Abstract: A high-strength steel sheet with excellent delayed fracture resistance, having a tensile strength of 1700 MPa or larger, including a predetermined component composition, having a martensite structure whose ratio accounts for 95 area % or more of the entire metallographic structure, and having a transition metal carbide whose ratio accounts for 0.8 volume % or more of the entire metallographic structure.

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: 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: 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: 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.