| 摘要 |
PROBLEM TO BE SOLVED: To obtain a thick steel plate for a welded structure which has excellent fatigue crack propagation resistance of a base material without depending on the addition of a large quantity of special or expensive alloy elements, or depending on a production method which is inferior in productivity or is complicated, or without the placement of large restrictions on tensile strength and steel plate thickness. SOLUTION: In the thick steel plate having a composition containing proper amounts of C, Si, Mn, Al and N, and, if required, containing Ni, Cu, Cr, Mo, W, Ti, V, Nb, Zr, Ta, B, Mg, Ca and rare earth metals, and having a structure consisting of ferrite and a hard secondary phase consisting of bainite or martensite or a mixed structure of both, by making the hard secondary phase in the cross-section parallel to the surface of the steel plate satisfy all conditions of a fraction of 20 to 80%, a mean Vickers hardness of 250 to 800, a diameter of the equivalent circle of 10 to 200μm, and the maximum spacing between the hard secondary phases of≤500μm, the fatigue crack propagation of the base material is remarkably delayed, so that the fatigue strength of a joint is improved. COPYRIGHT: (C)2003,JPO
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