Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Sumitomo Metal Corporation

Riki Okamoto

Nobuhiro Fujita

Kohichi Sano

Kazuaki Nakano

Kunio Hayashi

Hiroyuki Kawata

Manabu Takahashi

Hiroshi Yoshida

Takeshi Yamamoto

Tatsuo Yokoi

Hirokazu Taniguchi

Hiroshi Shuto

Takafumi Yokoyama

Takeshi Toyoda

Toshio Ogawa

Yuji Yamaguchi

Akihiro Uenishi

Yuri Toda

Chisato Wakabayashi

Yoshihiro Suwa

Kengo Takeda

Satoshi Uchida

Kaoru Kawasaki

Katsuya Nakano

Eisaku Sakurada

Natsuko Sugiura

Kohei Ueda

Genki Abukawa

Yuya Suzuki

Tetsuo Kishimoto

Tetsuya Hirashima

Kazuki Shiokawa

Masashi Fukuda

Yuichi Nakahira

Masahiro Nakata

Naoki Yoshinaga

Kazuo Hikida

Shinichiro Watanabe

Yukihisa Kuriyama

Osamu Kawano

Masaharu Oka

Yasuharu Sakuma

Yoshinari Ishida

Yutaka Mikazuki

Yuzo Takahashi

Eiji Isogai

Junichi Wakita

Hidesato Mabuchi

Satoshi Takeuchi

Kazuhito Ito

Naoki Maruyama

Hiroshi Sakai

Daisuke Maeda

Hiroyuki Okada

Masafumi Azuma

Koichi Goto

Jun Takahashi

Koutarou Hayashi

Toshimitsu Aso

Takayuki Nozaki

Nobusato Kojima

Jun Haga

Hidetoshi Shindo

Masaharu Kameda

Noriyuki Hishinuma

Yukiko Kobayashi

Kenichiroh Matsumura

Yoshinori Imai

Hidenori Miyata

Ryohta Niiya

A high-strength hot-rolled steel sheet: having a predetermined chemical composition; having a mean dislocation density of from 1×10to 1×10m, and containing at least bainitic ferrite, in which a total area ratio of the bainitic ferrite and ferrite is 70% or more and less than 90%, and in which a total area ratio of martensite and retained austenite is 5% or more and 30% or less, in which, in ferrite crystal grains and in bainitic ferrite crystal grains, a mean number density of TiC precipitates is from 1×10to 5×10[precipitates/cm], in which an amount of Ti present as a TiC precipitate precipitated in a matrix not on dislocations is 30 mass % or more of a total amount of Ti in the steel sheet, and in which a tensile strength is 850 MPa or more.

High-strength hot-rolled steel sheet

This steel sheet has a predetermined chemical composition, in which a steel structure of an inside of the steel sheet contains, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, a sum of pearlite and cementite: 0% to 10%, and a remainder includes hard ferrite, a number proportion of the retained austenite having an aspect ratio of 2.0 or more in the total retained austenite is 50% or more, a soft layer having a thickness of 1 to 100 &#x3bc;m from a surface in a sheet thickness direction is present, in ferrite contained in the soft layer, a volume fraction of grains having an aspect ratio of 3.0 or more is 50% or more, a volume fraction of retained austenite in the soft layer is 80% or less of the volume fraction of the retained austenite in the inside of the steel sheet, and a peak of an emission intensity at a wavelength indicating Si appears in a range of more than 0.2 &#x3bc;m and 10.0 &#x3bc;m or less from the surface.

Steel sheet and manufacturing method therefor

Steel sheet low in cost and improved in fatigue characteristics without causing a drop in the cold formability, characterized in that it comprises an inner layer and a hard layer on one or both surfaces of the inner layer, a thickness of the hard layer is 20 &#x3bc;m or more and 40% or less of the thickness of the steel sheet, an average micro-Vickers hardness of the hard layer is 240 HV or more and less than 400 HV, an amount of C of the hard layer is 0.4 mass % or less, an amount of N is 0.02 mass % or less, a variation of hardness measured by a nanoindenter at a depth of 10 from the surface of the hard layer is a standard deviation of 2.0 or less, an average micro-Vickers hardness of the inner layer is 80 HV or more and less than 400 HV, a volume rate of carbides contained in the inner layer is less than 2.00%, and the average micro-Vickers hardness of the hard layer is 1.05 times or more the average micro-Vickers hardness of the inner layer.

