| 摘要 |
This invention relates to steel materials suitable for use in welded structures, such as pipelines, bridges, and architectural structures, requiring structural safety and a method for manufacturing the same and particularly relates to improvement of resistance of ductile crack initiation from welded heat affected zone and a base material. Specifically, a steel material has a composition of C: 0.02 to 0.2%, Si: 0.01 to 0.5%, Mn: 0.5 to 2.5%, P: 0.05% or lower, S: 0.05% or lower, Al: 0.1% or lower, and N: 0.01% or lower and, as required, one or two or more elements selected from Cu: 0.01 to 2%, Ni: 0.01 to 5%, Cr: 0.01 to 3%, Mo: 0.01 to 2%, Nb: 0.1% or lower, V: 0.1% or lower, Ti: 0.1% or lower, B: 0.01% or lower, Ca: 0.01% or lower, and REM: 0.1 % or lower in terms of % by mass, and the balance Fe with inevitable impurities, in which the microstructure at the 1/4 position of the plate thickness contains ferrite and a hard phase, the area fraction of the hard phase is 50 to 90%, and the average aspect ratio of the ferrite is 1.5 or more. |
