| 摘要 |
There are provided a high strength galvanized steel sheet having a small thickness and excellent formability and a method for manufacturing thereof. Performed are a hot rolling step of subjecting a steel having a composition which includes, by mass%, 0.08 to 0.15% of C, 0.5 to 1.5% of Si, 0.5 to 1.5% of Mn, 0.01 to 0.1% of Al, and 0.005% or less of N, to hot rolling to form a hot rolled sheet; pickling of the hot rolled sheet; subsequently, without cold rolling, with a continuous galvanizing line, annealing in which the hot rolled sheet is held for 5 to 400 s in a first temperature region of an A c1 transformation point to an A c3 transformation point, cooling in which cooling from the first temperature region to 700°C is performed at an average cooling rate of 5°C/s or higher and a residence time in a second temperature region of 700°C to a temperature for entry into a galvanizing bath is in the range of 15 to 400 s, and subsequently galvanizing. As a result, it is possible to obtain a microstructure composed of a ferrite phase of which an area fraction with respect to the entire microstructure is in the range of 75 to 90% and a second phase including pearlite of which the area fraction is in the range of 10 to 25%, and thus a high strength galvanized steel sheet having excellent formability with a high strength, that is, a TS of 540 MPa or higher and excellent stretch flangeability is obtained. The pearlite occupies 70% or more in terms of an area fraction with respect to the entire second phase, and the average grain size of the pearlite is 5 µm or smaller. |
