| 摘要 |
Provided is a method that enables manufacture, with good productivity, of a high-strength cold-rolled steel with improved elongation (EL), stretch-flangeability (λ), bendability (R), and balance of these properties (TS×EL×λ/1000), outstanding composite workability as evaluated by Erichsen test, and a tensile strength of at least 980 MPa. A steel material satisfying a prescribed constituent composition is held and soaked for at least fifty seconds at a temperature at or above the Ac3 point, after which the steel material is cooled at an average cooling rate of at least 15° C./sec to a discretionary temperature (T) that satisfies expression (1) below, held for 5-180 seconds in a temperature range that satisfies expression (1) below, then heated to a temperature region that satisfies expression (2) below, held for at least 50 seconds in this temperature region, and then cooled.;300° C.≦T1 (° C.)<400° C. (1);400° C.≦T2 (° C.)<540° C. (2) |
| 主权项 |
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 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) |
