| 摘要 |
<p>A cold-rolled steel sheet which has a chemical composition, in mass %, where in C: 0.15 % or less, Al: 0.02 % or less and N: 0.0050 to 0.0250 %, and Si+Mn/5+10P and N/Al are adjusted to less than 0.44 or less and 0.3 or more respectively, and a structure containing a ferrite phase having an average crystal grain size of 15 .mu.m or less in 90 area % or more, the residue of the structure being a pealite phase, and which contains N in the state of solid solution in an amount of 0.0010 % or more and has a tensile strength o f 440 Mpa or less and a yield ratio of 70 % or less. The steel sheet is produc ed by a method which comprises subjecting a slab having the above composition t o a hot rolling at a FDT of 800~C or higher and reeling at a temperature of 650~C or lower, to prepare a hot-rolled sheet, subjecting the hot-rolled she et to a cold rolling, and then subjecting the resultant cold-rolled sheet to a continuous annealing at a temperature not lower than its recrystallization temperature and not higher than 950~C, to a primary cooling to a temperature region of 500~C or lower at a great cooling rate, and to an overaging treatment under conditions of a temperature of 350 to 400~C and a residence time of 30 s or less. The steel sheet is excellent in strain aging hardening properties and suitable for a body of an automobile.</p> |
