Development of Ultra High-strength TRIP-aided Forging Steels by Ausforming-austempering Process

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In order to develop ultra high-strength TRIP-aided forging steels with bainitic ferrite matrix (TBF steels), the effects of a new hot forging-heat-treatment process on (1) impact toughness, (2) fatigue strength and (3) hydrogen embrittlement performance of the steels were investigated. The main results are summarized as follows.(1) High toughness of a base steel (0.2%C-1.5%Si-1.5%Mn) was achieved when hot forging at 800-900℃ and subsequent austempering at 400℃ were conducted with small size specimens. Complex additions of 0.5%Al and 0.05%Nb considerably enhanced the strength-toughness balance, which was higher than that of conventional SCM440 steel quenched and tempered. This was associated with TRIP effect of retained austenite carbon-enriched by Al and uniform microstructure refined by Nb.(2) The TBF base steels exhibited the highest fatigue strength when austempered at 400℃ after annealing. Also, the steel were characterized by higher notched fatigue strength and lower notch sensitivity than the conventional martensitic steel. Complex addition of Al, Nb and Mo resulted in further good fatigue performance.(3) The TBF steels possessed higher delayed fracture strength than martensitic steel. Addition of 0.5%Al improved further the hydrogen embrittlement performance of the TBF steels, differing from addition of Mn. Al stabilized the retained austenite and consequently trapped most of hydrogen. On the other hand, Nb contributed to size down prior austenite grain.