Process and device for cooling molten steel

A process for cooling molten steel, in particular by continuous casting of hoop-steel. At least part of the molten mass that leaves a metallurgical vessel through a metal nozzle solidifies when contacting a cooling surface. A gaseous stream that forms a reducing atmosphere is directed onto the surface of the freely accessible liquid hoop-steel immediately after it leaves the metal nozzle and the surface of the hoop-steel is exposed to this gaseous atmosphere at least until it is completely solidified.

FIELD OF THE INVENTION 
The present invention relates to a process an apparatus for cooling molten 
steel, particularly continuous casting, in which at least a portion of the 
molten metal that emerges from a nozzle of a metallurgical vessel is 
solidified by means of contact with a cooling surface. 
BACKGROUND OF THE INVENTION 
In continuous or strand casting, the molten metal is directed into a cooled 
mold. Contact with the cooling mold causes a solidification front to form, 
beginning at the outside and moving toward the interior of the strand. In 
order to improve the quality of the metal blanks, it is known to supply 
them with an inert gas. 
For example, German Patent Publication DE OS 21 63 928 proposes that during 
the production of steel blanks by means of the continuous casting of a 
metal stream into a cooled mold, an inert gas be introduced over the metal 
at the upper part of the mold in the vicinity of the surface of the molten 
metal. The use of nitrogen or argon that has previously been liquified by 
compression and lowered temperature, which is applied in liquified state 
to the surface of the steel blanks, is suggested. The aforementioned 
document merely discloses exposing the molten metal to an inert gaseous 
atmosphere and directing the gaseous jet in such a way that the molten 
metal of the blanks is offset around a vertical axis in a rotational 
movement. 
From German Patent Publication DE 32 27 132 A1, it is known to surround a 
metal stream that emerges from a metering nozzle with a protective mantle 
of inert gas, e.g.; argon or nitrogen, in order to keep air away from the 
vicinity of the metal melt. This pressurized inert gas screens off the 
oxygen coming from the ambient air and in this way prevents reoxidation of 
the exposed metal melt meniscus. The expert in this document does not 
undertake more extensive influencing of the molten metal. Furthermore, the 
use of inert gas to treat metal strands or wires that are solidified 
already or only heated is known. For example, in German Patent Publication 
DE 35 06 597A1 a wire is exposed to a lightly reducing gas in the housing 
of a cooling column. The gas used in this case is supplied to the housing 
in an undirected fashion and serves exclusively to cool and, usually, to 
reduce scale formation. In the cited casting processes, the inert gas is 
brought into contact with the molten or the already solidified surface. In 
the case of continuous casting as known from German Patent Publication DE 
38 10 302, for example, the molten metal is deposited on a cooled 
continuous belt and the exposed surface of the strand cools during its 
transport on the belt, so that the exposed surface in the front area near 
the nozzle is still molten and solidifies later due to cooling. 
SUMMARY OF THE INVENTION 
An object of the invention is to create a process and a corresponding 
device that can influence the surface of a continuously cast metal strand 
in respect to both its form and its quality. 
According to the invention, a gaseous stream is directed onto the surface 
of a freely accessible molten steel strand directly after the latter 
emerges from a metal nozzle of a metallurgical vessel. The surface of the 
strand is thereby exposed to a gas that forms an inert atmosphere at least 
until the steel strand solidifies completely. Along with gases low in 
oxygen, e.g. flue gas, inert gases such as argon or nitrogen, in 
particular, can be used. 
The use of these gases intensely influences the surface of the steel 
strand; specifically, in the molten area as well as in the solidified area 
and the area of molten/solid transition. As a result, scaling is avoided. 
Furthermore, using the gas in the vicinity of the nozzle allows deliberate 
influence to be exercised on the heat extraction and surface tension. 
Depending on the desired quality of the steel strand or steel strip, the 
inventors propose to either heat the gas and in this way prevent 
solidification of the strand surface for a predeterminable segment or, in 
another embodiment, to cool the gas to such an extent that it is 
transported in liquid form. The temperature of the gas can be established 
in either of the two extreme ranges in predeterminable fashion. Of course, 
the gas can also be used at room temperature. 
In an advantageous further embodiment of the invention, is; the gas 
directed onto the surface of the steel strand not only at a temperature, 
but also in a quantity and at a speed that permit influence to be 
exercised on the form of the cast strand. First, the surface can be 
deliberately pressed upon and the entire strand, for example, given a 
profile in the form of a camber. However, it is also possible to direct 
the gas in such a way that the gaseous kinetics have a complementary 
positive influence in reducing bulge formation.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
FIG. 1 shows a metallurgical vessel 11, wherein a metal melt M flows out of 
a metal nozzle 12. 
The melt M is directed onto a transport belt 43, which is held as a 
continuous belt by a driving drum 41 and a guiding drum 42. On the 
underside of the carrying run of the transport belt 43, there is a cooling 
device 44 that cools the steel strand S, which is transported in the 
transport direction s. 
The metal strand S is surrounded by a housing 31, which surrounds the 
strand S at the exit 32 by a seal 33 in order to minimize gas leakage. 
Gas nozzles 25 are run through the cover of the housing 31. These gas 
nozzles 25 are arranged at an angle of between 0.degree. and 45.degree. 
relative to the steel strand S. The nozzles 25 are attached to gas 
distributors 26, which are connected to a compressor 21 via the supply 
lines 23. The gas nozzles 25 can be individually blocked by the blocking 
organs 24. 
Between the compressor 21 and the nozzles 25, there is a heat exchanger 22, 
which can be used to adjust the temperature of the gas that forms the 
reducing atmosphere or the temperature of the inert gas in predeterminable 
fashion. The compressor 21 is attached to a gas supply station 29. FIG. 1 
shows a connecting line 28 that connects the gas supply station 29 to the 
housing 31 in the area of the strand exit 32 via a collective gas line 27. 
Using the same item numbers as FIG. 1, FIG. 2 shows a cross-section through 
a continuous casting unit. FIG. 2 shows the arrangement of several gas 
nozzles 25 next to one another, each of which has a blocking organ 24 and 
is attached to the distributor 26, which has the supply line 23. 
In the upper area of the guiding drum 42, there is a seal 34, which 
minimizes leakages between the side walls of the housing 31 and the side 
shields of the drum 42.