| 摘要 |
PROBLEM TO BE SOLVED: To attain the increase of the fatigue strength of a steel plate without deteriorating its fracture toughness by arranging lamellar structures in which martensite as-quenched, residual austenite or their mixed structure occupies >=a specific ratio in a base phase contg. C, Si, Mn and Al of a specified compsn. and composed of ferrite or the like, at specified thickness and specified intervals in the plate thickness direction. SOLUTION: This steel plate is composed of, by weight, 0.02 to 0.35%, C, 0.02 to 2.0% Si, 0.30 to 2.5% Mn, 0.002 to 0.10% Al, the balance Fe with inevitable impurities. The base phase of the steel plate is composed of ferrite, pearlite, bainite and tempered martensite. In the base phase, lamellar structures in which martensite as-quenched, residual austenite or their mixed structure occupies >=50% are arranged at 2 to 30μm thickness and 20 to 200μm intervals in the plate thickness direction.
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