A dilatometric study of phase transformation for a Fe–C–Ni–Mo(−Si) sustainable advanced high strength steel

A novel Fe–C–Ni–Mo(−Si) sustainable AHSS-Gen3 steel with nickel replacing manganese to obtain a high recover rate of alloying elements from the steel scrap has been proposed. The phase transformation behavior of the steels (A717: Fe–0.19C–0.99Si–1.77Ni–0.23Mo and A718: Fe–0.19C–1.99Ni–0.23Mo), under various austenitizing temperatures (840°C/950°C) and cooling conditions (10–40°C/s, quenching) has been investigated by dilatometric method systematically, and verified by microstructure characterization. The results show that measured critical transformation temperatures of Ac1/Ac3 is 722°C/863°C for A717 and 703°C/840°C for A718 at 2°C/s, while the heating rate strongly affects Ac3. Silicon inhibits pearlite formation in A717, promoting granular bainite at 840°C and lath bainite at 950°C, while A718 exhibits overlapping ferrite–pearlite–bainite regions. High-temperature austenitizing coarsens grains and delays transformation (favoring lath bainite), while low-temperature austenitizing facilitates ferrite and granular bainite. A calculation method integrating continuous heating and isothermal kinetics at intercritical temperature to predict austenite fraction with an average relative error of 5.95% has been developed, along with a user-friendly GUI tool named “CritAust” for this application.