From the point of view of achieving weight reduction of automobile for the purpose of reduction in fuel consumption and ensuring safety for occupants of automobile, application of an ultra high strength cold-rolled steel sheet having a tensile strength of 980 MPa or more to a structural member of automobile has been studied. However, since such an ultra high strength cold-rolled steel sheet as described above is remarkably inferior to a mild cold-rolled steel sheet in stretch-flangeability and ductility, it is difficult to subject the ultra high strength cold-rolled steel sheet to press-forming.
In regard to formability of a high strength cold-rolled steel sheet, a number of prior arts have so far been disclosed, for example, in JP-B Nos. 7-59726, 55-22532, 55-51410, 1-35051, and 1-35052, Japanese Patent No. 2766693, and JP-B No. 8-30212.
However, except for a case in which C content is high, among these prior arts, there is no prior art which simultaneously achieves a tensile strength of 980 MPa or more and either excellent stretch-flangeability or ductility. In a case in which C content is high, there is a problem in that, since a spot-welded portion is liable to be fractured, sufficient joint strength can not be obtained.