HIGH TENSILE COLD-ROLLED STEEL SHEET HAVING EXCELLENT STRAIN AGING HARDENING PROPERTIES

摘要

A high tensile cold-rolled steel sheet which has a chemical composition in mass % wherein C: 0.15 % or less, Al: 0.02 % or less and N: 0.0050 to 0.0250 %, N/Al is adjusted to 0.3 or more, and the content of N in the state of solid solution is 0.0010 % or more, and a structure containing a ferrite phase having an average crystal grain size of 10 mu m or less in 50 area % or more and optionally a martensite phase as a second phase in 30 area % or more. The steel sheet is produced by a method which comprises subjecting a slab having the above composition to a hot rolling at a finish rolling output temperature of 800 DEG C or higher and reeling at a temperature of 750 DEG C or lower, to prepare a hot-rolled sheet, subjecting the hot-rolled sheet to a cold rolling, and then subjecting the resultant cold-rolled sheet to a continuous annealing under conditions of a temperature not lower than its recrystallization temperature and not higher than 900 DEG C and a holding time of 10 to 120 s, to a primary cooling to a temperature region of 500 DEG C or lower at a cooling rate of 10 to 300 DEG C/s, and optionally to a secondary cooling of a residence for 300 s or less at a temperature range not lower than a temperature of termination of the primary cooling and not lower than 350 DEG C. The high tensile steel sheet is excellent in formability, impact resistance characteristics and strain aging hardening properties.