| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel in which variation in HAZ toughness is reduced, and further, yield ratio is reduced to≤80%, and to provide a method for producing the same. SOLUTION: The steel contains (A) inclusions including REM and Zr. In the steel, (B) solid solution REM and solid solution Zr satisfy≤0.0010% (including 0%) solid solution REM and≤0.0010% (including 0%) solid solution Zr, and (C) a structure contains bainite and/or martensite, and ferrite, where the fraction of the ferrite occupied in the whole structure is 4 to 24 area%. When the metallic structure of the steel is observed by an EBSP (Electron Back Scattering Pattern) method, (D) inequality (1): 35≤D, is satisfied. In the inequality (1), D denotes the average equivalent circle diameter (μm) of the crystal grains surrounded by large angle boundaries with a crystal orientation difference of≥15°upon measurement of the orientation difference between the adjoining two crystals by an EBSP method. COPYRIGHT: (C)2010,JPO&INPIT
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