Abstract: A high-strength member having excellent delayed fracture resistance, a method for manufacturing the high-strength member, and a method for manufacturing a steel sheet for the high-strength member. The high-strength member has a bent ridge portion obtained by using a steel sheet having a tensile strength of 1470 MPa or more, an edge surface of the bent ridge portion has a residual stress of 800 MPa or less, and a longest crack among cracks that extend from the edge surface of the bent ridge portion in a bent ridge direction D1 has a length of 10 ?m or less.

Abstract: A steel sheet; a related member; and methods for manufacturing them are disclosed. The steel sheet has a chemical composition including specific amounts of C, Si, Mn, P, S, sol. Al, and N in mass %. The steel sheet has a specific ratio of the total of polygonal ferrite, upper bainite, retained ?, fresh martensite, tempered martensite, and lower bainite, and a specific ratio of a remaining microstructure. The steel sheet has a specific ratio of the number of fresh martensite grains and retained ? grains having an equivalent circular diameter of less than 1.2 ?m, and has a specific ratio of the number of fresh martensite grains and retained ? grains having an aspect ratio of 2.5 or more and an equivalent circular diameter of 1.2 ?m or more.

Abstract: A steel sheet; a related member; and methods for manufacturing them are disclosed. The steel sheet has a chemical composition including specific amounts of C, Si, Mn, P, S, sol. Al, and N in mass %. The steel sheet has a specific ratio of the total of polygonal ferrite, upper bainite, retained ?, fresh martensite, tempered martensite, and lower bainite, and a specific ratio of a remaining microstructure. The steel sheet has a specific ratio of the number of fresh martensite grains and retained ? grains having an equivalent circular diameter of less than 0.8 ?m, and has a specific ratio of the number of fresh martensite grains and retained ? grains having an aspect ratio of 2.0 or more and an equivalent circular diameter of 0.8 ?m or more.

Abstract: The steel sheet of the present invention has a steel microstructure containing, in area fraction, martensite: 20% to 100%, ferrite: 0% to 80%, and another metal phase: 5% or less, in which, on a surface of the steel sheet, a ratio of dislocation density in metal phases at a widthwise edge of the steel sheet to dislocation density in the metal phases at a widthwise center of the steel sheet is 100% to 140%, and, at a thicknesswise center of the steel sheet, a ratio of dislocation density in the metal phases at the widthwise edge of the steel sheet to dislocation density in the metal phases at the widthwise center of the steel sheet is 100% to 140%. The maximum amount of warpage of the steel sheet when the steel sheet is sheared to a length of 1 m in a rolling direction is 15 mm or less.

Abstract: Provided are a steel sheet; a related member; and methods for manufacturing the same. The steel sheet has a chemical composition including, in mass %, C: 0.06 to 0.25%, Si: 0.4 to 2.5%, Mn: 1.5 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol. Al: less than 1.0%, and N: less than 0.015%, the balance being Fe and incidental impurities, the steel sheet being such that the steel sheet includes a steel microstructure including, in area fraction, polygonal ferrite: 10% or less (including 0%), tempered martensite: 30% or more, fresh martensite: 20% or less (including 0%), lower bainite: 5 to 50%, and, in volume fraction, retained austenite: 5 to 20%, and the steel sheet has SC?0.5/SC?0.3×100 of 15% or more.

Abstract: A steel sheet contains, in mass %, C: 0.15 to 0.45%, Si: 1.5% or less, Mn: 1.7% or less, P: 0.03% or less, S: less than 0.0020%, sol. Al: 0.20% or less, N: 0.005% or less, B: 0.0015 to 0.0100%, and at least one of Nb and Ti in a total amount of 0.005 to 0.080%, with the balance being Fe and incidental impurities. The area fraction of martensite with respect to the total area of the microstructure is 95 to 100%, and the diameter of prior ? grains is less than 11.0 ?m. The number density A of precipitates having an equivalent circular diameter of 500 nm or more satisfies the formula: A (particles/mm2)?8.5×105×[B].

Abstract: A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ?: 95% to 100% by area percentage, and retained ?: 4% to 15% by volume percentage. Retained ?UB has a specified area percentage S?UB, retained ?LB has a specified distribution number N?LB, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained ? grains has a specified equivalent circular grain diameter and aspect ratio.

