Note: Descriptions are shown in the official language in which they were submitted.
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SI2JTE~D POI,YC~.YST~LLI~ cor~posITE MATl~RIALS BASED
OM BORON MITP~ID~
The invention relates to sintered polycrystalline composite
materials based on boron nitrid2 for use as breakrings in the
horizontal continuous casting of steel.
BA5KGROUr~D !:)F TF~E INVENTIOll:
In the field of the continuous casting of steel, in
addition to the vertical and billet casting plants, horizontal
casting plants have also been developed. The advan'ages of
horizontal continuous casting as compared with conventional
continuous casting are the ]ow overall height and construction
costs of the plant, protection of toe melt from re-oxidation and
freedom Erom deformation of the billet. horizontal continuous
casting plants are especially suitable for the casting of
relatively small melts, high-alloy special stee]s and simple
constructional steels used in quantities.
In the horizontal continuous casting of steel, the casting
mold is connected in an air-tight manner to a distributor vessel.
The solidification of the steel, that is the formation of the
billet shell in the casting mould, occurs not as in the case of
all other contlnuous casting plants at a phase interface between
liquid steel and the surrounding gas, but in a continuous liquid
phase. This requires special means to ensure that solidification
occurs at a specific point in the casting mold.
The function of the distributor is to ensure a constant
flow of molten metal and to distribute the molten metal evenly
among the billets. The transition to the casting mold is formed
of furnace brick which is firmly set into the furnace and on the
side towards the casting mo]d comprises a discharge nozzle which
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is generally formed from a high-grade refractory material such as,
for example, zirconium oxide, since it has to absorb high thermal
stresses. Adjacent the discharge nozzle is the break ring whicb
is a component that is characteristic of the horizontal continuous
casting process. The breakring has to ensure, in continuous
operation, that prom that point onwards the billet shel1 begins to
form. It thus serves precisely to determine the site of soli-
dification of the billet in relation to the casting mold. Since
it is unavoidable that a shell will also form on the break ring,
the shell must be easily removable and must not form any
interphase with the material of the break ring. In aadition, high
resistance to thermal shock and high abrasion resistance are
required. The lack of a suitable material which satisfies these
requirements -for the break ring is one of the main reasons why
horizontal continuous casting of steel has not hitherto won
recognition in practical operation, although the process per se
has been known for more than 100 years.
In plants for horizontal continuous casting of steel that
have been developed since 19~0, breakrings of ceramic material,
such as .silicon nitride, zirconium oxide, boron nitride or
aluminium nitride, have been used to solve the problem of
providing an operationally reliable transition prom the holding
furnace to the casting mold. It also became known, however, that
when, for example, silicon nitride brearings were used, typical
defects of horizontal continuous casting, such as transverse
cracks on the surface upon drawing the solidified billet out of
the casting mould, could not be prevented. on improvement in the
durability of the ceramic connection between the holding vessel
and casting mold is therefore considered to be imperative (cf. R.
Thielmann and I. Steffen in "Stahl und Eisen" 100 (1~80), Jo. 7,
pages ~0l-407).
~137~
Refractory composite materials based on oxides r"hich,
because ox the addition of graphite, are especially durable and
have therefore proved useful for immersion nozzles in conventional
continuous casting processes (cf. ~E-A-29 36 480 and DE 30 03 045
C 2) cannot, however, be used for breakrings in horizontal
contirluous casting. Breakrings containing graphite cannot be used
for horizontal continuous casting of high-alloy special steels
which are especially susceptible to the absorption of carbon.
Refractory materials consisting of sintered stabilized
zirconium oxide with a boron nitride content of not more than 20%
(cf. Jpn. Rokai Tokkyo Koho JP 59 21, 575, reE. in C.A. 100:
1967~e (198~)) are no more suitable for breakrings than those
base solely on oxides or nitrides since chipping and erosion of
the breakring cannot be avoided. This results not only in
increased wear of the breakrings themselves but also in the
embedd;rlg ox undesirable material in the steel which, in the case
of the long casting times required for high-alloy special steels,
leads to a marked diminution ln quality.
aterials based solely on boron nitride have the advantager
as compared with the other known materials, that they are
virtua]ly unwetted by steel and steel alloys and, in addition, are
easily worked. It is important that the material for the break-
ring can he easily worked since an accurate fit of the breakring
is requirea between the discharge nozzle and the mold.
