| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile cold rolled steel sheet with a composite structure containing a trace amount of Si, having tensile strength of≥590 MPa and satisfying TS×El of≥19,000 MPa×%. SOLUTION: In the method of producing the high tensile cold rolled steel sheet, a slab comprising 0.03 to 0.20% C,≤0.4% Si and 1.0 to 3.0% Mn, and comprising≤0.02% Al and 0.008 to 0.025% N so as to satisfy≥0.5 N/Al is heated to≥1,000°C, is thereafter subjected to finish rolling at an outlet side temperature of≥800°C, is coiled at≤750°C and is cold-rolled. The cold rolled sheet is heated to an annealing temperature in the range of (an Ac3 transformation point-50°C) to (an Ac3 transformation point+100°C) in such a manner that the average heating rate in an Ac1 transformation point or above is controlled to 0.5 to 3°C/s, is held for 10 to 120s, is subsequently cooled to a temperature region in a specified range at the average cooling rate of 30 to 100°C/s, and is retained for≥50s. The cooling from the annealing temperature may consist of two-step cooling composed of first cooling where cooling is performed to a temperature in the range of 550 to 750°C at the average cooling rate of 1 to 10°C/s, and secondary cooling where cooling is performed to a cooling stopping temperature Ts at the average cooling rate of 15 to 100°C/s. COPYRIGHT: (C)2005,JPO&NCIPI
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