Ferrite Recrystallization in Cold-Rolled Dual Phase Steel

The ferrite recrystallization of a cold rolled dual phase steel with 0.09%C-2.06%Mn was investigated by means of isothermal treatments at the temperatures of 600, 650, 680 and 710°C as well as during continuous heating. From the isothermal treatments the softening kinetics was mathematically adjusted to the AJMK model and the activation energy associated with the ferrite recrystallization process was estimated as 310 ± 22 kJ/mol. Vickers hardness measurements in the ferrite were carried out on samples subjected to heating at a constant rate from room temperature and quenched at temperatures up to 760°C. The results allowed to observe three main stages of the ferrite softening: (1) from room temperature to about 500°C the ferrite hardness remains constant; (2) between 550°C and 640°C, the hardness systematically increases with temperature. This effect was interpreted as being caused by the increase of carbon dissolved in the ferrite, coming from the dissolution of the adjacent pearlite colonies; (3) above 660°C the ferrite hardness decreased rapidly with temperature due to the recrystallization process. Analysis by means of SEM and EBSD indicated that the recrystallization process started mostly in regions severely deformed, which were predominantly orientated along the γ fiber ( parallel to the sheet normal direction).