A hot-dip galvanized steel sheet includes a base steel sheet and a hot-dip galvanized layer formed on at least one surface of the base steel sheet, in which the hot-dip galvanized layer includes Fe in a content of more than 0% to 5% or less, Al in a content of more than 0% to 1.0% or less, and columnar grains formed by a phase on the surface of the steel sheet, further, 20% or more of the entire interface between the hot-dip galvanized layer and the base steel sheet is coated with the phase, and a ratio of an interface formed between grains in which coarse oxides are present among grains and the base steel sheet with respect to the entire interface between the phase and the base steel sheet in the hot-dip galvanized layer is 50% or less, the base steel sheet has predetermined chemical components and a refined layer in direct contact with the interface between the base steel sheet and the hot-dip galvanized layer, an average thickness of the refined layer is 0.1 to 5.0 m, an average grain size of ferrite in the refined layer is 0.1 to 3.0 m, one or two or more of oxides of Si and Mn are contained in the refined layer, and a maximum size of the oxide is 0.01 to 0.4 m.

A steel sheet which uses steel sheet which contains the easily oxidizable elements Si and Mn as a base material and which is provided with a hot dip galvanized layer which is excellent in plating wettability and plating adhesion and a method of production of the same are provided. A hot dip galvanized steel sheet which is comprised of a steel sheet having a hot dip galvanized layer A on the surface of the steel sheet, characterized by having the following B layer right under the steel sheet surface and in the steel sheet: B layer: Layer which has thickness of 0.001 m to 0.5 m, which contains, based on mass of the B layer, one or more of Fe, Si, Mn, P, S, and Al oxides in a total of less than 50 mass %, which contains C, Si, Mn, P, S, and Al not in oxides in C: less than 0.05 mass %, Si: less than 0.1 mass %, Mn: less than 0.5 mass %, P: less than 0.001 mass %, S: less than 0.001 mass %, and Al: less than 0.005 mass % and which contains Fe not in oxides in 50 mass % or more.

A steel sheet containing manganese from 3.0 to 6.0% in weight which has both a good coatability by liquid zinc and a good LME resistance. The present invention also aims to make available an easy to implement method to obtain the steel sheet and an assembly which does not have LME issues after spot-welding.

There is provided a high-strength hot-dip galvanized steel sheet and the like excellent in mechanical cutting property, which are capable of obtaining high ductility while ensuring high strength with maximum tensile strength of 900 MPa or more. The high-strength hot-dip galvanized steel sheet has a sheet thickness of 0.6 to 5.0 mm and has a plating layer on a surface of a steel sheet with component compositions being set in appropriate ranges, in which the steel sheet structure contains a 40 to 90% ferrite phase and a 3% or more retained austenite phase by volume fraction. In the retained austenite phase, a solid solution carbon amount is 0.70 to 1.00%, an average grain diameter is 2.0 m or less, an average distance between grains is 0.1 to 5.0 m, a thickness of a decarburized layer in a steel sheet surface layer portion is 0.01 to 10.0 m, an average grain diameter of oxides contained in the steel, sheet surface layer portion is 30 to 120 nm and an average density thereof is 1.010.sup.12 oxides/m.sup.2 or more, and moreover, a work hardening coefficient (n value) during a 3 to 7% plastic deformation is 0.080 or more on average.

A steel product may include a metallic anticorrosion coating of an aluminum alloy. So that such a steel product possesses high media resistance, more particularly high acid resistance and corrosion resistance, the steel product may be immersed in a liquid-melt coating bath that includes an aluminum alloy. In addition to Aluminum and unavoidable impurities, the aluminum alloy may include at least one of 0.2-2% by weight Mn or 0.2-7% by weight Mg, 0.5-5% by weight Fe, and at least one of 0.05-0.4% by weight Ti or 0.05-0.4% by weight Zr. A method for producing such steel products may involve providing a steel product in a hot-rolled or cold-rolled state, activating a surface of the steel product to remove passive oxides, and coating the surface-activated steel product by immersion in a liquid-melt coating bath that comprises an aluminum alloy having the aforementioned composition.

