Jfe Steel Corporation

Noriaki Kohsaka

Yoshimasa Funakawa

Tatsuya Nakagaito

Lingling Yang

Yoshikazu Suzuki

Kenji Kawamura

Shinjiro Kaneko

Yasushi Tanaka

Yoshihiko Ono

Kazuhiro Seto

Michitaka Sakurai

Mai Aoyama

Yoshimasa Himei

Yasuhiro Nishimura

Tadachika Chiba

Yoshitsugu Suzuki

Hideyuki Kimura

Reiko Sugihara

Takuya Hirashima

Hidetaka Kawabe

Yuma Honda

Soshi Yoshimoto

Yasushi Kitani

Masahide Watanabe

Junya Tobata

Takeo Kikuchi

Tomohiro Hashimukai

Akinori Nakamura

Masahide Watabe

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 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.

A thin steel sheet has a steel structure which has a ferrite area fraction of 30% or less, a bainite area fraction of 5% or less, a martensite and tempered martensite area fraction of 70% or more, and a retained austenite area fraction of 2.0% or less and in which the ratio of the dislocation density in the range of 0 μm to 20 μm from a surface of the steel sheet to the dislocation density of a through-thickness central portion of the steel sheet is 90% to 110% and the average of the top 10% of the sizes of cementite grains located in a depth of up to 100 μm from a surface of the steel sheet is 300 nm or less. The maximum camber of the steel sheet sheared to a length of 1 m in a longitudinal direction of the steel sheet is 15 mm or less.

Thin steel sheet and method for manufacturing the same

Provided are a coated steel sheet and so forth, the coated steel sheet having a tensile strength of 590 MPa or more and good stretch-flangeability. The coated steel sheet includes a specific component composition, in which the area fraction of a ferrite phase is 80% or more and 98% or less, the area fraction of a martensite phase of 2% or more and 15% or less, ferrite grains have an average thickness of 3.0 &#x3bc;m or less in the sheet-thickness direction, the martensite phase has an average grain size of 2.0 &#x3bc;m or less, and a Nb-containing carbide precipitated in the ferrite grains has an average grain size of 8 nm or less, and in which the steel sheet has a tensile strength of 590 MPa or more.

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

Provided are a steel sheet having a tensile strength of 950 MPa or more and good toughness and a method for manufacturing the same. The steel sheet has a specific composition and a metallographic structure containing: a ferrite area fraction of 30% or less (including 0%), a tempered martensite area fraction of 70% or more (including 100%), and a retained austenite area fraction of 4.5% or less (including 0%), wherein the average aspect ratio of an iron based carbide, precipitated in tempered martensite grains, having a grain size in the largest 10% is 3.5 or more.

Steel sheet and method for manufacturing the same

Provided are a high-strength steel sheet having a yield strength of 550 MPa or higher and having a small amount of springback and width-direction uniformity in material properties as well as a manufacturing method therefor. The high-strength steel sheet has a yield strength (YP) of 550 MPa or higher and has a specific component composition and a microstructure containing a ferrite phase, 40 to 70% of a martensite phase in area ratio, and 5 to 30% of a bainite phase in area ratio, where: an average grain size of the martensite phase is 2 to 8 &#x3bc;m and an average grain size of the ferrite phase is 11 &#x3bc;m or less on a cross-section in the thickness direction and in a direction orthogonal to a rolling direction; and the average grain size of the ferrite phase is 3.0 times or less the average grain size of martensite.

High-strength steel sheet and manufacturing method therefor

A high-strength steel sheet contains, in mass %, C: 0.07 to 0.14%, Si: 0.65 to 1.65%, Mn: 1.8 to 2.6%, P: 0.05% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.0060% or less, Ti: 0.005 to 0.030%, B: 0.0002 to 0.0030%, and either or both of Cr: 0.01 to 0.40% and Mo: 0.01 to 0.50% and satisfies the expression (1); where: an average grain size of a ferrite phase is 1.5 &#x3bc;m or less; an area ratio of the ferrite phase is 2% or more and 15% or less; an area ratio of a tempered martensite phase is 75% or more and 96% or less; and a total length per unit area of an interface between an untempered martensite phase and the ferrite phase and an interface between the untempered martensite phase and the tempered martensite phase is 6.3&#xd7;10&#x3bc;m/mto 5.0&#xd7;10&#x3bc;m/m.

Provided are a high-strength coated steel sheet and a method for manufacturing the same. The high-strength coated steel sheet has a base steel sheet and a coating layer formed on a surface of the base steel sheet. The base steel sheet has a specified chemical composition and a microstructure, including a martensite phase and a ferrite phase. A volume fraction of the martensite phase is 50% to 80%. A volume fraction of tempered martensite with respect to the whole martensite phase is 50% or more and 85% or less. An average grain diameter of the ferrite phase is 13 &#x3bc;m or less. A volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more. Yield strength (YP) of the high-strength coated steel sheet is 550 MPa or more.

