Precipitation behavior and kinetics of second phases in Nb–V–Ti microalloyed steel during sub-rapid solidification

Microalloying is widely used to tailor precipitation and microstructure in high-strength steels, yet the cooling-rate dependence of secondary-phase precipitation in Nb-V-Ti microalloyed steels during sub-rapid solidification remains unclear. In this work, a 980 MPa-grade Nb-V-Ti microalloyed steel produced by MCCR was investigated at cooling rates of 0.1-20 °C/s. Thermo-Calc calculations, HT-CLSM, SEM-EDS, and EPMA were combined to reveal the precipitation sequence, morphology, size distribution, and kinetics of second phases, and precipitation-temperature-time curves were established for representative carbonitrides. As a result, the critical cooling rate of 1 °C/s in this system was determined. Below this rate, increasing undercooling enhances the nucleation driving force and raises the start and finish temperatures of precipitation; above it, diffusion limitation dominates and shifts precipitation to lower temperatures. Increasing cooling rate transforms secondary phases from coarse micrometer-scale particles to finer nanoscale precipitates, suppresses Al 2 O 3 -cored complex phases, and promotes MnS-containing low-temperature complexes. Ti(C, N)-AlN changes from coarse rod-like/coral-like morphologies to fine rod-like or elliptical precipitates. TiN number density and size reach maxima at 1 °C/s, whereas Ti(C, N)-AlN becomes continuously denser and finer. Low-temperature surface reliefs decrease sharply, with a characteristic burst at 900-1000 °C. Kinetic analysis indicates that Ti(C, N), Nb(C, N), and V(C, N) preferentially nucleate heterogeneously at grain boundaries, shortening incubation and shifting PTT curves to higher-temperature, shorter-time regions. These findings clarify the competition between nucleation driving force and diffusion kinetics, providing guidance for secondary cooling design and slab-quality control in MCCR processing.