Abstract: A high-strength steel sheet with a tensile strength of 1,180 MPa or more has a predetermined chemical composition and a steel microstructure in which the area fraction of ferrite is 5% or less, the area fraction of martensite is 2% to 10%, the area fraction of bainite is 5% to 37%, the area fraction of tempered martensite is 42% to 65%, the volume fraction of retained austenite is 3% to 15%, the average grain size of ferrite and bainite is 3 ?m or less, in a region extending 50 ?m from a surface of the steel sheet in a through-thickness direction, and the average grain size of prior austenite grains is 10 ?m or less, the average grain size of the prior austenite grains in the through-thickness direction is 0.9 or less of the average grain size thereof in a rolling direction.

Abstract: 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 ?m 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.

Abstract: A high-strength galvanized steel sheet having improved post-work impact resistance, a method for producing the high-strength galvanized steel sheet, and a high-strength member produced using the steel sheet. The high-strength galvanized steel sheet includes a steel sheet having a microstructure including ferrite and carbide-free bainite, martensite and carbide-containing bainite, and retained austenite, the total area fraction of ferrite and carbide-free bainite being 0% to 55%, the total area fraction of martensite and carbide-containing bainite being 45% to 100%, and the area fraction of retained austenite being 0% to 5%. Additionally, a galvanizing layer is disposed on the steel sheet. The density of gaps that cut across the entire thickness of the galvanizing layer in a cross section of the galvanizing layer, which is taken in the thickness direction so as to be perpendicular to the rolling direction, is 10 gaps/mm or more.

Abstract: To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 590 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 35% or more and 80% or less and martensite is 5% or more and 25% or less in area ratio, retained austenite is 8% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 6.0 ?m or less, 3.0 ?m or less and 3.0 ?m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Abstract: To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 980 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 15% or more and 55% or less and martensite is 15% or more and 30% or less in area ratio, retained austenite is 12% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 4.0 ?m or less, 2.0 ?m or less and 2.0 ?m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing of 1780 MPa or more, excellent delayed fracture resistance, and high cross tensile strength after resistance spot welding by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 8 ?m or less, a volume fraction of martensite is 90% or more, and at least 10 cementite grains having a grain size of 0.05 ?m or more are present on average per 200 ?m2 of a cross section parallel to a thickness direction of the member, and such that at least 10 Ti-based precipitates having a grain size of less than 0.10 ?m are present on average per 100 ?m2 of the cross section parallel to the thickness direction of the member in a range of 100 ?m in the thickness direction from a surface of the member.

Abstract: A high-strength steel sheet for warm working having excellent warm workability, and a method for manufacturing the steel sheet. The steel sheet has a chemical composition including, by mass %, C: 0.05% to 0.20%, Si: 3.0% or less, Mn: 3.5% to 8.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 3.0%, N: 0.010% or less, one or more selected from Nb: 0.005% to 0.20%, Ti: 0.005% to 0.20%, Mo: 0.005% to 1.0%, and V: 0.005% to 1.0%. The steel sheet has a microstructure including, in terms of area ratio, 10% to 60% of retained austenite, 10% to 80% of ferrite, 10% to 50% of martensite, and 0% to 5% of bainite, in which a C content in the retained austenite is less than 0.40 mass % and the average crystal grain diameter of the retained austenite, the martensite, and the ferrite is 2.0 ?m or less.

Abstract: 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 ?m are present on average per 100 ?m2 of a cross section parallel to a thickness direction of the member, a prior austenite average grain size is 8 ?m 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 ?m on a surface of the member is 40 or less.

Abstract: An object is to provide a high strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent formability and a method for manufacturing the steel sheet. A high strength steel sheet which is excellent in terms of formability, which is manufactured under optimized manufacturing conditions, and which has a predetermined chemical composition and a steel microstructure including, in terms of area fraction, 35% or more and 80% or less of ferrite, 5% or more and 35% or less of as-quenched martensite, 0.1% or more and less than 3.0% of tempered martensite, and 8% or more of retained austenite, in which the average grain size of the ferrite is 6 ?m or less, in which the average grain size of the retained austenite is 3 ?m or less, in which a value calculated by dividing the average Mn content in the retained austenite by the average Mn content in the ferrite is 1.

Abstract: Disclosed is a hot-pressed steel sheet member having a tensile strength of 1780 MPa or more and excellent bending collapsibility. The hot-pressed steel sheet member includes: a specific chemical composition; a microstructure in which an average grain size of prior austenite grains is 8 ?m or less, a volume fraction of martensite is 90% or more, and a solute C content is 25% or less of a total C content; and a tensile strength of 1780 MPa or more.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, excellent resistance to resistance welding cracking, and excellent delayed fracture resistance after resistance welding by having a specific chemical composition, and a microstructure such that a prior austenite average grain size is 7 ?m or less, a volume fraction of martensite is 90% or more, and at least 5 Nb-based precipitates having a grain size of less than 0.08 ?m are present on average per 100 ?m2 of a cross section parallel to a thickness direction of the member within a range of 100 ?m in the thickness direction from a surface of the member, and such that a Ni diffusion region having a thickness of 0.5 ?m or more is present in a surface layer of the member.

