| 摘要 |
PROBLEM TO BE SOLVED: To provide 600 MPa-class steel which has excellent HAZ (Heat Affected Zone) toughness, further has hardness in the HAZ, and satisfies TS (Tensile Strength) >=590 MPa even in welding of superhigh heat input. SOLUTION: The 600 MPa-class steel having excellent welding operability contains grains having prescribed components, also satisfying the condition that Pcm is <=0.21, and having a grain size of 0.005 to 2.0μm, particularly, 0.1 to 2.0μm, and having a composition at least containing Ca, Al and O, and in which, as the elements other than O, by weight, >=5% Ca and >=5% Al are contained, and the balance inevitable impurities are contained by the grain number of 100 to 3,000 pieces/mm<2> . The hardness (Hv) of the heat affected zone is >=80% of that of the base material.
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