Patent ID: 8182621

Claim:
A method of manufacturing a hot-rolled thin steel sheet having a structure of a bainitic ferrite single phase having an area ratio fraction of 95% or more, a tensile strength of 440 to 640 MPa, a thickness of less than 6 mm, high strength and excellent formability, and excellent strength and toughness after heat treatment, including: hot-rolling a steel base material having a composition containing, as mass %, C: 0.10 to 0.20%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.0%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.10%, N: 0.005% or less, Ti: 0.01 to 0.15%, B: 0.0005 to 0.0050%, the balance of Fe, and unavoidable impurities at a finishing temperature of finish rolling of 820 to 880° C. to produce a hot-rolled steel sheet with a thickness of less than 6 mm; cooling the hot-rolled steel sheet to a temperature range on a surface of the hot-rolled steel sheet to 550 to 650° C. at a surface cooling rate of 15 to 50° C. per second; and coiling the hot-rolled steel sheet at the temperature range.