Jfe Steel Corporation

Thi Huyen Doan

Hiroshi Hasegawa

Yoshihiko Ono

Hideyuki Kimura

Sota Goto

Kazuhiko Yamazaki

The high-strength steel sheet has a chemical composition with MSC value in the range of 2.7% to 3.8% by mass defined by a specific formula, wherein the high-strength steel sheet has a microstructure including specific microstructures in a surface layer region extending from the surface of the steel sheet to a depth of 100 μm and in an inner region other than the surface layer region. The high-strength steel sheet has the maximum height of the surface roughness of 30 μm or less, a tensile strength of 980 MPa or more, a uniform elongation of 6% or more, and a ratio of 10-cycle plane bending fatigue strength to tensile strength (fatigue limit ratio) of 0.45 or more.

High-strength steel sheet and method for manufacturing the same

Provided is a high-strength hot-rolled coated steel sheet having a TS of 980 MPa or more, excellent bulging formability and stretch flange formability, and excellent coatability. The high-strength hot-rolled coated steel sheet includes a steel sheet having a chemical composition containing, by mass %, C: 0.03% to 0.09%, Si: 0.01% to 1.60%, Mn: 2.20% to 3.60%, P: 0.100% or less, S: 0.0100% or less, Ti: 0.05% to 0.18%, B: 0.0005% to 0.0050%, Al: 0.005% to 0.40%, N: 0.010% or less, and a balance of Fe and inevitable impurities, in which a CSM value expressed by the equation 33.8[% C][% Mn]+12.4[% Si]/[% Mn] is 3.3 to 12.0, and a steel microstructure containing bainite in an amount of 85% or more in terms of area fraction and martensite in an amount of 2.0% or more and 15.0% or less in terms of area fraction and includes a coating layer or an alloyed coating layer on a surface of the steel sheet.

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Tokyo, Japan

Thi Huyen Doan