| 摘要 |
<p><P>PROBLEM TO BE SOLVED: To produce a cold rolled high tensile strength steel sheet having excellent ductility, work hardening properties, and stretch flange formability. <P>SOLUTION: A slab has chemical composition composed of, by mass, more than 0.020% and less than 0.30% of C, more than 0.10% and not more than 3.00% of Si, more than 1.00% and not more than 3.50% of Mn, not more than 0.10% of P, not more than 0.010% of S, not more than 2.00% of soluble Al, and not more than 0.010% of N. The slab is subjected to hot rolling, in which the rolling is completed in a temperature region of not less than the Ar<SB POS="POST">3</SB>point, cooled to a temperature region of not more than 780°C within 0.4 second after the rolling is completed, and rolled up in a temperature region of less than 400°C. The obtained hot rolled steel sheet is subjected to hot rolled sheet annealing, in which the obtained hot rolled steel sheet is heated to a temperature region of not less than 300°C, then to cold rolling, and then to soaking treatment in a temperature region of not less than (Ac<SB POS="POST">3</SB>point-40°C), then cooled to a temperature region of 300-500°C, and annealed by being held in the temperature region for not less than 30 seconds. Thus, the cold rolled steel sheet having metallic structure including a low temperature transformation-forming phase as the main phase and residual austenite in the second phase is produced. <P>COPYRIGHT: (C)2013,JPO&INPIT</p> |
