Abstract: A high-strength cold-rolled steel sheet has a specific chemical composition and has a steel microstructure meeting conditions: a total content of bainitic ferrite (BF) and tempered martensite (TM) is 65% (in area percent, hereinafter the same for steel microstructure) or more; a fresh martensite (M) content is 3% to 18%; a retained austenite content is 5% or more; and a polygonal ferrite (F) content is 5% or less. The steel sheet has a specific average KAM<1.00° of 0.50° or more and has a tensile strength of 980 MPa or more. The high-strength cold-rolled steel sheet excels in formability and shape fixability.

Abstract: Provided is an ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 2 area% or more of a region having a structure that includes 90% or more of martensite and 0.5% or more of residual austentite by area ratio relative to the entire structure, the local Mn concentration in said region being at least 1.1 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more.

Abstract: Provided is an ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 2 area % or more of a region having a structure that includes 90% or more of martensite and 0.5% or more of residual austentite by area ratio relative to the entire structure, the local Mn concentration in said region being at least 1.1 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more.

Abstract: An ultra-high-strength steel sheet having a component composition that includes specific amounts of each of C, Si, Mn, and Al and a remainder of iron and unavoidable impurities, and in which the amounts of each of P, S, and N among the unavoidable impurities are limited to a specific amount. The ultra-high-strength steel sheet includes 1 area % or more of a region in which martensite constitutes 90 area % or more, residual austentite constitutes 0.5 area % or more, and the local Mn concentration is at least 1.2 times that of the Mn content of the entire steel sheet. The ultra-high-strength steel sheet has a tensile strength of 1470 MPa or more, a yield ratio of 0.75 or more, and a total elongation of 10% or more.

Abstract: Disclosed is a high-strength steel sheet having a tensile strength of 1180 MPa or more and having satisfactory seam weldability. The steel sheet has a chemical composition of C: 0.12% to 0.40%, Si: 0.003% to 0.5%, Mn 0.01% to 1.5%, Al: 0.032% to 0.15%, N: 0.01% or less, P: 0.02% or less, S: 0.01% or less, Ti: 0.01% to 0.2% or less, and B: 0.0001% to 0.01%, with the remainder including iron and inevitable impurities, has a Ceq1 (=C+Mn/5+Si/13) of 0.50% or less, and has a steel structure of a martensite single-phase structure.

Abstract: The invention relates to a steel sheet having a tensile strength of 1180 MPa or more, which excels in workability and cold brittleness resistance. The high-strength steel sheet contains 0.10% to 0.30% of C, 1.40% to 3.0% of Si, 0.5% to 3.0% of Mn, 0.1% or less of P, 0.05% or less of S, 0.005% to 0.20% of Al, 0.01% or less of N, 0.01% or less of O, as well as Fe and inevitable impurities. The steel sheet has: (i) a ferrite volume fraction of 5% to 35% and a bainitic ferrite and/or tempered martensite volume fraction of 60% or more; (ii) a MA constituent volume fraction of 6% or less (excluding 0%); and (iii) a retained austenite volume fraction of 5% or more.

Abstract: Disclosed is an ultra high strength steel plate with at least 1100 MPa of tensile strength that has both an excellent strength-stretch balance and excellent bending workability, and a method for producing the same. The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite. The area of the aforementioned martensite constitutes 50% or more, the area of the aforementioned bainitic ferrite constitutes 15% or more, and the area of the aforementioned polygonal ferrite constitutes 5% or less (including 0%). When the circle-equivalent diameter of the aforementioned soft phases is measured, the coefficient of variation (standard deviation/mean value) is less or equal to 1.0. The ultra high strength steel plate has at least 1100 MPa of tensile strength.

Abstract: A high-strength steel sheet contains, by mass %, C: 0.12% to 0.40%, Si: 0% to 0.6%, Mn: more than 0% to 1.5%, Al: more than 0% to 0.15%, N: more than 0% to 0.01%, P: more than 0% to 0.02%, S: more than 0% to 0.01%, and has a martensite single-phase structure, wherein a region having a KAM value (Kernel Average Misorientation value) of 1° or more occupies 50% or more, and a maximum residual tensile stress in a surface layer region from a surface to a position at a depth of ¼ the sheet thickness is 80 MPa or less. As a result, the high-strength steel sheet excels in the resistance to delayed fracture of the cut end surface and the steel sheet base material.

