The high-strength hot-dip galvanized steel sheet, which includes a hot-dip galvanized coating layer on a surface of the steel sheet, has a component composition containing, in mass %, C: 0.07% to 0.20%, Si: 0.1% to 2.0%, Mn: 2.0% to 3.5%, P: 0.05% or less, S: 0.05% or less, and sol. Al: 0.005% to 0.1%, with the balance being Fe and incidental impurities; and has a steel microstructure containing, in area fraction, 60% or less of ferrite, 40% or more of tempered martensite, and 10% or less of fresh martensite and having a void number density of 1,500/mm.sup.2 or less in a bent portion in the VDA bending test.

A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 92% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [%Ti]+[%Nb]>0.007 and [%Ti]×[%Nb].sup.2≤7.5×10.sup.−6 are satisfied in the chemical composition.

The high-strength hot-dip galvanized steel sheet has a specific chemical composition and the following steel microstructure. In a region extending from 300 to 400 μm from a surface layer of the steel sheet in a thickness direction of the steel sheet, the total area fraction of martensite and bainite containing carbides is 60 to 100%, and the average grain size of prior austenite is 15 μm or less. In the region extending from 300 to 400 μm from the surface layer of the steel sheet in the thickness direction of the steel sheet, the ratio of the height of a peak of P in an Auger electron spectrum at a position 5 nm or more from a prior-austenite grain boundary to the height of a peak of P in an Auger electron spectrum at the prior-austenite grain boundary is 0.20 or more.

A method for producing a cold rolled steel sheet having a tensile strength≥1470 MPa and a total elongation TE≥19%, the method comprising the steps of annealing at an annealing temperature AT≥Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%<Cr≤0.5%, 0%<Mo≤0.3%, 0.01%≤Al≤0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT<Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 120 seconds.

A method for producing a cold rolled steel sheet having a tensile strength ≥1470 MPa and a total elongation TE≥19%, the method comprising the steps of annealing at an annealing temperature AT≥Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0<Cr≤0.5%, 0<Mo≤0.3%, 0.01%≤Al≤0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT<Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 120 seconds.

A flat steel product consisting of (in wt %) 0.1-0.5% C, 1.0-3.0% Mn, 0.9-1.5% Si, ≤1.5% Al, ≤0.008% N, ≤0.020% P, ≤0.005% S, 0.01-1% Cr and optionally one or more of: ≤0.2% Mo, ≤0.01% B, ≤0.5% Cu, ≤0.5% Ni and optionally a total of 0.005-0.2% microalloying elements, the remainder iron and unavoidable impurities. The steel has a structure consisting of ≥80 area % martensite, where ≥75 area % is tempered and ≤25 area % is non-tempered, ≥5 volume % residual austenite, 0.5-10 area % ferrite, ≤5 area % bainite, and carbides with a length of ≤250 nm, wherein in the phase boundary between tempered martensite and residual austenite, there is a low-Mn ferrite seam having a width of 4.Math.12 nm and a Mn content of ≤50% of the average Mn content. Also, a method for producing the flat steel in which the structural characteristics of the flat steel product are set by suitable heat treatment.

A steel sheet includes a hot-dip galvanized layer or a galvannealed layer on a surface of the steel sheet, the steel sheet including: in mass %, C: 0.06% or more and 0.22% or less; Si: 0.50% or more and 2.00% or less; Mn: 1.50% or more and 2.80% or less; Al: 0.02% or more and 1.00% or less; P: 0.001% or more and 0.100% or less; S: 0.0005% or more and 0.0100% or less; N: 0.0005% or more and 0.0100% or less; and a balance: Fe and impurities.

A method IS for producing a cold rolled steel sheet having a tensile strength ≥1470 MPa and a total elongation TE≥19%. The method includes the steps of annealing at an annealing temperature AT≥Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%<Cr≤0.7%, 0%≤Mo≤0.3%, 0.01%≤Al≤0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT<Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 250 seconds.

A method for manufacturing a quenched molding according to the present disclosure is a method including a first heat treatment process of heating a blanked steel material to a temperature higher than its Ac3 transformation point to perform austenite transformation, and then cooling to induce martensite transformation or bainite transformation, and a second heat treatment process of heating the steel material that has undergone the first heat treatment process to a temperature higher than the Ac3 transformation point to perform austenite transformation, and then cooling to induce martensite transformation. After the steel material has been heated to a temperature higher than the Ac3 transformation point in at least one process from out of the first heat treatment process or the second heat treatment process, molding is completed at a temperature higher than an Ar3 transformation point.

Provided is an Al—Fe-alloy plated steel sheet for hot forming, having excellent TWB welding characteristics since excellent hardness uniformity of a TWB weld zone after hot forming is obtained by suitably controlling a batch annealing condition, after plating Al, such that an Al—Fe-alloy layer is formed; a hot forming member; and manufacturing methods therefor.