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billets produced by continuous casting are affected by subsurface cracks that sometimes compromise the quality of the final product. For steels with a high sulfur content, these cracks are revealed in Baumann prints of billet cross sections like those shown in Figure 1, in which the chill zone is clearly visible and the radial cracks are located only where the chill zone thickness is thinner. This experimental finding induces the hypothesis that cracks are formed because of the unevenness in the mold heat extraction around the billet perimeter. Cracks start to open in the dendritic front in regions where the shell growth in the mold is slower. MARIA RITA RIDOLFI, Senior Researcher, STEFANO FRASCHETTI, Researcher, and ANDREA DE VITO, Researcher, are with Centro Sviluppo Materiali S.p.A., Rome 00128, Italy. Contact e-mail: m.ridolfi@c-s-m.it LUIS A. FERRO, Manager, is with Tenaris, Campana B2804, Argentina. Manuscript submitted February 10, 2010. Article published online September 4, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B

II.

BACKGROUND

The formation of longitudinal surface and subsurface cracks is one of the most common problems in continuous casting; a lot of research has been devoted to the study of longitudinal corner cracks,[1–3] whereas a smaller effort has been dedicated to round products.[4– 6] Longitudinal cracks are associated with hot tearing at the solidification front, and a common mechanism of formation can be raised both for corner cracks in square cross sections and for randomly located cracks in round cross sections. For these last items, the crack opening is associated with a local shell thinning, resulting from a reduced heat exchange in the same way that cracks form in the off-corner regions or in the middle of the corners, depending on the way that the shell detaches from the mold.[1] In both cases, there is an initial detachment of the shell from the mold, which results in a local shell thinning in which, later on, the presence of ferrostatic pressure, both inside the mold and below the mold exit, and/or excessive taper near the mold exit can generate tensile stresses applied to the fragile zone of the dendritic VOLUME 41B, DECEMBER 2010—1293

formulations, also take into account the dependence on d and c phases.[2,10,11] Hot tears form deep in the mushy zone where an opening of the dendritic front resulting from tensile deformation cannot be compensated for by the liquid. The following different kinds of hot tearing criteria have been proposed:

Fig. 1—Baumann prints showing subsurface defects: (a) small, and (b) severe.

front. Although for square sections, this event occurs in a preferential zone, in round sections, the detachment can be found everywhere; in this case, ferrostatic pressure and excessive taper also operate in an analogous way. The problem of generating tensile stresses at the dendritic front has been afforded by finite element modeling (FEM) with the aim of properly designing the mold taper and the corner radii,[1,3] which would eliminate the problem. To account for the nat