Effect of Hot Deformation Condition on Austenite Grain Refinement in Medium Carbon Steel and Related Properties of Ultrafine Grain Steel

Hot deformation conditions to obtain ultrafine grain austenite, the effect of austenite grain size on transformation behavior, and mechanical properties and the spheroidizing behavior of ultrafine grain medium carbon steels were studied by uniaxial compression and laboratory rolling of 0.45 C-0.71 Mn, 0.48 C-0.75 Mn-0.03 Nb, 0.44 C-1.58 Mn and 0.26 C-1.35 Mn steels. Manganese suppressed the recrystallization of austenite. Niobium addition or reduction of carbon content also slightly suppressed the recrystallization. Ultrafine asutenite grains under 10 μm were obtained by hot deformation of medium carbon-low manganese steels with an initial grain size of about 20 μm at deformation temperature of 850°C under a strain rate of 10 s-1. This suggests that a fine austenite grain of about 20 μm by rolling on the rough and the intermediate trains, low rolling temperature on the finishing train and appropriate strain rate are necessary conditions to obtain ultrafine austenite grain in practical rolling. The refinement of austenite grains induced an increase in the volume fraction of ferrite and a refinement of ferrite grains and pearlite nodules. Consequently, toughness and yield strength increased and better spheroidized microstructure was obtained in the ultrafine grain steels.