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

A high-strength hot-rolled steel sheet superior in stretch flange formability, comprising:

• 0.01-0.10 mass % of C;
• no more than 1.0 mass % of Si;
• no more than 2.5 mass % of Mn;
• no more than 0.08 mass % of P;
• no more than 0.005 mass % of S;
• 0.0 15-0.050 mass % of Al and
• 0.10-0.30 mass % of Ti,
• wherein a remainder is substantially Fe;
• wherein said hot-rolled steel sheet has a structure consisting of at least one of a polygonal ferrite structure, a granular bainitic ferrite structure and a bainitic ferrite structure;
• wherein a unit grain has an average particle diameter (d) no larger than 5 μm;
• and
• wherein said unit grain is defined such that adjacent grains which surround said unit grain differ from said unit grain in orientation more than 15°.

A high-strength hot-rolled steel sheet superior in stretch flange formability, comprising:

• 0.01-0.10 mass % of C;
• no more than 1.0 mass % of Si;
• no more than 2.5 mass % of Mn;
• no more than 0.08 mass % of P;
• no more than 0.005 mass % of S;
• 0.0 15-0.050 mass % of Al and
• 0.10-0.30 mass % of Ti,
• wherein a remainder is substantially Fe;
• wherein said hot-rolled steel sheet has structure consisting essentially of ferrite;
• wherein a unit grain has an average particle diameter (d) no larger than 5 μm;
• and
• wherein said unit grain is defined such that adjacent grains which surround said unit grain differ from said unit grain in orientation more than 15°.