A high-strength galvanized steel sheet comprising a cold-rolled steel sheet, an intermetallic compound formed on the cold-rolled steel sheet, and a galvanizing layer formed on the intermetallic compound, the cold-rolled steel sheet having a specific composition and a microstructure having a martensite area ratio of 7% or more and less than 25% and a ferrite area ratio of 50% or more and which includes a base metal surface portion in which the amount of internal oxides per single side is 0.05 g/m.sup.2 or less.

Provided is an austenitic, lightweight, high-strength steel sheet having a high yield ratio and ductility and a method for producing the same, and more particularly, to a high-strength steel sheet for automotive interior panels, exterior panels, and structural parts, and a method for producing the steel sheet. The steel sheet may be a hot-rolled steel sheet, a cold-rolled steel sheet, or a plated steel sheet. The steel sheet includes, by weight %, C: 0.6% to 1.0%, Si: 0.1% to 2.5%, Mn: 10% to 15%, P: 0.02% or less, S: 0.015% or less, Al: 5% to 8%, Ti: 0.01% to 0.20%, N: 0.02% or less, and the balance of Fe and inevitable impurities, wherein the steel sheet has a specific gravity of 7.4 g/cm.sup.3 and a Mn/Al ratio of 2 to 3.

The high strength steel sheet has a chemical composition including 0.08% to 0.20% of C, 0.3% or less of Si, 0.1% to 3.0% of Mn, 0.10% or less of P, 0.030% or less of S, 0.10% or less of Al, 0.010% or less of N, 0.20% to 0.80% of V, and the remainder composed of Fe and incidental impurities on a percent by mass basis, and a microstructure which includes 95% or more of ferrite phase on an area percentage basis, in which fine precipitates are dispersed having a distribution in such a way that the number density of precipitates having a particle size of less than 10 nm is 1.0×10.sup.5/μm.sup.3 or more and the standard deviation of natural logarithm values of precipitate particle sizes with respect to precipitates having a particle size of less than 10 nm is 1.5 or less.

In a hot stamped steel, when [C] represents an amount of C (mass %), [Si] represents an amount of Si (mass %), and [Mn] represents an amount of Mn (mass %), an expression of 5×[Si]+[Mn])/[C]>10 is satisfied, a metallographic structure includes 80% or more of a martensite in an area fraction, and optionally, further includes one or more of 10% or less of a pearlite in an area fraction, 5% or less of a retained austenite in a volume ratio, 20% or less of a ferrite in an area fraction, and less than 20% of a bainite in an area fraction, TS×λ, which is a product of TS that is a tensile strength and λ that is a hole expansion ratio is 50000 MPa.Math.% or more, and a hardness of the martensite measured with a nanoindenter satisfies H2/H1<1.10 and σHM<20.

A galvannealed steel sheet includes: a steel sheet; a coating layer on a surface of the steel sheet; and a mixed layer formed between the steel sheet and the coating layer, in which the mixed layer includes a base iron portion having fine grains having a size of greater than 0 μm and equal to or smaller than 2 μm, a Zn—Fe alloy phase, and oxides containing one or more types of Mn, Si, Al, and Cr, and in the mixed layer, the oxides and the Zn—Fe alloy phase are present in grain boundaries that form the fine grains and the Zn—Fe alloy phase is tangled with the base iron portion.
[Mn]+[Si]+[Al]+[Cr]≧0.4  (Expression 1)

A steel sheet having excellent fatigue resistance as a material for automobile parts and a TS of 590 MPa or more, and a method for producing the same. The steel sheet having a specified chemical composition and a microstructure of 50% or more and 90% or less of martensite, 10% to 50% of ferrite and bainite in total, in terms of an area ratio, and an average concentration of solute Mn in ferrite in a region from a surface of a base steel to a depth of 0.5 μm is 60% or more relative to an average concentration of solute Mn in ferrite at a location of ¼ in the thickness of the steel sheet.

High strength galvannealed steel sheet having, a) a composition consisting of (in wt. %): C 0.10-0.2, Mn 2.0-3.0, Si 0.2-0.5, Cr 0.1-0.7, Ti 0.01-0.07, Al≤0.2, Nb<0.05, Mo<0.1, optionally B 0.001-0.005, balance Fe apart from impurities, b) a multiphase microstructure comprising (in vol. %) retained austenite 4-20, martensite 5-25, bainitic ferrite ≤10, polygonal ferrite ≤10, balance bainite+tempered martensite 50-90, c) a tensile strength (Rm) 1180-1300 MPa, a yield strength (R.sub.p0.2) 800-970 MPa, an elongation (A50) ≥8%, or an elongation (A80) ≥6%, and d) a bendability value Ri/t of ≤4 for a sample having the size of 35 mm×100 mm, wherein Ri is the bending radius in mm and t is the thickness in mm of the steel sheet.

A coated steel sheet including a steel sheet and a coating layer provided on at least part of the surface of the steel sheet, in which the coating layer has a predetermined chemical composition in terms of % by mass, and the coating layer has a granular Mg.sub.2Sn phase-containing structure in an area fraction of from 5 to 65%, and a structure containing a solid solution of Zn and Al, and the granular Mg.sub.2Sn phase-containing structure is a structure constituted with a Zn phase and a granular Mg.sub.2Sn phase having a crystal grain size of less than 1 μm dispersed in the Zn phase.

A steel sheet includes, by mass %: C: 0.020% to 0.080%; Si: 0.01% to 0.10%; Mn: 0.80% to 1.80%; and Al: more than 0.10% and less than 0.40%; and further includes: Nb: 0.005% to 0.095%; and Ti: 0.005% to 0.095%, in which a total amount of Nb and Ti is 0.030% to 0.100%, and the steel sheet includes, as a metallographic structure, ferrite, bainite, and other phases, an area fraction of the ferrite is 80% to 95%, an area fraction of the bainite is 5% to 20%, a total fraction of the other phases is less than 3%, a tensile strength is 590 MPa or more, and a fatigue strength ratio as a fatigue strength to the tensile strength is 0.45 or more.

A warm-workable high-strength steel sheet having superior warm workability and residual ductility after warm working, and a method for manufacturing such steel sheets. The warm-workable high-strength steel sheet has a chemical composition including, in mass %, C: 0.05 to 0.20%, Si: not more than 3.0%, Mn: 3.5 to 8.0%, P: not more than 0.100%, S: not more than 0.02%, Al: 0.01 to 3.0% and N: not more than 0.010%, the balance being Fe and inevitable impurities. The steel sheet has a microstructure that includes, in area fractions, 10 to 60% retained austenite, 10 to 80% ferrite, 5 to 50% martensite and 0 to 5% bainite, the C content in the retained austenite being less than 0.40 mass %.