Abstract: Provided are a high-strength cold rolled steel sheet that has a tensile strength of 980 MPa or more, excellent ductility, and a low failure rate in a hole expanding test, and a method for manufacturing the same. A high-strength cold rolled steel sheet comprises a predetermined composition, wherein a total area ratio of ferrite and bainitic ferrite is 20% to 80%, an area ratio of retained austenite (RA) is more than 10% and 40% or less, an area ratio of tempered martensite is more than 0% and 50% or less, a ratio of RA with an aspect ratio of 0.5 or less is 75% or more, a ratio of RA with an aspect ratio of 0.5 or less in ferrite grain boundaries is 50% or more, and an average KAM value of bcc phase is 1° or less.

Abstract: To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 980 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 15% or more and 55% or less and martensite is 15% or more and 30% or less in area ratio, retained austenite is 12% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 4.0 ?m or less, 2.0 ?m or less and 2.0 ?m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Abstract: To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 590 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 35% or more and 80% or less and martensite is 5% or more and 25% or less in area ratio, retained austenite is 8% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 6.0 ?m or less, 3.0 ?m or less and 3.0 ?m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Abstract: Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, martensite: 5% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 ?m or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

Abstract: A high strength cold rolled steel sheet has a composition which contains, in terms of mass %, more than 0.15% and not more than 0.45% of C, 0.50-2.50% of Si, 1.50-3.00% of Mn, not more than 0.050% of P, not more than 0.0100% of S, 0.010-0.100% of Al and not more than 0.0100% of N, with the remainder including Fe and unavoidable impurities, has a total content of ferrite and bainitic ferrite of 20-80%, has a retained austenite content of more than 10% and not more than 40%, has a martensite content of more than 0% and not more than 50%, and is such that the proportion of retained austenite that has an aspect ratio of not more than 0.5 is not less than 75%, and the proportion of retained austenite having an aspect ratio of not more than 0.5 that is present at Bain group boundaries is not less than 50%.

Abstract: An object is to provide a high strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent formability and a method for manufacturing the steel sheet. A high strength steel sheet which is excellent in terms of formability, which is manufactured under optimized manufacturing conditions, and which has a predetermined chemical composition and a steel microstructure including, in terms of area fraction, 35% or more and 80% or less of ferrite, 5% or more and 35% or less of as-quenched martensite, 0.1% or more and less than 3.0% of tempered martensite, and 8% or more of retained austenite, in which the average grain size of the ferrite is 6 ?m or less, in which the average grain size of the retained austenite is 3 ?m or less, in which a value calculated by dividing the average Mn content in the retained austenite by the average Mn content in the ferrite is 1.

Abstract: Provided are a high-strength cold-rolled steel sheet that has a tensile strength of 1320 MPa or more, excellent ductility and stretch flangeability, and a low failure rate in a hole expanding test, and a method for manufacturing the same. A high-strength cold-rolled steel sheet comprises a specific composition, wherein a total area ratio of ferrite and bainitic ferrite is 10% to 50%, an area ratio of retained austenite (RA) is more than 15% and 50% or less, an area ratio of tempered martensite is more than 15% and 60% or less, a ratio of RA with an aspect ratio of 0.6 or less is 70% or more, a ratio of RA with an aspect ratio of 0.6 or less in ferrite grain boundaries is 50% or more, and an average KAM value of bcc phase is 1° or less.

Abstract: Provided is a steel material for high heat input welding having excellent welding heat affected zone low-temperature toughness at ?55° C. in the case that the steel material is welded by high heat input welding with a heat input of more than 80 kJ/cm. The steel material may be used in low-temperature storage tanks, for example. The steel material for high heat input welding includes, in mass %, C: 0.04 to 0.09%, Si: 0.15 to 0.25%, Mn: 1.40 to 2.00%, P: 0.015% or less, S: 0.0005 to 0.0040%, Al: 0.030 to 0.080%, Ti: 0.005 to 0.025%, B: 0.0003 to 0.0020%, Ca: 0.0005 to 0.0030%, N: 0.0030 to 0.0060%, O: 0.0040% or less, Nb: 0.005% or less, and Mo: 0.005% or less, the balance being Fe and incidental impurities.

Abstract: Provided is a steel material for high heat input welding having excellent welding heat affected zone low-temperature toughness at ?55° C. in the case that the steel material is welded by high heat input welding with a heat input of more than 80 kJ/cm. The steel material may be used in low-temperature storage tanks, for example. The steel material for high heat input welding includes, in mass %, C: 0.04 to 0.09%, Si: 0.15 to 0.25%, Mn: 1.40 to 2.00%, P: 0.015% or less, S: 0.0005 to 0.0040%, Al: 0.030 to 0.080%, Ti: 0.005 to 0.025%, B: 0.0003 to 0.0020%, Ca: 0.0005 to 0.0030%, N: 0.0030 to 0.0060%, O: 0.0040% or less, Nb: 0.005% or less, and Mo: 0.005% or less, the balance being Fe and incidental impurities.

