Nippon Steel Corporation

Kazuki Shiokawa

Takafumi Yokoyama

Hiroyuki Kawata

Riki Okamoto

Yuji Yamaguchi

Yuichi Nakahira

Masahiro Nakata

Satoshi Uchida

Kohichi Sano

Eisaku Sakurada

Kohei Ueda

Kunio Hayashi

Tooru Akashi

Takayuki Otsuka

Tatsuya Obuchi

Yuki Takeuchi

Hiroki Matsubara

Provided are a steel sheet having a predetermined chemical composition and having the following steel structure, and a method of manufacturing the steel sheet. (1) In area ratio %, ferrite: 0 to 5%, martensite: 90 to 100%, a ratio of tempered martensite to total martensite: 80 to 100%, and retained austenite: 0.5 to 6.0% are contained. (2) The number density of inclusions satisfying the maximum diameter≥3 μm is 40 inclusions/mmor less. (3) When the number density of the inclusions satisfying the maximum diameter≥3 μm in each section is calculated, the number density in the section where the number density of inclusions is in the top 10% is 80 inclusions/mmor less. (4) Formula (A) is satisfied.γ≥0.1  (A) Vγ: Initial retained austenite, Vγ′: Retained austenite after deep cooling at −196° C. (5) The tensile strength is 1470 MPa or more.

Steel sheet

A coil skid on which a hot-rolled coilobtained by coiling a hot-rolled steel sheet is placed includes: at least two or more layers of a metal plateand at least two or more layers of a heat insulating materialeach stacked alternately, in which the metal plateis in contact with the hot-rolled coil. The uneven cooling of an outer peripheral portion of the hot-rolled coil caused by the coil skid is prevented while ensuring the strength of the coil skid, to thereby inhibit the uneven hardness in the longitudinal direction of the hot-rolled coil resulting from this uneven cooling.

Coil skid and steel sheet manufacturing method

A steel sheet wherein a steel structure of an inside of the steel sheet contains, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, a sum of pearlite and cementite: 0% to 10%, and a remainder including hard ferrite. In the steel sheet, in a &#x215b; to &#x215c; thickness range, a proportion of retained austenite having an aspect ratio of 2.0 or more is 50% or more, and a soft layer having a thickness of 1 to 100 &#x3bc;m from a surface in a sheet thickness direction is present. When an emission intensity at a wavelength indicating Si is analyzed in the sheet thickness direction from the surface by a radio-frequency glow discharge analysis method, a peak of the emission intensity appears in a range of more than 0.2 &#x3bc;m and 5.0 &#x3bc;m or less from the surface.

Steel sheet and manufacturing method of therefor

A steel sheet has a chemical composition with a steel structure containing, by volume fraction, soft ferrite: 0-30%, retained austenite: 3-40%, fresh martensite: 0-30%, pearlite and cementite: 0-10%, and a remainder including hard ferrite. In the steel sheet, a number proportion of retained austenite having an aspect ratio of 2.0 or more in the total retained austenite is 50% or more, and a soft layer having a thickness of 1-100 &#x3bc;m is present. In the soft layer, a volume fraction of ferrite grains having an aspect ratio of less than 3.0 is 50% or more, and a volume fraction of retained austenite is 50% or more of the volume fraction of the retained austenite of the inside of the steel sheet. A peak of an emission intensity at a wavelength indicating Si appears in a range of more than 0.2 &#x3bc;m to 5 &#x3bc;m or less from the surface.

Steel sheet and manufacturing method therefor

A cold-rolled steel sheet is provided that has a tensile strength of 980 MPa or more, and has a prescribed chemical composition. The microstructure is composed of, in area %, ferrite: 1 to 29%, retained austenite: 5 to 20%, martensite: less than 10%, pearlite: less than 5%, and the balance: bainite and/or tempered martensite. The total sum of the lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 &#x3bc;m or more is 100 &#x3bc;m or less per 1000 &#x3bc;m. The cold-rolled steel sheet is excellent in workability and low-temperature toughness, and in particular is excellent in low-temperature toughness after introduction of plastic strain.

Cold-rolled steel sheet and hot-dip galvanized cold-rolled steel sheet

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Tokyo, Japan

Kazuki Shiokawa