| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-tensile hot-rolled steel sheet with a high tensile strength of 780-900 MPa excellent in blanking property and stretch flange formability, and a manufacturing method therefor.SOLUTION: Molten steel with a composition containing, by mass%, 0.03-0.06% C, 0.0020-0.0100% N, and 0.135-0.195% Ti with Si, Mn, and Al being adjusted to an appropriate amount is made into a steel raw material by a continuous casting method at a casting speed of 5 m/min or lower. The steel raw material is heated to a temperature of 1,250°C or higher, and rough rolling is started at a temperature of 1,200°C or higher. Hot rolling is finished at 900°C or higher. The steel raw material is cooled and is made into a wound hot rolled steel sheet at a winding temperature of 550°C or higher. The high-tensile hot-rolled steel sheet has a structure in which Ti carbide with a mean diameter of less than 6 nm is dispersed and precipitated in the ferrite crystal grain, the ferrite crystal grain occupies 95% or more of the metal structure in the area rate, TiN with a mean size of 20 nm or more is dispersed in the metal structure; and has a high tensile strength TS of 780-900 MPa and excellent blanking property and stretch flange formability. |