Abstract: A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80%, martensite being 5% to 35%, and tempered martensite being 0% to 5% in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 ?m or less; and the retained austenite being 3 ?m or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

Abstract: A high-strength steel includes a steel structure with: in area fraction, 60.0% to less than 90.0% of ferrite, 0% to less than 5.0% of unrecrystallized ferrite, 2.0% to 25.0% of martensite, 0% to 5.0% of carbide, and 0% to 3.0% of bainite; in volume fraction, more than 7.0% of retained austenite; in a cross-sectional view of 100 ?m×100 ?m, a value obtained by dividing number of retained austenite that are not adjacent to retained austenite whose crystal orientations are different by a total number of retained austenite being less than 0.80, an average crystal grain size of the ferrite being 6.0 ?m or less, an average crystal grain size of the retained austenite being 3.0 ?m or less, and a value obtained by dividing, by mass %, an average content of Mn in the retained austenite by an average content of Mn in steel being 1.50 or more.

Abstract: A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80% and tempered martensite being greater than 5% and 20% or less in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 ?m or less; and the retained austenite being 3 ?m or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

Abstract: Provided are a high-strength cold-rolled steel sheet that has a tensile strength of 1320 MPa or more, excellent ductility and stretch flangeability, and a low failure rate in a hole expanding test, and a method for manufacturing the same. A high-strength cold-rolled steel sheet comprises a specific composition, wherein a total area ratio of ferrite and bainitic ferrite is 10% to 50%, an area ratio of retained austenite (RA) is more than 15% and 50% or less, an area ratio of tempered martensite is more than 15% and 60% or less, a ratio of RA with an aspect ratio of 0.6 or less is 70% or more, a ratio of RA with an aspect ratio of 0.6 or less in ferrite grain boundaries is 50% or more, and an average KAM value of bcc phase is 1° or less.

Abstract: Provided are a high-strength cold rolled steel sheet that has a tensile strength of 980 MPa or more, excellent ductility, and a low failure rate in a hole expanding test, and a method for manufacturing the same. A high-strength cold rolled steel sheet comprises a predetermined composition, wherein a total area ratio of ferrite and bainitic ferrite is 20% to 80%, an area ratio of retained austenite (RA) is more than 10% and 40% or less, an area ratio of tempered martensite is more than 0% and 50% or less, a ratio of RA with an aspect ratio of 0.5 or less is 75% or more, a ratio of RA with an aspect ratio of 0.5 or less in ferrite grain boundaries is 50% or more, and an average KAM value of bcc phase is 1° or less.

Abstract: A high strength cold rolled steel sheet has a composition which contains, in terms of mass %, more than 0.15% and not more than 0.45% of C, 0.50-2.50% of Si, 1.50-3.00% of Mn, not more than 0.050% of P, not more than 0.0100% of S, 0.010-0.100% of Al and not more than 0.0100% of N, with the remainder including Fe and unavoidable impurities, has a total content of ferrite and bainitic ferrite of 20-80%, has a retained austenite content of more than 10% and not more than 40%, has a martensite content of more than 0% and not more than 50%, and is such that the proportion of retained austenite that has an aspect ratio of not more than 0.5 is not less than 75%, and the proportion of retained austenite having an aspect ratio of not more than 0.5 that is present at Bain group boundaries is not less than 50%.

Abstract: To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 980 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 15% or more and 55% or less and martensite is 15% or more and 30% or less in area ratio, retained austenite is 12% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 4.0 ?m or less, 2.0 ?m or less and 2.0 ?m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Abstract: To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 590 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 35% or more and 80% or less and martensite is 5% or more and 25% or less in area ratio, retained austenite is 8% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 6.0 ?m or less, 3.0 ?m or less and 3.0 ?m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Abstract: A high-strength cold-rolled steel sheet has a chemical composition containing, by mass %, C: 0.15% or more and 0.30% or less, Si: 0.8% or more and 2.4% or less, Mn: 2.4% or more and 3.5% or less, P: 0.08% or less, S: 0.005% or less, Al: 0.01% or more and 0.08% or less, N: 0.010% or less, Ti: 0.002% or more and 0.05% or less, B: 0.0002% or more and 0.0050% or less, and the balance being Fe and inevitable impurities, a microstructure including ferrite having an average grain diameter of 3 ?m or less and a volume fraction of 5% or less (including 0%), retained austenite having a volume fraction of 10% or more and 20% or less, martensite having an average grain diameter of 4 atm or less and a volume fraction of 20% or less (including 0%), and the balance including bainite and/or tempered martensite.

Abstract: A high strength cold rolled steel sheet with a low yield ratio has a chemical composition containing C: 0.05% to 0.10%, Si: 0.6% to 1.3%, Mn: 1.4% to 2.2%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, and the remainder being Fe and incidental impurities, on a percent by mass basis, and a microstructure in which the average grain size of ferrite is 15 ?m or less, the volume fraction of ferrite is 70% or more, the volume fraction of bainite is 3% or more, the volume fraction of retained austenite is 4% to 7%, the average grain size of martensite is 5 ?m or less, and the volume fraction of martensite is 1% to 6%, wherein the average C concentration (percent by mass) in the retained austenite is 0.30% to 0.70%, yield ratio is 64% or less, and the tensile strength is 590 MPa or more.

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.