A high-strength plated steel sheet sequentially includes an internal oxidized layer, a soft layer containing the internal oxidized layer, and a hard layer including a structure having metallic structure which contains a low-temperature-transformation produced phase in a proportion of 20 to 85% by area of the whole of the metallic structure, polygonal ferrite in a proportion more than 10% by area, and 70% or less by area of the whole of the metallic structure, and-retained austenite in a proportion of 5% or more by volume of the whole of the metallic structure. The high-strength plated steel sheet satisfies the average depth D of the soft layer being 20 μm or more, the average depth d of the internal oxidized layer being 4 μm or more and less than D, and a tensile strength being 980 MPa or more.

The invention relates to a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R.sub.e/R.sub.m ratio of less than 0.75 and an elongation at break of greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10%≦C≦0.25%; 1%≦Mn≦3%; Al≧0.015%; Si≦1.985%; Mo≦0.30%; Cr≦1.5%; S≦0.015%; P≦0.1%; Co≦1.5%; B≦0.005%; it being understood that 1%≦Si+Al≦2%; Cr +(3×Mo)≦0.3%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the steel consisting of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.

The invention relates to a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R.sub.e/R.sub.m ratio of less than 0.75 and an elongation at break of greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10%≦C≦0.25%; 1%≦Mn≦3%; Al≧0.015%; Si≦1.985%; Mo≦0.30%; Cr≦1.5%; S≦0.015%; P≦0.1%; Co≦1.5%; B≦0.005%; it being understood that 1%≦Si+Al≦2%; Cr +(3×Mo)≦0.3%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the steel consisting of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.

Disclosed herein is a high-strength plated steel sheet having a plated layer on the surface of a base steel sheet and containing predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet; and a hard layer consisting of a structure which is mainly composed of martensite and bainite and in which the average grain size of prior austenite is 20 μm or less. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

The high-strength plated steel sheet of the present invention has a plated layer on the surface of a base steel sheet and contains predetermined steel components. The steel sheet includes, in the order from the interface of the base steel sheet and the plated layer towards the base steel sheet: a soft layer having a Vickers hardness that is 90% or less of the Vickers hardness at a portion t/4 of the base steel sheet, where t is a sheet thickness of the base steel sheet: and a hard layer containing martensite, bainite, and ferrite in predetermined ranges. The average depth D of the soft layer is 20 μm or greater, and the average depth d of an internal oxide layer is 4 μm or greater and smaller than D.

Disclosed herein is a high-strength plated steel sheet containing an internal oxidized layer, a soft layer including the internal oxidized layer, and a hard layer including a structure having metallic structure containing a low-temperature-transformation produced phase in a proportion of 70% or more by area of the whole of the metallic structure, in which polygonal ferrite is in a proportion of 0% or more by area, and 10% or less by area of the same, and retained austenite is in a proportion of 5% or more by volume of the same. The high-strength plated steel sheet satisfies the average depth D of the soft layer is 20 μm or more, the average depth d of the internal oxidized layer is 4 μm or more and less than D, and a tensile strength of 980 MPa or more.

The present invention relates to material utilized for heavy construction machinery, vehicle frames, reinforcing members, and the like, and more specifically to a high-strength steel sheet having excellent impact resistance and a method for manufacturing same.

An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).

An oriented electrical steel sheet according to an embodiment of the present invention includes, in a unit of wt %, Si at 1.0 wt % to 5.0 wt %, C at 0.005 wt % or less (excluding 0 wt %), Mn at 0.001 wt % to 0.1 wt %, Cu at 0.001 wt % to 0.1 wt %, S at 0.001 wt % to 0.020 wt %, Se at 0.001 wt % to 0.050 wt %, Al at 0.0005 wt % to 0.010 wt %, N at 0.0005 wt % to 0.005 wt %, and the remainder of Fe and inevitable impurities.

The oriented electrical steel sheet according to the embodiment of the present invention satisfies Equation 1.

16≤(10×[Mn]+[Cu])/([S]+[Se])+(0.02−[Al])/[N]≤20  [Equation 1]

(In Equation 1, [Mn], [Cu], [S], [Se], [Al], and [N] represent contents (wt %) of Mn, Cu, S, Se, Al, and N, respectively.)

A hot-rolled strip steel product having a chemical composition consisting of, in terms of weight percentages (wt. %): 0.025%-0.070% C, 0%-1.10% Si, 0.50%-2.0% Mn, <0.020% P, <0.050% S, <0.010% N, 0%-0.60% Cr, 0%-0.20% Ni, 0%-0.25% Cu, 0%-0.20% Mo, 0%-0.15% Al, 0%-0.050% Nb, 0.020%-0.20-% V, 0.020%-0.15% Ti, 0%-0.0010% B, remainder Fe and inevitable impurities, wherein the strip steel product has a microstructure comprising of, in terms of volume percentages (vol. %), ferrite ≥90%, wherein the ferrite structure comprises 10%-50% quasi-polygonal ferrite and a reminder of ferrite structure is polygonal ferrite and/or bainite: and wherein the steel strip product has an average ferrite grain size of <10 μm, an average hole expansion ratio of ≥50%, a yield strength (Rp0.2%) longitudinal to rolling direction of ≥660 MPa and a tensile strength of ≥760 MPa.