| 摘要 |
PROBLEM TO BE SOLVED: To provide 600 MPa class steel which has low YR(yield ratio) property (YR<=80%), and in which excellent HAZ(heat affected zone) toughness has been realized even on welding of superhigh heat input in which heat input is >=500 kJ/cm. SOLUTION: The 600 MPa class steel having excellent low YR property and superhigh heat input welded joint toughness contains prescribed components. Also, oxide grains of 0.005 to 2.0μm by a diameter of the equivalent circle are contained in 100 to 3,000 pieces/mm<2> by piece density per unit area in the steel. The composition of the oxide grains at least contains Ca, Al and O, and, as the elements other than Ow, by mass, >=5% Ca and >=5% Al are contained. Also, in the microstructure of the steel, the ferrite fraction in the two phases of ferrite and bainite is 25 to 75%.
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