This study unravels the microstructural mechanisms controlling the mechanical behaviour and austenite mechanical stability in ultrafine grained austenite/martensite (α′/γ) microstructures, created varying the austenitisation heating rate (0.1–10 °C/s) and temperature within the start-finish austenite formation temperatures (A_S − A_F) in a cold-rolled semi-austenitic stainless steel. A wide spectrum of strength-ductility combinations; i.e. strengths of 900–2100 MPa and elongations up to 25%, were characterised by sub-size tensile testing. The nanoprecipitation of Ni₃(Ti,Al) in martensite during heating to low austenitisation temperatures rises the strength, while the martensite recovery, enhanced at low heating rates, improves the work-hardening. The high strength of martensite partially suppresses the formation of mechanically-induced martensite during loading, which is enabled with the increase of austenite volume fraction and contributes positively to the work-hardening. The heating rate barely affects the mechanical properties of microstructures austenitised close to A_F. The austenite ultrafine grain size controls the yield strength, while the decrease in austenite mechanical stability and the α′/γ composite effect increase remarkably the work-hardening with respect to dual (α′/γ) microstructures with larger martensite volume fractions and fully austenitised microstructures. These results will enable the design of microstructures with controlled mechanical behaviour for a wider spread use of similar steel grades.