Patent ID: 8298356

Claim:
A steel sheet, comprising C: 0.05-0.25%; Si: 1.00-2.5%; Mn: 2.0-4.0%; P: 0.1% or below; S: 0.05% or below; Al: 0.01-0.15%; Ti: 0.003-0.10%; N: 0.01% or below; and the balance comprising iron with inevitable impurities, wherein: % means mass %; the steel sheet is a composite structure steel sheet comprising ferrite and martensite; ferrite is 10-50 area % and martensite is 50 area % or above; the average circle-equivalent grain diameter of ferrite grains is 2.0 μm or below and the average aspect ratio of ferrite grains is 2.0 or below; and the tensile strength is 1,100 MPa or above.