Influence of cold working on recovery and recrystallization of lath martensite in 0.2%C steel

The effect of cold working on the recovery and recrystallization behavior of lath martensitic structure has been investigated in a 0.2%C low alloy steel by means of optical and transmission electron microscopy. Cold working in the lath martensitic steel causes an intrusion of slip bands into the matrix, and resulting in the destruction of the lath martensitic structure and the formation of a dislocation cell structure around slip bands. The volume fraction of such a damaged martensite increases with increasing deformation rate. In the specimens subjected to heavy cold rolling above 80% reduction, the undamaged lath martensitic structure could rarely be observed and the dislocation density becomes one order of magnitude higher. On annealing at 973 K (below A_c1 temperature), there is a large difference in the recovery and recrystallization behavior between the damaged and undamaged areas. Recrystallized ferrite grains preferentially nucleate within the damaged area and encroach on the undamaged area where the speed of recovery is slower than that in the damaged area. With increasing prior deformation rate, the recovery and recrystallization of the matrix are markedly promoted, corresponding to an increase in volume fraction of the damaged martensite and in dislocation density. Thus, it is concluded that cold working before annealing plays a role in destroying the lath martensitic structure and increasing the dislocation density, leading to increases in both of nucleation and growth rate of recrystallized ferrite grains.