High strength steel sheet and method of producing same

A hot-rolled high strength steel sheet comprising a chemical composition containing, in mass %:

• C:
• 0.10% to 0.20%,
• Si:
• 0.7% to 1.4%,
• Mn:
• 2.3% to 4.0%,
• P:
• 0.10% or less,
• S:
• 0.03% or less,
• Al:
• 0.001% to 2.0%,
• N:
• 0.01% or less,
• O:
• 0.01% or less, and
• B:
• 0.0005% to 0.010%,
• with the balance being Fe and inevitable impurities, and
• with a MSC defined by the following formula (1) of 3.0 mass % to 4.2 mass %, the hot-rolled high strength steel sheet having a microstructure including:
• upper bainite in an area fraction of 70% to 93% as a main phase, and
• fresh martensite and retained austenite in a total area fraction of 7% to 30%,
• with the retained austenite having an area fraction of 2% to 7%, and the hot-rolled high strength steel sheet having a mechanical property with a uniform elongation of 6% to 10% and a tensile strength of 1180 MPa to 1400 MPa:
• MSC (mass %)=Mn+0.2×Si+1.7×Cr+2.5×Mo (1)
• where each element symbol in the formula (1) represents a content, in mass %, of a corresponding element and is taken to be 0 when the corresponding element is not contained.