Steel sheet and method for producing same

A steel sheet includes: a predetermined chemical composition; and a steel structure represented by, in area %, first martensite in which two or more iron carbides each having a circle-equivalent diameter of 2 nm to 500 nm are contained in each lath: 20% to 95%, ferrite: 15% or less, retained austenite: 15% or less, and the balance: bainite, or second martensite in which less than two iron carbides each having a circle-equivalent diameter of 2 nm to 500 nm are contained in each lath, or the both of these, in which the total area fraction of ND//&#x3c;111&#x3e; orientation grains and ND//&#x3c;100&#x3e; orientation grains is 40% or less, and the content of solid-solution C is 0.44 ppm or more.

Steel sheet

A steel sheet wherein a steel structure of an inside of the steel sheet contains, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%, a sum of pearlite and cementite: 0% to 10%, and a remainder including hard ferrite. In the steel sheet, in a &#x215b; to &#x215c; thickness range, a proportion of retained austenite having an aspect ratio of 2.0 or more is 50% or more, and a soft layer having a thickness of 1 to 100 &#x3bc;m from a surface in a sheet thickness direction is present. When an emission intensity at a wavelength indicating Si is analyzed in the sheet thickness direction from the surface by a radio-frequency glow discharge analysis method, a peak of the emission intensity appears in a range of more than 0.2 &#x3bc;m and 5.0 &#x3bc;m or less from the surface.

Steel sheet and manufacturing method of therefor

The hot-rolled steel sheet includes, in % by mass, 0.10% or more and 0.50% or less of C; 0.10% or more and 3.0% or less of Si; 0.5% or more and 3.0% or less of Mn; 0.10% or less of P; 0.010% or less of S; 1.00% or less of Al; 0.010% or less of N; 0% or more and 0.20% or less of Ti; 0% or more and 0.100% or less of Nb; 0% or more and 0.0060% or less of Ca; 0% or more and 0.50% or less of Mo; and 0% or more and 1.00% or less of Cr; with the balance comprising Fe and impurities, and an average grain size of prior austenite in a structure is 0.1 &#x3bc;m or larger and 3.0 &#x3bc;m or smaller, and a sheet crown quantity corresponding to a thickness difference between a width center portion and a portion away, by 10 mm, from a width edge portion in the widthwise direction toward the width center portion is 80 &#x3bc;m or smaller.

Hot-rolled steel sheet

There is provided a steel sheet including an inner layer and a hard layer formed on one or both surfaces of the inner layer, wherein each content of C and Mn in the hard layer is more than each content of C and Mn in the inner layer, a thickness of the hard layer is 20 &#x3bc;m or more and a total of the thickness of the hard layer is &#x2156; or less of the entire sheet thickness, an average micro-Vickers hardness of the hard layer is 400 HV or more and less than 800 HV, an average micro-Vickers hardness of the inner layer is 350 HV or more and is 50 HV or more smaller than a hardness of the hard layer, and a screw dislocation density of the inner layer is 2.0&#xd7;10m/mor more.

A steel sheet has a chemical composition with a steel structure containing, by volume fraction, soft ferrite: 0-30%, retained austenite: 3-40%, fresh martensite: 0-30%, pearlite and cementite: 0-10%, and a remainder including hard ferrite. In the steel sheet, a number proportion of retained austenite having an aspect ratio of 2.0 or more in the total retained austenite is 50% or more, and a soft layer having a thickness of 1-100 &#x3bc;m is present. In the soft layer, a volume fraction of ferrite grains having an aspect ratio of less than 3.0 is 50% or more, and a volume fraction of retained austenite is 50% or more of the volume fraction of the retained austenite of the inside of the steel sheet. A peak of an emission intensity at a wavelength indicating Si appears in a range of more than 0.2 &#x3bc;m to 5 &#x3bc;m or less from the surface.

A cold-rolled steel sheet is provided that has a tensile strength of 980 MPa or more, and has a prescribed chemical composition. The microstructure is composed of, in area %, ferrite: 1 to 29%, retained austenite: 5 to 20%, martensite: less than 10%, pearlite: less than 5%, and the balance: bainite and/or tempered martensite. The total sum of the lengths of phase boundaries where ferrite comes in contact with martensite or retained austenite having a circle-equivalent radius of 1 &#x3bc;m or more is 100 &#x3bc;m or less per 1000 &#x3bc;m. The cold-rolled steel sheet is excellent in workability and low-temperature toughness, and in particular is excellent in low-temperature toughness after introduction of plastic strain.

