Abstract: A high-strength galvannealed steel sheet has a chemical composition containing, by mass %, C: 0.14% or more and 0.24% or less, Si: 0.8% or more and 1.8% or less, Mn: 1.0% or more and 3.0% or less, P: 0.020% or less, S: 0.0040% or less, Al: 0.01% or more and 0.1% or less, N: 0.01% or less, Ca: 0.0001% or more and 0.

Abstract: A cold-rolled steel sheet having a steel composition comprising, by mass %, 0.12% to 0.22% C, 0.8% to 1.8% Si, 1.8% to 2.8% Mn, 0.020% or less P, 0.0040% or less S, 0.005% to 0.08% Al, 0.008% or less N, 0.001% to 0.040% Ti, 0.0001% to 0.0020% B, 0.0001% to 0.0020% Ca, and Fe and incidental impurities. The steel sheet includes a microstructure in which ferrite and bainite phases are 50% to 70% of the total area, the average grain size of the ferrite and bainite phase is 1 to 3 ?m, a tempered martensite phase is 25% to 45% of the total area, the average grain size of the tempered martensite phase is 1 to 3 ?m, and a retained austenite phase is 2% to 10% of the total area.

Abstract: This disclosure provides a high-strength thin steel sheet excellent in both tensile strength and elongation with small elongation anisotropy. The high-strength thin steel sheet has a specific chemical composition and a microstructure where a total area ratio of ferrite, tempered bainitic ferrite and bainitic ferrite is 40% or more and 70% or less, an area ratio of martensite is 5% or more and 30% or less, an area ratio of retained austenite is 10% or more and 35% or less, an average equivalent circular diameter of martensite and retained austenite (secondary phase) grains is 2.0 ?m or less, an area ratio of secondary phase grains having an equivalent circular diameter of 2.0 ?m or more is 10% or less, and an average minor axis length of secondary phase grains is 0.40 ?m or less.

Abstract: A steel sheet has a microstructure that contains ferrite in an area ratio of 20% or more, martensite in an area ratio of 5% or more, and tempered martensite in an area ratio of 5% or more. The ferrite has a mean grain size of 20.0 ?m or less. An inverse intensity ratio of ?-fiber to ?-fiber in the ferrite is 1.00 or more and an inverse intensity ratio of ?-fiber to ?-fiber in the martensite and the tempered martensite is 1.00 or more.

Abstract: A high-strength steel sheet with excellent formability and high yield ratio that has TS of 590 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 35 to 80% of polygonal ferrite, 5% or more of non-recrystallized ferrite, and 5 to 25% of martensite, and that contains, in volume fraction, 8% or more of retained austenite, in which the polygonal ferrite has a mean grain size of 6 ?m or less, the martensite has a mean grain size of 3 ?m or less, the retained austenite has a mean grain size of 3 ?m or less, and a value obtained by dividing an Mn content in the retained austenite (in mass %) by an Mn content in the polygonal ferrite (in mass %) equals 2.0 or more.

Abstract: Disclosed is a steel sheet having a predetermined chemical composition and a steel microstructure that contains, in area ratio, 35% or more and 80% or less of polygonal ferrite and 5% or more and 25% or less of martensite, and that contains, in volume fraction, 8% or more of retained austenite, in which the polygonal ferrite, the martensite, and the retained austenite have a mean grain size of 6 ?m or less, 3 ?m or less, and 3 ?m or less, respectively, and each have a mean grain aspect ratio of 2.0 or less, and in which a value obtained by dividing an Mn content in the retained austenite in mass % by an Mn content in the polygonal ferrite in mass % equals 2.0 or more.

