Provided in the present invention is a 120-kg-grade ultrahigh-strength galvanized steel sheet with a good resistance spot welding performance. The galvanized steel sheet comprises: 0.18-0.24% of C, 2.3-3.0% of Mn, 0.5-1.7% of Si, 0.02-1.0% of Al, 0.55<Si+Al1.75%, C+Si/30+Mn/200.395%, and at least one selected from Nb, Ti, B, Cr, Mo and REM, with the balance being Fe and inevitable impurities, wherein the thickness of the steel sheet is set as t, and the resistivity of the steel sheet is 0<R.sub.155 .Math.cm; in the direction starting from an interface between a plating and a steel sheet substrate and towards the steel sheet substrate, the resistivity of the steel sheet within the range of 0.025 t or more to no more than 0.05 t is 0<R.sub.215 .Math.cm, and the resistivity of the material within the range of 0.01 t or more to no more than 0.015 t is 0<R.sub.335 .Math.cm; and the resistivities satisfy: 1.5R.sub.1.sup.1/2-0.1R.sub.2-0.25R.sub.3>0.

Provided are a plated steel sheet and a method for manufacturing same, the plated steel sheet comprising: a base steel sheet; a ZnMgAl plating layer provided on at least one surface of the base steel sheet; and an FeAl inhibition layer provided between the base steel sheet and the ZnMgAl plating layer. The plating layer comprises, by weight %, 4 to 10% of Mg and 5.1-25% of Al and the remainder being Zn and unavoidable impurities with respect to components not including iron (Fe) diffused from the base steel sheet. The plating layer comprises a 24-50% MgZn.sub.2 phase in phase fraction. In the MgZn.sub.2 phase, an Al single phase is present in the ratio of 1-30% relative to the cross-sectional area of the total MgZn.sub.2 phase.

A hot dip galvannealed steel sheet comprising a base steel sheet and a plating layer. The base steel sheet has a predetermined chemical composition by mass %. An existence ratio ER of internal oxides identified under predetermined conditions by observation of a cross-section of the base steel sheet is 40 % or less and an interval I of internal oxides is 300 m or more. The hot dip galvannealed steel sheet is excellent in appearance.

This Zn-plated hot stamped product includes a steel, a Zn-based plating layer, and an oxide layer, in which an upper layer which is a region on a surface side of the Zn-based plating layer has a two-phase structure of a phase and an FeZn solid solution, and a lower layer which is a region of the Zn-based plating layer excluding the upper layer has a single-phase structure of an FeZn solid solution, an upper layer thickness and a lower layer thickness satisfy the following expression, a Mn content ratio of Max. Mn/Min. Mn, which is a ratio of a maximum value Max. Mn to a minimum value Min. Mn of an Mn content on a surface of the Zn-plated hot stamped product, is 10.0 or less, and an average value Ave. Mn is 0.5 to 7.5% by mass %.
0.20upper layer thickness/(upper layer thickness+lower layer thickness)0.80

A low-carbon, low-alloy and high-formability dual-phase steel having a tensile strength of greater than or equal to 590 MPa, a hot-dip galvanized dual-phase steel, and a manufacturing method therefor. The steel comprises the following components, in percentage by mass: 0.04-0.12% of C, 0.1-0.5% of Si, 1.0-2.0% of Mn, P0.02%, S0.015%, 0.02-0.06% of Al, and can further comprise one or two of Cr, Mo, Ti, Nb and V, wherein Cr+Mo+Ti+Nb+V0.5%, and the balance is Fe and other unavoidable impurities. The manufacturing method includes smelting, casting, hot rolling, cold rolling, and rapid heat treatment processes. With the present invention, the recovery of a deformed structure, recrystallization and austenite transformation processes are changed, the nucleation rate is increased, the grain growth time is shortened, grains are refined, the strength and n value of the material are improved, and the interval range of the material performance is expanded.

This steel sheet includes: a base steel sheet having a predetermined chemical composition; and a zinc-plated layer formed on a surface of the base steel sheet, in which, when a sheet thickness of the base steel sheet is denoted by t, a metallographic structure at a t/4 position, which is a position at t/4 from the surface in a cross section in a sheet thickness direction of the base steel sheet, contains, by volume percentage, tempered martensite: 85% or more, residual austenite: 7% or more, and one or more selected from ferrite, pearlite, bainite, and fresh martensite: 0% or more and 8% or less, a metallographic structure in a surface layer region, which is a range from the surface to a position of 50 m in the cross section in the sheet thickness direction, contains, by volume percentage, 30% or more of bainite, and a remainder including one or more selected from ferrite, pearlite, tempered martensite, fresh martensite, and residual austenite, in the surface layer region, a diameter of prior austenite grains in the sheet thickness direction is 10.0 m or less, and a tensile strength of the steel sheet is 1,470 MPa or more.

A clad steel plate having tensile strength (TS) of 780 MPa or more, excellent ductility, bendability, collision resistance, and LME resistance. The clad steel plate having a base metal and a cladding metal on front and back surfaces of the base metal, and the chemical composition and microstructure of the base metal and the cladding metal being appropriately controlled so that the average Vickers hardness (HVL) of the cladding metal is 260 or less, the average Vickers hardness (HVL) of the cladding metal divided by the average Vickers hardness (HVB) of the base metal is 0.80 or less, the boundary roughness between the base metal and the cladding metal is 50 m or less at the maximum height Ry, and the number of voids at the boundary between the base metal and the cladding metal is controlled to 20 or fewer per 10 mm length of the boundary.

When a hot-dip galvanizing treatment is performed on a steel sheet containing Si in an amount of 0.2 mass % or more by using a continuous hot-dip galvanizing apparatus including an annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a snout adjacent to the cooling zone, and hot-dip galvanizing equipment, a humidified nitrogen-hydrogen gas mixture containing moisture in such a manner that a certain expression is satisfied is supplied into a region on the downstream side of the soaking zone, gas nozzles are arranged over the entire perimeter of an inner wall of the snout, nitrogen gas or a nitrogen-hydrogen gas mixture is supplied through the gas nozzles downward along the inner wall, and the dew point in the snout is controlled to be 50 C. to 35 C.