Jfe Steel Corporation

Yoshie Obata

Katsutoshi Takashima

Takeshi Yokota

Hiroshi Matsuda

Minoru Tanaka

Shinjiro Kaneko

Yoshiyasu Kawasaki

Kazuhiro Seto

Takashi Kobayashi

Yuji Tanaka

Shin Ishikawa

Takeshi Nishiyama

Masaki Tada

Yuki Toji

Michitaka Sakurai

Takako Yamashita

Yusuke Kimata

Keiji Ueda

Shinsuke Komine

Naoya Kiyokane

Yoshiko Nakahara

A high-strength cold-rolled steel sheet comprises: a chemical composition that contains C, Si, Mn, P, S, N, Al, Ti, Nb, and B with a balance consisting of Fe and inevitable impurities, and satisfies [mol % N]/[mol % Ti]<1; and a steel microstructure in which: an area fraction of ferrite is 12% or more and less than 30%; a total area fraction of tempered martensite and bainite is 55% or more and 85% or less; an area fraction of quenched martensite is 15% or less; an area fraction of retained austenite is 1% or more and 10% or less; an area fraction of low-Mn ferrite having a Mn concentration of 0.8×[% Mn] or less is 5% or more and 20% or less; a result of subtracting the area fraction of the low-Mn ferrite from the area fraction of the ferrite is 10% or more; an area fraction of a residual microstructure is less than 3%; and an average grain size of the low-Mn ferrite is 10 μm or less.

High-strength cold-rolled steel sheet, high-strength coated or plated steel sheet, method of producing high-strength cold-rolled steel sheet, and method of producing high-strength coated or plated steel sheet

A high-strength cold rolled steel sheet has a specific composition and a steel structure that is, by volume %, 10%-70% ferrite, 1%-10% retained austenite, 10%-60% bainite, and 2%-50% martensite, the average crystal grain size of the ferrite being no more than 6.0 μm, the average crystal grain size of the retained austenite being no more than 4.0 μm, the average crystal grain size of the bainite being no more than 6.0 μm, and the average crystal grain size of the martensite being no more than 4.0 μm. The concentration ratio of the average concentration of Si to a depth of 10 μm from the surface of the high-strength cold-rolled steel sheet to the average concentration of Si throughout the high-strength cold-rolled steel sheet is, by mass ratio, greater than 1.00 but less than 1.30.

High-strength cold-rolled steel sheet and production method for same

Provided is a hot-pressed member that combines both high strength of 1850 MPa or more in TS and excellent delayed fracture resistance. A hot-pressed member comprises: a predetermined chemical composition; a microstructure in which a prior austenite average grain size is 8 μm or less, a volume fraction of martensite is 95% or more, and a volume fraction of granular carbide of 0.1 μm or more in grain size is 0.10% to 4.0%; a Ni diffusion region of 2.0 μm or more in a depth direction in a surface layer; and a tensile strength of 1850 MPa or more.

Hot-pressed member, cold-rolled steel sheet for hot pressing, and manufacturing methods therefor

A hot press member includes excellent indentation peel strength which has a tensile strength of 1780 MPa or more. A plating layer has at a surface thereof a 10-point average roughness Rzjis of 25 μm or less, and a steel sheet contains, in mass %, not less than 0.25% but less than 0.50% of C, 1.5% or less of Si, 1.1-2.4% of Mn, 0.05% or less of P, 0.005% or less of S, 0.01-0.50% of Al, 0.010% or less of N, 0.001-0.020% of Sb, 0.005-0.15% of Nb, and 0.005-0.15% of Ti, the balance being Fe and incidental impurities. The average crystal grain size of prior austenite is 7 μm or less and the volume proportion of martensite is 90% or more, within 50 μm in the thickness direction from the surface of the steel sheet excluding the plating layer.

Hot press member, production method for steel sheet for hot press, and production method for hot press member

A photographing condition determining method includes: photographing a part of a metal structure of a metal material subjected to predetermined sample preparation under a predetermined photographing condition; assigning, to pixels corresponding to one or a plurality of predetermined phases of the metal structure, labels of respective phases for a photographed image; calculating one or more feature values for a pixel to which a label of one of the assigned phases; classifying the phases of the metal structure of the image by inputting a calculated feature value to a model, which has been learned in advance using feature values assigned with labels of respective phases as input and labels of the respective phases as output, and acquiring a label of a phase of a pixel corresponding to the input feature value; and determining a photographing condition when other parts of the metal structure are photographed based on a classification result.

Photographing condition determining method for metal structure, photographing method for metal structure, phase classification method for metal structure, photographing condition determining device for metal structure, photographing device for metal structure, phase classification device for metal structure, material property estimating method for metal material, and material property estimating device for metal material

A high-strength cold rolled steel sheet having a specified chemical composition and a method for producing the same. The steel sheet having a microstructure including, in terms of volume fraction, 35% or less of ferrite, 1% or more and 10% or less of retained austenite, 2% or more and 12% or less of as-quenched martensite, and a total of 25% to 70% of bainite and tempered martensite. The microstructure has an average crystal grain diameter of the ferrite: 5.0 &#x3bc;m or less, an average crystal grain diameter of the retained austenite: 2.0 &#x3bc;m or less, an average crystal grain diameter of the as-quenched martensite: 3.0 &#x3bc;m or less, an average crystal grain diameter of the bainite and the tempered martensite phase: 4.0 &#x3bc;m or less, and an average intergrain distance of the as-quenched martensite of 1.0 &#x3bc;m or more.

High-strength cold rolled steel sheet and method for producing the same

A steel having a composition containing C: more than 0.20% and 0.45% or less, Si: 0.50% to 2.50%, Mn: 2.00% or more and less than 3.50%, and one or two selected from Ti: 0.005% to 0.100% and Nb: 0.005% to 0.100% is hot-rolled and cold-rolled. The steel sheet is heated to 800&#xb0; C. to 950&#xb0; C. and cooled to a cooling-end temperature of 350&#xb0; C. to 500&#xb0; C. at a cooling rate of 5&#xb0; C./s or more to form a steel sheet having a microstructure including martensite and bainite phases such that the total proportion of the martensite and bainite phases is 80% or more by volume. The steel sheet is heated to 700&#xb0; C. to 840&#xb0; C. and maintained at 700&#xb0; C. to 840&#xb0; C., cooled to a cooling-end temperature of 350&#xb0; C. to 500&#xb0; C. at a cooling rate of 5 to 50&#xb0; C./s, and maintained within the above temperature range for 10 to 1800 s.

Thin high-strength cold-rolled steel sheet and method of producing the same

Disclosed is a high-strength steel sheet having a predetermined chemical composition, satisfying the condition that Mn content divided by B content equals 2100 or less, and a steel microstructure that contains, by area, 25-80% of ferrite and bainitic ferrite in total, 3-20% of martensite, and that contains, by volume, 10% or more of retained austenite, in which the retained austenite has a mean grain size of 2 &#x3bc;m or less, a mean Mn content in the retained austenite in mass % is at least 1.2 times the Mn content in the steel sheet in mass %, and an aggregate of retained austenite formed by seven or more identically-oriented retained austenite grains accounts for 60% or more by area of the entire retained austenite.

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Tokyo, Japan

Yoshie Obata