Abstract: A steel having a composition containing C: more than 0.20% and 0.45% or less, Si: 0.50% to 2.50%, Mn: 2.00% or more and less than 3.50%, and one or two selected from Ti: 0.005% to 0.100% and Nb: 0.005% to 0.100% is hot-rolled and cold-rolled. The steel sheet is heated to 800° C. to 950° C. and cooled to a cooling-end temperature of 350° C. to 500° C. at a cooling rate of 5° C./s or more to form a steel sheet having a microstructure including martensite and bainite phases such that the total proportion of the martensite and bainite phases is 80% or more by volume. The steel sheet is heated to 700° C. to 840° C. and maintained at 700° C. to 840° C., cooled to a cooling-end temperature of 350° C. to 500° C. at a cooling rate of 5 to 50° C./s, and maintained within the above temperature range for 10 to 1800 s.

Abstract: A steel sheet has a microstructure that contains ferrite in an area ratio of 20% or more, martensite in an area ratio of 5% or more, and tempered martensite in an area ratio of 5% or more. The ferrite has a mean grain size of 20.0 ?m or less. An inverse intensity ratio of ?-fiber to ?-fiber in the ferrite is 1.00 or more and an inverse intensity ratio of ?-fiber to ?-fiber in the martensite and the tempered martensite is 1.00 or more.

Abstract: A steel sheet has a microstructure that contains ferrite in an area ratio of 20% or more, martensite in an area ratio of 5% or more, and tempered martensite in an area ratio of 5% or more. The ferrite has a mean grain size of 20.0 ?m or less. An inverse intensity ratio of ?-fiber to ?-fiber in the ferrite is 1.00 or more and an inverse intensity ratio of ?-fiber to ?-fiber in the martensite and the tempered martensite is 1.00 or more.

Abstract: A steel sheet has a microstructure that contains ferrite in an area ratio of 20% or more, martensite in an area ratio of 5% or more, and tempered martensite in an area ratio of 5% or more. The ferrite has a mean grain size of 20.0 ?m or less. An inverse intensity ratio of ?-fiber to ?-fiber in the ferrite is 1.00 or more and an inverse intensity ratio of ?-fiber to ?-fiber in the martensite and the tempered martensite is 1.00 or more.

Abstract: A high-strength steel sheet with excellent formability and high yield ratio that has TS of 980 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 15 to 55% of polygonal ferrite, 8% or more of non-recrystallized ferrite, and 15 to 30% of martensite, and that contains, in volume fraction, 12% or more of retained austenite, in which the polygonal ferrite has a mean grain size of 4 ?m or less, the martensite has a mean grain size of 2 ?m or less, the retained austenite has a mean grain size of 2 ?m or less, and a value obtained by dividing an Mn content in the retained austenite (in mass %) by an Mn content in the polygonal ferrite (in mass %) equals 2.0 or more.

Abstract: Disclosed is a steel sheet having a predetermined chemical composition and a steel microstructure that contains, in area ratio, 35% or more and 80% or less of polygonal ferrite and 5% or more and 25% or less of martensite, and that contains, in volume fraction, 8% or more of retained austenite, in which the polygonal ferrite, the martensite, and the retained austenite have a mean grain size of 6 ?m or less, 3 ?m or less, and 3 ?m or less, respectively, and each have a mean grain aspect ratio of 2.0 or less, and in which a value obtained by dividing an Mn content in the retained austenite in mass % by an Mn content in the polygonal ferrite in mass % equals 2.0 or more.

Abstract: A high-strength steel sheet with excellent formability and high yield ratio that has TS of 590 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 35 to 80% of polygonal ferrite, 5% or more of non-recrystallized ferrite, and 5 to 25% of martensite, and that contains, in volume fraction, 8% or more of retained austenite, in which the polygonal ferrite has a mean grain size of 6 ?m or less, the martensite has a mean grain size of 3 ?m or less, the retained austenite has a mean grain size of 3 ?m or less, and a value obtained by dividing an Mn content in the retained austenite (in mass %) by an Mn content in the polygonal ferrite (in mass %) equals 2.0 or more.

