Jfe Steel Corporation

Mazda Motor Corporation

Yoshihiko Ono

Shimpei Yoshioka

Yoshimasa Funakawa

Kenji Takahashi

Kaneharu Okuda

Yuma Honda

Shinjiro Kaneko

Nobuyuki Nakamura

Kenji Kawamura

Katsutoshi Takashima

Takuya Hirashima

Shoichiro Taira

Kohei Hasegawa

Yusuke Fushiwaki

Nobusuke Kariya

Michitaka Sakurai

Hiroyuki Akimoto

Junya Tobata

Hiroshi Hasegawa

Hiroshi Matsuda

Tatsuya Nakagaito

Reiko Sugihara

Yusuke Kimata

Hiromi Yoshitomi

Kazuma Mori

Yoichiro Matsui

Takeshi Fujita

Kentaro Sato

Hideyuki Kimura

Hiroyuki Masuoka

Noriaki Kohsaka

Yasuaki Okita

Masaki Koba

Takayuki Futatsuka

Yasushi Kitani

Ryosuke Yamaguchi

Tadachika Chiba

Koichi Taniguchi

Yasushi Tanaka

Kozo Harada

Yasunobu Nagataki

Saiji Matsuoka

Yuka Miyamoto

Sota Goto

Shinichi Furuya

Soshi Yoshimoto

Kazuhiko Yamazaki

Yu Hashimoto

Yasuhiro Sakurai

Tomohiro Hashimukai

Takeshi Murai

Akinori Nakamura

Thi Huyen Doan

Kenta Hidaka

Tetsuya Ueoka

Norifumi Takasuka

Hisanori Ando

The high strength steel sheet of the present invention has a specific chemical composition, and contains, in terms of area fraction relative to an entire steel microstructure, 30% or more and 100% or less ferrite, 0% or more and 70% or less martensite, and less than 20% in total of pearlite, bainite and retained austenite, and the ferrite contains, in terms of area fraction relative to an entire microstructure, 0% or more and 10% or less non-recrystallized ferrite, with a difference of the area fraction of the non-recrystallized ferrite in the longitudinal direction of the steel sheet of 5% or smaller.

High strength steel sheet, high strength member, and methods for manufacturing the same

A steel sheet having a steel microstructure including, by area %, martensite in a range of 20% or more and 100% or less, ferrite in a range of 0% or more and 80% or less, and the balance being 5% or less. A residual stress generated at the transverse center of the steel sheet when the steel sheet is subjected to a V-bending process is 800 MPa or less. A residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is 90% or more and 110% or less of the residual stress generated at the transverse center of the steel sheet. The maximum amount of warpage of the steel sheet sheared to a length of 1 m in the longitudinal direction is 15 mm or less.

Steel sheet, member, and methods for producing the same

A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 5% or more and 60% or less, as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an area fraction of 5% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 15% and less than 85%; BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an aspect ratio of 2.5 or higher is present in an area fraction of 5% or more and 70% or less; and a top 10% of retained austenite in terms of an equivalent circular diameter has an average aspect ratio of 2.5 or higher.

Thin steel sheet and method for manufacturing same

A high-carbon hot-rolled steel sheet and a method for manufacturing the steel sheet are provided. The high-carbon hot-rolled steel sheet has a particular chemical composition. The microstructure of the steel sheet includes ferrite, cementite, and pearlite that accounts for 6.5% or less of the entire microstructure by area fraction. The proportion of the number of cementite grains having an equivalent circle diameter of 0.1 μm or less to the total number of cementite grains is 20% or less, the average cementite grain size is 2.5 μm or less, and the cementite accounts for 1.0% or more and less than 3.5% of the entire microstructure by area fraction. The average concentration of solute B in a region extending from a surface layer to a depth of 100 μm is 10 mass ppm or more. The average concentration of N present as AlN in the region is 70 mass ppm or less.

High-carbon hot-rolled steel sheet and method for manufacturing the same

A thin steel sheet has a specific chemical composition. The thin steel sheet has a microstructure in which ferrite is present in an area fraction of 4% or less (including 0%), as-quenched martensite is present in an area fraction of 10% or less (including 0%), retained austenite is present in an amount of 7% or more and 20% or less, and upper bainite, lower bainite, and tempered martensite are present in a total area fraction of more than 71% and less than 93%; and BCC iron that has a misorientation of 1° or less and surrounds retained austenite having an equivalent circular diameter of 1 μm or less is present in an area fraction of 4% or more and 50% or less, and BCC iron that has a misorientation of more than 1° is present in an area fraction of 25% or more and 85% or less.

The steel sheet has a steel microstructure containing ferrite: 6% to 90% by area, a microstructure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 10% to 94% by area in total, and retained γ: 3% to 20% by volume, a ratio (S/S)×100(%) of an area ratio Sof an upper bainite with a width in the range of 0.8 to 7 μm, a length in the range of 2 to 15 μm, and an aspect ratio of 2.2 or more in contact with retained γwith a grain width in the range of 0.17 to 0.80 μm and an aspect ratio in the range of 4 to 25 to an area ratio Sof the microstructure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ ranges from 2.0% to 15%.

