Mechanism of Grain Refinement of Microalloyed Steel via “Forging–Annealing” Cycle

A new “forging–annealing” cycle heat working process was developed, which was employed in heat treatment experiment on Ti−Nb−V−B microalloyed steel. The effects of “forging−annealing” cycle on the grain size and mechanical property of Ti−Nb−V−B microalloyed steel were studied. The mechanism of grain refinement of Ti−Nb−V−B microalloyed steel was explored by transmission electron microscopy observation and thermodynamic calculations, and its grain refinement model was established. The results show that the “forging−annealing” cycle heat treatment can significantly refine the grain size of Ti−Nb−V−B microalloyed steel. After the “three forging and three annealing” treatment, the grain size is refined from 2.5–4.0 levels before forging to 9.5–11.5 levels. The comprehensive mechanical properties of the “three forging and three retreating” specimens are significantly improved, compared to “one forging and one retreating”, its tensile strength (R_m), yield strength (R_el), elongation after fracture (A), reduction of area (Z), and Charpy impact energy (K_V) increase by 10.6%, 9.9%, 18.8%, 30.2%, and 135.0%, respectively. The precipitation temperatures of NbC, TiC, VC and Cr₂₃C₆ in Ti−Nb−V−B microalloyed steel are 1 202, 1 187, 897, 672 ℃, respectively, and the dissolution of VC and Cr₂₃C₆ occurs during the pre-forging heating and forging process, while annealing promotes the precipitation of the second phase, and the precipitation of NbC and TiC occurs during the “forging−annealing” cycle. The synergistic effect of austenite recrystallization before forging, forging recrystallization, and second-phase precipitation by annealing after forging is the key mechanism for obtaining fine grains in the Ti−Nb−V−B microalloyed steel.