A cold rolled DP-grade (TS 600 MPa), coiled at 550°C, was compared
with a steel of approximately the same analysis but with the
addition of 0.015 wt% Nb, coiled at 520 and 600°C respectively. The
Nb-addition refines the ferrite grain size substantially and a more
uniform grain shape is achieved. The smaller grain size of the ferrite
and martensite phases, results in a material with better formability.
Recrystallization and phase transformations were studied with the
laser ultrasonic technique. This technique could be implemented on
industrial annealing lines.