Abstract: Provided is a steel sheet having high strength, high YS, excellent ductility, and excellent bendability. A steel sheet comprises a specified chemical composition and steel microstructure. In particular, a grain number density of retained austenite, a ratio, average C concentration, average intergrain distance, and maximum intergrain distance of grains with an aspect ratio of 3 or less, and [Mn]C/[Mn] in the steel microstructure are appropriately controlled.

Abstract: A steel sheet having high strength, excellent ductility, high YR, and excellent bendability. The steel sheet includes a defined chemical composition, the steel microstructure includes: area ratio of ferrite: 5% or more and 65% or less, area ratio of martensite: 10% or more and 60% or less, area ratio of bainite: 10% or more and 60% or less, and area ratio of retained austenite: 5% or more. The relationship in the following Formula (1) is satisfied. Average solute C concentration of the retained austenite [C]? is 0.5 mass % or more, and standard deviation of C concentration distribution in the retained austenite is 0.250 mass % or less. [ Mn ] ? ? / [ Mn ] ? 1.

Abstract: A steel sheet production method includes: a hot rolling process in which a steel slab is heated, rolled, cooled, and coiled to obtain a hot-rolled sheet; a cold rolling process in which the hot-rolled sheet is cold rolled to obtain a cold-rolled sheet; and an annealing process in which the cold-rolled sheet is annealed to obtain a steel sheet. In the annealing process, the cold-rolled sheet is subjected to: heating at 1 to 7° C./sec from 600° C. to an annealing temperature; annealing from (AC1 point+50° C.) to (AC3 point+20° C.) for 1 10 to 35 sec; cooling at 10 to 50° C./sec from the annealing temperature to a primary cooling stop temperature of 450 to 600° C.; and cooling for 20 to 100 sec from the primary cooling stop temperature to a temperature of 400 to 500° C.

Abstract: Provided is a steel sheet having excellent workability while ensuring excellent strength. The steel sheet comprises: a predetermined chemical composition; and a steel microstructure that contains ferrite: 45% to 90%, martensite: 5% to 30%, bainite: 1% to 25%, and retained austenite: 3% or more and in which [Mn]M/[Mn] is 1.00 to 1.15 and [Mn]M/[Mn]F is 1.00 to 1.30, wherein TS×El is 16000 MPa·% or more.

Abstract: A steel sheet having high strength, excellent ductility, high YR, and excellent bendability is provided. The steel sheet includes a defined chemical composition, the steel microstructure includes: area ratio of ferrite: 5% or more and 65% or less, area ratio of martensite: 10% or more and 60% or less, area ratio of bainite: 10% or more and 60% or less, and area ratio of retained austenite: 5% or more. The relationship in the following Formula (1) is satisfied. Average solute C concentration of the retained austenite [C]? is 0.5 mass % or more, and standard deviation of C concentration distribution in the retained austenite is 0.250 mass % or less. [ Mn ] ? ? / [ Mn ] ? 1.

Abstract: Provided is a steel sheet having excellent workability while ensuring excellent strength. The steel sheet comprises: a predetermined chemical composition; and a steel microstructure that contains ferrite: 45% to 90%, martensite: 5% to 30%, bainite: 1% to 25%, and retained austenite: 3% or more and in which [Mn]M/[Mn] is 1.00 to 1.15 and [Mn]M/[Mn]F is 1.00 to 1.30, wherein TS×El is 16000 MPa·% or more.

Abstract: Provided are a high-yield-ratio high-strength galvanized steel sheet and a method for manufacturing thereof. The high-yield-ratio high-strength galvanized steel sheet has a steel sheet having a specified chemical composition and a metallographic structure including, in terms of area ratio, in terms of area ratio, 15% or less of ferrite, 20% or more and 50% or less of martensite, and bainite and tempered martensite in a total amount of 30% or more, and a galvanized layer formed on the steel sheet having a coating weight of 20 g/m2 to 120 g/m2 per side, in which a yield strength ratio is 65% or more, a tensile strength is 950 MPa or more, and Mn oxides are contained in the galvanized layer in an amount of 0.015 g/m2 to 0.050 g/m2.

Abstract: Provided are a high-yield-ratio high-strength galvanized steel sheet and a method for manufacturing thereof. The high-yield-ratio high-strength galvanized steel sheet has a steel sheet having a specified chemical composition and a metallographic structure including, in terms of area ratio, in terms of area ratio, 15% or less of ferrite, 20% or more and 50% or less of martensite, and bainite and tempered martensite in a total amount of 30% or more, and a galvanized layer formed on the steel sheet having a coating weight of 20 g/m2 to 120 g/m2 per side, in which a yield strength ratio is 65% or more, a tensile strength is 950 MPa or more, and Mn oxides are contained in the galvanized layer in an amount of 0.015 g/m2 to 0.050 g/m2.

Abstract: Provided are a high-yield-ratio high-strength galvanized steel sheet that includes a Mn-containing steel sheet serving as a base material, and a production method therefor. The high-yield-ratio high-strength galvanized steel sheet includes a steel sheet having a specific composition and a metallographic structure including, by area percentage, 20% or less ferrite, 40% or more in total of bainite and tempered martensite, and 60% or less as-quenched martensite, the bainite having an average grain size of 6.0 ?m or less, and a coated layer arranged on the steel sheet, the coated layer having a coating weight per side of 20 to 120 g/m2 and having a Mn content of 0.05 g/m2 or less, in which the high-yield-ratio high-strength galvanized steel sheet has a yield ratio of 65% or more and a tensile strength of 950 MPa or more.