Cold-rolled steel sheet having excellent strain aging hardening properties and method for producing the same

A method for manufacturing a cold-rolled steel sheet having superior strain-aging hardenability and exhibiting a tensile strength of less than 440 Mpa and yield ratio YR of less than 70%, the method comprising:

• a hot-rolling process comprising:
• heating a steel slab to a temperature of not less than 1,000° C., the steel slab comprising, by mass:
• not more than 0.15% carbon;
• not more than 0.4% silicon;
• not more than 2.0% manganese;
• not more than 0.04% phosphorous;
• not more than 0.02% sulfur;
• greater than 0.005% and not more than 0.02% aluminum;
• and
• 0.0050% to 0.025% nitrogen,
• wherein the Si content, the Mn content, and the P content satisfy relationship (1):
• Si+Mn/5+10P<0.44 where Si, Mn, P represent contents, in terms of percent by mass, of corresponding elements, the ratio of N/Al being not less than 0.3;
• rough-rolling the heated slab into a sheet bar;
• finish-rolling the sheet bar at a finish-rolling delivery temperature of not less than 800° C.;
• and coiling the resulting sheet bar at a temperature of not more than 650° C. into a coiled hot-rolled sheet,
• a cold-rolling process comprising pickling and cold-rolling the resulting hot-rolled sheet to prepare a cold-rolled sheet, and
• a cold-rolled sheet annealing process comprising:
• annealing the resulting cold-rolled sheet at a temperature between the recrystallization temperature and 950° C. for a holding time of 10 to 120 seconds;
• cooling at a cooling rate of 10 to 300° C./s to a temperature zone of not more than 500° C.;
• and optionally overaging in the temperature zone of 350° C. to 500° C. for a retention time of not less than 20 seconds,
• wherein the hot-rolling process, the cold-rolling process, and the cold-rolled sheet annealing process are sequentially performed.