Abstract:
A permanent chill mold for the continuous casting of metals. At least one partial region of the outer surface of the permanent chill mold ( 1 ) is provided with cooling channels ( 2, 2   a,    2   b,    2   c ). The depth and/or the width of the cooling channels ( 2, 2   a,    2   b,    2   c ) are greatest in the middle of a sidewall of the permanent chill mold ( 1 ), and they decrease in the direction of the corner region ( 4 ) of the sidewall. Because of this, a uniform heat dissipation can be implemented, and thereby higher casting speeds are possible.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a permanent chill mold for the continuous casting of metals.  
       DESCRIPTION OF RELATED ART  
       [0002]     Tube-shaped chill molds made of copper or copper alloys, for casting profiles made of steel or other metals having a high melting point have been described many times in the related art. Permanent chill tubes usually have a uniform wall thickness in a horizontal cross sectional plane, which increases in the direction of the billet because of the inner conicity of the chill tube. The inner conicity is adapted to the solidification response of the billet and the continuous casting parameters. Shortly after the setting in of the solidification of the continuous casting material, that is, directly below the casting bath level, because of the heat transfer that is three-dimensionally characteristic over the cross section, there is a greatly different characteristic cooling response. Because particularly great quantities of heat are dissipated in the corners of the permanent chill tube, based on the geometric ratios, that region demonstrates an especially great strand shell growth, and therewith an especially great shrinkage. At the sidewalls of the permanent chill tube, the heat dissipation is less, as a rule, although at that location a greater heat flow is imposed at the same time. The result of the locally different cooling is a non-uniform strand shell growth, which is able to lead to material stresses and cracks in the strand shell, and thereby increases the risk of a billet break-out.  
         [0003]     A series of suggestions have been made in the past for achieving as homogeneous as possible a heat dissipation, and therewith also to create the prerequisite for a greater casting output. For instance, a permanent chill mold is known from DE 36 21 A1 in which only the arch-shaped sidewalls, but not the corner regions, are provided with cooling channels. The cooling should, above all, be increased in the region of the casting bath level, and this is indeed described in DE 34 11 359 A1. EP 1 468 760 B1 also concerns itself with improvement in the cooling performance and increase in the casting speed, and the document proposes that the cooling channels take up 65% to 95% of the outer surface of the copper tube, the copper tube at the same time being provided with a supporting jacket over the entire circumference and essentially over the entire length. DE 195 81 547 C2 proposes, in the case of vertically oscillating continuous casting permanent chill molds, to provide the inner surface with recesses or depressions that are situated at a clearance of 15 mm to 200 mm below a casting bath level, as measured in a stable operating state. Stable casting at high speed is also supposed to be made possible thereby. All these attempts do not sufficiently take into account the real heat flow distribution.  
       SUMMARY OF THE INVENTION  
       [0004]     It is an object of the invention to provide a permanent chill mold wherein the homogeneity of the billet cooling is able to be increased even further, so that as a result one may implement higher casting outputs and a better billet quality, and which, in addition, contributes to reducing stresses inside the permanent chill mold wall.  
         [0005]     This and other objects of the invention are attained by a permanent chill mold for the continuous casting of metals, wherein at least one partial region of the outer surface ( 3 ) of the permanent chill mold is provided with cooling channels ( 2 ,  2   c ), and wherein the depth and/or the width of the cooling channels ( 2 ,  2   a ,  2   b ,  2   c ) are greatest in the middle of a sidewall of the permanent chill mold ( 1 ), and they decrease in the direction of the corner regions of the sidewall. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention is described in greater detail below, using an exemplary embodiment represented in the drawings.  
         [0007]      FIG. 1   a  is a perspective view of a permanent chill mold  1  which is positioned in a cooling-water tank that is not shown in detail.  
         [0008]      FIG. 1   b  is an enlarged perspective view of a portion of the chill mold illustrated in  FIG. 1   a.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0009]     What is important in the permanent chill mold according to the present invention is that the cooling effect of the permanent chill mold is optimized in such a way that it is equivalent to the heat supply of the billet, so that uniform cooling may be achieved. This is achieved in that the depth and/or width of the cooling channels are greatest in the middle of a sidewall of the permanent chill mold, and becomes less in the direction of the corner regions of the sidewall. What is decisive is that the cross sectional area of the cooling channels in the middle area of a sidewall is greater than at the edge region of a sidewall. It has turned out that, by introducing the cooling channels in the manner according to the present invention, the maximum effective stresses occurring in the sidewall are clearly reduced. Ideally elastic strength calculations have confirmed that the effective stress is able to be reduced by more than 30%, from 504 MPa to 348 MPa. This information relates to a permanent chill mold cross section of 130×130 mm, a mold tube without channels being compared here to a mold tube designed using the channels according to the present invention. The reduction, achieved in this manner, of the stresses in the mold tube has an advantageous effect on the service life, and reduces the thermally conditioned distortion of the mold tube. The mold tube, according to the present invention, has, in this calculation, eight channels on each sidewall at a clearance of 5 mm, having a length of 200 mm extending in the casting direction. The middle channels have a depth of 5 mm, whereas the outer channels have a depth of 4 mm, at a width of 12 mm or 8 mm. There are no channels situated in the corner regions of the sidewall.  
