Patent Publication Number: US-2012037331-A1

Title: Method and Device for Guiding and Orienting a Strand in a Continuous Casting Facility for Large-Sized Round Profiles

Description:
The invention relates to a method and a device for guiding and orienting a strand in a continuous casting facility for large-sized round profiles made of steel or a similar material. 
     Round profiles are products with an approximately round or oval cross-section which have a continuously curved outer surface and so, unlike profiles with a polygonal cross-section, are free from edges. As a result, there are different requirements for said round profiles than for profiles with a polygonal strand cross-section. With the latter it is mainly problematic that the casting strand cools down more quickly at the edges than on the wide sides during the solidification process, and this is associated with the risk of cracks forming in the edge regions. This risk is largely independent of the format size of the profile, and it arises in particular at the cross-over from the radius to the extension of the strand. In order to avoid the formation of cracks it is necessary to heat the edge regions of the profile by specifically introducing heat energy along the whole guide track. This type of method is disclosed in WO 2007/131584 A1. 
     This problem does not occur with edge-free profiles because their mass is distributed evenly over the circumference, and there are no edges which are cooled from two sides. In this regard the continuous casting of round or oval profiles is in principle non-problematic. With large-sized round profiles the problem does arise, however, in the strand guide that with said profiles the cooling and solidification of the molten mass takes place more quickly on the outer surface of the strand in comparison to the core of the strand. Specifically, the strand must on the one hand be cooled down very quickly so that the core of the strand is also cooled and solidified sufficiently. In this way, however, the outer surface layer of the strand is cooled down all the more strongly. 
     The strand passing out of the die is bent via one or more orienting points from the perpendicular direction at the die exit into the horizontal direction. In order to keep the space requirement and the facility costs small, the smallest possible curvature radius is sought. During the orienting process this leads to high compressive stresses on the lower side and to corresponding tensile stresses on the upper side of the casting strand. In particular, with large-sized profiles, due to the temperature ratios, as described, within the strand cross-section, there is a risk that the tensile stresses cause cracks to form on the upper side of the strand surface although these regions themselves are subjected to the same even cooling as the rest of the strand surface. Moreover, with very large round profiles the casting speeds are very low. In this way the risk of crack formation is increased because the surface temperature of the strand is lower due to the slow casting. 
     The object forming the basis of the invention is to avoid these disadvantages and to provide a method and a device of the type specified at the start which also guarantees crack-free bending of the casting strand along the orienting track, even with large-sized round profiles. 
     This object is achieved according to the invention in that the strand surface within the orienting track is heated at least on the upper side. 
     In this way, in particular the tensile stresses occurring here can be broken down to such an extent that the strand bears up without cracks forming while the latter is oriented, while the core of the strand retains the respectively achieved degree of solidification largely unchanged. The strand surface is largely stress-free at the side. 
     Nevertheless, for the purpose of a simple mode of operation the invention makes provision to heat the whole of the strand surface. 
     According to the invention, the heating power can be adapted to the stresses occurring locally in the strand surface along the orienting track. In this way it is possible to optimise the mode of operation of the facility procedurally. 
     Within this context provision is also made according to the invention such that the strand surface is heated in a number of preferably evenly arranged partial zones of the orienting track. In this way the heating power can be adapted to the different requirements in the individual orienting track sections. 
     The stresses occurring while orienting the strand are by nature particularly large in the region of the extension of the strand guide. For this reason the invention makes provision to heat the strand surface preferably in the whole orienting track section. 
     For the purpose of even, sensitively controllable heating which is gentle on the strand surface, the invention also makes provision such that the strand surface is heated by burning an air/gas mixture in a porous structure and blowing the hot exhaust gasses produced onto the strand surface. This results in flame-free, volumetric burning with easily controllable and stable burning of the energy carrier, a high degree of conversion of the energy into radiation energy, and a large control range of the heat energy introduced and exhaust gasses flowing out over a large area. 
     The device according to the invention for implementing the method has at least one porous burner with a reactor filled with ceramic foam or similar structures, the hot exhaust gasses of which flow around the whole of the strand surface. Since, as known from experience, the risk of cracks on the upper side of the strand surface is greater, it is advantageous to orient the outlet opening of the reactor specifically so that the hot exhaust gasses can initially heat the region of the strand at greatest risk of cracking. 
     Since the risk of cracking is at its greatest in the strand guide region on the outlet side, in particular within the orienting driver device, the invention makes provision to equip the device with a number of preferably evenly distributed porous burners which are placed in the spaces between the pairs of straightening rolls of the orienting driver device. This type of device is on the one hand space-saving, and on the other hand enables finally tuned heating of the casting strand. 
     With particularly large sizes it is advantageous within the context of the invention to provide the device additionally with at least one porous burner disposed in front of the orienting driver device. The risk of crack formation is thus prevented over a longer section of the strand guide. 
    
