Abstract: A cold-rolled steel sheet of DP (Dual Phase) type with a specific composition meets the requirements: (I) In the surface of the steel sheet, there exist Si—Mn complex oxides no larger than 5 ?m in diameter of the equivalent circle as many as 10 or more per 100 ?m2 and the coverage of oxides composed mainly of Si on the surface of steel sheet is no more than 10% of surface area, and/or (II) The cross section near the surface of the steel sheet does not show cracks with a width no larger than 3 ?m and a depth no smaller than 5 ?m in arbitrary ten fields of observation under an SEM with a magnification of 2000. A high-strength cold-rolled steel sheet excellent in coating film adhesion and having a tensile strength no lower than 550 MPa is provided.

Abstract: A superhigh-strength dual-phase steel sheet containing ferritic microstructure and a martensitic microstructure—containing composite-phase steel sheet containing: C: 0.08-0.20% (mass% here and hereinafter), Si: 0.5% or less (inclusive of 0%) Mn: 3.0% or less (exclusive of 0%) P: 0.02% or less (inclusive of 0%) S: 0.02% or less (inclusive of 0%), and Al: 0.001-0.15%, and further containing Mo: 0.05-1.5%, and Cr: 0.05-1.5%, and which satisfying that: the average Vickers hardness of the ferritic microstructure is 150 Hv or more and the average Vickers hardness of the martensitic microstructure is 500 Hv or more, the superhigh-strength dual-phase steel sheets being of excellent fatigue characteristic in a spot welded joint.

Abstract: A superhigh-strength dual-phase steel sheet containing ferritic microstructure and a martensitic microstructure—containing composite-phase steel sheet containing: