Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Sony Corporation

Sumitomo Metal Corporation

Kohichi Sano

Riki Okamoto

Hiroyuki Kawata

Nobuhiro Fujita

Kunio Hayashi

Yuji Yamaguchi

Kazuaki Nakano

Masahiro Nakata

Natsuko Sugiura

Hiroshi Yoshida

Eisaku Sakurada

Takafumi Yokoyama

Chisato Wakabayashi

Toshio Ogawa

Akihiro Uenishi

Yuri Toda

Koutarou Hayashi

Jun Haga

Makoto Uno

Masaharu Kameda

Ryoichi Nishiyama

Satoshi Uchida

Kaoru Kawasaki

Kohei Ueda

Yoshihiro Suwa

Toshitada Hayashi

Masafumi Azuma

Naoki Yoshinaga

Kazuki Shiokawa

Heihachi Ide

Yuichi Nakahira

Takeshi Yamamoto

Kengo Takeda

Manabu Takahashi

Shinichiro Watanabe

Takayuki Nozaki

Yoshinori Imai

A high-strength steel sheet excellent in formability, toughness and weldability has a chemical composition including: by mass %, C: 0.05 to 0.30%, Si: 2.50% or less, Mn: 0.50 to 3.50%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 2.500%, N: 0.0150% or less, O: 0.0050% or less, and the balance consisting of Fe and inevitable impurities. The high-strength steel sheet has a microstructure in a region from ⅛ t (t: sheet thickness) to ⅜ t (t: sheet thickness) from a steel sheet surface, the microstructure including: by volume %, acicular ferrite (): 20% or more, and martensite (): 10% or more, polygonal ferrite: 20% or less, residual austenite: 2.0% or less, and the martensite satisfies a formula (A),

High-strength steel plate having excellent formability, toughness and weldability, and production method of same

A steel sheet of the present invention is a steel sheet having a predetermined chemical composition and containing at least ferrite, residual austenite, and/or martensite in a microstructure, and furthermore, is a steel sheet in which, in a plane parallel to a rolled surface, an average distance between centers of high Mn regions adjacent to each other is 1.00 mm or less, a density Dof the high Mn regions at a sheet width center portion and a density Dof the high Mn regions at a ¼ position from a sheet width end portion satisfy a relationship of 0.77≤D/D≤1.30, a ratio of an average hardness of the high Mn regions to an average hardness of the low Mn regions is 1.1 to 2.0, and a difference between an average of a top 5% and an average of a bottom 5% of Mn contents in the low Mn regions is 0.3% or more.

Steel sheet and method for producing same

This steel sheet has a predetermined chemical composition, in which the area ratio of plate martensite is 10% or more, the average grain size of prior austenite grains is 2.0 μm to 10.0 μm, the maximum diameter thereof is 20.0 μm or less, the amount of solid solution C in martensite is 0.20 mass % or less, the average carbide size is 0.25 μm or less, the crystal orientation difference between plate martensite and another martensite adjacent thereto in the same prior austenite grain is 10.0° or less, and the P concentration at grain boundaries of the prior austenite grains is 4.0 at % or less.

Steel sheet and method for manufacturing same

A high-strength steel sheet includes a chemical composition including: by mass %, C: 0.080 to 0.500%, Si: 2.50% or less, Mn: 0.50 to 5.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 2.500%, N: 0.0150% or less, O: 0.0050% or less, and the balance: Fe and inevitable impurities. The high-strength steel sheet satisfying a predetermined formula has a microstructure in a region from ⅛t to ⅜t from a steel sheet surface. The microstructure includes: by volume %, 20% or more of acicular ferrite, 20% or more of an island-shaped hard structure including residual austenite, 2% to 25% of residual austenite, and 20% or less of aggregated ferrite.

High-strength steel sheet having excellent moldability and impact resistance, and method for manufacturing high-strength steel sheet having excellent moldability and impact resistance

In a metal coated steel sheet, a chemical composition contains, in mass %, at least C: 0.03% to 0.70%, Si: 0.25% to 2.50%, Mn: 1.00% to 5.00%, P: 0.100% or less, S: 0.010% or less, sol. Al: 0.001% to 2.500, N: 0.020% or less, and a balance composed of iron and impurities, a metal structure contains greater than 5.0 vol % of retained austenite and greater than 5.0 vol % of tempered martensite, and satisfies a C content in the retained austenite being 0.85 mass % or more.

Metal coated steel sheet, manufacturing method of hot-dip galvanized steel sheet, and manufacturing method of alloyed galvannealed 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

A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 5 to 60%, and bainite: 40 to 95%. When a region that is surrounded by a grain boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. A precipitate density of Ti(C,N) and Nb(C,N) each having a circle-equivalent diameter of 10 nm or less is 10precipitates/mmor more. A ratio (Hvs/Hvc) of a hardness at 20 &#x3bc;m in depth from a surface (Hvs) to a hardness of the center of a sheet thickness (Hvc) is 0.85 or more.

Steel sheet and plated 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

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 steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 30 to 95%, and bainite: 5 to 70%. When a region that is surrounded by a grain boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. An average aspect ratio of ellipses equivalent to the crystal grains is 5 or less. An average distribution density of the total of Ti-based carbides and Nb-based carbides each having a grain size of 20 nm or more on ferrite grain boundaries is 10 carbides/&#x3bc;m or less.

A steel sheet has a specific chemical composition and has a structure represented by, by area ratio, ferrite: 0 to 30%, and bainite: 70 to 100%. When a region that is surrounded by a grain boundary having a misorientation of 15&#xb0; or more and has a circle-equivalent diameter of 0.3 &#x3bc;m or more is defined as a crystal grain, the proportion of crystal grains each having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. A grain boundary number density of solid-solution C or a grain boundary number density of the total of solid-solution C and solid-solution B is 1 piece/nmor more and 4.5 pieces/nmor less. An average grain size of cementite precipitated at grain boundaries is 2 &#x3bc;m or less.

A hot dip galvanized steel sheet and hot dip galvannealed steel sheet improved in uniform ductility and local ductility, yield strength and tensile strength, and low temperature impact property, characterized by having a predetermined chemical composition, having a metal structure containing, by volume %, retained austenite: over 5.0% and tempered martensite: over 5.0%, having retained austenite containing C: 0.85 mass % or more, and having a ratio [C]/[P]of an amount of segregation of C (number of atoms/nm): [C]to an amount of segregation of P (number of atoms/nm): [P]at prior austenite grain boundaries of 4.0 or more.

Hot dip galvanized steel sheet and hot dip galvannealed steel sheet

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 having a specific chemical composition and structure represented by, by area ratio, ferrite: 5 to 95%, and bainite: 5 to 95%. The proportion of crystal grains having an intragranular misorientation of 5 to 14&#xb0; to all crystal grains is 20 to 100% by area ratio. Hard crystal grains A in which precipitates or clusters with a maximum diameter of 8 nm or less are dispersed in the crystal grains with a number density of 1&#xd7;10to 1&#xd7;10pieces/cm. Soft crystal grains B in which precipitates or clusters with a maximum diameter of 8 nm or less are dispersed in the crystal grains with a number density of 1&#xd7;10pieces/cmor less are contained. The volume % of the hard crystal grains A/(the volume % of the hard crystal grains A+the volume % of the soft crystal grains B) is 0.1 to 0.9.

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.

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Tokyo, Japan

Kohichi Sano