Abstract
Ultra high-strength TRIP-aided steel consisting of bainitic ferrite matrix and interlath retained austenite films (TBF steel) possesses high toughness and fatigue strength, as well as high resistance against hydrogen embrittlement. In this study, to improve further these mechanical properties, the effects of hot forging and subsequent isothermal transformation holding process (FIT process) on microstructure, retained austenite characteristics, tensile properties and toughness of the TBF steel with chemical composition of 0.4%C, 1.5%Si, 1.5%Mn, 0.5%Cr, 0.2%Mo, 0.05%Nb and 0.5%Al (mass%) were investigated. The FIT process brought on an excellent combination of tensile strength of 1350-1550 MPa and Charpy impact absorbed value of 100-110 J/cm2 in the developed TBF steel, exceeding so much that of SCM440 steel. The excellent combination was mainly caused by (i) refined mixed structure of bainitic ferrite and retained austenite and (ii) the increased mechanical stability of retained austenite due to the FIT process.
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