Abstract Nb microalloying is of significance to the grain refinement and mechanical properties of DP steels. In the present study, two DP steels without/with addition of Nb were produced by the TMCP process and the effect of Nb on the microstructure and mechanical properties was comparatively investigated by characterization methods of optical microscopy (OM), electron back-scattered diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The results show that the addition of 0.02 wt% Nb in the DP steels, the volume fraction of martensite increases from 43 to 85.2 vol%, and the equiaxed grain and elongated grains are obtained with grain size decreasing from 1.88 to 1.66 μm during the hot rolling process. Furthermore, during the tempering, the martensite lath of the DP steel with Nb grows slower (from 127.4 to 139.3 nm). The DP steel with addition of Nb exhibits a better combination of strength (∼1,281 MPa tensile strength) and ductility (∼16.0 % total elongation), attributed to grain boundary strengthening, dislocation strengthening and solid solution strengthening.
Fu et al. (Thu,) studied this question.