Influence of niobium and coiling temperature on the mechanical properties of a cold rolled dual phase steel

Dual-phase (ferrite-martensite) low-carbon sheet steels are produced by annealing in the intercritical temperature range to produce ferrite-austenite mixtures, followed by accelerated cooling to transform the austenite phase into martensite. Dual-phase steels have gained considerable importance over the conventional HSLA steels due to the high strength and formability achieved. A good formability is achieved by high deformation hardening, due to a fine distribution of martensite. A cold rolled DP-grade (TS 600 Wa), coiled at 550 degrees 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 degrees 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. It appears that coiling at 520 degrees C results in a slightly better formability than coiling at 600 degrees C. The material shows a lower (alpha ->gamma start temperature, probably due to easier dissolution of thinner carbides, compared to the strip coiled at 600 degrees C. The recrystallization and phase transformation behavior was studied using the laser ultrasonic technique (LUS). Developing the LUS-technique for use in-line during annealing would open new ways of optimizing and controlling the mechanical properties of the final strip.