Characterization and Control of Secondary Phase Precipitation of Nb-V-Ti Microalloyed Steel during Continuous Casting Process

The strand surface and subsurface cracks could be prevented through the control of the strand surface microstructure, which correlates with the precipitation behavior of carbonitrides in the microalloyed steel. In this study, the carbonitride precipitation behavior was characterized in-situ with a high-temperature confocal laser scanning microscope, and the effects of cooling rate on the morphology and distribution of precipitates was investigated. The results show that the carbonitride precipitation process is usually accompanied by the formation of dark particles due to the volume expansion of the solute depletion region. The evolution of dark particles suggests that carbonitrides mainly precipitate between 910 degrees C and 1 085 degrees C, and the "fast-growing region" ranges from 910 degrees C to 960 degrees C. As the cooling rate increases, the size and volume fraction of carbonitrides decrease. Meanwhile, the nucleation location changes from grain boundary to grain interior. To quantify the pinning capacity of the carbonitrides, the pinning force factor sigma is defined according to the classical Gladman equation, which is a power function of the cooling rate, namely sigma = 0.43 - 0.1(1+ 2.76Vc(3.48))(-1). Combining the above research, a new secondary cooling method is proposed and has been applied on an actual caster, which improves the crack resistance of the bloom surface microstructures.