PROBLEM TO BE SOLVED: To provide a steel plate which does not increase manufacturing costs, satisfies high strength (tensile strength is 490 MPa or more) even when plate thickness exceeds 50 mm, satisfies 3,500N/mm3/2 at a Kca value at -10°C, and whose fatigue crack development speed (da/dn) is 5.0×10-6 mm/cycle orless under specific stress (ΔK=10mPa√m) environment.;SOLUTION: Chemical component composition is appropriately adjusted, average area ratio of pseudopolygonal ferrite is 30-85% in microstructure at a position with the depth of t/4-t/2 (t represents the plate thickness, as follows) from the surface, average crystal grain size in an area from a position with the depth of 2 mm from the surface to a position with the depth of t/4 is 50 μm or less, and when relation between crystal grain size of steel structure at the position with the depth of 2 mm from the surface and crystal grain size to be counted by the crystal grain size is expressed by histogram, difference between two crystal grain size in which the number of crystal grains takes maximum is 10-30 μm.;COPYRIGHT: (C)2011,JPO&INPIT
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