Cold rolled steel sheet and galvanized steel sheet having strain age hardenability and method of producing the same

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 Ac1 transformation point to Ac3 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.