The solidification processes of super- and hyper-duplex stainless steels (i.e., UNS S32750 and S 33 207) are investigated in situ by a high-temperature confocal laser scanning microscope (HT-CLSM) and differential scanning calorimetry (DSC). The variations of δ-ferrite phase fraction during solidification are measured quantitatively. The results show that liquid L→δ-ferrite transformation first occurs at a certain degree of supercooling during the solidification process of steel. UNS S3DSS 3207 with a higher Cr content can result in a higher nucleation temperature and faster growth of δ-ferrite compared to those of UNS S332750 steel. Moreover, both the liquidus (TL) and solidus temperatures (TS) are increased with the increasing Cr content, while TL increases greater than TS. Electron microscopies are used to quantify the fraction and composition of each phase. Scheil equation is employed to predict the distribution behavior of the main alloying elements in the solidification process, and the predicted results are consistent with the experimental findings. This study aims to provide real-time experimental insights into the solidification kinetics of state-of-the-art high-alloy-grade duplex steels and benefits for controlling the casting process in the real production of stainless steels.