Abstract: A high-strength hot-pressed part having a specified chemical composition, a microstructure including, in terms of volume fraction, 80% or more of a martensite phase, in a range of 3.0% to 20.0% of a retained austenite phase, a tensile strength TS of 1500 MPa or more, and a uniform elongation uEl of 6.0% or more. A method for manufacturing the high-strength hot-pressed part, the method comprising performing a heating process and a hot press forming process on a raw material steel sheet in order to obtain a hot-pressed part having a specified shape.

Abstract: Disclosed is a steel sheet having a predetermined chemical composition and a steel microstructure that contains, in area ratio, 15% or more and 55% or less of polygonal ferrite and 15% or more and 30% or less of martensite, and that contains, in volume fraction, 12% or more of retained austenite, in which the polygonal ferrite, the martensite, and the retained austenite have a mean grain size of 4 ?m or less, 2 ?m or less, and 2 ?m or less, respectively, and each have a mean grain aspect ratio of 2.0 or less, and in which a value obtained by dividing an Mn content in the retained austenite in mass % by an Mn content in the polygonal ferrite in mass % equals 2.0 or more.

Abstract: Disclosed is a method comprising: preparing a steel slab with a predetermined chemical composition; subjecting the steel slab to hot rolling by heating it to a temperature of 1100-1300° C., hot rolling it with a finisher delivery temperature of 800-1000° C. to form a hot-rolled steel sheet, and coiling the steel sheet at a mean coiling temperature of 200-500° C.; subjecting the steel sheet to pickling treatment; and subjecting the steel sheet to annealing by retaining the steel sheet at a temperature of 740-840° C. for 10-900 s, then cooling the steel sheet at a mean cooling rate of 5-50° C./s to a cooling stop temperature of higher than 350° C. and 550° C. or lower, and retaining the steel sheet in a temperature range of higher than 350° C. to 550° C. for 10 s or more.

Abstract: Disclosed is a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, fatigue properties, stretch flangeability, surface characteristics, and sheet passage ability that can be obtained by providing a predetermined chemical composition and a steel microstructure that contains, by area, 20-50% of ferrite, 5-25% of bainitic ferrite, 1-10% of martensite, and 5-15% of tempered martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 ?m or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, the retained austenite has a mean free path of 1.2 ?m or less, and the tempered martensite has a mean free path of 1.2 ?m or less.

Abstract: Disclosed is a high-strength steel sheet having a tensile strength (TS) of 780 MPa or more and excellent in ductility, fatigue properties, balance between high strength and ductility, surface characteristics, and sheet passage ability that can be obtained by providing a predetermined chemical composition and a steel microstructure that contains, by area, 20-50% of ferrite, 5-25% of bainitic ferrite, and 5-20% of martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 ?m or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, and the retained austenite has a mean free path of 1.2 ?m or less.

Abstract: A cold-rolled steel sheet having excellent spot weldability suitable for use in vehicles, electric appliances, etc. is provided. The cold-rolled steel sheet has a steel composition containing, in mass%: C: 0.05% to 0.13%; Si: 0.05% to 2.0%; Mn: 1.5% to 4.0%; P: 0.05% or less; S: 0.005% or less; Al: 0.01% to 0.10%; Cr: 0.05% to 1.0%; Nb: 0.010% to 0.070%; Ti: 0.005% to 0.040%; and N: 0.0005% to 0.0065%, with a mass ratio Ti/N of Ti and N being 2.5 or more and 7.5 or less, and a balance being Fe and incidental impurities, wherein 70 mass % or more of Ti in steel exists as a precipitate, and 15 mass % or more of Nb in the steel exists as solute Nb, and a tensile strength is 980 MPa or more.

Abstract: Disclosed is a high-strength steel sheet having a predetermined chemical composition and a steel microstructure that contains, by area, 25-80% of ferrite and bainitic ferrite in total, and 3-20% of martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 ?m or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, an area ratio of retained austenite having a mean C content in mass % at least 2.1 times the C content in the steel sheet in mass % is 60% or more of an area ratio of the entire retained austenite.