A method for producing a high-strength steel sheet having high ductility, formability and weldability includes providing a cold-rolled sheet, with a composition containing: 0.15% ≤C≤0.23%, 1.4% ≤Mn≤2.6%, 0.6% ≤Si≤1.3%, with C+Si/10≤0.30%, 0.4% ≤Al≤1.0%, with Al≥6(C+Mn/10)−2.5%, 0.010% ≤Nb≤0.035%, 0.1% ≤Mo≤0.5%, annealing the sheet at 860° C.-900° C. to obtain a structure consisting of at least 90% austenite and at least 2% intercritical ferrite, quenching to a temperature between Ms-10° C. and Ms-60° C. at a rate Vc higher than 30° C./s, heating to a temperature PT between 410° C. and 470° C. for 60 s to 130 s, hot-dip coating the sheet, and cooling to room temperature. The microstructure includes 45% to 68% of martensite, consisting of 85% to 95% partitioned martensite having a C content of at most 0.45%, and fresh martensite; 10% to 15% retained austenite; 2% to 10% intercritical ferrite; 20% to 30% lower bainite.

A ferritic stainless steel sheet comprises a chemical composition containing, in mass %, C: 0.030% or less, Si: 3.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.010% or less, Cr: 11.0% to 30.0%, Al: 8.0% to 20.0%, Ni: 0.05% to 0.50%, N: 0.020% or less, and at least one selected from the group consisting of Zr: 0.01% to 0.20% and Hf: 0.01% to 0.20%, with a balance consisting of Fe and inevitable impurities.

A galvannealed steel sheet according to one aspect of the present invention has a hot-dip galvannealed layer on at least one surface of the steel sheet, and the steel sheet has a predetermined chemical composition, in which the steel sheet contains 10% or more and 90% or less of ferrite, and 10% or more of tempered martensite and tempered bainite in terms of an area ratio, a sum of the ferrite, the tempered martensite, and the tempered bainite is 90% or more, carbides having a major axis of 50 nm or more and 300 nm or less are contained in grains of the ferrite in a number density of 20/μm.sup.2 or more, and a two-dimensional homogeneous dispersion ratio S of Mn is 0.75 or more and 1.30 or less.

The present invention provides: a plated steel sheet for hot press forming having excellent impact properties after hot press forming; a hot press formed member manufactured using the plated steel sheet for hot press forming; and manufacturing methods thereof. The plated steel sheet comprises: a base steel sheet containing, by weight, 0.15-0.4% of C, 0.1-1% of Si, 0.6-8% of Mn, 0.001-0.05% of P, 0.0001-0.02% of S, 0.01-0.1% of Al, 0.001-0.02% of N, and 0.01-0.5% of Cr, with the remainder comprising Fe and miscellaneous impurities; and a plating layer formed on the surface of the base steel sheet and composed of zinc, aluminum, or an alloy containing zinc and aluminum, wherein the ratio (C S/C B) of the content (C S) of C in a surface layer to the content (C B) of C in the base steel sheet is 0.6 or less, and the ratio ((Mn S+Cr S)/(Mn B+Cr B)) of the total content (Mn S+Cr S) of Mn and Cr in the surface layer to the total content (Mn B+Cr B) of Mn and Cr in the base steel sheet is 0.8 or more.

This steel sheet has a predetermined chemical composition and the steel sheet in which, at a ¼ depth position of a sheet thickness from a surface, an average grain size is 15.0 μm or less, a total grain boundary number density of solute C and solute B is 1.0 solute/nm.sup.2 or more and 12.0 solutes/nm.sup.2 or less, a total of pole densities of {211}<011> and {332}<113> in a thickness middle portion is 12.0 or less, and a tensile strength is 780 MPa or more is adopted.

A cold rolled and heat-treated steel sheet having a composition including, by weight percent: C 0.3-0.4%, Mn 2.0-2.6%, Si: 0.8-1.6%, Al 0.01-0.6%, Mo 0.15-0.5%, Cr 0.3-1.0%, Nb≤0.06%, Ti≤0.06%, Ni≤0.8%, S≤0.010%, P≤0.020% and N≤0.008%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction: between 15% and 30% of retained austenite, said retained austenite having a carbon content of at least 0.7%, between 70% and 85% of tempered martensite, at most 5% of fresh martensite and at most 5% of bainite. It also deals with a manufacturing method thereof.

A method for producing a high-strength hot-dip galvannealed steel sheet, in which a high-strength steel sheet is used as a base material, includes a rolling step (x) of rolling a hot-dip galvannealed steel sheet with a coating layer having an Fe concentration of 8% to 17% by mass, and a heat treatment step (y) of heating the coated steel sheet which has been subjected to the rolling step (x) under the conditions satisfying the following formulae (1) and (2):

(273+T)×(20+2×log.sub.10(t))≥8000  (1)

40≤T≤160  (2) where T: heating temperature (° C.) of the coated steel sheet, and t: holding time (hr) at the heating temperature T.

A hot dip galvanized steel sheet and hot dip galvannealed steel sheet improved in uniform ductility and local ductility, yield strength and tensile strength, and low temperature impact property, characterized by having a predetermined chemical composition, having a metal structure containing, by volume %, retained austenite: over 5.0% and tempered martensite: over 5.0%, having retained austenite containing C: 0.85 mass % or more, and having a ratio [C].sub.γgb/[P].sub.γgb of an amount of segregation of C (number of atoms/nm.sup.2): [C].sub.γgb to an amount of segregation of P (number of atoms/nm.sup.2): [P].sub.γgb at prior austenite grain boundaries of 4.0 or more.

The present invention has as its object the provision of a coated steel member and coated steel sheet excellent in hydrogen embrittlement resistance in a corrosive environment and methods for manufacturing the same. The coated steel member of the present invention is provided on its surface with an Al-Fe-based coating containing Cu and one or more of Mo, Ni, Mn, and Cr in a total by mass % of 0.12% or more by heating, cooling, and manufacturing a coated steel sheet having a layer containing Cu on its surface under predetermined conditions.

The present invention relates to steel used for members of chassis parts and wheel discs of commercial vehicles, or the like and, more specifically, to a high-strength steel with excellent durability and a method for manufacturing same.