| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel plate which has low yield ratio characteristic hardly causing brittle fracture even if applied, in particular, to uses where the steel plate is exposed to low temperature conditions and also has improved base-material toughness and weld heat-affected zone toughness. SOLUTION: The steel plate having low yield ratio and excellent in toughness and welded joint toughness is composed of steel having a composition containing 0.02 to 0.10% C, 0.002 to 0.010% N, 0.003 to 0.025% Nb and 0.005 to 0.025% Ti. The microstructure appearing in the longitudinal section of the steel plate consists of ferrite and pearlite or ferrite, pearlite and bainite. In the ferrite, elongated ferrite of larger aspect ratio and equiaxed ferrite of smaller aspect ratio exist as a mixture in a specific proportion. Further, the number of compound precipitates of Ti nitrides and Nb carbonitrides dispersed in the cross section is≥100 pieces/mm<SP>2</SP>. COPYRIGHT: (C)2006,JPO&NCIPI
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