High strength steel sheet having excellent formability and method for production thereof

A high strength steel sheet superior in formability,

• (1) containing the following chemical components in mass %:
• C:
• 0.06 to 0.25%
• Si+Al:
• 0.5 to 3%
• Mn:
• 0.5 to 3%
• P:
• 0.15% or less (not including 0%)
• S:
• 0.02% or less (not including 0%), and
• (2) having a structure comprising:
• (2-1) a base phase structure, the base phase structure being tempered martensite or tempered bainite and accounting for 50% or more in terms of a space factor relative to the whole structure, or the base structure comprising tempered martensite or tempered bainite which accounts for 15% or more in terms of a space factor relative to the whole structure and further comprising ferrite,
• the tempered martensite or the tempered bainite having a hardness which satisfies the relation of:
• Vickers hardness (Hv)≧500[C]+30[Si]+3[Mn]+50
• where [ ] represents the content (mass %) of each element;
• and
• (2-2) a second phase structure comprising retained austenite which accounts for 3 to 30% in terms of a space factor relative to the whole structure and optionally further comprising bainite and/or martensite, the retained austenite having a C concentration (CγR) of 0.8% or more, wherein the second phase structure satisfies the following expression (1) to enhance the fatigue characteristic:
• (S1/S)×100≧20 (1)
• where S stands for a total area of the second phase structure, and S1 stands for a total area of coarse second phase crystal grains (Sb) contained in the second phase structure, the Sb corresponding to three times or more as large as an average crystal grain area (Sm) of the second phase structure.