Jfe Steel Corporation

Jfe Steel Corporatin

Yoshimasa Funakawa

Hiroshi Hasegawa

Takashi Kobayashi

Yoshihiko Ono

Yuki Toji

Noriaki Kohsaka

Hiroshi Matsuda

Yasushi Tanaka

Seiji Nakajima

Hidekazu Minami

Hiroshi Nakano

Kaoru Sato

Katsutoshi Takashima

Nobuyuki Nakamura

Yoshiyasu Kawasaki

Yoshitsugu Suzuki

Tatsuya Nakagaito

Kozo Harada

Yoichi Makimizu

Yoshikazu Suzuki

Reiko Sugihara

Kenji Kawamura

Nobusuke Kariya

Takeshi Yokota

Tomohiro Ishii

Shinjiro Kaneko

Takumi Ujiro

Kazuhiro Seto

Hideyuki Kimura

Chikaumi Sawanishi

Masayasu Ueno

Kunio Fukuda

Takashi Ueno

Satoshi Maeda

Kohei Hasegawa

Akio Kobayashi

Saiji Matsuoka

Kaneharu Okuda

Kentaro Sato

Toru Hoshi

Kunikazu Tomita

Takanobu Saito

Tsuyoshi Shiozaki

Yuka Miyamoto

Michitaka Sakurai

Shimpei Yoshioka

Hiroshi Nakata

Mai Aoyama

Katsumi Nakajima

Hayato Saito

Yuma Honda

Taro Kizu

Kazuma Mori

Kazuki Endo

Tetsuo Yamamoto

Takako Yamashita

Takayuki Murata

Lingling Yang

Yoshimasa Himei

Noriaki Moriyasu

Hidekazu Ookubo

Tokunori Kanemura

Hiroshi Uchomae

Gentaro Takeda

Takayuki Futatsuka

Hiromi Yoshitomi

Shoji Kasai

Yukihiro Matsubara

Masayuki Ohta

Tetsuya Mega

Eiji Maeda

Minoru Suwa

Masato Shigemi

Kazuki Endoh

Kazutaka Okimoto

Noriaki Kosaka

Takahiko Ogura

Ryosuke Yamaguchi

Tamako Ariga

Nobuyuki Kageyama

Toshihiro Kasamo

Katsuhiro Kobori

Yusuke Yasufuku

Hiroshi Owada

Muneka Iwasawa

Shinji Yamazaki

Reiko Mizuno

Shuji Okada

Yasuhiro Murao

Yasunobu Uchida

Mayumi Ojima

Hidekazu Okubo

A high-strength steel sheet with a tensile strength of 1,180 MPa or more has a predetermined chemical composition and a steel microstructure in which the area fraction of ferrite is 5% or less, the area fraction of martensite is 2% to 10%, the area fraction of bainite is 5% to 37%, the area fraction of tempered martensite is 42% to 65%, the volume fraction of retained austenite is 3% to 15%, the average grain size of ferrite and bainite is 3 μm or less, in a region extending 50 μm from a surface of the steel sheet in a through-thickness direction, and the average grain size of prior austenite grains is 10 μm or less, the average grain size of the prior austenite grains in the through-thickness direction is 0.9 or less of the average grain size thereof in a rolling direction.

High-strength steel sheet and method for manufacturing the same

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.

High strength steel sheet and method for manufacturing the same

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 method of producing a hot-rolled steel sheet and a method for producing a cold-rolled full hard steel sheet are provided. The methods comprising heating a steel slab having a composition containing, in terms of mass %, C: 0.010% or more and 0.150% or less, Si: 0.20% or less, Mn: 1.00% or less, P: 0.100% or less, S: 0.0500% or less, Al: 0.001% or more and 0.100% or less, N: 0.0100% or less, and the balance being Fe and unavoidable impurities, in which 0.002%≤[% P]+[% S]≤0.070% ([% M] denotes a content (mass %) of M element in steel) is satisfied.

Method for producing hot-rolled steel sheet and method for producing cold-rolled full hard steel sheet

A steel sheet is provided, having a tensile strength of 590 MPa or more, a particular composition and a steel structure that contains, in terms of area fraction, particular amounts of ferrite and martensite, in which the ferrite average crystal grain size is 20 μm or less, the martensite average size is 15 μm or less, the ratio of the average crystal grain size of the ferrite to the average size of the martensite (ferrite average crystal grain size/martensite average size) is 0.5 to 10.0, the ratio of the hardness of the martensite to the hardness of the ferrite (martensite hardness/ferrite hardness) is 1.0 or more and 5.0 or less, and, in the texture of the ferrite, the inverse intensity ratio of γ-fiber to the α-fiber is 0.8 or more and 7.0 or less.

