Jfe Steel Corporation

Yuki Toji

Yoshimasa Funakawa

Takashi Kobayashi

Takeshi Yokota

Shinjiro Kaneko

Yoshiyasu Kawasaki

Kohei Hasegawa

Katsutoshi Takashima

Kazuki Endoh

Mai Aoyama

Takaaki Tanaka

Kazuki Endo

Masayasu Ueno

Takako Yamashita

Muneka Iwasawa

Yoshitsugu Suzuki

Tatsuya Nakagaito

Yuji Tanaka

Satoshi Maeda

Kazuhiro Seto

Takeshi Nishiyama

Kazukuni Hase

Masaki Tada

Saiji Matsuoka

Kentaro Sato

Hideyuki Kimura

Hidekazu Minami

Nobusuke Kariya

Katsuyuki Ichimiya

Shunsuke Yamamoto

Yusuke Kimata

Lingling Yang

Takayuki Futatsuka

Yusuke Wada

Akio Ohmori

Shinya Yamaguchi

Shinsuke Komine

Tomomi Kanazawa

Yoshie Obata

Junya Tobata

Koichi Nakashima

Fusae Shiimori

Michitoshi Saeki

Ryo Arao

A high strength steel sheet with a chemical composition containing C, Si, Mn, P, S, Al, N, Nb, Ti, and B, where [% Si]/[% Mn] is 0.10 or more and 0.60 or less, and free Ti content determined from Formula (1), below, is 0.001 mass % or more, with the balance being Fe and inevitable impurity. In a steel sheet surface layer, a total amount of Nb solute and 100 nm or smaller precipitate is 0.002 mass % or more.

High strength steel sheet, high strength coated or plated steel sheet, methods of producing same, and member

A high strength galvanized steel sheet is disclosed having a tensile strength of 1,320 MPa or more. A chemical composition of a steel sheet contains, by mass, C: 0.150 to 0.450%, Si: 0.80 to 3.00%, Mn: 2.00 to 4.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 0.100%, O: not more than 0.0100%, and N: not more than 0.0100%, with a balance of Fe and inevitable impurities. An amount of diffusible hydrogen in steel is not more than 0.60 mass ppm. A total of tempered martensite and bainite is 60 to 95%, retained austenite is 5 to 30%, and retained austenite having an aspect ratio of 5.5 or higher with respect to an entire retained austenite is not more than 50%, and fresh martensite having a diameter of 2.0 μm or less is not more than 20%.

High-strength galvanized steel sheet and member, and method for manufacturing same

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

Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, martensite: 5% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

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

Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

A high strength steel sheet and method for manufacturing the same. The steel sheet has a specified chemical composition and a specified microstructure. A value calculated by dividing the average Mn content in retained austenite by the average Mn content in ferrite is 1.5 or more, a value calculated by dividing the sum of the area fraction of as-quenched martensite having a circle-equivalent grain size of 3 μm or more and the area fraction of retained austenite having a circle-equivalent grain size of 3 μm or more by the sum of the area fraction of all the as-quenched martensite and the area fraction of all the retained austenite is less than 0.4, and a value calculated by dividing the area fraction of retained austenite grains adjacent to three or more ferrite grains having different crystal orientations by the area fraction of all the retained austenite is less than 0.6.

A high strength steel sheet has a yield-point elongation of 1.0% or greater and a tensile strength of 980 MPa or greater. The high strength steel sheet has a specific chemical composition and microstructure. The ferrite has an average grain size of 5.0 μm or less, the retained austenite has an average grain size of 2.0 μm or less, a value obtained by dividing a Mn content of the retained austenite by a Mn content of steel is 1.50 or greater, 15% or more of all retained austenite grains in the retained austenite have an aspect ratio of 3.0 or greater, and 15% or more of all the retained austenite grains in the retained austenite have an aspect ratio of less than 2.0.

High strength steel sheet, impact absorbing member, and method for manufacturing high strength steel sheet

A high strength steel sheet has a yield-point elongation of 1% or greater and a tensile strength of 980 MPa or greater. The high strength steel sheet has a specific chemical composition and microstructure. A ratio of retained austenite grains adjoining a retained austenite grain having a different crystal orientation to total retained austenite grains is 0.60 or greater, the ferrite has an average grain size of 5.0 μm or less, and the retained austenite has an average grain size of 2.0 μm or less. A value obtained by dividing a volume fraction Vγa by a volume fraction Vγb is 0.40 or greater, where the volume fraction Vγa is a volume fraction of retained austenite in a fractured portion of a tensile test specimen after a warm tensile test at 150° C., and the volume fraction Vγb is a volume fraction of retained austenite before the warm tensile test at 150° C.

A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80% and tempered martensite being greater than 5% and 20% or less in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 &#x3bc;m or less; and the retained austenite being 3 &#x3bc;m or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

High-strength steel sheet and method for manufacturing same

A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80%, martensite being 5% to 35%, and tempered martensite being 0% to 5% in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 &#x3bc;m or less; and the retained austenite being 3 &#x3bc;m or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

A high-strength steel includes a steel structure with: in area fraction, 60.0% to less than 90.0% of ferrite, 0% to less than 5.0% of unrecrystallized ferrite, 2.0% to 25.0% of martensite, 0% to 5.0% of carbide, and 0% to 3.0% of bainite; in volume fraction, more than 7.0% of retained austenite; in a cross-sectional view of 100 &#x3bc;m&#xd7;100 &#x3bc;m, a value obtained by dividing number of retained austenite that are not adjacent to retained austenite whose crystal orientations are different by a total number of retained austenite being less than 0.80, an average crystal grain size of the ferrite being 6.0 &#x3bc;m or less, an average crystal grain size of the retained austenite being 3.0 &#x3bc;m or less, and a value obtained by dividing, by mass %, an average content of Mn in the retained austenite by an average content of Mn in steel being 1.50 or more.

High-strength steel sheet and production method thereof

Provided are a high-strength cold rolled steel sheet that has a tensile strength of 980 MPa or more, excellent ductility, and a low failure rate in a hole expanding test, and a method for manufacturing the same. A high-strength cold rolled steel sheet comprises a predetermined composition, wherein a total area ratio of ferrite and bainitic ferrite is 20% to 80%, an area ratio of retained austenite (RA) is more than 10% and 40% or less, an area ratio of tempered martensite is more than 0% and 50% or less, a ratio of RA with an aspect ratio of 0.5 or less is 75% or more, a ratio of RA with an aspect ratio of 0.5 or less in ferrite grain boundaries is 50% or more, and an average KAM value of bcc phase is 1&#xb0; or less.

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

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Tokyo, Japan

Yuki Toji