Abstract: A steel sheet contains, in mass %, C: 0.15 to 0.45%, Si: 1.5% or less, Mn: more than 1.7%, P: 0.03% or less, S: less than 0.0040%, sol .Al: 0.20% or less, N: 0.005% or less, B: 0.0015 to 0.0100%, and at least one of Nb and Ti in a total amount of 0.005 to 0.080%, with the balance being Fe and incidental impurities. The area fraction of martensite with respect to the total area of the microstructure is 95 to 100%, and the diameter of prior y grains is less than 11.2 ?m. The number density A of precipitates having an equivalent circular diameter of 500 nm or more satisfies the formula: A (particles/mm2)?8.5×105×[B].

Abstract: A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ?: 95% to 100%, and retained ?: 5% to 20%. Retained ?UB has a specified area percentage S?UB, retained ?LB has a specified distribution number N?LB, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained ? grains has a specified equivalent circular grain diameter and aspect ratio.

Abstract: A high-ductility, high-strength electrolytic zinc-based coated steel sheet includes an electrolytic zinc-based coating on a base steel sheet, in which the base steel sheet has a predetermined composition and a steel microstructure in which the total area percentage of one or two of martensite containing a carbide having an average particle size of 50 nm or less and bainite containing a carbide having an average particle size of 50 nm or less is 90% or more in the entire steel microstructure, the total area percentage of one or two of the martensite containing a carbide having an average particle size of 50 nm or less and the bainite containing a carbide having an average particle size of 50 nm or less is 80% or more in a region extending from the surface of the base steel sheet to a depth of ? of the thickness of the base steel sheet.

Abstract: A steel sheet has a certain chemical composition and a steel microstructure containing, in area fraction, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and retained austenite: 2% or more. Carbides in tempered martensite in the martensite have an average particle size of 0.40 ?m or less, an average amount of C in the retained austenite is 0.5% by mass or more, a Si concentration in a region within 100 ?m in a thickness direction from a surface of the steel sheet is 1.3% by mass or less, and a tensile strength is 1,470 MPa or more.

Abstract: A steel sheet has a chemical composition containing, by mass %, C: 0.20% or more and 0.40% or less, Si: more than 1.0% and 3.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.40% or less (not including 0%), and N: 0.0100% or less, with the balance being Fe and incidental impurities; and a steel microstructure containing, in area fraction, tempered martensite: 45% or more and 83% or less, bainite: 15% or more and 53% or less, and retained austenite: 2% or more. Carbides in the tempered martensite have an average particle size of 0.40 ?m or less, an average amount of C in the retained austenite is 0.5% by mass or more, and a tensile strength is 1,470 MPa or more.

Abstract: To provide a steel sheet with a tensile strength of 1310 MPa or more that can achieve excellent press formability in a steel having a martensite-dominant microstructure with excellent delayed fracture resistance properties. Disclosed is a steel sheet including: a chemical composition containing, by mass %, C: 0.12-0.40%, Si: 1.5% or less, Mn: more than 1.7% and 3.5% or less, P: 0.05% or less, S: 0.010% or less, sol.Al: 1.00% or less, N: 0.010% or less, Ti: 0.002-0.080%, and B: 0.0002-0.0050%, with the balance being Fe and inevitable impurities; a metallic structure in which an area ratio of martensite to an entire microstructure is 85% or more, and a ratio LS/LB satisfies a predetermined formula (1), where LS denotes a length of a sub-block boundary and LB denotes a length of a block boundary; and a tensile strength of 1310 MPa or more.

Abstract: To provide a steel sheet with a tensile strength of 1310 MPa or more that can achieve excellent press formability in a steel having a martensite-dominant microstructure with excellent delayed fracture resistance properties. Disclosed is a steel sheet including: a chemical composition containing, by mass %, C: 0.12-0.40%, Si: 1.5% or less, Mn: 1.7% or less, P: 0.05% or less, S: 0.010% or less, sol.Al: 1.00% or less, N: 0.010% or less, Ti: 0.002-0.080%, and B: 0.0002-0.0050%, with the balance being Fe and inevitable impurities; a metallic structure in which an area ratio of martensite to an entire microstructure is 95% or more, and a ratio LS/LB satisfies a predetermined formula (1), where LS denotes a length of a sub-block boundary and LB denotes a length of a block boundary; and a tensile strength of 1310 MPa or more.

