Method for manufacturing high-strength cold-rolled steel sheet with outstanding workability

A method of manufacturing a high-strength cold-rolled steel sheet having excellent workability, the steel sheet satisfying, by mass percent,

• C:
• 0.10 to 0.3%,
• Si:
• 1.0 to 3%,
• Mn:
• 1.5 to 3%,
• Al:
• 0.005 to 3%,
• P:
• 0.1% or less, and
• S:
• 0.05% or less,
• the remainder consisting of iron and inevitable impurities,
• a microstructure of the steel sheet including bainite, retained austenite, and tempered martensite, wherein
• (1) when the microstructure is observed by a scanning electron microscope,
• the bainite is composed of a composite structure of
• high-temperature-region-formed bainite, in which an average distance between adjacent retained-austenite grains and/or carbide particles is 1 μm or more, and
• low-temperature-region-formed bainite, in which an average distance between adjacent retained-austenite grains and/or carbide particles is less than 1 μm, and
• when an area fraction of the high-temperature-region-formed bainite in the entire microstructure is denoted as “a”, and
• when a total area fraction of the low-temperature-region-formed bainite and the tempered martensite in the entire microstructure is denoted as “b”,
• a:
• 20 to 80%, b:
• 20 to 80%, and a+b:
• 70% or more are satisfied, and
• (2) a volume fraction of the retained austenite determined by saturation magnetization measurement is 3% or more with respect to the entire microstructure, the method being characterized in that
• steel satisfying the above-described constituent composition is held for 50 sec or more at a temperature equal to or higher than a Ac3 point so as to be soaked, and then
• cooled to an appropriate temperature T satisfying Formula (1) at an average cooling rate of 15° C./sec or higher, and is held in a temperature region satisfying Formula (1) for 5 to 180 sec, and then
• heated into a temperature region satisfying Formula (2) and held in the temperature region for 50 sec or more, and is then cooled.
• 300° C.≦T1 (° C.)<400° C. (1)
• 400° C.≦T2 (° C.)≦540° C. (2)