Method and apparatus for cooling continuously cast metal strands

A method and apparatus for the cooling of a continuously cast metal strand, comprising a plurality of support rollers, located in the second zone of cooling. The support rollers are cooled from their interior by the passage therethrough of cooling fluid. The rollers have interior walls which display a wave-like shape to present a great surface area for superior cooling while still supporting the loading stress due to the weight of the metallic strand.

BACKGROUND OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART 
The present invention relates to a method for cooling continuously 
cast-steel strands. In addition, the present invention relates to a 
cooling apparatus capable of carrying out the method disclosed herein. 
Continuously cast-steel strands have been traditionally cooled in a 
spray-water cooling process, in which, essentially, the heat required for 
vaporization of the cooling water is absorbed, to thereby carry away the 
heat of the metallic cast strand. In the spray-water cooling process, the 
cooling water does not only extract the heat from the hot cast strand but, 
in addition, simultaneously protects the support elements (rollers) and 
other structural elements of the continuous-casting apparatus against 
undesirable overheating (Manual of Strand Casting 1958, 188, by Dr. 
Hermann). 
From DE-B2-913 697 it is known to use internally cooled rollers in 
continuous rubber casting. Rubber casting, however, is significantly and 
completely different in various aspects from continuous steel casting. One 
of these essential differences lies in the fact that, in the processing of 
rubber, heat must first be added to the strand and the temperature then 
downwardly adjusted by cooling. 
SUMMARY OF THE INVENTION 
The basic object of the present invention is to reduce the temperature of 
the metallic casting strand, in a dry procedure, using internally cooled 
roller elements and to thereby completely bypass the conventional water 
spray cooling process. 
The solution, according to the invention, is that the heat of the 
continuously cast metal strand is eliminated exclusively through 
structural metal roller elements, which are cooled from the inside. The 
cooling occurs, by location of the hollow rollers, on the metallic cast 
strand in the lower cooling section or secondary cooling zone, as defined 
in the Detailed Description of the Drawings. The structural roller 
elements surround, support and guide the cast strand during cooling. 
During the cooling process the cooling fluid, preferably in the form of 
liquid, is circulated and itself cooled. The invention substantially 
avoids the disadvantages of the water spray cooling process, namely 
non-uniform cooling of the strand surface, disproportionately high thermal 
stress in areas of the strand, undercooling of the edges, uncontrollable 
quantities of cooling water on the strand surface, an otherwise required 
preparation of water spray, an otherwise large clearance for the strand 
support because the water spray jets are arranged between adjacent support 
elements, the stress due to thermal shock on the support roller elements 
themselves and distortion of the support elements because of the heat 
generated by the process. 
One basic advantage of the present invention is the provision of uniform 
cooling of the surface of the cast strand so that thermal stress may be 
kept low in critical areas of the cast strand. Also, by eliminating the 
water spray jets, the spacing of adjacent support roller elements may be 
kept smaller so that the undesirable bulges of the cast strand, which 
otherwise occur due to the, not as yet, sufficiently solidified inside, 
are avoided. 
The present invention is also based on the cooling circulation process, in 
which the cooling fluid, preferably a liquid, circulates within the 
structural roller elements, absorbs heat, and is close-circuit 
subsequently cooled by having the now-heated fluid pass outside of the 
structural roller elements. 
One of the problems with the mentioned cooling apparatus of the present 
invention lies in the difficulty of providing a high-heat flow, i.e., good 
heat transfer through the wall of the hollow structural roller elements. 
It is desirable that a large amount of the heat of the cast strand be 
withdrawn from the strand. This presupposes a low heat resistance of the 
wall of the structural roller element. In attempting to solve this 
problem, the wall of the structural element could be very thinly made 
(thereby decreasing heat wall resistance and increasing heat flow through 
the wall) which, however, is contrary to the customary principles of high 
mechanical stress capacity of the structural roller elements. Where 
support rollers of extended length, for example, are used, thin walls of 
structural roller elements cannot be used because they could not withstand 
the otherwise present stress of the cast strand. 
The present invention, therefore, operates without a too-thin structural 
roller wall thickness and, nevertheless, a low heat resistance value for 
the wall is obtained, i.e., a high heat-transfer performance is provided. 
