High-yield-ratio and high-strength steel sheet excellent in workability

A steel sheet, which has a chemical composition comprising:

• C:
• 0.05% or more and less than 0.12% provided that the “%”s each mean “% by mass” and hereinafter the same matter is applied to any “%” described in connection with the chemical composition,
• Si:
• 0.1% or less, which is not 0%,
• Mn:
• 2.0 to 3.5%,
• at least one selected from the group consisting of Ti, Nb, and V:
• 0.01 to 0.2% in total,
• B:
• 0.0003 to 0.005%,
• P:
• 0.05% or less,
• S:
• 0.05% or less,
• Al:
• 0.1% or less,
• N:
• 0.015% or less, and
• Fe and one or more inevitable impurities as the balance of the composition;
• which has a metal structure comprising:
• bainite:
• 42 to 85%,
• martensite:
• 15 to 50%,
• ferrite:
• 5% or less, and
• entire microstructure of the balance of the metal structure other than bainite, martensite and ferrite:
• 3% or less provided that these proportions are each the proportion by area of one of these microstructures in the whole of the metal structure, and further satisfying a requirement that bainite has an average crystal grain diameter of 7 μm or less;
• and
• which has a tensile strength of 980 MPa or more.