Abstract: A high-strength steel sheet of the present invention is a steel sheet satisfying a predetermined component composition. A metal structure of the steel sheet is composed of polygonal ferrite, high-temperature region generated bainite, low-temperature region generated bainite and retained austenite each having a predetermined area percent, and a distribution using each average IQ of predetermined crystal grains determined by electron backscatter diffraction satisfies Equations (1) and (2) below. According to the present invention, a high-strength steel sheet having excellent ductility and low-temperature toughness can be realized even at a tensile strength of 780 MPa or more. (IQave?IQmin)/(IQmax?IQmin)?0.40??(1) (?IQ)/(IQmax?IQmin)?0.

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: Provided is a high strength cold rolled steel sheet having a tensile strength of 980 MPa or higher and excellent ductility and bendability. The high strength cold rolled steel sheet has a specified component composition. The structures at a position of ¼ sheet thickness in the steel sheet has certain area ratio of ferrite relative to the entire structure, certain area ratio of a mixed structure of fresh martensite and retained austenite relative to the entire structure, certain region in which the concentration of Mn is enriched, certain standard deviation of the fraction of a region in which the concentration of Mn is enriched, and certain concentration of Mn in a ferrite phase.

Abstract: A high-strength steel sheet of the present invention is a steel sheet satisfying a predetermined component composition. A metal structure of the steel sheet is composed of polygonal ferrite, high-temperature region generated bainite, low-temperature region generated bainite and retained austenite each having a predetermined area percent, and a distribution using each average IQ of predetermined crystal grains determined by electron backscatter diffraction satisfies Equations (1) and (2) below. According to the present invention, a high-strength steel sheet having excellent formability and low-temperature toughness can be realized even at a tensile strength of 590 MPa or more. (IQave?IQmin)/(IQmax?IQmin)?0.40??(1) (?IQ)/(IQmax?IQmin)?0.

Abstract: A high-strength steel sheet of the present invention is a steel sheet satisfying a predetermined component composition. A metal structure of the steel sheet is composed of polygonal ferrite, high-temperature region generated bainite, low-temperature region generated bainite and retained austenite each having a predetermined area percent, and a distribution using each average IQ of predetermined crystal grains determined by electron backscatter diffraction satisfies Equations (1) and (2) below. According to the present invention, a high-strength steel sheet having excellent ductility and low-temperature toughness can be realized even at a tensile strength of 780 MPa or more. (IQave?IQmin)/(IQmax?IQmin)?0.40??(1) (?IQ)/(IQmax?IQmin)?0.

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: 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: 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 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.