The quality of continuously cast steels is governed by events that occur during the initial stage of solidification in the mold, where the first layer of steel shell forms. In peritectic steels, solidification is influenced by the nature of the ferrite to austenite transformation, which is an antecedent for shell buckling and non-uniform growth during continuous casting of these steel grades. A dip test, which employs a copper mold instrumented with distributed fiber-optic sensors, has been developed to map closely spaced thermal features that are induced by the peritectic transformation. The magnitude of unevenness in shell thickness was also obtained.