Patent ID: 8795442

Claim:
A cold rolled steel sheet wherein a maximum depth (Ry) of unevenness existing on the surface of the steel sheet is 10 μm or more, a average spacing (Sm) of unevenness is 30 μm or less, a load length ratio (tp40) of unevenness of the surface is 20% or less, and a crack of 3 μm or less width and 5 μm or more depth does not exist, wherein the steel sheet satisfies C:0.05-1.0 wt %, Si:2.0 wt % or less, Mn:0.3-4.0 wt %, Al:0.005-3.0 wt %, the balance is substantially iron, wherein a metal structure is a two-phase structure of ferrite and tempered martensite, and the tensile strength is 780 MPa or above.