Steel sheet, coated steel sheet, method for producing hot-rolled steel sheet, method for producing cold-rolled full hard steel sheet, method for producing heat-treated steel sheet, method for producing steel sheet, and method for producing coated steel sheet

A method for producing a hot-rolled steel sheet, a method for producing a cold-rolled full-hard steel sheet, and a method for producing a heat-treated sheet are provided herein. The methods comprising hot rolling a steel material of a composition comprising, in mass %, C: 0.05 to 0.12%, Si: 0.80% or less, Mn: 1.30 to 2.10%, P: 0.001 to 0.050%, S: 0.005% or less, Al: 0.01 to 0.10%, N: 0.010% or less, one or more selected from Cr in an amount of 0.05 to 0.50%, and Mo in an amount of 0.05 to 0.50%, one or more selected from Ti in an amount of 0.01 to 0.10%, Nb in an amount of 0.01 to 0.10%, and V in an amount of 0.01 to 0.10%, and the balance Fe and unavoidable impurities.

Method for producing hot-rolled steel sheet, method for producing cold-rolled full-hard steel sheet, and method for producing heat-treated sheet

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

A steel sheet having excellent fatigue resistance as a material for automobile parts and a TS of 590 MPa or more, and a method for producing the same. The steel sheet having a specified chemical composition and a microstructure of 50% or more and 90% or less of martensite, 10% to 50% of ferrite and bainite in total, in terms of an area ratio, and an average concentration of solute Mn in ferrite in a region from a surface of a base steel to a depth of 0.5 &#x3bc;m is 60% or more relative to an average concentration of solute Mn in ferrite at a location of &#xbc; in the thickness of the steel sheet.

Steel sheet, plated steel sheet, method for producing hot-rolled steel sheet, method for producing cold-rolled full hard steel sheet, method for producing steel sheet, and method for producing plated steel sheet

A high-strength galvanized steel sheet having improved post-work impact resistance, a method for producing the high-strength galvanized steel sheet, and a high-strength member produced using the steel sheet. The high-strength galvanized steel sheet includes a steel sheet having a microstructure including ferrite and carbide-free bainite, martensite and carbide-containing bainite, and retained austenite, the total area fraction of ferrite and carbide-free bainite being 0% to 55%, the total area fraction of martensite and carbide-containing bainite being 45% to 100%, and the area fraction of retained austenite being 0% to 5%. Additionally, a galvanizing layer is disposed on the steel sheet. The density of gaps that cut across the entire thickness of the galvanizing layer in a cross section of the galvanizing layer, which is taken in the thickness direction so as to be perpendicular to the rolling direction, is 10 gaps/mm or more.

High-strength galvanized steel sheet, high-strength member, and method for producing high-strength galvanized steel sheet

To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 590 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 35% or more and 80% or less and martensite is 5% or more and 25% or less in area ratio, retained austenite is 8% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 6.0 &#x3bc;m or less, 3.0 &#x3bc;m or less and 3.0 &#x3bc;m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

High-strength steel sheet and method for producing same

To provide a high-strength steel sheet with excellent ductility and hole expansion formability, a yield ratio of less than 68%, and a tensile strength of 980 MPa or more, by having a predetermined chemical composition and a microstructure where ferrite is 15% or more and 55% or less and martensite is 15% or more and 30% or less in area ratio, retained austenite is 12% or more in volume fraction, the average grain size of ferrite, martensite and retained austenite is 4.0 &#x3bc;m or less, 2.0 &#x3bc;m or less and 2.0 &#x3bc;m or less respectively, the average aspect ratio of crystal grain of ferrite, martensite and retained austenite is each more than 2.0 and 15.0 or less, and the value obtained by dividing the Mn content (mass %) in retained austenite by the Mn content (mass %) in ferrite is 2.0 or more.

Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing of 1780 MPa or more, excellent delayed fracture resistance, and high cross tensile strength after resistance spot welding by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 8 &#x3bc;m or less, a volume fraction of martensite is 90% or more, and at least 10 cementite grains having a grain size of 0.05 &#x3bc;m or more are present on average per 200 &#x3bc;mof a cross section parallel to a thickness direction of the member, and such that at least 10 Ti-based precipitates having a grain size of less than 0.10 &#x3bc;m are present on average per 100 &#x3bc;mof the cross section parallel to the thickness direction of the member in a range of 100 &#x3bc;m in the thickness direction from a surface of the member.

Hot-pressed member and method for manufacturing same, and cold-rolled steel sheet for hot pressing and method for manufacturing same

A high-strength steel sheet for warm working having excellent warm workability, and a method for manufacturing the steel sheet. The steel sheet has a chemical composition including, by mass %, C: 0.05% to 0.20%, Si: 3.0% or less, Mn: 3.5% to 8.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 3.0%, N: 0.010% or less, one or more selected from Nb: 0.005% to 0.20%, Ti: 0.005% to 0.20%, Mo: 0.005% to 1.0%, and V: 0.005% to 1.0%. The steel sheet has a microstructure including, in terms of area ratio, 10% to 60% of retained austenite, 10% to 80% of ferrite, 10% to 50% of martensite, and 0% to 5% of bainite, in which a C content in the retained austenite is less than 0.40 mass % and the average crystal grain diameter of the retained austenite, the martensite, and the ferrite is 2.0 &#x3bc;m or less.

High-strength steel sheet for warm working and method for manufacturing the same

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Tokyo, Japan

Yoshimasa Funakawa