Cold-rolled steel sheet and hot-dip galvanized cold-rolled steel sheet

A hot rolled steel sheet is provided, which is excellent in collision characteristics, excellent in anisotropy of toughness, and high in strength. The hot rolled steel sheet is characterized by containing, by mass %, C: 0.10% to 0.50%, Si: 0.10% to 3.00%, Mn: 0.5% to 3.0%, P: 0.100% or less, S: 0.010% or less, Al: 1.00% or less, N: 0.010% or less and a balance of Fe and impurities, wherein a metal structure at position of &#xbc; thickness from surface in L-cross-section of the steel sheet comprises prior austenite grains of average value of aspect ratios of 2.0 or less, average grain size of 0.1 &#x3bc;m to 3.0 &#x3bc;m, and coefficient of variation of a standard deviation of grain size distribution/average grain size of 0.40 or more, and a texture with X-ray diffraction intensity ratio of {001}&#x3c;110&#x3e;orientation for random samples of 2.0 or more, and the steel sheet has tensile strength of 1180 MPa or more.

Hot rolled steel sheet and method for producing same

Provided is a hot rolled steel sheet comprising a predetermined composition wherein the hot rolled steel sheet comprises a dual structure of, by area fraction, a structural fraction of a martensite phase of 10 to 40% and a structural fraction of a ferrite phase of 60% or more, has an average grain size of ferrite grains of 5.0 &#x3bc;m or less, and has a coverage rate of martensite grains by ferrite grains of more than 60%. Also provided is a method for producing a hot rolled steel sheet comprising rolling a steel sheet wherein the respective rolling loads of the final three rolling stands are 80% or more of an immediately previous rolling stand and an average value of these rolling temperatures is 800 to 950&#xb0; C., and forcibly cooling, then coiling the steel sheet wherein the forcibly cooling includes cooling started within 1.5 seconds after the rolling ends and cooling the steel sheet by a 30&#xb0; C./second or more average cooling rate down to 600 to 750&#xb0; C., natural cooling for 3 seconds or more and 10 seconds or less, and cooling by a 30&#xb0; C./second or more average cooling rate down to 200&#xb0; C. or less.

A high-strength steel sheet having a microstructure represented by, in area %, martensite: 5% or more; ferrite: 20% or more; and pearlite: 5% or less. A ratio of the number of bulging type martensite grains to the number of martensite grains on grain boundary triple points of a matrix is 70% or more, wherein: the bulging type martensite grain is on one of the grain boundary triple points of the matrix; and at least one of grain boundaries of the bulging type martensite grain, the grain boundaries connecting two adjacent grain boundary triple points of the bulging type martensite grain and grains of the matrix, has a convex curvature to an outer side with respect to line segments connecting the two adjacent grain boundary triple points. An area ratio VM/Ais 1.0 or more.

High-strength steel sheet and method of manufacturing the same

A steel sheet includes a hot-dip galvanized layer or a galvannealed layer on a surface of the steel sheet, the steel sheet including: in mass %, C: 0.06% or more and 0.22% or less; Si: 0.50% or more and 2.00% or less; Mn: 1.50% or more and 2.80% or less; Al: 0.02% or more and 1.00% or less; P: 0.001% or more and 0.100% or less; S: 0.0005% or more and 0.0100% or less; N: 0.0005% or more and 0.0100% or less; and a balance: Fe and impurities.

Galvannealed steel sheet

A steel sheet including an inner layer and a hard layer at one or both surfaces of the inner layer wherein the hard layer and the inner layer have predetermined compositions, each hard layer has a thickness of 20 &#x3bc;m or more and &#x2156; of the total sheet thickness or less, the hard layer has an average micro-Vickers hardness of 400 HV or more and less than 700 HV, the hard layer has an N amount of 0.02% or less, the inner layer has an average micro-Vickers hardness of 80 HV or more and less than 400 HV, the inner layer has a carbide volume ratio of less than 2.00%, and the hard layer has a nanohardness standard deviation of 2.00 or less is provided.

A steel sheet for galvannealed steel contains, by mass %, C: 0.25 to 0.70%, Si: 0.25 to 2.50%, Mn: 1.00 to 5.00%, Al: 0.005 to 3.50%, P: 0.15% or less, S: 0.03% or less, N &#xa3; 0.02%, O &#xa3; 0.01%, and optionally one or more selected from Ti, Nb, V, Cr, Mo, Cu, Ni, B, Ca, REM, Mg, W, Zr, Sb, Sn, As, Te, Y, Hf and Co, a balance being Fe and impurities. The microstructure consists of, by vol. %, retained g: 10.0 to 60.0%, high-temperature tempered martensite5.0%, low-temperature tempered martensite5.0%, fresh martensite &#xa3; 10.0%, a: 0 to 15.0%, P: 0 to 10.0%, a balance being bainite. Total volume ratio of tempered martensite and bainite is 30.0% or more, tensile strength is 1470 MPa or more, tensile strength&#xd7;uniform elongation is 13000 MPa % or more, and tensile strength&#xd7;local elongation is 5000 MPa % or more.

Galvannealed steel sheet and method for producing the same

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Tokyo, Japan

Riki Okamoto