| 摘要 |
<p>A high-strength hot-rolled steel sheet superior in stretch flange formability which comprises C (0.01-0.10 mass%), si (no more than 1.0 mass%), Mn (no more than 2.5 mass%), P (no more than 0.08 mass%), S (no more than 0.005 mass%), Al (0.015-0.050 mass%), and Ti (0.10-0.30 mass%), with the remainder being substantially Fe, said hot-rolled steel sheet having a structure composed mainly of ferrite in which the unit grain is surrounded by grains such that adjacent grains differ in orientation more than 15 DEG , said unit grain having an average particle diameter (d) no larger than 5 mu m. This steel sheet is produced by the steps of heating, rolling, cooling, and coiling under the following conditions. Heating temperature: 1150-1300 DEG C; reduction in rolling at 900-840 DEG C: no less than 70%; cooling rate: no less than 60 DEG C/s; and coiling temperature: 300-500 DEG C or 600-750 DEG C.</p> |
