| 摘要 |
PROBLEM TO BE SOLVED: To provide low yield ratio high strength steel which has tensile strength of 490 to 590 MPa, and is excellent in base material toughness and weldability. SOLUTION: The high strength steel has a composition comprising, by mass, 0.01 to 0.15% C,≤0.8% Si, 1.0 to 2.0% Mn,≤0.05% Al, 0.005 to 0.050% Ti and 0.0003 to 0.0050% B, and the balance Fe with impurities, and has a structure consisting of polygonal ferrite in 5 to 30 vol% and MA (a mixture of martensite and austenite) in 3 to 15 vol%, and the balance bainite, and in which the average size of the MA is≤5μm. In addition to the fundamental components, one or more elements selected from each group of (1) prescribed amounts of one or more kinds of metals selected from Cu, Ni, Cr and Mo and (2) prescribed amounts of one or two kinds of metals selected from Nb and V can further be added individually or in combination. COPYRIGHT: (C)2005,JPO&NCIPI
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