Abstract: Provided is a high-strength steel sheet having a tensile strength of 1180 MPa or more. The high-strength steel sheet has a specific chemical composition, includes a microstructure meeting all conditions (1) to (4) as measured by specific methods at a position corresponding to one-fourth the thickness of the steel sheet, has excellent workability and low-temperature toughness, and has a tensile strength of 1180 MPa or more. (1) The microstructure includes ferrite in a volume fraction of 5% to 35% and at least one of bainitic ferrite and tempered martensite in a volume fraction of 50% or more relative to the entire microstructure. (2) The microstructure includes MA constituent in a volume fraction of 20% or less (excluding 0%) relative to the entire microstructure, where the MA constituent is a mixed phase of fresh martensite and retained austenite. (3) The microstructure includes retained austenite in a volume fraction of 5% or more relative to the entire microstructure.

Abstract: Provided is a high-strength steel sheet having a tensile strength of 1180 MPa or more. The high-strength steel sheet has a specific chemical composition, includes a microstructure meeting all conditions (1) to (4) as measured by specific methods at a position corresponding to one-fourth the thickness of the steel sheet, has excellent workability and low-temperature toughness, and has a tensile strength of 1180 MPa or more. (1) The microstructure includes ferrite in a volume fraction of 5% to 35% and at least one of bainitic ferrite and tempered martensite in a volume fraction of 50% or more relative to the entire microstructure. (2) The microstructure includes MA constituent in a volume fraction of 20% or less (excluding 0%) relative to the entire microstructure, where the MA constituent is a mixed phase of fresh martensite and retained austenite. (3) The microstructure includes retained austenite in a volume fraction of 5% or more relative to the entire microstructure.

Abstract: A high-strength cold-rolled steel sheet has a specific chemical composition and has a steel microstructure meeting conditions: a total content of bainitic ferrite (BF) and tempered martensite (TM) is 65% (in area percent, hereinafter the same for steel microstructure) or more; a fresh martensite (M) content is 3% to 18%; a retained austenite content is 5% or more; and a polygonal ferrite (F) content is 5% or less. The steel sheet has a specific average KAM<1.00° of 0.50° or more and has a tensile strength of 980 MPa or more. The high-strength cold-rolled steel sheet excels in formability and shape fixability.

Abstract: Disclosed is a high-strength steel plate with excellent warm workability that has a component composition comprising, in mass %, 0.05 to 0.4% C, 0.5 to 3% Si+Al, 0.5 to 3% Mn, no more than 0.15% P (not including 0%), and no more than 0.02% S (including 0%), with the remainder comprising iron and impurities, and a composition that includes a total of 45 to 80% martensite and/or bainitic ferrite in terms of the area ratio relative to the entire composition, 5 to 40% polygonal ferrite in terms of the area ratio relative to the entire composition, and 5 to 20% retained austenite in terms of the area ratio relative to the entire composition, wherein the C concentration (C?R) within said residual austenite is in the range of 0.6 mass % to less than 1.0 mass %, and that furthermore may include bainite. In the high-strength steel plate, TRIP effects are achieved to the fullest extent in warm working, and increased ductility over prior steel plates is reliably achieved.

Abstract: Disclosed is a cold-rolled steel sheet having a specific steel composition and having a composite steel structure including a ferrite structure and a martensite-containing second phase. In a surface region of the steel sheet from the surface to a depth one-tenth the gage, the number density of n-ary groups of inclusions determined by specific n-th determinations is 120 or less per 100 cm2 of a rolling plane, in which the distance in steel sheet rolling direction between outermost surfaces of two outermost particles of the group of inclusions is 80 ?m or more. Also disclosed is a cold-rolled steel sheet having a specific steel composition and having a steel structure of a martensite single-phase structure. In the surface region, the number density of groups of inclusions, in which the distance between the outermost surfaces is 100 ?m or more, is 120 or less per 100 cm2 of a rolling plane.

Abstract: A cold-rolled steel sheet has a chemical composition of C: 0.12% to 0.3%, Si: 0.5% or less, Mn: less than 1.5%, Al: 0.15% or less, N: 0.01% or less, P: 0.02% or less, and S: 0.01% or less, with the remainder including iron and inevitable impurities and has a martensite single-phase structure as its steel microstructure. In a surface region of the steel sheet from the surface to a depth one-tenth the gauge, the number density of n-ary groups of inclusions determined by specific n-th determinations is 120 or less per 100 cm2 of a rolling plane, where the distance in steel sheet rolling direction between outermost surfaces of two outermost particles of the group of inclusions is 100 ?m or more. The steel sheet is a high-strength cold-rolled steel sheet which has a sufficiently minimized rate of bending fracture starting from inclusions and thereby has excellent bending workability.

