Serious defects in steel continuous casting, including cracks and depressions, are often related to thermal-mechanical behavior during solidification in the mold. Defect samples have been collected, categorized, and analyzed; and a finite-element model has been developed to simulate the temperature, shape, and stress of the steel shell, as it travels through the mold to explore formation mechanisms and behavior of these defects. The model simulates transient heat transfer in the solidifying steel, between the shell and mold, and is coupled with a stress model that features temperature-, composition-, and phase-dependent elastic-visco-plastic constitutive behavior of the steel, accounting for liquid, delta-ferrite, and gamma-austenite phases. Depressions form when the shell is subjected to either excessive compression or tension, but appearances vary, and the shape of the depression changes when cracks are present. These defects are not mutually exclusive, and samples indicate that sub-surface cracks may be present without obvious surface indications.