Continuous casting mold for casting a billet with a polygonal cross section has side walls delimiting a mold space with a polygonal cross section and the side walls have a center region extending from an open top end to an open bottom or exit end of the mold with a first degree of taper and, at the sides of the center regions, abutting side regions with a lesser degree of taper than the first degree. In order to obtain even growth of the casting shell with low frictional forces, the center region has a degree of taper which is greater than the amount from the billet contraction and the width of the side regions increases as the distance from the exit end of the continuous casting mold decreases.
A continuous casting mold of this type is known from EP-A - 0 179 364. According to this document, the interfacial angle between adjacent side walls of the continuous casting mold diminishes in the moving direction of the strand, i.e. in the casting direction, provided that the tensile stresses in the edge region which are caused by the shrinkage of the strand shell continuously diminish and/or compensate each other. Hereby, detaching of the strand shell from the cooled mold wall in the region of the comers is to be avoided with a view to achieving uniform shell growth, particularly a shell having a uniform thickness. This is, however, disadvantageous, namely for the following reason:
In conventional continuous casting molds, a particularly pronounced shell growth results in the edge or corner region of the strand already in the initial solidification phase of the strand, and thus directly below the meniscus, due to the two-dimensional heat transport taking place in the edge region. Hereby, the rigidity of the strand shell in the edge region increases to such an extent that the ferrostatic pressure inside the strand is no longer sufficient for pressing the strand shell against the mold side walls in the edge region. Hence contact loss in the edge region will ensue. Due to this contact loss, further cooling of the strand in the edge region can only be effected by heat radiation, but no longer by heat conduction.
As a consequence, the shell growth will immediately fall short of that of adjacent strand zones which rest against the side walls of the continuous casting mold. Directly at the edge of the strand the cessation of heat transmission is, however, compensated for by heat conduction through two-dimensionally acting heat radiation. Thus, there form zones of weak spots having slighter shell thickness, each closely adjacent to the edges of the strand, said zones of weak spots extending in the longitudinal direction of the strand. These local shortfalls in shell growth lead to the strand shell being inhomogeneous and thus richer in tension and more susceptible to cracking and results in a risk of breakout. As the strand passes through the mold, these weak spots move slightly away from the comer regions of the mold toward the center of the side walls.
It is thus rendered feasible to selectively release the edge regions of the strand within the continuous casting mold, whereby frictional forces are reduced and jamming of the strand is reliably avoided. It has emerged that by the membranous bending behavior of the strand shell in the central regions where the strand shell is in contact in the center regions of the side walls of the continuous casting mold, elastic recession of the strand shell is enabled, without, however, entailing a heavy increase in the frictional forces acting between the strand shell and the side walls of the continuous casting mold. The construction according to the invention of the side regions in combination with the center region of the side walls of the continuous casting mold not only makes possible the selective release of the edge region of the strand but also permits to achieve contact of the strand shell in those regions where the above-mentioned local weak spots are incurred in conventional continuous casting molds.
Preferably, the center region extends from the end of the continuous casting mold at least into the meniscus region, wherein suitably the center region is formed by a flat surface and has a constant taper throughout its length.
A heavy increase in the frictional forces acting between the strand shell and the mold side walls is reliably avoided if the center region has a taper in the region of 1.5 to 2.5%/m mold length, preferably in the region of 2 to 2.5%/m mold length.
Preferably, the side regions from the end of the continuous casting mold extend to a point below the meniscus region but into the upper half of the mold, i.e. it is sufficient if the side regions extend only until the approximate point where lifting-off of the strand shell in the edge region occurs for the first time.