Unevenness and cracks of solidifying shell are typical defects in the continuous casting of peritectic steels. This study focuses on a transformation from the ferrite (BCC) – austenite (FCC) during solidification and the influence of transformation mode on the deformation of solidifying shell. In-situ X-ray imaging techniques (radiography, time-resolved tomography, and diffraction) were extensively used to observe how the transformation proceeded and how the microstructure evolved and to measure the physical properties required for quantitative modeling. The observation proved that the gamma hardly nucleated even below the peritectic temperature, and consequently the delta massively transformed to the gamma (massive-like transformation). Simultaneously the austenite rapidly solidified in the remaining liquid. The volume changes due to the ferrite-austenite transformation (0.7%), the solidification (3.5%), and the thermal expansion were also measured using tomography. In addition, the austenite nucleation (a trigger of the massive-like transformation) and the moving velocity of the ferrite-austenite interface were measured. The austenite nucleation significantly depended on specimen size. The size effect suggested that the ferrite-austenite transformation occurred inhomogeneously in the solidifying shell. Based on the observations/measurements, the strains induced in the solidifying shell and the deformation will be discussed in this paper.