The problem is, to improve the material based on boron
nitride by selecting suitable additives so that the composite
satisfies the requirements for use as a breakring in the hori-
zontal contirluous casting of steel.
BRIEF ~ESCRIPTIO~ OF TOE INVENTION
_ _ . ... .
According to the invention, this problem is solved by
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forming breakrings from sintered polycrystalline composite
materials based on boron nitride that have been manuactured by
hot~pressing powder mixtures of hexagonal boron nitride and from 1
to 70~ by weight of at least one oxide based on the total weight
of the mixture, selected from the group consisting of zirconium
oxide, magneslum oxide, aluminium oxide and silicon oxide, at
temperatures in the range of from 1,600 to 2,000C under pressure
of from 7 to 20 ~.~PaO
The starting materials employed for the manufacture of the
composite materials used in accordance with the invention are
preferably hexagonal boron nitride powders having an oxygen
content of from 5 to 10% by weight and a specific surface area of
from 5 to 30 m /g (measured according to the ~r5T method) and the
oxide powders, individually or in admixture, each having 3
purity of at least 7 97~.
The powders are homogeneollsly mixed ina manner known per se
in a conventional. mixing apparatus, optionally with the use of a
temporary binder, and then hot-pressed in graphite molds under
biaxial app7ication of pressure at temperatures of pre.erably from
about l700 to 1~00C under a pressure of preferably from about 9
to ll Pa The breakrirlgs are cut from the resulting blocks in
the desired dimensions and worked into their final shape.
The composite materials useful i.n the practice of the
invention are predominarltly of a hexagonal boron nitride phase,
and the oxide phases, the boron nitride phase being bonded to the
boron oxide present in the starting powder (expressed by the
oxygen content in the given range, which acts as a sintering aid
he].ieved to be partly formed from boroxinitride phases.
Composite materials in which the hexagonal boron nitride
portion is from ~0 to 60~ by weight and the oxide portion consists
of zirconium oxide or magnesium oxide, and which have a density of
at least 94% of the theoretically possible density (based on the
boron nitride and oxide mixture), have proved especially useful as
breakrings for contirluous horizontal casting processes.
In comparison with breakrings formed from a sintered
polycrystalline materal consistirlg solely ox boron nitride, the
composite materials used according to the invention exhibited
considerably improved wear behavior when used as breakrings in the
horizontal continuous casting of various types of steel. The
subject matter of the invention is described in detail in the
fol]owing examples.
~xam~le l
a) A powder mixture of 30% by weight zirconium oxide
(purity: 98~), 68% by weight boron nitride t2 content 7~ by
weight, speci-fic surface area 10 m2/g) and 2% by weight of a
temporary binder was hot-pressed in graphite molds at 1800C and
lD Pupa to form a sintered body. The block size was d 200 and
length 200 mm. The density obtained was 9~% of the theoretical
density (TD).
b) Under the same conditions, a powder mixture of 98% by
weight of the boron nitride powder and 2% by weight of the
temporary binder was hot-pressed. density obtained: 94~ To.
Rings having the dimensions do 65 x di ~5 x 12.7 mm and do
165 x di 120 x 12.7 mm and samples for further tests were worked
from these bodies. Table 1 shows the results obtained in compari-
son with boron nitride. The ring (do 1~5 x di 120 x 12.7 mm) was
used to cast stainless steel. With this quality of steel, erosion
at the exit side of the breakring occurred to an increased extent
when pure boron nitride breakrings were used. Table 2 aives the
results obtained in comparison.
~0~1~
The ring do 65 x di 45 x 12.7 mm was used to cast high
cobalt-alloy steel. In this case also, the usual erosion at the
exit side of the pure boron nitride breakring occurred. Table 2
shows the results obtained.
In the examples, do refers to outside diameter and di,
refers to inside diameter of the breakring.
Example 2:
powder mixture of 30~ by weight magnesium oxide (purity:
98~), 68 % by weight boron nit,irde from example la) and 2% by
weight oE a temporary binder was hot-pressed at 1700r and 10 MPa
to form a sintered body. The block size was d 180 and length 200
mm. The density obtained was 94~ of the theoretical density.
Breakrings having the dimension do 165 x di 120 x 12.7 mm
were likewise worked from this body. Table 1 shows the measured
va]ues obtained in comparison with boron nitride. stainless steel
was cast using the breakring. Table 2 shows the results obtained
in comparison with boron nitride.
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