Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Teijin Chemicals Limited

Hiroshi Shuto

Tatsuo Yokoi

Akifumi Sakakibara

Nobuhiro Fujita

Koutarou Hayashi

Riki Okamoto

Kazuaki Nakano

Yuuki Kanzawa

Takeshi Yamamoto

Jun Ando

Natsuko Sugiura

Kazumasa Tsutsui

Masafumi Azuma

Mitsuru Yoshida

Masayuki Wakita

Daisuke Maeda

Eisaku Sakurada

Genki Abukawa

Mutsumi Sakakibara

Hiroshi Kaido

Shinsuke Kai

Hiroshi Okamoto

Toshiyuki Miyake

Takeshi Toyoda

Teruki Hayashida

Tetsu Asato

Takayuki Ozaki

Shota Endo

Hiroyuki Okada

Akihiro Uenishi

Shinichiro Watanabe

Ken Kimura

Shinya Saitoh

Toshimasa Tokuda

Shohei Yabu

Kazuya Ootsuka

Koki Ikeda

Yoji Ohira

Shunsuke Taniguchi

Shunsuke Kobayashi

Manabu Matsui

Kazushi Tando

Maiko Murai

Kenta Morisawa

Ryohta Niiya

A hot-rolled steel sheet includes, as a chemical composition, by mass %: C: 0.030% to 0.250%; Si: 0.05% to 2.50%; Mn: 1.00% to 4.00%; Sol. Al: 0.001% to 2.000%; P: 0.100% or less; S: 0.0200% or less; N: 0.01000% or less; Ti: 0% to 0.20%; Nb: 0% to 0.20; B: 0% to 0.010%; V: 0% to 1.0%; Cr: 0% to 1.0%; Mo: 0% to 1.0%; Cu: 0% to 1.0%; Co: 0% to 1.0%; W: 0% to 1.0%; Ni: 0% to 1.0%; Ca: 0% to 0.01%; Mg: 0% to 0.01%; REM: 0% to 0.01%; Zr: 0% to 0.01%; and a remainder of Fe and impurities, in which an area ratio of a scale damage portion, which is a portion lower than an average height of a surface of the steel sheet by 10 μm or more, is 20% or less, and a tensile strength of the sheet is 780 MPa or more.

Hot-rolled steel sheet

This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L/Lof a length Lof a grain boundary having a crystal misorientation of 60° to a length Lof a grain boundary having a crystal misorientation of 7° about a <> direction of 0.60 or more, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition, and in a case where the thickness is denoted by t, the metallographic structure at a t/4 position from the surface contains one or both of tempered martensite and lower bainite at a volume percentage of 90% or more, the tensile strength is 980 MPa or more, and the average Ni concentration on the surface is 7.0% or more.

Hot-rolled steel sheet and method for manufacturing same

This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, more than 92.0% and 100.0% or less of martensite and tempered martensite in total, less than 3.0% of residual austenite, and less than 5.0% of ferrite, has a ratio S/S, which is a ratio of a density Sof a length of a grain boundary having a crystal misorientation of 60° to a density Sof a length of a grain boundary having a crystal misorientation of 7° about a <110> direction, of more than 0.34 and less than 0.60, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition, in which a microstructure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L/Lof a length Lof a grain boundary having a crystal misorientation of 60° to a length Lof a grain boundary having a crystal misorientation of 7° about a <110> direction of less than 0.60, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, 3.0% or more of retained austenite, has a ratio L/Lof a length Lof a grain boundary having a crystal misorientation of 52° to a length Lof a grain boundary having a crystal misorientation of 7° about a <110> direction of more than 0.18, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 1180 MPa or more.

A hot-rolled steel sheet according to the present invention has a predetermined chemical composition, in which, when a height profile of a surface of the hot-rolled steel sheet is measured in each of five measurement ranges in a rolling direction and a direction perpendicular to the rolling direction, a distance in a height direction from an average height position which is an average of a height position of a point having a highest height position and a height position of a recessed part which is a point having a lowest height position to the recessed part is indicated as R(μm) in each of the height profiles, and an average of heights of two measurement points away from the recessed part in the rolling direction or the direction perpendicular to the rolling direction by 5 μm is indicated as R(μm), an average value of radii of curvature r represented by Expression (1) is 10 μm or more, and a tensile strength of the hot-rolled steel sheet is 500 MPa or more. r=(25+|R−R|)/2|R−R| . . . (1)

A high strength steel sheet according to the present invention contains a predetermined chemical composition, in a metallographic structure, the total area ratio of tempered martensite and bainite is 80% or more, at a sheet thickness ¼ position of a cross section parallel to a rolling direction and perpendicular to a rolled surface, the standard deviation of number densities of precipitates having a diameter of 10 nm or less and including one or both of Ti and Nb is less than 5×10numbers/mm, and the tensile strength is 780 MPa or more.

