Effects of the Addition of Cr, Mo and Ni on the Microstructure and Retained Austenite Characteristics of 0.2% C-Si-Mn-Nb Ultrahigh-strength TRIP-aided Bainitic Ferrite Steels

Cr, Mo and/or Ni were added to TRIP-aided bainitic ferrite (TBF) steel (0.2% C, 1.5% Si, 1.5% Mn and 0.05% Nb ultrahigh-strength TBF steel) in order to increase its hardenability. In addition, the effects of the alloying elements on the Vickers hardness, microstructure and retained austenite characteristics of the TBF steels were investigated. When the TBF steels were austempered at temperatures between M-S and M-f, the Vickers hardness increased from HV300 to HV430 with increasing hardenability. The microstructure consisted of martensite and bainitic ferrite lath structures and retained austenite phases and the volume fraction of retained austenite increased with increasing hardenability. Conversely, the carbon concentration of the retained austenite decreased with increasing hardenability. Simultaneously, the quantity of the hard blocky martensite phase (M-A constituent) with refined interlath retained austenite films increased with increasing hardenability. These characteristics are mainly caused by the delayed bainite transformation during austempering through the addition of Cr, Mo and/or Ni. The addition of Ni lowered the To line further. The retained austenite phases of Cr- and/or Mo-bearing TBF steels were relatively stable against straining, despite their low carbon concentrations.