Patent ID: 7780797

Claim:
High strength hot rolled steel sheet excellent in burring, elongation, and ability of phosphate coating characterized by being a steel composition containing, by mass%, C: 0.02 to 0.08%, Si: 0.25% or less, Mn: 0.50 to 3.50%, P: 0.03% or less, S: 0.01% or less, Al: 0.40 to 2.0%, Ti: 0.003 to 0.20%, and the balance of iron and unavoidable impurities, satisfying the following formula (1), the steel sheet being hot-rolled at a rolling end temperature of the Ar3point or more, then cooled by a cooling rate of 20° C./sec or more to 650 to 800° C., then air-cooled for 2 to 15 seconds, then further cooled by a cooling rate of 20° C./sec or more to 350 to 600° C. and coiled; and the steel sheet having a microstructure of said steel sheet consisting essentially of a two phase structure of ferrite, having a grain diameter of 2 μm or more, and bainite, and having a ratio of ferrite of a grain size of 2 μm or more of at least 40%, and having precipitations of TiC and with a tensile strength of at least 590 N/mm 2 : and having a hole expandability ratio of 62% or higher, Mn+0.5×Al<4 (1).