Abstract: Provided is a high-strength steel sheet having a tensile strength (TS) of 750 MPa or more and excellent in ductility and stretch flangeability, in which the steel sheet has a predetermined chemical composition and a microstructure containing, in area ratio, ferrite: 50% to 90%, quenched martensite: 1% to 8%, tempered martensite: 3% to 40%, and retained austenite: 6% to 15%, the quenched martensite has an average grain size of 2.5 ?m or less, the quenched martensite has an average circularity index of 0.50 or more, the circularity index being defined as 4 ?M/D2, where D is a perimeter of the quenched martensite and M is an area of the quenched martensite, and the steel sheet has a ratio of an area ratio of the quenched martensite fM to a total area ratio of the quenched martensite and the tempered martensite fM+TM, fM/fM+TM, of 50% or less.

Abstract: A high-strength cold-rolled steel sheet or high-strength coated steel sheet that has a tensile strength (TS) of 780 MPa or more and has high ductility, stretch-flangeability, and in-plane stability of stretch-flangeability and methods for producing the same. The high-strength cold-rolled steel sheet has a specified chemical composition and a microstructure comprising, by area fraction, in a range of 50% to 80% of ferrite, 8% or less of martensite with an average grain size of 2.5 ?m or less, in a range of 6% to 15% of retained austenite, and in a range of 3% to 40% of tempered martensite. A ratio fM/fM+TM being 50% or less, where fM denotes the area fraction of martensite and fM+TM denotes the total area fraction of martensite and tempered martensite, and a standard deviation of the grain size of martensite at certain portions being 0.7 ?m or less.

Abstract: A high-strength cold-rolled steel sheet or high-strength coated steel sheet that has a tensile strength (TS) of 780 MPa or more and has high ductility, stretch-flangeability, and in-plane stability of stretch-flangeability and methods for producing the same. The high-strength cold-rolled steel sheet has a specified chemical composition and a microstructure comprising, by area fraction, in a range of 50% to 80% of ferrite, 8% or less of martensite with an average grain size of 2.5 ?m or less, in a range of 6% to 15% of retained austenite, and in a range of 3% to 40% of tempered martensite. A ratio fM/fM+TM being 50% or less, where fM denotes the area fraction of martensite and fM+TM denotes the total area fraction of martensite and tempered martensite, and a standard deviation of the grain size of martensite at certain portions being 0.7 ?m or less.

Abstract: Provided is a high-strength steel sheet having a tensile strength (TS) of 750 MPa or more and excellent in ductility and stretch flangeability, in which the steel sheet has a predetermined chemical composition and a microstructure containing, in area ratio, ferrite: 50% to 90%, quenched martensite: 1% to 8%, tempered martensite: 3% to 40%, and retained austenite: 6% to 15%, the quenched martensite has an average grain size of 2.5 ?m or less, the quenched martensite has an average circularity index of 0.50 or more, the circularity index being defined as 4 ?M/D2, where D is a perimeter of the quenched martensite and M is an area of the quenched martensite, and the steel sheet has a ratio of an area ratio of the quenched martensite fM to a total area ratio of the quenched martensite and the tempered martensite fM+TM, fM/fM+TM, of 50% or less.