High-strength hot-rolled steel sheet superior in stretch flange formability and method for production thereof

摘要

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°, the unit grain having an average particle diameter (d) no larger than 5 &mgr;m. This steel sheet is produced by the steps of heating, rolling, cooling, and coiling under the following conditions. Heating temperature: 1150-1300° C.; reduction in rolling at 900-840° C.: no less than 70%; cooling rate: no less than 60° C./s; and coiling temperature: 300-500° C. or 600-750° C.