Patent ID: 9631266
Date: 2017-04-25
CPC Classifications: C21D,C22C,C23C,C25D

Claim:
1. 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 comprising 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 Ac cooled to a temperature T1 satisfying Formula (1) at an average cooling rate of 20° C./sec or higher, and 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 then cooled;