Effects of Nb on Microstructural Evolution and Mechanical Properties of Low-Carbon Cold-Rolled Dual-Phase Steels

The effects of Nb additions, up to 0.06%, on the microstructure and properties of dual-phase (DP) steels containing 0.08% C and 2.2% Mn, were evaluated after hot rolling at different coiling temperatures, and after intercritical annealing of cold-rolled steels. During hot rolling, Nb precipitation was promoted by higher coiling temperature. During annealing of cold-rolled steels, microstructural characterization revealed that recovery and recrystallization processes overlapped with austenite formation. The addition of Nb increased the ferrite recrystallization start temperature, but had no significant effect on the start temperature of austenite formation during heating. Nb additions accelerated austenite formation once the transformation started, and also resulted in the formation of a finer and more homogeneous microstructure after annealing. In comparison with the steel without Nb, the ultimate tensile strength increased with the addition of up to 0.04% Nb, whereas addition of 0.06% Nb did not further increase the strength level of cold-rolled DP steels. Hot rolling at lower coiling temperatures improved both the strength and the ductility of the Nb-containing DP steels after annealing.