| 摘要 |
PROBLEM TO BE SOLVED: To provide a steel plate excellent in fatigue crack propagation characteristics, and used for a welded structural member of a building, a ship, a bridge, a construction machine, a marine structure and the like, and to provide a method for production thereof. SOLUTION: The steel plate excellent in fatigue crack propagation characteristics is characterized in that it has a chemical composition, in mass%, that C: 0.03 to 0.2%, Si: 0.01 to 1.6%, Mn: 0.5 to 2%, P: 0.02% or less, S: 0.02% or less, Al: 0.001 to 0.1%, N: 0.001 to 0.008%, and the balance Fe and inevitable impurities are included, that a microstructure of a base material is a layer structure comprising a ferrite having a Vickers hardness of 150 or more, as a base phase, and a flat martensite having a Vickers hardness of 400 to 900, an area proportion of 5 to 30% and an aspect ratio (long axis/short axis) of three or more, as a second phase, that an average spacing between the ferrite and the martensite in the thickness direction is 3 to 50μm, and that a fatigue crack propagation rate (da/dN) in the case, where a stress magnification coefficientΔK is 20 MPa√m at stress ratio of 0.1, is 10<SP>-8</SP>m/cycle or less. COPYRIGHT: (C)2006,JPO&NCIPI
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