Jfe Steel Corporation

Seigo Tsuchihashi

Hidekazu Minami

Takashi Kobayashi

Tatsuya Nakagaito

Shinsuke Komine

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 &#x3bc;m or less, the quenched martensite has an average circularity index of 0.50 or more, the circularity index being defined as 4 &#x3c0;M/D, 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 fto a total area ratio of the quenched martensite and the tempered martensite f, f/f, of 50% or less.

High-strength cold-rolled steel sheet and method for manufacturing same

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 &#x3bc;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 f/fbeing 50% or less, where fdenotes the area fraction of martensite and fdenotes 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 &#x3bc;m or less.

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Tokyo, Japan

Seigo Tsuchihashi