| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel material having a structure consisting mainly of a hard phase and having excellent resistance to fatigue crack propagation and to provide its manufacturing method. SOLUTION: The steel material is composed mainly of bainite/martensite, and a minimum short side length of a lathlike structure is≤1.3μm, and a proportion of a M-A structure contained in a bainite structure and having an aspect ratio of≥5 is less than 5% as an area ratio, and further an aspect ratio of prior austenite grain are≤1.2. The steel has a chemical composition of 0.01-0.15% C, 0.03-0.6% Si, 0.5-2.0% Mn, 0.005-0.1% sol.Al, 0.0005-0.008% N and also 4.0-6.0% Ft (3Mn+Cu+1.5Cr+1.8Ni+1.5Mo). The steel may further contain≤0.0030% B and have 3.5-5.5% Ft. One or more elements selected among Cu, Ni, Cr, Mo, Nb, Ti and V may be contained. The steel is manufactured by cooling after hot rolling. COPYRIGHT: (C)2004,JPO
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