changes, wherein volatile constituents escape and deoxidation
products are received. The softening temperature is usually
determined according to DIN 51730~
The surface temperature of the strand in the casting
plant may be measured in various ways, e.g. pyrometrically.
The coordination of the strand surface temperature
with the slag softening temperature or vice versa does not
constitute any problem in practice. The strand surface
temperature may be set over a relatively wide range; if the
softening temperature of the slag is used as a standard size
one can choose the suitable casting powder from the large
variety of commercially available casting powders having
different sotening temperatures, or one can with a given
casting powder vary the softening temperature by adding
fluxing agentsO
The method of the invention is of great advantage
particularly in the casting o~ slabs having a width of more
than 800 mm, as said slabs are especially liable to transverse
cracks. The reason for it lies in the fact that with larger
strand widths higher tensions occur in the strand skin than
with smaller strand widths. As has been mentioned before the
tensions promote the chemical attack. The method of the
invention is suited for casting alloyed and unalloyed steelsO
The method of the invention and its advantages as
against the known method will now be illustrated by the
1046731
~ollowing example.
In a continuous slab casting plant 50 metric tons
of steel were cast, which had been melted in an hD melting pot
and which had the following composition:
C Si ~ P S ~1
~05 ~05 ~35 ~013 oO17 ~035 J~
The casting plant had a straight moldO The strand was bent
to form an arc in a bending zone and was guided into the
horizontal in a straightening zone. The mold had Ihe dimensions
1300 x 225 mm. The lowering speed was 1.2 m/minO In the tundish
the steel had a temperature of 1 555C~ From the tundish the
steel was passed into the mold using immersion tubes. Onto
the casting level in the mold casting powder having the
following chemical composition was applied.
Fe23 MnO SiO2 A123 MgO CaO Na20
2 ~36 0~05 27072 1.38 0096 26 ~35 8 .20
~2 CaF2 C(total) ignition loss B2o3
0~40 10~80 4~42 12.26 4~19 %
From the mold a slag sample was taken, whose chemical
composition was the following:
Fe23 MnO SiO2 Al23 MgO CaO Na20
1~70 0~10 31~80 11.20 1022 29.38 7.20
K20 CaF2 C(total) ignition loss B2o3
0~90 10~19 1~20 100 2045 %
The softening temperature of the slag was 985C according
30 to DIN 51 730o The watex used for cooling the strand in the
- 6 -
1t~4673~
secondary cooling zone was applied onto the strand by means
of ~lat spray nozzles~ The amount of cooling water used was
0.7 l/kg of steel. For 4 minutes the strand was exposed to a
temperature between 1135 and 1170C over a strand length of 5 m.
In this method of operation both the 150-temperature
limit and the maximum exceeding duration of 2 minutes suggested
by the method of the invention were exceeded. As a consequence
thereof cracks occurred in the oscillation marks of the slabs
produced, which cracks had to be eliminated by scarfing
entailing a loss of 3~.
In contrast thereto slabs were cast as previously
described Prom steel of the same composition on the same
plant at the same casting temperature and at the same
lowering speed, wherein while using the same casting powder
with the same softening temperature of the slag cooling was
increased by spraying an amount of cooling water of 0.75 l/
kg of steel onto the strand. Th~reby the highest temperature
of the strand surface came to lie at 1125C, which lies below
the critical 150-temperature limit. The slabs obtained were
free from cracks and did not require scarfing.