Abstract: Provided are a coated steel sheet and so forth, the coated steel sheet having a tensile strength of 590 MPa or more and good stretch-flangeability. The coated steel sheet includes a specific component composition, in which the area fraction of a ferrite phase is 80% or more and 98% or less, the area fraction of a martensite phase of 2% or more and 15% or less, ferrite grains have an average thickness of 3.0 ?m or less in the sheet-thickness direction, the martensite phase has an average grain size of 2.0 ?m or less, and a Nb-containing carbide precipitated in the ferrite grains has an average grain size of 8 nm or less, and in which the steel sheet has a tensile strength of 590 MPa or more.

Abstract: 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 %.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent indentation peeling strength at projection welds by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7 ?m or less within a range of 50 ?m or less in a thickness direction from a surface of the member, a volume fraction of martensite is 90% or more, and an average intergrain distance of Nb and Ti carbonitrides having a grain size of less than 0.10 ?m within a depth range of 20 ?m to 100 ?m in the thickness direction from the surface of the member is 5 ?m or less.

Abstract: 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 has a composition comprising, by mass %, C: 0.04% or more and 0.15% or less, Si: 0.3% or less, Mn: 1.0% or more and 2.6% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.01% or more and 0.1% or less, N: 0.015% or less, one or two of Ti and Nb: 0.01% or more and 0.2% or less in a total, and the balance being Fe and unavoidable impurities. The steel sheet has 50% or more of ferrite and 10% or more and 50% or less of martensite in terms of an area ratio, and a microstructure in which a standard deviation of nano-hardness is 1.50 GPa or less and tensile strength of 590 MPa or more.

Abstract: A high-strength steel sheet having a chemical composition containing, by mass %, C: 0.15% to 0.25%, Si: 1.00% to 2.20%, Mn: 2.00% to 3.50%, P: 0.05% or less, S: 0.005% or less, Al: 0.01% to 0.50%, N: 0.010% or less, B: 0.0003% to 0.0050%, one, two, or more selected from Ti: 0.005% to 0.05%, Cu: 0.003% to 0.50%, Ni: 0.003% to 0.50%, Sn: 0.003% to 0.50%, Co: 0.003% to 0.05%, and Mo: 0.003% to 0.50%, and the balance being Fe and inevitable impurities and a microstructure including, in terms of volume fraction, 15% or less (including 0%) of ferrite, 2% to 15% of retained austenite, 10% or less (including 0%) of martensite, and the balance being bainite and tempered martensite, in which the average number of cementite grains having a grain diameter of 0.04 ?m or more existing in the bainite and tempered martensite grains is 10 or more.

Abstract: Provided are a high-strength coated steel sheet and a method for manufacturing the same. The high-strength coated steel sheet has a base steel sheet and a coating layer formed on a surface of the base steel sheet. The base steel sheet has a specified chemical composition and a microstructure, including a martensite phase and a ferrite phase. A volume fraction of the martensite phase is 50% to 80%. A volume fraction of tempered martensite with respect to the whole martensite phase is 50% or more and 85% or less. An average grain diameter of the ferrite phase is 13 ?m or less. A volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more. Yield strength (YP) of the high-strength coated steel sheet is 550 MPa or more.

Abstract: A steel sheet, a plated steel sheet, and methods for producing a hot-rolled steel sheet, a cold-rolled full hard steel sheet, and a steel sheet. The steel sheet has a specified composition and a microstructure including 0 to 80% of polygonal ferrite and 20 to 100% of a total of martensite, bainite, and residual austenite in terms of an area ratio within 20 ?m of the steel sheet surface. The content of Mn in martensite present within 20 ?m of the steel sheet surface ([Mn]SM) and the content of Mn in a bulk ([Mn]B) satisfy [Mn]SM/[Mn]B?1.5. At a location 300 ?m from the steel sheet surface, an area ratio of the martensite is in a range of 20 to 50%.

Abstract: A steel sheet, a plated steel sheet, and methods for producing a hot-rolled steel sheet, a cold-rolled full hard steel sheet, and a steel sheet. The steel sheet has a specified composition and a microstructure including 0 to 60% of polygonal ferrite and 40 to 100% of a total of martensite, bainite, and residual austenite in terms of an area ratio within 20 ?m from the steel sheet surface. The content of Mn in martensite present within 20 ?m of the steel sheet surface ([Mn]SM) and the content of Mn in a bulk ([Mn]B) satisfy [Mn]SM/[Mn]B?1.5. At a location 300 ?m from the steel sheet surface, an area ratio of the martensite is in a range of 40 to 80%, and the polygonal ferrite and the bainite have an average crystal grain size of less than 15 ?m.

Abstract: Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent resistance to resistance welding cracking by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7.5 ?m or less, a volume fraction of martensite is 95% or more, and at least 10 Nb-based and Ti-based precipitates having a grain size of less than 0.10 ?m are present on average per 100 ?m2 of a cross section parallel to a thickness direction of the member within a range of 100 ?m or less in the thickness direction from the surface of the member, and such that a B concentration in prior austenite grain boundaries is at least 3.0 times a B concentration at a position 5 nm away from the grain boundaries.