Abstract: A steel sheet has a chemical composition containing, by mass %, C: 0.20% or more and 0.40% or less, Si: more than 1.0% and 3.0% or less, Mn: 1.5% or more and 3.5% or less, P: 0.002% or more and 0.010% or less, S: 0.0002% or more and 0.0020% or less, sol. Al: 0.40% or less (not including 0%), and N: 0.0100% or less, with the balance being Fe and incidental impurities; and a steel microstructure containing, in area fraction, tempered martensite: 45% or more and 83% or less, bainite: 15% or more and 53% or less, and retained austenite: 2% or more. Carbides in the tempered martensite have an average particle size of 0.40 ?m or less, an average amount of C in the retained austenite is 0.5% by mass or more, and a tensile strength is 1,470 MPa or more.

Abstract: A steel sheet has a certain chemical composition and a steel microstructure containing, in area fraction, martensite: 40% or more and 78% or less, bainite: 20% or more and 58% or less, and retained austenite: 2% or more. Carbides in tempered martensite in the martensite have an average particle size of 0.40 ?m or less, an average amount of C in the retained austenite is 0.5% by mass or more, a Si concentration in a region within 100 ?m in a thickness direction from a surface of the steel sheet is 1.3% by mass or less, and a tensile strength is 1,470 MPa or more.

Abstract: A steel sheet is provided, having a tensile strength of 590 MPa or more, a particular composition and a steel structure that contains, in terms of area fraction, particular amounts of ferrite and martensite, in which the ferrite average crystal grain size is 20 ?m or less, the martensite average size is 15 ?m or less, the ratio of the average crystal grain size of the ferrite to the average size of the martensite (ferrite average crystal grain size/martensite average size) is 0.5 to 10.0, the ratio of the hardness of the martensite to the hardness of the ferrite (martensite hardness/ferrite hardness) is 1.0 or more and 5.0 or less, and, in the texture of the ferrite, the inverse intensity ratio of ?-fiber to the ?-fiber is 0.8 or more and 7.0 or less.

Abstract: Disclosed herein are a method for producing a hot-rolled steel sheet, a method for producing a cold-rolled full hard steel sheet, and methods for producing a heat-treated steel sheet that serve as the methods for producing intermediate products for obtaining a steel sheet having a tensile strength of 590 MPa or more, a particular composition and a particular steel structure.

Abstract: Provided are a steel sheet having high strength and high delayed fracture resistance and a method for manufacturing the steel sheet. The steel sheet has a specific chemical composition and a microstructure in which the area fraction of martensite is 95% to 100%, with the balance being one or more of bainite, ferrite, and retained austenite. In the steel sheet, prior-austenite grains have an average grain size of 18 ?m or less, 90 mass % or more of the total content of Nb and Ti contained is present as a carbonitride having an equivalent circular diameter of 100 nm or more, and a Nb carbonitride and a Ti carbonitride, having an equivalent circular diameter of 1.0 ?m or more, are present at a rate of 800 pieces/mm2 or less in total. The steel sheet has a tensile strength of 1310 MPa or more.

Abstract: Provided are a steel sheet and a member, having high strength and high delayed fracture resistance, and methods for manufacturing the steel sheet and the member. The steel sheet has a specific chemical composition and a microstructure in which the area fraction of martensite is 95% to 100%, with the balance being one or more of bainite, ferrite, and retained austenite. In the steel sheet, prior-austenite grains have an average grain size of 18 ?m or less, 90 mass % or more of the total content of Nb and Ti contained is present as a carbonitride having an equivalent circular diameter of 100 nm or more, and a Nb carbonitride and a Ti carbonitride, having an equivalent circular diameter of 1.0 ?m or more, are present at a rate of 800 pieces/mm2 or less in total. The steel sheet has a tensile strength of 1310 MPa or more.