         [0010]     With respect to their depth and width, it is decisive for the specific execution of the cooling channels that the cooling geometry corresponds as well as possible to the heat flow applied from the inside, and because of that, a largely homogeneous temperature field can be achieved, which, up to now, has only succeeded in an unsatisfactory manner. It is important that the cooling channels be executed deeper and/or wider in the middle of the sidewall, where the heat supply is at its highest, that is, that they have a greater cross sectional area than in the regions that are close to the corner radii.  
         [0011]     Preferably, no cooling channels are provided in the sidewall at a distance of 10 mm to 15 mm from the radius corner region, so as not to increase the cooling at this position, and not unnecessarily to weaken the rigidity of the permanent chill mold. The best results may be achieved if the cooling channels have a depth of 3 mm-6 mm. In this context, a residual wall thickness should not fall below 6 mm between the deepest part of the cooling channels and the mold tube interior.  
         [0012]     The width of the cooling channels is preferably to be selected between 5 mm and 20 mm.  
         [0013]     In order to adjust the number of cooling channels to different formats/dimensions of the mold tubes, it has proven favorable, for the stated channel dimensions, to have a number of 4-10 cooling channels per 100 mm of side surface of the mold tube.  
         [0014]     Width and depth ratios of the cooling channels between 1 and 4 are regarded as particularly favorable from a flow technology point of view. Ratios deviating from this have unfavorable influences on the flow relationships, and thus also on the cooling performance as well as the rigidity of the mold tube in the region of the bath level. The cooling channels are provided with a small transition radius to the channel walls, at the base of the channels, in order to avoid stress peaks there.  
         [0015]     At the run-in region and the run-out region, the cooling channels ideally have a radius which contributes to flow optimization of the cooling water and to the reduction in pressure losses.  
         [0016]     In one positioning of the cooling channels regarded as favorable, their mutual clearance, measured from the middle of the channel, amounts to between 10 mm and 25 mm. A ratio of average channel clearance to the width of a cooling channel between 1.2 to 3 provides surprisingly good results.  
         [0017]     Basically, one tries to achieve that the width of the cooling channels becomes greater going towards the middle of the sidewall, and in addition, that the depth also increases going towards the middle. The different cooling channel geometry is able to be produced either by metal-cutting processing of the permanent chill mold or even by non-cutting processing, in reshaping the permanent chill mold.  
         [0018]     It is favorable if the cooling channels are situated in a region that begins approximately 50 mm above the casting bath level setpoint position and extends to about 300 mm below the casting bath level setpoint position, since in this region the greatest heat flow densities occur, and, with that, the stresses in the sidewall of the permanent chill mold are at a maximum. Regions lying lower in the casting direction, that is, regions at a distance greater than 300 mm below the casting bath level setpoint position, have also to be cooled, to be sure, but, because of the strand shell that has already formed, the temperature non-homogeneity is not so great that the channels designed according to the present invention are absolutely necessary in these lower regions. Superior results are achieved already if the channels designed according to the present invention begin approximately 50 mm above the casting bath level setpoint position and extend to 300 mm below the casting bath level setpoint position.  
         [0019]     Chill tube  1  is provided with especially configured cooling channels  2  that are formed on the outer surface  3  of the chill tube  1 . Cooling channels  2  do not extend over the entire length of chill tube  1 , but are located exclusively in the upper, pouring-end region of chill tube  1 . In this exemplary embodiment, cooling channels  2  have a length of 200 mm. Cooling channels  2  are located in the region of the casting bath level setpoint position, the latter lying in the upper one-quarter of cooling channels  2  shown. The special thing, with regard to cooling channels  2  of this chill tube, is that they are not all equally wide and deep, but differ in both width and depth. In this exemplary embodiment, cooling channels  2   a  and  2   b , that face corner regions  4 , are narrower than cooling channels  2   c  that lie in the middle region of the respective sidewall. Whereas middle cooling channels  2   c , for example, have a width of 12 mm, the four outer cooling channels  2   a  and  2   b  may, for instance, have a width of 8 mm. All cooling channels  2   a ,  2   b ,  2   c  are of the same length. However, not only does the width of cooling channels  2   a ,  2   b ,  2   c  vary, but also their depth. This may be seen in that cooling channels  2   a ,  2   b ,  2   c  in each case have a radius  5  in the run-in and the run-out region. The transition of radius  5  to the deepest part of individual cooling channels  2   a ,  2   b ,  2   c  may be recognized by a horizontal line. In middle cooling channels  2   c , the depth is recognizably at its greatest. The depth of cooling channels  2   b  that are closest together on the outside is somewhat smaller. In the case of outside cooling channels  2   c  that face corner regions  4 , the depth is the least.  
         [0020]     Corner regions  4  are not provided with cooling channels. The chill tube is fastened in the cooling water tank using a sheet metal water deflector, that is not shown in detail, so that the cooling water is pushed into individual cooling channels  2   a ,  2   b ,  2   c . The sheet metal water deflectors are positioned in such a way that the chill tube is held concentrically in the water gap.