    
     
       In the following the invention is described in more detail by means of an exemplary embodiment with reference to the drawings. 
         FIG. 1  shows the guide and orientation device according to the invention, shown diagrammatically in the side view, and 
         FIG. 2  shows a porous burner for the device from  FIG. 1 , also shown diagrammatically as a section and enlarged. 
     
    
    
     The guide and orientation device  1  shown in  FIG. 1  is part of a continuous casting facility for producing large-sized round profiles made of steel or a similar material. The molten steel is introduced into a die  2  to the die outlet  3  of which a strand guide  4  is connected. The casting strand  5  passing out of the die  2  is guided in the upper section of the guide track through elongate tunnel elements  6   a  to  6   d  in which the casting strand  5  is, as the case may be, insulated or heated. These tunnel elements  6   a  to  6   d  can be closed all around or advantageously designed to be open at the bottom. The first three tunnel elements  6   a  to  6   c  can be formed, for example, such that cooling of the strand running through the latter is implemented, with a subsequent tunnel element  6   d,  however, insulation being brought about without cooling or even heating of the strand. 
     There is disposed in the adjoining section of the guide track an orienting driver device  7  with pairs of straightening rolls  7   a  to  7   h.  Further narrow tunnel elements  8   a  to  8   g  are fitted between the latter. The casting strand  5  that has meanwhile hardened on the outside is oriented here with the pairs of straightening rolls  7   a  to  7   h  and introduced to an adjoining reducing roller plant  9 . 
     The tunnel elements  8   b,    8   d  and  8   f  are equipped with porous burners  10 . One of these burners is illustrated diagrammatically in  FIG. 2 . The tunnel elements  8   b,    8   d  and  8   f  have an upper top wall  11  on which the respective porous burner  10  is placed. Preferably, a wall, not detailed, which is advantageously provided with openings or the like, is also respectively provided on the lower side of these tunnel elements  8   b,    8   d  and  8   f  so that these tunnel elements insulate better. 
     This porous burner  10  substantially consists of a burnable gas and air supply connection  12  and a reactor cell  13  connected to the latter and which is filled with a filler  14  made of porous ceramic foam. Here the combustion reaction of the pre-mixed burnable gas/air mixture runs off resulting in flame-free volumetric combustion. The exhaust gas flowing out of the reactor cell  13  flows around the whole of the casting strand  5  and advantageously heats the strand surface  15  in the upper region more than on the lower side without heating the core of the strand  16  at the same time to an appreciable degree. 
     The porous burners  10  enable very easily controllable and stable combustion of the energy carrier, a very high degree of conversion of the energy into radiation energy, an exhaust gas flowing out evenly and over a large area, as well as a very high adjustment range of the heat energy introduced so that the heating power of the individual burners can easily be adapted to the local requirements. 
     An additional porous burner  17  is disposed in the elongate tunnel element  6   d  of the strand guide in front of the orienting driver device  7 . It is formed in exactly the same way as the porous burners  10  of the orienting driver device  7 , and like the latter also has the function of heating the strand surface  15  in order to avoid the risk of crack formation here too. Since this risk increases as the solidification process progresses, it is in any case advantageous to arrange the porous burners  10  and  17  in the outlet side region of the strand guide in which the strand surface has already hardened to a considerable degree. Within this context other burner arrangements are needless to say conceivable inside and outside of the orienting driver device, respectively adapted to the respective conditions. 
     It is also possible within the framework of the invention to design the burners differently from the embodiment described in so far as they are suitable for heating the whole of the upper and lower side of the strand surface evenly. In this regard the described embodiment has the advantage that the hot exhaust gases flowing out of the reactor  13  heat the upper side of the strand which is most greatly at risk of cracks, whereas the lower side which is less at risk of cracks is only heated subsequently by exhaust gases which are not quite as hot. 
     With the individual burners the burner output is controlled dependently upon their position in the orienting track. For this purpose temperature sensors  18  are provided along the strand guide. 
     The facility described is equipped with a strand guide continuously bent from the die outlet  3  to the outlet from the orienting driver device  7 . 
     However, the invention can also needless to say be applied to facilities the strand guide of which, after the die outlet, initially runs perpendicularly so as only then to pass into a bent guide track. 
     Theoretically, heating of the strand surface could also only take place on its upper side in order to reduce these tensile stresses occurring when stretching the strand.