High strength steel sheet

This hot-rolled steel sheet has a predetermined chemical composition, in which a metal microstructure contains, by area %, 3.0% or more of residual austenite, has a ratio L/Lof a length Lof a grain boundary having a crystal orientation difference of 52° to a length Lof a grain boundary having a crystal orientation difference of 7° about a <110> direction of 0.10 or more and 0.18 or less, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, by area %, ferrite is less than 15.0%, residual austenite is less than 3.0%, L/L, which is a ratio of a length Lof a grain boundary having a crystal orientation difference of 52° to a length Lof a grain boundary having a crystal orientation difference of 7° about a <110> direction is 0.10 to 0.18, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition. The metallographic structure at a sheet thickness ¼ depth from a surface and at a center position in a sheet width direction in a sheet width cross section parallel to a rolling direction contains, by area %, 77.0% to 97.0% of bainite and tempered martensite in total, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite. The average grain size of the metallographic structure excluding the residual austenite is 7.0 μm or less. The C concentration in the residual austenite is 0.5 mass % or more. The number density of iron-based carbides having a diameter of 20 nm or more is 1.0×10carbides/mmor more.

This hot-rolled steel sheet has a predetermined chemical composition, in which in a case where the thickness is denoted by t, a metallographic structure at a t/4 position from the surface includes, by area fraction, 77.0% to 97.0% of bainite or tempered martensite, 0% to 5.0% of ferrite, 0% to 5.0% of pearlite, 3.0% or more of residual austenite, and 0% to 10.0% of martensite, in the metallographic structure, the average grain size excluding the residual austenite is 7.0 &#x3bc;m or less, the average number density of iron-based carbides having a diameter of 20 nm or more is 1.0&#xd7;10carbides/mmor more, a tensile strength is 980 MPa or more, and an average Ni concentration on the surface is 7.0% or more.

A hot-rolled steel sheet includes a specific chemical composition, and includes a microstructure represented by, in vol %: retained austenite: 2% to 30%; ferrite: 20% to 85%; bainite: 10% to 60%; pearlite: 5% or less; and martensite: 10% or less. A proportion of grains having an intragranular misorientation of 5&#xb0; to 14&#xb0; in all grains is 5% to 50% by area ratio, the grain being defined as an area which is surrounded by a boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more.

A hot-rolled steel sheet includes, by mass %, C: 0.030% or more and less than 0.075%, Si+Al: 0.08% to 0.40%, Mn: 0.5% to 2.0%, and Ti: 0.020% to 0.150%, and includes a microstructure having a ferrite and a martensite. The hot-rolled steel sheet includes a microstructure having, by area %, 90% to 98% of the ferrite, 2% to 10% of the martensite, 0% to 3% of a bainite, and 0% to 3% of a pearlite. In the martensite, the number proportion of martensite grains having a hardness of 10.0 GPa or more is 10% or less, and a ratio N1/N2 of the number N1 of martensite grains having a hardness of 8.0 GPa or more and less than 10.0 GPa to the number N2 of martensite grains having a hardness of less than 8.0 GPa is 0.8 to 1.2.

There is provided a hot-rolled steel sheet in which a composition contains: in mass %, C: 0.01% to 0.2%; Si: 2.5% or less; Mn: 4.0% or less; P: 0.10% or less; S: 0.03% or less; Al: 0.001% to 2.0%; N: 0.01% or less; O: 0.01% or less; Ti: 0.01 to 0.30%; and the balance being composed of iron and impurities and a structure is composed of by volume fraction, 90% or more of tempered martensite with an average aspect ratio of 2 or less, or 90% or more in total of both tempered martensite and lower bainite.

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Tokyo, Japan

Hiroshi Shuto