Strengthening of Low Carbon Steel by Nano-sized Vanadium Carbide in Ferrite and Tempered Martensite

The precipitation of nano-sized alloy carbides in steels with a large amount of strengthening can be obtained by conventional tempering of martensite or interphase precipitation occurring during isothermal ferrite transformation. In this study, a vanadium-microalloyed low carbon steel with a composition of Fe-0.1C-0.4V-1.5Mn-0.05Si (mass%) was either isothermally transformed or quenched and tempered at 923 K for various periods, to comparatively investigate the precipitation behaviors of vanadium carbide and the resultant strengthening effects in ferrite and tempered martensite. When compared under the same conditions, tempered martensite is characterized by a higher yield strength and a smaller uniform elongation than that of ferrite. The quantification of microstructural features via multiple characterization techniques demonstrates a finer crystallographic grain size, a higher dislocation density of tempered martensite compared with that of ferrite, together with a relatively coarser dispersion of nano-sized precipitates due to its lower nucleation rate and higher growth rate than interphase precipitation. The strengthening amounts of all these factors are estimated using the theoretical models, the summation of which can well reproduce the yield strength of both ferrite and tempered martensite in the microalloyed low carbon steel.