Microstructural evolution and precipitation kinetics in Nb–V microalloyed HSLA steel welded joints

High-strength low-alloy (HSLA) steel develops significant microstructural gradients after girth welding thermal cycles, increasing embrittlement and softening risks in the joint. This study examines microstructural characteristics and precipitation dynamics in welded joints of two Nb–V variant HSLA steels, correlating them with microhardness distribution and oscillographic impact curves. Results show that the HAZ of 0.051 wt% Nb–0.053 wt% V (5Nb5V) steel, designed by reducing Nb and adding V, exhibits better softening resistance and higher impact toughness than 0.092 wt% Nb (9Nb) steel, due to finer grains and lower M-A constituent content. Thermodynamic and kinetic calculations reveal similar austenitization temperatures for both steels. Above the Ae 3 temperature, V atoms largely dissolve into the austenite, while a small fraction combines with C atoms to form precipitates and replaces Nb atoms in NbC, which then dissolve into the austenite. This ensures precipitation strengthening while enhancing solid solution strengthening. (Nb,V)C has a higher nucleation energy threshold and lower precipitation free energy, resulting in fewer precipitate particles. The primary mechanism is the solute drag effect caused by the dissolution of Nb and V in austenite, which suppresses austenite grain growth. Additionally, the solid solution of Nb and V induces lattice distortion in the ferrite matrix, hindering dislocation motion and thereby improving the softening resistance of the welded joint. This work proposes a novel 0.051 wt% Nb–0.053 wt% V system via Nb reduction and V addition, demonstrating through combined experiment and theory that this design offers cost efficiency and an excellent strength–toughness balance.