Effect of precipitation on the mechanical behavior of vanadium micro-alloyed HSLA steel investigated by microstructural evolution and strength modeling

Abstract

In the current work, the relationship between the microstructural evolution and strengthening mechanisms in a vanadium micro-alloyed high strength low alloy (HSLA) martensitic steel has been systematically investigated by multi-scale characterizations and modeling. Two heat treatment routes, i.e., solution quenching plus tempering (SQT) and solution quenching plus isothermal annealing (QIA) are applied to produce a martensitic matrix combined with considerable nano-scale MC-type carbides (M = V, Mo, Cr) in the experimental steels. Theoretic calculations of the strengthening contributions are conducted based on quantitative characterizations of microstructural parameters, the overall modeling results agree quite well with the experimental results for estimating yield strength of the specimens. Precipitation and dislocation strengthening are found to be the main factors causing strength evolution with different isothermal holding time (IHT) at 600 °C. In both SQT and QIA specimens, the prolonged IHT contributes to an increased particle size and volume fraction of MC precipitates but a decreased dislocation density, and the most excellent mechanical properties are achieved with IHT of 2 h. The precipitation strengthening of SQT specimens is higher than that of QIA specimens because of its finer particle size and higher volume fraction of the MC precipitates, resulting in the higher yield strength of the SQT specimens than that of the QIA specimens. This can be explained by the hierarchy lath martensite that is highly dislocated with numerous interfaces, providing considerable nucleation points and high-speed atomic diffusion paths for precipitation, leading to the significantly refined MC particle size in the SQT specimens. Moreover, the MC precipitates exhibit no significantly coarsening during isothermal holding at 600 °C, which is a result of the presence of Mo/Cr atoms in MC precipitates that yields precipitate size stabilization due to its higher chemical potentials than that of V, thus maintaining a stable precipitation strengthening after IHT exceeding 2 h.