The present invention solves the dilemma of wall thickness being thin for 
high heat transfer and wall thickness being thick for support of the 
strand, by providing recesses in the interior wall of the structural 
roller element which recesses face the cast strand. Thus, the heat flow 
resistance of the roller walls, in its inner surface, is kept to a minimum 
without sacrificing structural rigidity. The inner wall surfaces of the 
rollers consist of a plurality of grooves or recesses of a width and depth 
of less than one millimeter. 
The cooling process of the present invention, according to the invention, 
may alternatively also be carried out with a cooling apparatus designed 
such that the inner wall surface of the structural roller element consists 
of a capillary-type structure similar to that used in heat transfer pipes. 
The cooling apparatus, according to the invention, has a structural roller 
element consisting of a support roller which is rotatably supported on a 
hollow journal pin for feeding in and allowing for outflow of cooling 
fluid, preferably liquid. Alternatively, vaporous cooling fluid can be 
used. Between adjacent support rollers, a distance of between three and 
five millimeters is preferably maintained. The hollow support rollers 
allow for substantial uniform cooling of the metallic cast strands. 
Furthermore, the support rollers, themselves, are cooled without the 
mentioned disadvantages of the water spray cooling mechanism. 
A high density heat flow and a simultaneous high mechanical stress capacity 
is obtained when the recesses in the interior wall of the structural 
roller element extend in a quasi wave-like manner along the longitudinal 
cross section. Located within the wave-like cross section are 
capillary-type structures. The recesses advantageously reinforce the flow 
of heat from the cooling metallic cast strand through the structural 
roller element wall. 
A complete cooling system for a section of the support roller frame of a 
continuous-casting metallurgical mill can be provided by having a 
plurality of the interior cooled support rollers connected, through their 
hollow draining bearing pins, to a cooling device at a heat-emitting area, 
and the hollow infeed bearing pins of the roller elements connected to a 
different cooling device. 
A closed circuit cooling loop substantially corresponding to the structure 
of the continuous-casting apparatus can also be designed such that the 
first cooling device extends basically parallel to the cast strand. 
The conventional method for cooling cast-steel strands works according to 
the water spray cooling process, in which the heat vaporizing the cooling 
water on the surface of the hot cast strand, is used to cool the strand. 
The conventional method necessitates a minimum distance for location of 
the spray jets between the support rollers. However, in order to avoid 
non-uniform cooling of the strand surface, high thermal stress in the 
strand, undercooling of the strand's edges, uncontrollable quantities of 
cooling water on the strand surface, as well as an otherwise required 
water spray preparation, in which the spray water, besides, causes 
stresses by thermal shock on the roller support elements and heat 
distortion of the roller support elements, it is suggested, in the present 
invention, that the heat be eliminated from the cast strand, in the 
secondary cooling zone of the casting guide, exclusively through 
structural metal roller elements, which are cooled from the inside. The 
roller elements surround the cast strand at short adjacent distances 
between rollers and support and guide the strand, while the cooling fluid 
circulates within the rollers. 
The cooling apparatus required in order to carry out the cooling process is 
designed in such a manner that the structural roller element interior 
walls have recesses in their inner surfaces. This results in the 
decreasing of the heat resistance of the wall. The inner surface can be 
provided with a plurality of grooves or recesses of a width and depth 
smaller than one millimeter. It is furthermore proposed that the inner 
surface of the structural roller element wall have a fin-like structure 
which is similar to the structure used in heat transfer pipes.

DETAILED DESCRIPTION OF THE DRAWINGS 
The continuous-casting apparatus shown in FIG. 1 displays a 
continuous-casting cast-iron mold 1 and a strand guide comprising an upper 
cooling roller section 2 and a lower cooling roller section or secondary 
cooling zone 3. The individual support rollers 4 are separated by a 
distance 5 which, according to the invention, is set at approximately 
three to five millimeters. 
The structural roller elements, according to the invention, are formed by 
the support rollers 4. These structural roller elements surround the 
metallic cast strand 6 at a gap clearance which does not interfere with 
the movement of the cast strand 6, i.e., the support rollers 4 support and 
guide the cast strand 6 by being in direct contact therewith but do not 
inhibit the controlled movement of the cast strand 6. 