Abstract: Disclosed is a high-strength steel sheet having a tensile strength of 1180 MPa or more and having satisfactory seam weldability. The steel sheet has a chemical composition of C: 0.12% to 0.40%, Si: 0.003% to 0.5%, Mn 0.01% to 1.5%, Al: 0.032% to 0.15%, N: 0.01% or less, P: 0.02% or less, S: 0.01% or less, Ti: 0.01% to 0.2% or less, and B: 0.0001% to 0.01%, with the remainder including iron and inevitable impurities, has a Ceq1 (=C+Mn/5+Si/13) of 0.50% or less, and has a steel structure of a martensite single-phase structure.

Abstract: Disclosed is a high-strength steel plate with excellent warm workability that has a component composition comprising, in mass %, 0.05 to 0.4% C, 0.5 to 3% Si+Al, 0.5 to 3% Mn, no more than 0.15% P (not including 0%), and no more than 0.02% S (including 0%), with the remainder comprising iron and impurities, and a composition that includes a total of 45 to 80% martensite and/or bainitic ferrite in terms of the area ratio relative to the entire composition, 5 to 40% polygonal ferrite in terms of the area ratio relative to the entire composition, and 5 to 20% retained austenite in terms of the area ratio relative to the entire composition, wherein the C concentration (C?R) within said residual austenite is in the range of 0.6 mass % to less than 1.0 mass %, and that furthermore may include bainite. In the high-strength steel plate, TRIP effects are achieved to the fullest extent in warm working, and increased ductility over prior steel plates is reliably achieved.

Abstract: Disclosed is an ultra high strength steel plate with at least 1100 MPa of tensile strength that has both an excellent strength-stretch balance and excellent bending workability, and a method for producing the same. The metal structure of the steel plate has martensite, and the soft phases of bainitic ferrite and polygonal ferrite. The area of the aforementioned martensite constitutes 50% or more, the area of the aforementioned bainitic ferrite constitutes 15% or more, and the area of the aforementioned polygonal ferrite constitutes 5% or less (including 0%). When the circle-equivalent diameter of the aforementioned soft phases is measured, the coefficient of variation (standard deviation/mean value) is less or equal to 1.0. The ultra high strength steel plate has at least 1100 MPa of tensile strength.

Abstract: The invention relates to a steel sheet having a tensile strength of 1180 MPa or more, which excels in workability and cold brittleness resistance. The high-strength steel sheet contains 0.10% to 0.30% of C, 1.40% to 3.0% of Si, 0.5% to 3.0% of Mn, 0.1% or less of P, 0.05% or less of S, 0.005% to 0.20% of Al, 0.01% or less of N, 0.01% or less of O, as well as Fe and inevitable impurities. The steel sheet has: (i) a ferrite volume fraction of 5% to 35% and a bainitic ferrite and/or tempered martensite volume fraction of 60% or more; (ii) a MA constituent volume fraction of 6% or less (excluding 0%); and (iii) a retained austenite volume fraction of 5% or more.

Abstract: Disclosed is a high-strength electric resistance welded steel pipe which is manufacture from a steel sheet containing, on mass basis, 0.2% to 0.4% C, 0.05% to 0.5% Si, 0.5% to 2.5% Mn, 0.025% or less P, 0.01% or less S, 0.01% to 0.15% or Al, 0.01% to 2% Cu, 0.05% to 2% Cr, 0.005% to 0.2% Ti, and 0.0002% to 0.005% B, with the remainder being iron and inevitable impurities. The steel pipe has a tensile strength of 1750 N/mm2 or more, a 0.1%-proof stress of 1320 N/mm2 or more, and a Charpy impact value at a testing temperature of minus 40° C. of 50 J/cm2 or more.

Abstract: The ultrahigh-strength steel sheet contains, in mass %, C: 0.05-0.25%, Si: 1.00-2.5%, Mn: 2.0-4.0%, P: 0.1% or below (not inclusive of 0%), S: 0.05% or below (not inclusive of 0%), Al: 0.01-0.15%, Ti: 0.003-0.10%, N: 0.01% or below (not inclusive of 0%), the balance comprising iron with inevitable impurities, and is a composite structure steel sheet comprising 10-50 area % ferrite and 50 area % or above martensite, in which the average circle-equivalent grain diameter of ferrite grains is 2.0 ?m or below, the average aspect ratio of ferrite grains is 2.0 or below, and the tensile strength is 1,100 MPa or above. Accordingly, the ultrahigh-strength steel sheet is excellent in hydrogen embrittlement resistance and workability.