Kobe Steel, Ltd.

Hirokazu Natsumeda

Toshio Murakami

Kenji Saito

Tadao Murata

Disclosed is a high-strength sheet containing: C: 0.15% by mass to 0.35% by mass, a total of Si and Al: 0.5% by mass to 3.0% by mass, Al: 0.01% by mass or more, N: 0.01% by mass or less, Mn: 1.0% by mass to 4.0% by mass, P: 0.05% by mass or less, and S: 0.01% by mass or less, with the balance being Fe and inevitable impurities, wherein the steel structure satisfies that: a ferrite fraction is 5% or less, the total fraction of tempered martensite and tempered bainite is 60% or more, the amount of retained austenite is 10% or more, MA has an average size of 1.0 &#x3bc;m or less, retained austenite has an average size of 1.0 &#x3bc;m or less, retained austenite having a size of 1.5 &#x3bc;m or more accounts for 2% or more of the total amount of retained austenite, and the amount of solute nitrogen in a steel sheet is 0.002% by mass or less.

High-strength steel sheet and method for producing same

Disclosed is a high-strength sheet containing: C: 0.15% by mass to 0.35% by mass, a total of Si and Al: 0.5% by mass to 3.0% by mass, Mn: 1.0% by mass to 4.0% by mass, P: 0.05% by mass or less (including 0% by mass), S: 0.01% by mass or less (including 0% by mass), and Ti: 0.01% by mass to 0.2% by mass, with the balance being Fe and inevitable impurities, wherein the steel structure satisfies that: a ferrite fraction is 5% or less, a total fraction of tempered martensite and tempered bainite is 60% or more, a retained austenite fraction is 10% or more, a fresh martensite fraction is 5% or less, retained austenite has an average grain size of 0.5 &#x3bc;m or less, retained austenite having a grain size of 1.0 &#x3bc;m or more accounts for 2% or more of the total amount of retained austenite, and a prior austenite grain size is 10 &#x3bc;m or less.

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Kobe, Japan

Hirokazu Natsumeda