GRAIN GROWTH AND HIGH-TEMPERATURE HOT ROLLING BEHAVIOR OF LOW-ALLOY STEEL AUSTENITE

The aim of this investigation was to study the recrystallization of austenite which accompanies high temperature hot rolling. The influence of reheated microstructure on recrystallization behavior was of primary concern. Three low-alloy steels were used. One of these contained Nb; one Nb plus a high N level. Isothermal grain growth experiments were conducted from 900 to 1250(DEGREES)C. Hot rolling was performed between 1050 and 1250(DEGREES)C. Microstructural analysis by optical and transmission electron microscopy was supplemented by x-ray diffraction and microchemical techniques.;Abnormal grain growth was the dominant grain growth process in the Nb steels, and resulted from the presence of niobium carbonitride (NbCN) in the austenite. Differences in grain growth behavior between the Nb steels was attributed to a difference in NbCN particle composition. NbCN particles in the high N steel had a higher N content making them more stable with respect to coarsening and dissolution in austenite. The Nb steels were more resistant to recrystallization than the C steel. When recrystallization was complete in these steels, recrystallized grain size was a function of rolling temperature alone. Additional recrystallized grain size refinement was observed for specimens containing undissolved NbCN particles prior to rolling. Recrystallization of the Nb steels was inhibited only when a substantial proportion of the Nb was supersaturated with respect to the precipitation of NbCN in austenite.;Normal grain growth in the C steel was sluggish at 1100(DEGREES)C and above. Below 1100(DEGREES)C abnormal grain growth was observed. Solute segregation was the apparent cause of the abnormal growth stage. For every rolling condition the C steel was fully recrystallized. The recrystallized grain size was a function of deformation temperature only.