Patent ID: 7252724

Claim:
A method for producing a high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a BH of 80 MPa or more, a ΔTS of 40 MPa or more, and a tensile strength of 440 MPa or more comprising the steps of: heating a steel slab to 1,000° C. or more, the steel slab comprising: in percent by mass, 0.15% or less of C; 2.0% or less of Si; 3.0% or less of Mn; 0.08% or less of P; 0.02% or less of S; 0.02% or less of Al; 0.0050% to 0.0250% of N; and optionally further comprising at least one selected from the group consisting of the following Group a to Group d, the ratio N (mass %)/Al (mass %) being 0.3 or more: Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, and Mo Group b: 0.1% or less in total of at least one of Nb, Ti, and V Group c: 0.0030% or less of B Group d: 0.0010% to 0.10% in total of at least one of Ca and REM; rough-rolling the steel slab to form a sheet bar; finish-rolling the sheet bar at a finishing temperature of 800° C. or more; cooling at a cooling rate of 20° C./s or more within 0.5 second after the finish-rolling; slow cooling for 1 to 5 seconds at a rate of 10° C./s or less in the temperature range of 700 to 800° C.; cooling at a rate of 20° C./s or more; and coiling at a temperature of 450° C. or less.