溶接缶向けNb-B複合添加極低炭素鋼の熱間延性に及ぼす窒素の影響

摘要

Nb-B複合添加極低炭素鋼の熱間延性に及ぼす窒素量の影響を実機製造鋼を用いて調査した。0.0048mass％の窒素を添加したNb-B極低炭素鋼のAr_3変態点直上の950℃近傍の熱間延性劣化を明らかにした。窒素量を0.0032mass％以下まで減らすと950℃近傍の熱間延性は改善した。試験片の破面は粒界延性破面と粒内脆性破面が観察された。MnS, BNとAlN析出物は粒界に，Nb(C, N)析出物はフェライト粒内で観察された。これらの結果は，Nb-B複合添加極低炭素鋼の熱間延性がMnS, BNとAlN析出物による粒界延性破壊とNb(C, N)析出物による粒内脆性破壊が複合することにより低下することを示す。%The influence of nitrogen (N) content on the hot-ductility of combined niobium (Nb) and boron (B) added ultra low-carbon steel was investigated with use of the related commercial steel materials. Nb and B added ultra low-carbon steel in which nitrogen was 0.0048 mass% showed the bottom of ductility around 950℃, which was slightly higher than the Ar_3 transformation temperature. Reducing N content less than 0.0032 mass% improved the hot-ductility around the temperature of 950℃. Both grain boundary ductile fracture and intragranular 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 combined Nb and B added ultra low-carbon steel is deteriorated by both the grain boundary ductile fracture dominated by the MnS, BN, AlN precipitates and the transgranular brittle fracture by the Nb (C, N) precipitates.