Jfe Steel Corporation

Junya Tobata

Yoshihiko Ono

Shinjiro Kaneko

Hiroyuki Akimoto

Hidekazu Minami

Yoichiro Matsui

Takashi Kobayashi

Takeshi Yokota

Yuji Tanaka

Yuki Toji

Noriaki Kohsaka

Yasushi Kitani

Tadachika Chiba

A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below:

High strength steel sheet and method for manufacturing the same

There is provided a high-strength steel sheet in which content amounts of a specific steel composition are limited and manufacturing conditions are optimized and that has a steel structure that contains ferrite and a harder phase and in which ferrite accounts for 30% to 70% in terms of area ratio, the harder phase contains bainitic ferrite, tempered martensite, martensite (including 0%), and retained austenite (including 0%), the maximum grain diameter of ferrite dis 2.5 μm or less, and the area ratio of ferrite Aand the average grain diameter of ferrite dsatisfy A[%]/d[βm]≥15.

High-strength steel sheet and method for manufacturing the same

A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 4% or less (including 0%), as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an amount of 7% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 71% and less than 93%; and BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an equivalent circular diameter of 1 μm or less is present in an area fraction of 4% or more and 50% or less, and BCC iron that has a misorientation of more than 1° is present in an area fraction of 25% or more and 85% or less.

Thin steel sheet and method for manufacturing same

A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100% by area percentage, and retained γ: 4% to 15% by volume percentage. Retained γhas a specified area percentage S, retained γhas a specified distribution number N, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained γ grains has a specified equivalent circular grain diameter and aspect ratio.

Steel sheet and method for producing the same

A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100%, and retained γ: 5% to 20%. Retained γhas a specified area percentage S, retained γhas a specified distribution number N, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained γ grains has a specified equivalent circular grain diameter and aspect ratio.

A high-strength steel sheet having a tensile strength of 1,180 MPa or more, and specified chemical composition. The steel sheet includes a steel structure in which an area fraction of martensite having a carbon concentration of more than 0.7×[% C] and less than 1.5×[% C] is 55% or more, an area fraction of tempered martensite having a carbon concentration of 0.7×[% C] or less is 5% or more and 40% or less, a ratio of a carbon concentration in retained austenite to a volume fraction of retained austenite is 0.05 or more and 0.40 or less, and the martensite and the tempered martensite each have an average grain size of 5.3 μm or less, where [% C] represents the content, by mass %, of compositional element C in steel.

High-strength steel sheet and method for manufacturing same

A steel sheet and a manufacturing method therefor are provided. A steel sheet has a component composition and contains 6 to 80% of ferrite and 20 to 94% of a microstructure composed of one or two or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained &#x3b3;, and contains 7 to 20% of retained &#x3b3;, where: an area ratio (S) of retained &#x3b3; having a particle width of 0.18 to 0.60 &#x3bc;m, a particle length of 1.7 to 7.0 &#x3bc;m, and an aspect ratio of 5 to 15 is 0.2 to 5%; and a total area ratio (S) of fresh martensite having an equivalent circle diameter of 1.5 to 15 &#x3bc;m and an aspect ratio of 3 or less and/or retained &#x3b3; particles having an equivalent circle diameter of 1.5 to 15 &#x3bc;m and an aspect ratio of 3 or less is 3% or less (including 0%).

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Tokyo, Japan

Junya Tobata