The high strength steel sheet of the present invention has a specific chemical composition, and contains, in terms of area fraction relative to an entire steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total of pearlite, bainite and retained austenite, a total content of Nb and Ti contained in a precipitate having a particle size of smaller than 20 nm is 25 mass ppm or more and 220 mass ppm or less, and the difference between the maximum value and the minimum value of the total content of Nb and Ti contained in the precipitate having a particle size of smaller than 20 nm, in the longitudinal direction of the steel sheet, is smaller than 20 mass ppm.

The high-strength thin steel sheet has a chemical composition containing C, Si, Mn, P, S, Al, and N, with the balance being Fe and inevitable impurities, and a complex structure containing ferrite, tempered martensite, and bainite, where a volume fraction of a total of tempered martensite and bainite containing five or more carbides with a particle size of 0.1 μm or more and 1.0 μm or less in a grain with respect to a total of the tempered martensite and the bainite is 85% or more, and C mass % and Mn mass % in a region of 20 μm or less in a thickness direction from a surface of the steel sheet are each 20% or less with respect to C mass % and Mn mass % in a region of 100 μm or more and 200 μm or less from the surface of the steel sheet.

A method for producing a molded article includes the steps of: preparing a first steel plate and a second steel plate. The first steel plate and the second steel plate are joined to each other, thereby preparing a joined steel plate. The joined steel plate is heated at a temperature between 910° C. and 950° C. The heated joined steel plate is then subjected to hot-press molding, thereby preparing an intermediate molded article; and cooling the intermediate molded article, wherein the first steel plate has a tensile strength (TS) higher than that of the second steel plate.

A cold rolled martensitic steel sheet including the following elements, expressed in percentage by weight: 0.1%≤C≤0.2%; 1.5%≤Mn≤2.5%; 0.1%≤Si≤0.25%; 0.1%≤Cr≤1%; 0.01%≤Al≤0.1%; 0.001%≤Ti≤0.1%; 0%≤S≤0.09%; 0%≤P≤0.09%; 0%≤N≤0.09%; and can contain one or more of the following optional elements 0%≤Ni≤1%; 0%≤Cu≤1%; 0%≤Mo≤0.4%; 0%≤Nb≤0.1%; 0%≤V≤0.1%; 0%≤B≤0.05%; 0%≤Sn≤0.1%; 0%≤Pb≤0.1%; 0%≤Sb≤0.1%; 0.001%≤Ca≤0.01%; the remainder composition being composed of iron and unavoidable impurities caused by processing, the microstructure of the steel including, by area percentage, at least 95% of martensite, a cumulated amount of ferrite and bainite between 1% and 5%, and an optional amount of residual austenite between 0% and 2%.

A manufacturing method of a steel sheet includes: a step of performing continuous casting of molten steel having a Si content of 0.4 mass % to 3.0 mass % to obtain a slab; a step of performing hot rolling of the slab to obtain a hot-rolled steel sheet; a step of performing cold rolling of the hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of performing cold-rolled sheet annealing of the cold-rolled steel sheet; a step of performing pickling after the cold-rolled sheet annealing; a step of performing water washing after the pickling; and a step of performing drying after the water washing. A dew point is set to −35° C. or lower in the cold-rolled sheet annealing, an electrical conductivity of a rinse water to be used in the water washing is set to 5.0 mS/m or less, a water-washing time is set to 15 seconds or less in the water washing, and the drying is started within 60 seconds from an end of the water washing.

Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing, excellent delayed fracture resistance, and high tensile shear stress after resistance spot welding by properly adjusting its chemical composition and its microstructure such that at least 20 Nb-based precipitates having a grain size of less than 0.10 μm are present on average per 100 μm.sup.2 of a cross section parallel to a thickness direction of the member, a prior austenite average grain size is 8 μm or less, an average aspect ratio of prior austenite grains is 2.5 or less, and a volume fraction of martensite is 90% or more, and such that a standard deviation of Vickers hardness measured every 200 μm on a surface of the member is 40 or less.

A high strength member according to the present invention is the high strength member having a bending ridge line portion formed from a steel sheet, the member having a tensile strength of 1470 MPa or higher, a residual stress of 300 MPa or lower in an end surface of the bending ridge line portion, and a Vickers hardness (HV) of 200 or higher and 450 or lower in the end surface of the bending ridge line portion.

A high-strength hot-dip galvanized steel sheet has a hot-dip galvanized layer on a surface of the steel sheet, a specific component composition, and a steel microstructure containing, on an area percentage basis, 0% to 15% of ferrite and upper bainite in total, 80% to 100% of lower bainite and martensite in total, and 0% to 10% of retained austenite, and containing precipitates having a particle size of 100 to 2,000 nm in an amount of 10.sup.9 to 10.sup.12 particles/m.sup.2 in a region extending to a position 100 to 300 μm from the surface layer of the steel sheet, in which the ratio of the average amount of C at a position 5 μm from the surface layer of the steel sheet to the average amount of C at a position 70 μm from the surface layer of the steel sheet in the thickness direction is 0.20 to 0.80.

Provided is a cold-rolled high-strength steel having a tensile strength of not less than 1500 MPa and an excellent formability, the chemical elements thereof having the following mass percent ratios: 0.25%-0.40% of C, 1.50%-2.50% of Si, 2.0%-3.0% of Mn, 0.03%-0.06% of Al, P≤0.02%, S≤0.01%, N≤0.01% and at least one of 0.1%-1.0% of Cr and 0.1%-0.5% of Mo, with the balance being Fe and other unavoidable impurities. The microstructure of the cold-rolled high-strength steel has 5%-20% of residual austenite and 70%-90% of martensite, and the carbon concentration ratio of the residual austenite to the martensite is greater than 3.5 and less than 15. The cold-rolled high-strength steel sheet has a high strength and an excellent formability through a rational ingredient design and microstructure control.

A cold rolled steel strip or sheet has a composition consisting of (in wt. %): C 0.15-0.25; Si 0.5-1.6; Mn 2.2-3.2; Cr≤0.8; Mo≤0.2; Al 0.03-1.0; Nb≤0.04; V≤0.04; Ti 0.01-0.04; B 0.001-0.010; TUB 5-30; Cu≤0.15; Ni≤0.15; Ca≤0.01; balance Fe apart from impurities, where the cold rolled steel has a multiphase microstructure comprising a matrix mainly composed of tempered martensite and has a tensile strength (R.sub.m) of at least 1380 MPa.