| 摘要 |
The present invention provides a cold-rolled sheet exhibiting superior strain-aging hardenability which is suitable for use in automobile bodies, and a method for manufacturing the same. In particular, a slab including, by mass, not more than 0.15% carbon, not more than 0.02% Al, and 0.0050% to 0.0250% nitrogen in which Si+Mn/5+10P is adjusted to less than 0.44 and the ratio N/Al is adjusted to not less than 0.3 is hot-rolled at a finish-rolling delivery temperature FDT of not less than 800° C., and then coiled at a temperature of not more than 650° C. Next, after cold-rolling, continuous-annealing at a temperature between the recrystallization temperature and 950° C., primary cooling for cooling to a temperature zone of not more than 500° C., and overaging in the temperature zone of 350° C. to 450° C. for a retention time of not more than 30 seconds are performed to prepare a steel sheet containing not less than 0.0010% of solute nitrogen and having a structure in which the ferrite phase with the grain size of not more than 15 mum is included at a ratio of not less than 90%, the remainder of the structure being the pearlite phase. The resulting steel sheet has superior strain-aging hardenability in which the tensile strength is less than 440 MPa and the yield ratio is less than 70%.
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