Static Restoration after Hot Deformation of a 2.9% Si Steel

Abstract

Static restoration process was investigated quantitatively for the 2.9% Si steel deformed in tension for the temperature range from 1 003 K to 1 173 K and for the strain rate range from 0.02 s^-1 to 18 s^-1. Recrystallization is strongly accelerated by increasing strain in the low strain region. The acceleration results not only from the increase of dislocation density with increasing strain but also from the progressive formation of uniform subgrain structures by dynamic recovery. These results suggest that recrystallization in dynamically recovered structure is faster than that in the strain hardened structure at low strains or the structure deformed at low temperatures, if dislocation densities in the both are about the same.
Most of recrystallized grains occured "in colonies" along the original grain boundaries when the fraction of recrystallization, X, is 0.03. With an increase in X the number of nuclei increased when X≅0.1 and then decreased to about half of the maximum value (at X≅0.1), which was almost constant in the range of X from 0.2 to 0.6. When X≤0.6 the retarding effect of recrystallization by static recovery was almost negligible, but when X≥0.6 it appeared clearly. This effect is caused by the decrease of dislocation density in subgrains and the growth of subgrains in the latter stage of recrystallization.
Temperature dependence of the rate of recrystallization was measured under the same flow stress in high strains. The activation energy for recrystallization is 215 kJ/mol and that for grain growth is 230 kJ/mol. Both the values are almost the same with that for self-diffusion of iron atom in a 2.9% Si steel. The rate of recrystallization is concluded, therefore, to be controlled mainly by the growing velocity of new grains.