| 摘要 |
PROBLEM TO BE SOLVED: To provide a method of producing a high tensile strength cold rolled steel sheet which has excellent aging resistance and strain aging hardening property. SOLUTION: A slab containing <=0.15% C and 0.0050 to 0.0250% N, and satisfying N/Al: >=0.3 is heated at >=1,000 deg.C, and thereafter is rough-rolled and is finish-rolled at an outlet side temperature of >=800 deg.C. The slab is coiled at <=750 deg.C, and is subjected to a cold rolling stage. The rolling stock is further subjected sequentially to a continuous annealing stage so as to be annealed at Ac3 to (Ac3 +50 deg.C), and to be cooled in the temperature of 600 to 250 deg.C at a cooling rate of CR defined by the contents of Mn, Mo, Cr and Si or higher to <=50 deg.C. Further, it is possible that the cold rolled sheet is annealed at Ac3 to (Ac3 +100 deg.C), and is thereafter cooled in the range of Ac3 to Ac1 at a cooling rate of 3 to 25 deg.C/s, and, further, in the range of Ac1 to 250 deg.C at a cooling rate of CR or higher, or is cooled in the range of 600 to 450 deg.C at a mean cooling rate of CR or higher, and, in the range of 450 to 250 deg.C at a mean cooling rate of 1 to 5 deg.C/s. Further, in a componential system in which the contents of C and Mn are reduced, the annealing temperature can be controlled in (Ac3 -50 deg.C) to (Ac3 +50 deg.C).
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