To provide a steel sheet with a low yield ratio, high strength and high toughness, which does not increase a material cost through adding a large amount of alloy elements or the like, does not deteriorate the toughness of a weld heat-affected zone, and is manufactured at high manufacturing efficiency and at a low cost; and to provide a manufacturing method therefor.
The steel sheet with the low yield ratio, high strength and high toughness comprises, by mass%, 0.03-0.1% C, 0.01-0.5% Si, 1.2-2.5% Mn, 0.05-0.4% Mo, 0.005-0.04% Ti, 0.08% or less Al, while controlling C/(Mo+Ti) to 1.2 to 3.0, and the balance substantially Fe; and has a metallographic structure of a three-phase structure consisting substantially of ferrite, bainite and island martensite, in which an area fraction of the island martensite is 3 to 20%. The steel sheet preferably further includes, by mass%, 0.005-0.07% Nb and/or 0.005-0.1% V, and controls C/(Mo+Ti+Nb+V) to 1.2 to 3.0.
ENDO SHIGERU
ISHIKAWA NOBUYUKI
MURAOKA RYUJI
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