The steel shell is prone to a variety of distortion and crack problems, owing to its creep at elevated temperature, combined with metallurgical embrittlement and thermal stress. Shell solidification makes those phenomena more complex.
Solidification occurs at the meniscus and continues in the mold and after the mold exit.
As solidification progresses, microsegregation of alloying elements occurs between the dentrites as they grow outward to form columnar grains. The rejected solute lower the local solidification temperature, leaving a thin layer of liquid steel along the grain boundaries, which may later form embrittling precipitates. When liquid feeding cannot compensate for shrinkage due to solidification, thermal contraction, phase transformation, and mechanical forces, then tensile stresses are generated. When the tensile stresses are high enough to nucleate an interface from dissolved gases, then a crack will form. After the shell exits the mold and moves between successive rolls in the spray zones, it is subject to large surface temperature fluctuations, which cause phase transformations and other microstructural changes that affect its strength and ductility.