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: An object of the present invention is to provide a hot-dip galvanized steel sheet which is excellent in adhesion with a hot-dip galvanized zinc layer, and has high tensile strength and good formability even when a steel sheet containing Si and Mn in relatively larger amounts is used as a basis steel sheet. The hot-dip galvanized steel sheet is composed of: a basis steel sheet containing Si in an amount of 0.05 to 2.5% and Mn in an amount of 0.2 to 3%, by mass; a Fe plated layer formed on the basis steel sheet; and a hot-dip galvanized zinc layer formed on the surface layer of the basis steel sheet via the Fe plated layer, wherein oxides containing Si and/or Mn are discontinuously dispersed in the vicinity of the interface between the basis steel sheet and the Fe plated layer.

Abstract: An object of the present invention is to provide a hot-dip galvanized steel sheet which is excellent in adhesion with a hot-dip galvanized zinc layer, and has high tensile strength and good formability even when a steel sheet containing Si and Mn in relatively larger amounts is used as a basis steel sheet. The hot-dip galvanized steel sheet is composed of: a basis steel sheet containing Si in an amount of 0.05 to 2.5% and Mn in an amount of 0.2 to 3%, by mass; a Fe plated layer formed on the basis steel sheet; and a hot-dip galvanized zinc layer formed on the surface layer of the basis steel sheet via the Fe plated layer, wherein oxides containing Si and/or Mn are discontinuously dispersed in the vicinity of the interface between the basis steel sheet and the Fe plated layer.

Abstract: A hot-dip galvanized steel sheet composed of a basis steel sheet containing Si in an amount of 0.05-2.5 mass % and Mn in an amount of 0.2-3 mass % and a hot-dip galvanized zinc layer formed on the surface thereof, wherein said hot-dip galvanized zinc layer is formed in such a way that there is an Si—Mn enriched phase which is found, by observation under a scanning electron microscope or a transmission electron microscope, in the vicinity of the interface in a region no shorter than 50 &mgr;m in the cross section perpendicular to the interface between the basis steel sheet and the hot-dip galvanized zinc layer, said Si—Mn enriched phase containing more than twice as much Si and/or Mn as the basis steel sheet and extending over a length no more than 80% of the length of the interface observed. This hot-dip galvanized steel sheet is free of bare spots even in the case where the basis steel sheet contains Si and Mn in a comparatively large amount and hence is liable to suffering bare spots.

Abstract: A hot-dip galvanized steel sheet composed of a basis steel sheet containing Si in an amount of 0.05-2.5 mass % and Mn in an amount of 0.2-3 mass % and a hot-dip galvanized zinc layer formed on the surface thereof, wherein said hot-dip galvanized zinc layer is formed in such a way that there is an Si-Mn enriched phase which is found, by observation under a scanning electron microscope or a transmission electron microscope, in the vicinity of the interface in a region no shorter than 50 &mgr;m in the cross section perpendicular to the interface between the basis steel sheet and the hot-dip galvanized zinc layer, said Si-Mn enriched phase containing more than twice as much Si and/or Mn as the basis steel sheet and extending over a length no more than 80% of the length of the interface observed. This hot-dip galvanized steel sheet is free of bare spots even in the case where the basis steel sheet contains Si and Mn in a comparatively large amount and hence is liable to suffering bare spots.