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: 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 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 and a member excellent in cold workability, hardenability, and post-quenching surface layer hardness, and methods for manufacturing the steel sheet and the member. The steel sheet has a specified chemical composition and a microstructure containing ferrite and carbides, a ratio of the volume of ferrite and carbides to the volume of the entire microstructure is 90% or more, a ratio of the volume of proeutectoid ferrite to the volume of the entire microstructure is 20% or more and 80% or less, a Mn concentration in the carbides is 0.10 mass % or more and 0.50 mass % or less, and a ratio of the number of carbides with particle diameters of 1 ?m or more to the total number of carbides is 30% or more and 60% or less.

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 steel sheet has a specified chemical composition and a specified steel microstructure. An average grain size of ferrite and/or bainitic ferrite is 7.0 ?m or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the grain size of the ferrite and/or the bainitic ferrite to the average grain size of the ferrite and/or the bainitic ferrite is 10% or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the area fraction of as-quenched martensite to the area fraction of the as-quenched martensite is 10% or less. On the basis of a distribution in the width direction of the steel sheet, a ratio of a deviation of the area fraction of retained austenite to the area fraction of the retained austenite is 10% or less.

Abstract: The steel sheet has a steel microstructure containing ferrite: 6% to 90% by area, a microstructure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ?: 10% to 94% by area in total, and retained ?: 3% to 20% by volume, a ratio (SUB/S2nd)×100(%) of an area ratio SUB of an upper bainite with a width in the range of 0.8 to 7 ?m, a length in the range of 2 to 15 ?m, and an aspect ratio of 2.2 or more in contact with retained ?UB with a grain width in the range of 0.17 to 0.80 ?m and an aspect ratio in the range of 4 to 25 to an area ratio S2nd of the microstructure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ? ranges from 2.0% to 15%.

Abstract: A high-carbon cold rolled steel sheet having a specified chemical composition, and a method for manufacturing the same. The method includes forming a hot rolled steel sheet, performing cooling at an average cooling rate of 30° C./s or more and 70° C./s or less through a temperature range of a finish rolling end temperature to 660° C., coiling a hot rolled steel sheet at a temperature of 500° C. or more and 660° C. or less, and, optionally, pickling the coiled hot rolled steel sheet, and then performing a first box-annealing of holding at an annealing temperature in a temperature range of 650 to 720° C., then performing cold rolling at a rolling reduction ratio of 20 to 50%, and then performing a second box-annealing of holding at an annealing temperature in a temperature range of 650 to 720° C.

Abstract: A steel sheet and a member excellent in cold workability, hardenability, and post-quenching surface layer hardness, and methods for manufacturing the steel sheet and the member. The steel sheet has a specified chemical composition and a microstructure containing ferrite and carbides, a ratio of the volume of ferrite and carbides to the volume of the entire microstructure is 90% or more, a ratio of the volume of proeutectoid ferrite to the volume of the entire microstructure is 20% or more and 80% or less, a Mn concentration in the carbides is 0.10 mass % or more and 0.50 mass % or less, and a ratio of the number of carbides with particle diameters of 1 ?m or more to the total number of carbides is 30% or more and 60% or less.

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 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 having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 6 to 90% in terms of an area fraction, and a constituent formed of at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained ?: 10 to 94% in terms of an area fraction, and retained ?: 3 to 15% in terms of a volume fraction. A total area fraction of SC-enriched is 0.1 to 5%, a total area fraction of S?Block of at least one of (i) fresh martensite having a specified equivalent circular grain diameter and aspect ratio and (ii) retained ? grains having a specified equivalent circular grain diameter and aspect ratio is 5% or less, and prior ? in a surface layer has a grain diameter of 2 to 12 ?m.

Abstract: A high-carbon cold rolled steel sheet having a specified chemical composition, and a method for manufacturing the same. The method includes forming a hot rolled steel sheet, performing cooling at an average cooling rate of 30° C./s or more and 70° C./s or less through a temperature range of a finish rolling end temperature to 660° C., coiling a hot rolled steel sheet at a temperature of 500° C. or more and 660° C. or less, and, optionally, pickling the coiled hot rolled steel sheet, and then performing a first box-annealing of holding at an annealing temperature in a temperature range of 650 to 720° C., then performing cold rolling at a rolling reduction ratio of 20 to 50%, and then performing a second box-annealing of holding at an annealing temperature in a temperature range of 650 to 720° C.