Effect of Rolling Strain Rate on the Grain Size of Austenite Recrystallized Immediately after Hot Rolling

Abstract

The effect of rolling strain rate on the grain size of austenite recrystallized immediatelly after hot rolling was investigated in the temperature range of 1 000 to 1 200°C. An increase in roll speed and/or a decrease in the specimen thickness before reduction was found to contribute to the refinement of recrystallized austenite grains. It suggested that austenite grain size also changed with roll radius. These results indicate that austenite grain size depends on roll radius, roll speed, and the slab thickness, in addition to generally known factors such as rolling temperature, rolling reduction, and initial grain size. It is known that recrystallized austenite grain size after hot rolling is well related with rolling reduction when rolling temperature and initial grain size are held constant. In this experiment where the specimen thickness before reduction, roll speed, and roll radius were variable factors, a good correlation was obtained between austenite grain size and rolling strain rate. We found that austenite grain size number (N) was hardly affected by the alloying elements such as C, Mn, Nb, V, Mo, and Ti, but was approximately related to rolling strain rate (ε) by :
N=-8.3ε^-1/2+0.33N₀+0.010(1 200-T)+8.8
where N₀ is initial grain size number and T is rolling temperature.
ε=2πn/60√r·√R/H₀·ln(1/1-r)
where n is roll speed (rpm), r is rolling reduction, R is roll radius (mm), and H₀ is thickness before reduction (mm).