Billets produced by continuous casting sometimes show the presence of subsurface cracks that can compromise the quality of the final product. The presence of these cracks is revealed by Baumann prints of billet cross sections in which the chill zone is visible and the short radial cracks are located only where the chill zone thickness is thinner. This experimental finding induces the hypothesis that cracks are formed as a result of the presence of 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. The study presented in this article focused on steels with a sulfur content of about 300 ppm. The Baumann prints taken from billet samples of numerous different heats allowed detecting the presence of subsurface cracks and their location nearby visible chill zone thinning areas. To understand the mechanisms of crack formation and to define the possible corrections, a modeling activity has been carried out using the finite element technique on 148-mm diameter billets continuously cast at TenarisDalmine (Dalmine, Italy). The model performs a two-dimensional thermomechanical analysis of the solidification in the mold and within about 4 cm below the mold exit, along which the shell surface is cooled only by radiation to the environment, before the sprays of the first ring impact on the strand. The model includes the contact of the shell with the mold inner surface, which moves according to taper and distortion (this last part is calculated by means of a separate mold model); the steel creep behavior; the calculation of the heat transfer through the gap depending on the local mutual distance between the two surfaces; the effect of the liquid steel fluid dynamics on the solidification growth as a result of the temperature distribution; and the calculation of a hot tearing indicator represented by the porosity fraction caused by mechanical strains applied at the dendrite roots. From the simulation results, it is concluded that subsurface cracks are generated in the space between the mold exit and the first cooling ring; the involved mechanisms of formation also are withdrawn. Nucleation of MnS precipitates of large dimensions is an additional cause of defectiveness in controlled sulfur steels. As a final conclusion of this work, the most important actions to eliminate subsurface cracks are derived.

A study has been performed on the microstructural properties of steels in the dendritic region with the scope of defining some key parameters denoting the tendency to form cracks during the solidification process, depending on the alloy composition. This study has been coupled to the thermomechanical modeling of the steel solidification inside the mold in order to explain the generation of stresses in the brittle dendritic region. The study has been carried out with the aid of data from round billet casting at Tenaris Siderca, Tamsa, and Dalmine. The linear thermal expansion coefficient calculated at the solidus temperature and the brittle zone width have been defined as the two parameters giving information about the tendency of an alloy to form cracks. The linear thermal expansion coefficient at the solidus temperature, above which steels have peritectic behavior, has also been specified. Some real cases of problems related to cracks and breakouts have been analyzed by means of coupled modeling, and solutions are proposed in terms of alloy composition and mold taper optimization.

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