Effects of Carbon Content and Cooling Path on the Microstructure and Properties of TRIP-aided Ultra-High Strength Steels

The effects of carbon content in the range 0.2–0.4 wt% and thermomechanical treatment parameters on the mechanical properties and microstructures of steels containing 0.5Si–2.0Mn–1.0Al–0.6Cr have been studied in order to help develop an efficient processing route for ultra-high-strength TRIP-aided martensitic-bainitic structural steels. The microstructures consist of granular bainite, lath-like bainite, martensite and up to 23 volume % retained austenite as granular islands and lath-like films. Microsegregation of chromium and manganese results in the formation of martensite-rich bands, the volume fraction of which increases with carbon content. Bands of martensite, induced by the segregation of chromium and manganese, become more prominent with increasing carbon content. Good combinations of tensile strength, elongation and impact toughness can be achieved with 0.2 or 0.3 wt% carbon by thermomechanical rolling followed by water quenching to 150–350°C and isothermal holding for 2 h at the quenching stop temperature. The steel with 0.4 wt% carbon shows high strength and good uniform elongation due to high contents of retained austenite, but its impact toughness is clearly inferior to its lower carbon counterparts. Annealing to alleviate microstructural banding produced a marked improvement in impact toughness.