Effect of Nitrogen Content on Hot-ductility of Nb and B-added Extra Low-carbon Steel

Abstract

The effect of nitrogen(N) content on the hot-ductility of niobium(Nb) and boron(B) combined added extra low-carbon steel was investigated by using commercial steels. With containing N of 0.0048 mass%, Nb and B added extra low-carbon steel showed the bottom of ductility around 950ºC, which was slightly higher than the Ar₃ transformation temperature. Reducing N content down to 0.0032 mass% or less improved the hot-ductility around the temperature of 950ºC.
Both grain boundary ductile fracture and transgranular brittle fracture were observed in the fractured surface of the tested specimen. MnS,BN and AlN precipitates were observed at the grain boundaries, and the Nb(C, N) precipitates were observed in the ferrite matrix.
These results suggest that the hot-ductility of Nb and B combined added extra low-carbon steel is deteriorated by both the grain boundary ductile fracture dominatied by the MnS, BN, AlN precipitates and the transgranular brittle fracture by the Nb(C, N) precipitates.