Effect of Boron on the Hot Ductility of Low-Carbon Nb-Ti-Microalloyed Steel

Hot tensile tests were performed to examine the effect of boron on the hot ductility of Nb-Ti-microalloyed steels. The equilibrium precipitation in the steel was predicted by Thermo-Calc calculation. The microstructure, fracture surface and precipitates in the deformed steel were examined. The results show that boron addition is favorable to improving the hot ductility of Nb-Ti-microalloyed steel. This beneficial effect is caused by the soluble boron instead of coarse BN in the steel. The hot ductility of the steel decreases less from 1000 degrees C with increasing boron addition. The hot ductility trough shifts toward lower temperatures because ferrite formation was restrained with increasing boron content of the steel. The formation of NbC, TiN and thin film-like ferrite along austenite grain boundaries lead to the decrease in the hot ductility of the steel. Boron addition has negligible influence on the precipitation temperature and amount of TiN and NbC precipitates in Nb-Ti-microalloyed steel. The amount of NbC precipitates is largest in the steel, followed by TiN and BN. The precipitation temperature of BN increases considerably with further increasing the boron content. The fracture mode of Nb-Ti-microalloyed steel tends to be more ductile with the increase in the boron content of the steel.