Nippon Steel Corporation

Eriko Tsukamoto

Kengo Takeda

Katsuya Nakano

Yuya Suzuki

Provided is a steel sheet having a predetermined chemical composition, wherein precipitates having a diameter of less than 0.1 μm are present in a number density of 10 to 200/μmin a depth region of 1 to 10 μm from a surface, an amount of dissolved C in a depth region of 10 to 60 μm from the surface is less than 0.20 mass %, and a tensile strength is 1200 MPa or more. Further, provided is a method for producing a steel sheet comprising a step of hot rolling a steel slab having a predetermined chemical composition, then coiling it at 580° C. or less, a step of pickling the hot rolled steel sheet to remove oxide scale and remove the surface layer of the hot rolled steel sheet down to at least 5 μm, and a step of cold rolling the hot rolled steel sheet, then holding it in an atmosphere of a dew point of −20 to 20° C. at a temperature region of 200 to 400° C. for 20 to 180 seconds and holding it at a temperature region of 740 to 900° C. for 40 to 300 seconds.

Steel sheet and method for producing same

This steel sheet has a predetermined chemical composition, the metallurgical structure at a thickness ¼ portion is, by area ratio, martensite: 40% to 97%, ferrite+bainite: 50% or less, residual austenite: 3% to 20% and a remainder in microstructure: 5% or less, the aspect ratio of residual austenite having an aspect ratio of 3 or more is 80% or more with respect to the total area of residual austenite, and the number of carbides having a grain diameter of 8 to 40 nm per square micrometer is five or more in the residual austenite.

Steel sheet and manufacturing method thereof

This steel sheet has a predetermined chemical composition, the metallographic structure includes, by an area ratio, ferrite, bainite and pearlite: 0 to 10% in total, residual austenite: 1 to 15%, and a remainder that is martensite, and a proportion of martensite or residual austenite having a width of 50 nm to 2 μm present at a prior austenite grain boundary is 70% or more.

Steel sheet and method for manufacturing same

This steel sheet has a predetermined chemical composition, and includes, as a metallographic structure, ferrite, bainite, and pearlite in a total volume percentage of 0% or more and 50% or less, residual austenite in a volume percentage of 3% or more and 20% or less, and a remainder of fresh martensite and tempered martensite, in which residual austenite having an aspect ratio of 3.0 or more occupies 80% or more of a total residual austenite by area ratio, the steel sheet includes an internal oxide layer having a thickness of 4.0 μm or more from a surface of the steel sheet and a decarburized layer having a thickness of 10 μm or more and 100 μm or less from the surface of the steel sheet, and an amount of diffusible hydrogen included in the steel sheet is 1.00 ppm or less on a mass basis.

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Tokyo, Japan

Eriko Tsukamoto