The exemplary embodiment of the present invention, as shown in FIG. 1, does 
not, therefore, require the conventional water spray cooling system (not 
illustrated). The disadvantages of the conventional water spray cooling 
process were previously explained, in brief detail, 
In brief review, however, the cooling water emanating from the conventional 
spray jets can and do cause the metallic cast strand surface to cool 
non-uniformly, in which disproportionately high thermal stress cracks or 
other defects may form in the strand. Also, undercooling of the edges may 
result from water spray. In addition, conventional water spray techniques 
may require a water spray preparation procedure. The conventional spray 
cooling process, furthermore, causes thermal shock stress and heat 
distortion on the support elements, themselves. 
According to the invention, the support rollers 4 are in fluid connection 
through their hollow journal pins 25, with a cooling device 26 (FIG. 2). 
Journal pins 25 drain the fluid from the rollers 4. The fluid connection 
is at the heat dissipating area 27 of the cooling device 26. The hollow 
journal pins 28 feed the cooling liquid into the rollers and are connected 
with a pump 30 in the low-temperature areas 29 of the cooling device 26. 
The cooling device 26, itself, is cooled by a process which operates 
countercurrent to the heat yielded by the cast strand 6 by means of the 
entering cooling conduit 31, which is part of a gaseous or, if applicable, 
liquid jacket 32, and the now heated cooling liquid leaving the cooling 
device 26 through exit conduit 31a. The cooling device 26 extends 
approximately parallel to the metallic cast strand 6 and is arranged next 
to, below or above the cast strand 6. 
The support rollers 4 (see FIG. 3) comprise separate support roller 
sections 4a and 4b having individual support bearings 11a, 11b, 11c and 
11d. 
The cooling apparatus, according to the invention, conveys cooling fluid 
14, for example cooling water, in an open or closed cooling circuit into 
the central pipe 12 in the direction of arrow 14a, with the cooling fluid 
flowing out of the mouth 12a of pipe 12, in the direction of arrow 14b. 
From there the fluid is guided through the cooling zone 13. The fluid 
exits the support rollers 4 in the direction of the arrow 14c, after 
becoming heated up (as vapor or as a mixture of vapor and fluid). After 
leaving roller 4, the fluid passes through the cooling device 26, in order 
to be cooled off and reused. 
In the illustrated exemplary embodiment, the structural roller element wall 
15 is the interior wall of the support roller 4. In the area of the 
cooling zone 13, the thickness of the wall 16 (see FIG. 3) is steadily 
reduced from zone to zone. A zone 17 of this step-like structure does not 
represent a significant mechanical weakening of the remaining wall 
thickness 16. 
The capacity of the support roller 4 to withstand stress is assured by the 
inner surface 18. Two features of the inner surface 18 facilitate heat 
transfer. More specifically, the inner shape of surface 18 is in the form 
of an archway and, therefore, enlarges the surface area of surface 18. 
Added to this is the fin-like structure 19 of inner surface 18, consisting 
of step-like recesses (best shown in FIG. 5), which is similar to the 
structure shown by heat transfer pipes, i.e., a large surface area is 
displayed to enhance heat flow. 
A heat transfer pipe of this type is described in DE-AS 12 64 461. There, 
however, it is a closed structural element. Here, however, the support 
roller 4 may, in principle, have the exterior shape of such a heat 
transfer pipe. 
The inner surface 18 is provided with recesses or steps 22, extending as a 
quasi wave-like pattern 24 in the longitudinal cross section 23. The quasi 
wave-like pattern 24 avoids the formation of stress peaks in the work 
material of the support rollers 4. The wavy-like pattern 24 may also be 
very long and drawn out or even flat, so that only one zone 17 is present 
along the length of the support roller sections 4a and 4b. 
The high temperature of the metallic cast strand 6, which may be 
approximately 700.degree. to 1,100.degree. C., causes the cooling liquid 
14 to heat up to its vaporizing point. It is, therefore, appropriate to 
eliminate the superheated steam because of the very high heat content 
within the cooling zone 13, and to substitute therefor cooler, unsaturated 
steam of the "fresh" cooling liquid and/or to continuously substitute a 
mixture of liquid and vaporous cooling liquid. The introduction of fresh 
coolant can be done in accord with the cooling curve of the 
slowly-solidifying metallic cast strand 6. 
The teachings of the attached copy of the corresponding German application, 
upon which this application claims priority, is herein specifically 
incorporated by reference. 
It should be understood, of course, that the specific form of the invention 
herein illustrated and described is intended to be representative only, as 
certain changes may be made therein without departing from the clear 
teachings of the disclosure. Accordingly, reference should be made to the 
following appended claims in determining the full scope of the invention.