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.