A base steel sheet has a hot-dip galvanized layer formed on a surface thereof, in which, in a steel sheet structure in a range of thickness to thickness centered around thickness of a sheet thickness from a surface, a volume fraction of a retained austenite phase is 5% or less, and a total volume fraction of phases of bainite, bainitic ferrite, fresh martensite, and tempered martensite is 40% or more, an average effective crystal grain diameter is 5.0 m or less, a maximum effective crystal grain diameter is 20 m or less, and a decarburized layer with a thickness of 0.01 m to 10.0 m is formed on a surface layer portion, in which a density of oxides dispersed in the decarburized layer is 1.010.sup.12 to 1.010.sup.16 oxides/m.sup.2, and an average grain diameter of the oxides is 500 nm or less.

A high strength galvanized steel sheet excellent in coating adhesiveness is made from a base material that is a high strength steel sheet containing Si, Mn, and Cr. A method includes performing an oxidation treatment on steel containing Si, Mn, and Cr in an oxidation furnace under the condition that a selected exit temperature T, reduction annealing and a galvanizing treatment, or optionally, further an alloying treatment under conditions that heating is performed at a temperature of 460 C. or higher and 600 C. or lower for an alloying treatment time of 10 seconds or more and 60 seconds or less.

A method for manufacturing a galvanized steel sheet includes heating a base steel sheet in a heating zone such that the surface of the base steel sheet is heated at a temperature of 600 C. or higher and 790 C. or lower while a furnace temperature T C. in the heating zone of an annealing furnace is controlled based on the water vapor partial pressure P.sub.H2O.sup.in Air of air fed into the heating zone, the base steel sheet having a chemical composition consisting of, by mass %, C: 0.05% or more and 0.25% or less, Si: 0.1% or more and 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.001% or more and 0.10% or less, Al: 0.01% or more and 3.00% or less, S: 0.200% or less, and the balance being Fe and inevitable impurities, heating the base steel sheet in the heating zone such that the surface of the base steel sheet is heated at a temperature of 630 C. or higher and 850 C. or lower in an atmosphere containing hydrogen gas having a partial pressure P.sub.H2O of 1000 Pa or more and 50000 Pa or less, water vapor gas having a partial pressure P.sub.H2O of 610 Pa or less, and the balance being N.sub.2 and inevitable impurities, and galvanizing the base steel sheet.

A high-strength surface-coated steel sheet includes a base steel sheet having a composition by mass of C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.10 to 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Nb: 0 to 0.10%, Mo: 0 to 0.50%, Cr: 0 to 0.50%, Al: 0.01 to 0.10%, and the balance of Fe and inevitable impurities; and a ZnAlMg-based coating layer disposed on a surface of the base steel sheet, The steel sheet has a diffusible hydrogen concentration in the base steel sheet of 0.30 ppm or less and having a time until occurrence of red rust of 7000 hours or more as measured by a salt spray test. The high-strength hot-dip ZnAlMg-based-plated steel sheet has a significantly lowered in-steel concentration of hydrogen which has entered the steel in a plating line and exhibits excellent corrosion resistance.

A welded member includes: a first steel sheet including a base steel sheet and a zinc-based plating layer formed on a surface of the base steel sheet; and a second steel sheet spot-welded on the first steel sheet and facing the zinc-based plating layer of the first steel sheet, a spot-welded zone being formed between the first steel sheet and the second steel sheet. A surface layer of the base steel sheet has a decarburization ratio of 30% or more. The decarburization ratio is represented by equation: Decarburization ratio (%) of surface layer=(1average carbon concentration in surface layer/bulk carbon concentration)*100 where the surface layer refers to a region of the base steel sheet from the surface thereof to a depth of 35 m. A shoulder portion of the spot-welded zone includes a B-type crack having a length of 100 m or less, and no C-type crack.