A pre-coated steel sheet wherein at least a region at the periphery (7) of at least one (6a,6b) of the opposite faces (6a,6b) of said pre-coated sheet (1,1′) is coated with an additional coating (8) selected for increasing the vapor pressure between the pre-coating (2) and said additional coating (8) during a laser welding method up to a critical pressure at which the pre-coating (2) is ejected away from the weld (14). Preferably, the vaporization temperature of the additional coating (8) is greater than the vaporization temperature of the pre-coating (2) and the additional coating includes gammagene elements like carbon and/or nickel. A steel part obtained by laser welding, preferably butt laser welding, of at least a first and second pre-coated steel sheet (1,1′) as above indicated is also provided.

A pre-coated steel sheet wherein at least a region at the periphery (7) of at least one (6a,6b) of the opposite faces (6a,6b) of said pre-coated sheet (1,1′) is coated with an additional coating (8) selected for increasing the vapor pressure between the pre-coating (2) and said additional coating (8) during a laser welding method up to a critical pressure at which the pre-coating (2) is ejected away from the weld (14). Preferably, the vaporization temperature of the additional coating (8) is greater than the vaporization temperature of the pre-coating (2) and the additional coating includes gammagene elements like carbon and/or nickel. A steel part obtained by laser welding, preferably butt laser welding, of at least a first and second pre-coated steel sheet (1,1′) as above indicated is also provided.

The present invention relates to a coated steel sheet having a thin aluminium alloy coating and a coating method thereof The coated steel sheet of the present invention is used for hot stamping. The coating thickness of the coated steel sheet is 5˜14 μm, wherein the aluminium alloy coating comprises a FeAlSi inhibitive layer adjacent to a substrate steel sheet and an Al alloy layer outside the FeAlSi inhibitive layer, wherein the thickness of the FeAlSi inhibitive layer is no more than 60% of the coating thickness and is 1.5˜6.0 μm. The diameters of Kirkendall voids within 2 μm from an interface between the FeAlSi inhibitive layer and the substrate steel to the interior of the substrate steel are no more than 2.5 μm, wherein the number of Kirkendall voids with a diameter of no less than 0.5 μm and no more than 2.5 μm does not exceed 15 per 35 μm. The present invention also discloses a coating method for coating a thin aluminium alloy coating on a substrate steel sheet for hot stamping. It can eliminate skip coating and enable a hot stamped component obtained by the coated steel sheet to have excellent resistance spot welding performance.

The present invention relates to a coated steel sheet having a thin aluminium alloy coating and a coating method thereof The coated steel sheet of the present invention is used for hot stamping. The coating thickness of the coated steel sheet is 5˜14 μm, wherein the aluminium alloy coating comprises a FeAlSi inhibitive layer adjacent to a substrate steel sheet and an Al alloy layer outside the FeAlSi inhibitive layer, wherein the thickness of the FeAlSi inhibitive layer is no more than 60% of the coating thickness and is 1.5˜6.0 μm. The diameters of Kirkendall voids within 2 μm from an interface between the FeAlSi inhibitive layer and the substrate steel to the interior of the substrate steel are no more than 2.5 μm, wherein the number of Kirkendall voids with a diameter of no less than 0.5 μm and no more than 2.5 μm does not exceed 15 per 35 μm. The present invention also discloses a coating method for coating a thin aluminium alloy coating on a substrate steel sheet for hot stamping. It can eliminate skip coating and enable a hot stamped component obtained by the coated steel sheet to have excellent resistance spot welding performance.

Provided are a plated steel sheet and a method for manufacturing same, the plated steel sheet comprising: a base steel sheet; a Zn—Mg—Al based steel sheet plating layer provided on at least one surface of the base steel sheet; and an Fe—Al based inhibition layer provided between the base steel sheet and the Zn—Mg—Al based plating layer, wherein the plating layer comprises, in wt %: 4% or more of Mg; 2.1 times or more of Mg content and 14.2% or less of Al; 0.2% or less (including 0%) of Si; 0.1% or less (including 0%) of Sn; the remainder Zn; and unavoidable impurities.

A high-strength cold-rolled steel sheet comprises: a chemical composition that contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1; and a steel microstructure in which: an area fraction of ferrite is 30% or more and 60% or less; a total area fraction of tempered martensite and bainite is 35% or more and 65% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 40% or less; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.

A high-strength cold-rolled steel sheet comprises: a chemical composition that contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1; and a steel microstructure in which: an area fraction of ferrite is 30% or more and 60% or less; a total area fraction of tempered martensite and bainite is 35% or more and 65% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 40% or less; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.

An exemplary embodiment in the present disclosure provides a hot-dip Zn—Al—Mg-based alloy-plated steel material having excellent corrosion resistance in a processed portion, and a method for manufacturing the same. The steel material includes: an iron substrate; and a hot-dip alloy-plated layer formed on the iron substrate, wherein the hot-dip alloy-plated layer contains, by wt %, more than 8% to 25% of Al, more than 4% to 12% of Mg, and a balance of Zn and inevitable impurities, a fraction of a MgZn2 phase in the hot-dip alloy-plated layer is 10 to 45 area %, cracks are formed inside the MgZn2 phase, and the number of cracks present per 100 μm in a direction perpendicular to a thickness direction of a steel sheet in a field of view in which the cracks are observed based on a cross section in the thickness direction of the steel sheet is 3 to 80.

A plated steel includes: a steel; and a plating layer that is provided on a surface of the steel, in which the plating layer includes, by mass %, Al: 5.00% to 35.00%, Mg: 2.50% to 13.00%, Fe: 5.00% to 40.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, and a remainder of Zn and impurities, and in a cross section of the plating layer, the area fraction of a Zn solid-solution Fe.sub.2Al.sub.5 phase in which 5% or more of Zn is solid-soluted is 10% to 60% and the area fraction of a MgZn.sub.2 phase is 10% to 90%.

A plated steel includes: a steel; and a plating layer that is provided on a surface of the steel, in which the plating layer includes, by mass %, Al: 5.00% to 35.00%, Mg: 2.50% to 13.00%, Fe: 5.00% to 40.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, and a remainder of Zn and impurities, and in a cross section of the plating layer, the area fraction of a Zn solid-solution Fe.sub.2Al.sub.5 phase in which 5% or more of Zn is solid-soluted is 10% to 60% and the area fraction of a MgZn.sub.2 phase is 10% to 90%.