| 摘要 |
<p>A steel plate excellent in fatigue crack propagation characteristics, 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, 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 50 mum, and that a fatigue crack propagation rate (da/dN) in the case, where a stress magnification coefficient (DeltaK) is 20 MPa√m at a stress ratio of 0.1, is 10-8 m/cycle or less; and a method for producing the above steel plate. The steel plate can be advantageously used for a welded structural member of a building, a ship, a bridge, a construction machine, a marine structure and the like.</p> |