High-strength coated steel sheet and method for manufacturing the same

Provided are a high-strength steel sheet having high strength of a yield strength of 550 MPa or more and a method for manufacturing the same. The high-strength steel sheet has a specified chemical composition and a microstructure, where observed in a cross section in a thickness direction perpendicular to a rolling direction, including a martensite phase having a volume fraction of 50% to 80%, and a ferrite phase having an average grain diameter of 13 &#x3bc;m or less, wherein a volume fraction of ferrite grains having an aspect ratio of 2.0 or less with respect to the whole ferrite phase is 70% or more, and wherein an average length in a longitudinal direction (in a width direction of the steel sheet) of the ferrite grains is 20 &#x3bc;m or less, and a yield strength (YP) of 550 MPa or more.

High-strength steel sheet and method for manufacturing the same

Provided are a steel sheet which has a tensile strength of 900 MPa or higher and excellent weldability, and a method for producing the steel sheet. The steel sheet has a specific composition and a microstructure containing, in terms of area fraction, ferrite of 30% or less (including 0%), tempered martensite of 70% or more (including 100%), and the balance other than ferrite and the tempered martensite of 10% or less (including 0%) in total, the tempered martensite having an average grain size thereof is 5 &#x3bc;m or lower, the tempered martensite having iron-based carbides, which have average particle sizes of 100 nm or less, precipitated on grain boundaries, and the tempered martensite containing, in terms of atomic concentration, 5 atom % or more in total of Si and Mn on the grain boundaries of the tempered martensite. The steel sheet has a tensile strength of 900 MPa or higher.

Steel sheet and method for producing the same

Provided are coated steel sheets, production methods therefor, and so forth, the coated steel sheets having a tensile strength of 440 MPa or more, good formability, and good aging resistance. A steel sheet of the present invention includes a specific component composition and a steel microstructure having an area fraction of a ferrite phase of 80% or more and 95% or less, an area fraction of pearlite of 5% or more and 20% or less, and an average ferrite grain size of 5 &#x3bc;m or more and 20 &#x3bc;m or less, in which in a ferrite grain size histogram, the average grain size of the largest 20% of ferrite grains in terms of grain size is 10 &#x3bc;m or more, and the pearlite has an average lamellar spacing of 200 nm or less, the area fraction, the average ferrite grain size, and the lamellar spacing being determined by microstructure observation.

A high-strength steel sheet having a tensile strength of at least 780 MPa and excellent punchability comprises a ferrite phase, a martensite phase, and a bainite phase. The area ratio of the martensite phase is 20%-50%; the area ratio of crystal grains having an average crystal grain diameter of less than 1.0 &#x3bc;m in the martensite phase is 5%-30%; the area ratio of crystal grains having an average crystal grain diameter of 1.0-4.0 &#x3bc;m is 70%-95%; and the area ratio of crystal grains having an average crystal grain diameter of more than 4.0 &#x3bc;m is less than 5%.

High-strength steel sheet and production method therefor

A steel sheet having low yield ratio, tensile strength of 780 MPa or more, and good bending fatigue properties. The steel sheet includes a specific chemical composition and a steel microstructure having an area percentage of a ferrite phase of 20% or more and 80% or less and an area percentage of a martensite phase of 20% or more and 80% or less, the area percentage being determined by microstructure observation, in which a surface layer portion of the steel sheet has an average ferrite grain size of 5.0 &#x3bc;m or less and an inclusion density of 200 particles/mmor less, and in which the steel sheet has a surface hardness of 95% or more when the steel sheet has a hardness of 100% at a position &#xbd;t, where t represents the thickness of the steel sheet, away from a surface of the steel sheet in the thickness direction.

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

Provided are a steel sheet with excellent weldability, and a production method therefor. The steel sheet is characterized by having a specific composition and a metallographic structure containing, in terms of an area ratio, ferrite of 25% or more and 65% or less, martensite having iron-based carbides precipitated in the grains of 35% or more and 75% or less, and the balance structure other than the ferrite and the martensite of 20% or less (including 0%) in total, the average grain diameters of the ferrite and the martensite being respectively 5 &#x3bc;m or lower, the total of concentration of Si and Mn at interface between the ferrite and the martensite being, in terms of an atomic concentration, 5% or more, and having a tensile strength of 900 MPa or higher.

Steel sheet, and production method therefor

Provided are a cold-rolled steel sheet which can preferably be used for manufacturing a high-strength galvanized steel sheet and methods for manufacturing the steel sheets. The cold-rolled steel sheet has a specified chemical composition, in which the Mn concentration in a surface layer of the steel sheet satisfies relational expression (1) and relational expression (2) below. 8&#x2264;(C/C)&#xd7;Mn . . . (1) (C/C)&#xd7;Mn&#x2264;2.5 . . . (2) where C: maximum Mn concentration in a region within 0.5 &#x3bc;m of the surface of a steel sheet in the thickness direction; C: average Mn concentration in a region from a position located 5 &#x3bc;m from a surface of a steel sheet in the thickness direction to a position located 5 &#x3bc;m from an opposite surface in the thickness direction; C: minimum Mn concentration in a region from 0.5 &#x3bc;m to 5 &#x3bc;m from the surface of a steel sheet in the thickness direction; and Mn: Mn content (mass %).

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Tokyo, Japan

Noriaki Kohsaka