Retained austenite in dual-phase silicon steels and its effect on mechanical properties

Steels of constant manganese and carbon content with 0.26–1.96 wt.% silicon content were cast in a hot mould. The as-cast steels were then directly hot rolled at 1150°C in a single pass to reduce the cast ingot thickness from 15 to 11 mm, air cooled to room temperature and cold rolled to 5 mm thickness. Dual-phase microstructures were obtained by intercritical annealing of the steels to produce 50% austenite, with interrupted quenched into a salt bath, and holding at the salt bath temperature for 5 min before cooling to room temperature. The amount of retained austenite was optimised by varying the silicon content of the steels and the isothermal holding temperature. Greater amounts of retained austenite were observed after interrupted quenching than after continuous cooling. The two types of retained austenite observed after interrupted quenching were isolated volumes and interlath austenite. After interrupted quenching, the amount of retained austenite, the tensile strength and uniform elongation were optimised at 1.44% Si. The higher strength and ductility at this silicon content was found to be due to increased stability of the retained austenite and its continued contribution to work hardening by the TRIP mechanism to higher strains.