The invention relates to a method for producing an anti-corrosion coating for hardenable sheet steels, wherein at least two metal layers are deposited one after another onto the steel substrate; the one metal layer is a zinc layer or zinc-based layer and the other layer is a layer composed of a metal that forms baser intermetallic phases with Zn or Fe and has a higher oxidation potential than Zn, namely Ni, Cu, Co, Mn, or Mo, or a layer based on these metals; and an anti-corrosion coating for hardenable sheet steels.

A high-strength cold-rolled steel sheet having a specified chemical composition and a microstructure including ferrite having an average crystal grain diameter of 2 μm or less in an amount of 10% to 25% in terms of volume fraction, retained austenite in an amount of 5% to 20% in terms of volume fraction, martensite having an average crystal grain diameter of 2 μm or less in an amount of 5% to 15% in terms of volume fraction, and the balance being a multi-phase structure including bainite and tempered martensite having an average crystal grain diameter of 5 μm or less, in which a relational expression, 0.35≦V2/V1≦0.75 (1), is satisfied, where V1 is a volume fraction of phases which are different from ferrite and V2 is a volume fraction of tempered martensite.

A high-strength cold-rolled steel sheet having excellent bending workability includes, by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chromium (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N), the remainder of Fe and inevitable impurities, and comprises, by area fraction, 3-25% of ferrite, 20-40% of martensite, and 5-20% of retained austenite, in which a nickel-rich layer formed of nickel (Ni) introduced from the outside is provided on a surface layer portion, and the concentration of nickel (Ni) at a depth of 1 μm from the surface may be greater than or equal to 0.15 wt %.

Provided is a low-cost cold rolled steel sheet for zirconium-based chemical conversion treatment that contains Si and Mn and has excellent zirconium-based chemical convertibility even when a relatively large amount of Si-based oxide and Si—Mn-based oxide exists on the steel sheet surface. A cold rolled steel sheet for zirconium-based chemical conversion treatment comprises a chemical composition containing, in mass %, C: 0.05% to 0.30%, Si: 0.01% to 1.4%, Mn: 0.14% to 3.2%, P: 0.10% or less, and S: 0.01% or less, with a balance consisting of Fe and inevitable impurities, wherein a Si/Mn mass ratio in steel is 0.10 to 0.7, and a surface coating ratio of steel sheet surface layer oxides having Si content of 10 mass % or more is 40% or less.

A hot press member includes excellent indentation peel strength which has a tensile strength of 1780 MPa or more. A plating layer has at a surface thereof a 10-point average roughness Rzjis of 25 μm or less, and a steel sheet contains, in mass %, not less than 0.25% but less than 0.50% of C, 1.5% or less of Si, 1.1-2.4% of Mn, 0.05% or less of P, 0.005% or less of S, 0.01-0.50% of Al, 0.010% or less of N, 0.001-0.020% of Sb, 0.005-0.15% of Nb, and 0.005-0.15% of Ti, the balance being Fe and incidental impurities. The average crystal grain size of prior austenite is 7 μm or less and the volume proportion of martensite is 90% or more, within 50 μm in the thickness direction from the surface of the steel sheet excluding the plating layer.

Provided are a hot dip galvanized steel sheet comprising a base steel sheet wherein the base steel sheet has a predetermined chemical composition, and contains ferrite: 50% or less, retained austenite: 30% or less, tempered martensite: 5% or more, fresh martensite: 10% or less, and pearlite and cementite in total: 5% or less, remaining structures consist of bainite, and a number ratio of tempered martensite with a Mn concentration profile satisfying [Mn].sub.b/[Mn].sub.a>1.2 and [Mn].sub.a/[Mn]<2.0 ([Mn] is the Mn content in the base steel sheet, [Mn].sub.a is the average Mn concentration in the tempered martensite, and [Mn].sub.b is the Mn concentration at the interfaces of different phases of the tempered martensite and ferrite phase and bainite phase) is 0.2 or more with respect to the total number of tempered martensite, and a method for producing the same.