Steel sheet and method for producing the same

A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100% by area percentage, and retained γ: 4% to 15% by volume percentage. Retained γhas a specified area percentage S, retained γhas a specified distribution number N, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained γ grains has a specified equivalent circular grain diameter and aspect ratio.

A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 95% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [% Ti]+[% Nb]>0.007 and [% Ti]×[% Nb]≤7.5×10are satisfied in the chemical composition.

Steel sheet, member, and methods for producing them

A steel sheet having a specified chemical composition and a tensile strength of 1,320 MPa or more, and methods for producing the steel sheet. The steel sheet has a specified microstructure including martensite and bainite, the total area fraction of the martensite and the bainite being 92% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite. The forumulae [% Ti]+[% Nb]>0.007 and [% Ti]×[% Nb]≤7.5×10are satisfied in the chemical composition.

A high-yield-ratio high-strength electrogalvanized steel sheet having an electrogalvanized coating layer formed on a surface of a base steel sheet, in which the base steel sheet has a certain chemical composition, and a steel microstructure, in which a total area fraction of one or both of bainite containing carbides having an average grain diameter of 50 nm or less and tempered martensite containing carbides having an average grain diameter of 50 nm or less is 90% or more in the whole of the steel microstructure, and in which a total area fraction of one or both of the bainite containing and the tempered martensite containing carbides is 80% or more in a region from the surface of the base steel sheet to a position located at ⅛ of a thickness of the base steel sheet, and diffusible hydrogen in steel in an amount of 0.20 mass ppm or less.

High-yield-ratio high-strength electrogalvanized steel sheet and method for manufacturing the same

A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure comprising ferrite: 5% or less, and at least one of upper bainite, fresh martensite, tempered martensite, lower bainite, and retained γ: 95% to 100%, and retained γ: 5% to 20%. Retained γhas a specified area percentage S, retained γhas a specified distribution number N, and at least one of (i) fresh martensite has a specified equivalent circular grain diameter and aspect ratio and (ii) retained γ grains has a specified equivalent circular grain diameter and aspect ratio.

A high-strength steel sheet of the present invention has a specific chemical composition. Furthermore, in the steel sheet, a degree of Mn segregation in a specific region is 1.5 or less; a maximum P concentration in a specific region is 0.08 mass % or less; in a specific region, at least one specific MnS particle group is present, the number of specific MnS particle groups is 2.0 or fewer per 1 mm, and the number of specific oxide-based inclusions is 8 or fewer per 1 mm; of all oxide-based inclusions, oxide-based inclusions having a specific composition are present in a number ratio of 80% or greater; the microstructure includes, in terms of a volume fraction, 30 to 95% martensite, 5 to 70% ferrite phase, less than 30% (and 0% or greater) bainite, and less than 2.0% (and 0% or greater) austenite phase; and a tensile strength is 980 MPa or greater.

High-strength steel sheet and method for producing same

A high-strength steel sheet according to the present invention includes a specific chemical composition, and a steel structure in which a total area fraction of martensite and bainite in a position of ¼ of a sheet thickness is 95% or more and 100% or less, the balance in a case where the total area fraction is not 100% contains retained austenite, and an area fraction of ferrite in a region extending up to 10 μm in a sheet thickness direction from a surface is 10% or more and 40% or less, in which a tensile strength is 1320 MPa or more, and a Vickers hardness in a position of 15 μm in the sheet thickness direction from the surface satisfies a specified formula.

High-strength steel sheet and method for manufacturing same

The high-strength steel sheet according to the present invention includes a specific chemical composition, a steel structure in which a total area fraction of martensite and bainite in a position of ¼ of a sheet thickness is 92% or more and 100% or less, the balance in a case where the total area fraction is not 100% contains retained austenite, and an area fraction of ferrite in a region extending up to 10 μm in a sheet thickness direction from a surface is 10% or more and 40% or less, in which a tensile strength is 1320 MPa or more, and a Vickers hardness in a position of 15 μm in the sheet thickness direction from the surface satisfies a specified formula.

A high-strength galvanized steel sheet of the present invention includes a steel sheet having a specific chemical composition, and a steel structure containing, in terms of area ratio, 4% or more and 20% or less of retained austenite, 30% or less (including 0%) of ferrite, 40% or more of martensite, and 10% or more and 50% or less of bainite; and a galvanized layer provided on the steel sheet, in which an amount of diffusible hydrogen in the steel is less than 0.20 mass ppm, a tensile strength is 1100 MPa or more, a relationship between a tensile strength TS (MPa), an elongation El (%), and a sheet thickness t (mm) satisfies a (1) formula below, and a yield ratio YR is 67% or more.TS&#xd7;(El+3&#x2212;2.5)&#x2265;13000 &#x2003;&#x2003;(1)

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

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Tokyo, Japan

Yoshihiko Ono