| 摘要 |
PROBLEM TO BE SOLVED: To provide a non-heattreated thick steel plate having high strength, a low yield ratio, and high toughness, and to provide a production method therefor. SOLUTION: The steel plate contains 0.005 to 0.03% C, 0.05 to 0.5% Si, 1.5 to 3.0% Mn, 0.01 to 0.08% Al, 0.05 to 1.00% Mo, and 0.005 to 0.08% Nb, and a structure containing a bainitic ferrite phase in >=80% by an area ratio, or further containing insular martensite in <=2% by an area ratio, and the packet size of the bainitic ferrite is controlled to <=15μm. Thus, a tensile strength of >700 MPa, a yield ratio of <=80%, and a fracture appearance transition temperature vTrs of <=-20 deg.C can be obtained. The steel plate can further contain one or more kinds selected from Cu, Ni, Cr, Ti, V and B, and/or one or more kinds selected from Ca% and rare earth metals.
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