Provided is a high-strength hot-rolled coated steel sheet having a TS of 980 MPa or more, excellent bulging formability and stretch flange formability, and excellent coatability. The high-strength hot-rolled coated steel sheet includes a steel sheet having a chemical composition containing, by mass %, C: 0.03% to 0.09%, Si: 0.01% to 1.60%, Mn: 2.20% to 3.60%, P: 0.100% or less, S: 0.0100% or less, Ti: 0.05% to 0.18%, B: 0.0005% to 0.0050%, Al: 0.005% to 0.40%, N: 0.010% or less, and a balance of Fe and inevitable impurities, in which a CSM value expressed by the equation 33.8[% C][% Mn]+12.4[% Si]/[% Mn] is 3.3 to 12.0, and a steel microstructure containing bainite in an amount of 85% or more in terms of area fraction and martensite in an amount of 2.0% or more and 15.0% or less in terms of area fraction and includes a coating layer or an alloyed coating layer on a surface of the steel sheet.

A rolled steel sheet, for press hardening is provided, having a chemical composition where Ti/N>3.42, and the carbon, manganese, chromium and silicon contents satisfy:

[00001]$2.6C+\frac{\mathrm{Mn}}{5.3}+\frac{\mathrm{Cr}}{13}+\frac{\mathrm{Si}}{15}\ge 1.1\%.$

The sheet has a nickel content Ni.sub.surf at any point of the steel in the vicinity of the surface over a depth Δ, such that: Ni.sub.surf>Ni.sub.nom, Ni.sub.nom denoting the nominal nickel content of the steel, and such that, Ni.sub.max denoting the maximum nickel content within Δ:

[00002]$\frac{\left({\mathrm{Ni}}_{\max }+{\mathrm{Ni}}_{\mathrm{nom}}\right)}{2}\times \left(\Delta \right)\ge 0.6,$

and such that:

[00003]$\frac{\left({\mathrm{Ni}}_{\max }-{\mathrm{Ni}}_{\mathrm{nom}}\right)}{\Delta }\ge 0.01$

and the surface density of all of the particles D.sub.i , and the surface density of the particles D.sub.(>2 μm) larger than 2 micrometers satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said sheet:

D.sub.i+6.75 D.sub.(>2 μm)<270

D.sub.i and D.sub.(>2 μm) being expressed as number of particles per square millimeter, and said particles denoting all the oxides, sulfides, and nitrides, either pure or combined such as oxysulfides and carbonitrides, present in the steel matrix.

A flat steel product is provided, including a steel substrate made of a steel which consists of, in % by weight, 0.07-0.4% C, 1.0-2.5% Mn, 0.06-0.9% Si, ≤0.03% P, ≤0.01% S, ≤0.1% Al, ≤0.15% Ti, ≤0.6% Nb, ≤0.005% B, ≤0.5% Cr, 50.5% Mo, where the sum of Cr and Mo is ≤0.5%, and, as the remainder, Fe and unavoidable impurities, and including an anti-corrosion coating which is formed from, in % by weight, ≤15% Si, ≤5% Fe, ≤5% of at least one alkaline earth or transition metal and, as the remainder, Al and unavoidable impurities. If the anti-corrosion coating contains ≤0.1% by weight of the alkaline earth or transition metal, a solution containing at least one alkaline earth or transition metal is applied to the anti-corrosion coating of the flat steel product. Then, the flat steel product is flexibly cold-rolled to produce the portions of different thicknesses. Then, it is heated to 800-1000° C. in an atmosphere with >15% by volume O.sub.2 until an amount of thermal energy Js of >44,000 kJs and ≤400,000 kJs has been introduced.

A high strength steel sheet comprising a center part of sheet thickness and a surface soft part formed at one side or two sides of the center part of sheet thickness, having metal structures of the center part of sheet thickness comprising, by area ratio, tempered martensite: 85% or more, one or more of ferrite, bainite, pearlite, and retained austenite: total of less than 15%, and as-quenched martensite: less than 5%, and additionally having metal structures of the surface soft part comprising, by area ratio, ferrite: 65% or more, pearlite: 5% or more and less than 20%, one or more of tempered martensite, bainite, and retained austenite: total of less than 10%, and as-quenched martensite: less than 5%, wherein a Vickers hardness (Hc) of the center part of sheet thickness and a Vickers hardness (Hs) of the surface soft part satisfy 0.50≤Hs/Hc≤0.75.