Patent ID: 7067023

Claim:
A method of producing a high-tensile-strength cold-rolled steel sheet having a r value of as high as 1.2 or more, and excellent strain age hardenability and natural aging resistance comprising: a hot-rolling step of roughly rolling a steel slab by heating to a slab heating temperature of 1000° C. or more to form a sheet bar, finish-rolling the sheet bar so that the finisher delivery temperature is 800° C. or more, and coiling the finish-rolled sheet at a coiling temperature of 800° C. or less to form a hot-rolled sheet; a cold rolling step of pickling and cold-rolling the hot-rolled sheet to form a cold-rolled sheet; and a cold-rolled sheet annealing step of box-annealing the cold-rolled sheet at an annealing temperature of the recrystallization temperature to 800° C., then continuously annealing the annealed sheet at an annealing temperature of Ac 1 transformation point to Ac 3 transformation point −20° C.), and then cooling the sheet to the temperature region of 500° C. or less at a cooling rate of 10 to 300° C./s; wherein the steel slab has a composition, by mass %, comprising at least one of: C: 0.025 to 0.15%; Si: 1.0% or less; Mn: 2.0% or less; P: 0.08% or less; S: 0.02% or less; Al: 0.0% or less; and N: 0.0050 to 0.0250%; wherein N/Al is 0.30 or more.