The primary objective of this work is to obtain fundamental insights on phase transformations, with focus on the reaustenitization process (¿¿¿¿ transformation), of a cold-rolled (CR) semi-austenitic metastable stainless steel upon different isochronal conditions (0.1, 1, 10 and 100 °C/s). For this purpose, an exhaustive microstructural characterization has been performed by using complementary experimental such as scanning and transmission electron microscopy (SEM and TEM), electron backscattered diffraction (EBSD), electron probe microanalysis (EPMA), micro-hardness Vickers and magnetization measurements. It has been detected that all microstructural changes shift to higher temperatures as the heating rate increases. The reaustenitization occurs in two-steps for all heating rates, which is attributed to the chemical banding present in the CR state. The ¿¿¿¿ transformation is controlled by the migration of substitutional alloying elements across the austenite/martensite (¿/¿¿) interface, which finally leads to ultrafine-grained reaustenitized microstructures (440¿280 nm). The morphology of the martensite phase in the CR state has been found to be the responsible for such a grain refinement, along with the presence of ¿-phase and nanometric Ni3(Ti,Al) precipitates that pin the austenite grain growth, especially upon slowly heating at 0.1 °C/s. CCC and DSM would like to acknowledge the financial support from the Ministerio de Economía y Competitividad (project No. MAT2010-19522). CCC would like to thank the financial support from the Consejo Superior de Investigaciones Científicas (CSIC) (JAEPre_2011_01167) in the form of a JAE-predoc grant, co-funded by the European Social Fund and to the National Science Council of Taiwan for the provision of a grant under the program “Summer Program in Taiwan 2013 for Spanish Graduate Students” to undergo a research internship in the National Taiwan University (NTU). Peer Reviewed