Effects of prior austenite and primary carbides on mechanical properties of a novel 2.5 GPa grade ultra-high strength steel

The effects of prior austenite and primary carbides on the mechanical properties of a novel 2.5 GPa grade steel were investigated by treating at various solid-solution temperatures. The ultimate tensile strength and Charpy U-notch impact energy initially increased and subsequently decreased as the solid-solution temperature rose, while the yield strength consistently decreased. The size of prior austenite grain and martensite block always increased with rising the solid-solution temperature, and austenite grain growth activation energy is 274,969 J/mol. The growth of prior austenite was restricted by primary carbides M₆C and MC. The dissolution of the primary carbides not only enhanced solid-solution strengthening and secondary hardening effects but also increased the volume fraction of retained austenite. The increase in the ultimate tensile strength and Charpy U-notch impact energy was primarily attributed to the dissolution of the primary carbides M₆C and MC, while the decrease was due to the increase in the size of prior austenite grain and martensite block. Exceptional combination of strength, ductility and toughness with ultimate tensile strength of 2511 MPa, yield strength of 1920 MPa, elongation of 9.5%, reduction of area of 41% and Charpy U-notch impact energy of 19.5 J was obtained when experimental steel was solid-solution treated at 1020 °C.