| 摘要 |
A thin high strength steel sheet having excellent formability and a method for manufacturing the sheet are provided. A hot rolling step of hot-rolling 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 form a hot rolled sheet is conducted. After the hot-rolled sheet is pickled, the hot rolled sheet is subjected to an annealing treatment of holding the hot rolled sheet in a first temperature region of an A c1 transformation point to an A c3 transformation point for 5 to 400 s by a continuous annealing line without cold rolling; and to a cooling treatment of cooling the sheet at an average cooling rate of 5°C/s or higher from the first temperature region to 700°C and adjusting a residence time in a second temperature region of 700°C to 400°C in the range of 30 to 400 s. As a result, a microstructure composed of a ferrite phase having an area fraction in the range of 75 to 90% with respect to the entire microstructure and a second phase including pearlite, the second phase having an area fraction in the range of 10 to 25% can be obtained. The pearlite accounts for 70% or more in terms of the area fraction with respect to the entire second phase and the average grain size of the pearlite is 5 µm or smaller. Thus, a high strength steel sheet with excellent formability having a high strength, i.e., a tensile strength TS of 540 MPa or more, excellent elongation, and excellent stretch flangeability can be obtained. |
