Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Yoshihiro Suwa

Koichi Sato

Riki Okamoto

Kazuo Hikida

Shinichiro Tabata

Kunio Hayashi

Masafumi Azuma

Nobuhiro Fujita

Kohichi Sano

Kazuaki Nakano

Daisuke Maeda

Kenichi Yamamoto

Toshiki Nonaka

Yoshifumi Kobayashi

Manabu Naruse

Yasunori Iwasa

Naoki Yoshinaga

Ichiro Tanaka

Takashi Kataoka

Koutarou Hayashi

Satoshi Hirose

Takafumi Yokoyama

Kazuhisa Kusumi

Hideaki Yamamura

Osamu Kawano

Nobusato Morishige

Kazumi Mizukami

Yusuke Kondo

Ryutaro Yamagata

Yoshinari Ishida

Junji Haji

Yuzo Takahashi

Genichi Shigesato

Haruhiko Atsumi

Kohsuke Kume

Takahiko Kohtake

A steel sheet has a predetermined chemical composition containing, in mass %, C: more than 0.18% and less than 0.30%, Mn: more than 2.50% and 4.00% or less, and other elements. The steel micro-structure at a position at a depth of ¼ of the sheet thickness from the surface in an L cross section of the steel sheet is, in area %, tempered martensite: 25 to 90%, ferrite: 5% or less, retained austenite: 10 to 50%, and bainite: 5% or less. At a position at a depth of ¼ of the sheet thickness from the surface of the L cross section, the proportion of a total area of retained austenite grains having an area of 1 μmor more and having a grain shape circularity of 0.1 or more is less than 50% with respect to the entire area of retained austenite. The steel sheet satisfies the formula C/C≥1.2.

Steel sheet and method for producing same

Grain-oriented electrical steel sheet and its method of manufacture are provided. The method includes a process of heating to 1280° C. or more and hot rolling a slab containing, by mass %, Bi and a predetermined composition of constituents and having a balance of Fe and impurities to obtain hot rolled steel sheet. After hot rolling annealing, the hot rolled steel sheet is cold rolled one time or more with process annealing performed interposed so as to obtain cold rolled steel sheet, followed by a process of rapidly heating then decarburization annealing the steel sheet, a process of coating the surface of the cold rolled steel sheet after decarburization annealing with an annealing separator comprising sulfates or sulfides and having MgO as its main constituent, then performing finish annealing, a process of performing strictly controlled finish annealing, and a process of coating an insulating coating, then performing flattening annealing.

Method for manufacturing grain-oriented electrical steel sheet and grain-oriented electrical steel sheet

A steel member according to an aspect of the present invention has a predetermined chemical composition, in which a metallographic structure includes, by a volume %, 60.0% to 85.0% of martensite, 10.0% to 30.0% of bainite, 5.0% to 15.0% of residual austenite, and 0% to 4.0% of a remainder in microstructure. A length of a maximum minor axis of the residual austenite is 30 nm or longer. A number density of a carbide which exist in the steel member and has a circle equivalent diameter of 0.1 &#x3bc;m or more and an aspect ratio of 2.5 or less is 4.0&#xd7;10pieces/mmor less.

Steel member and method of manufacturing same

A cold-rolled steel according to the present invention has a predetermined chemical composition, satisfies (5&#xd7;[Si]+[Mn])/[C]&#x3e;10 when [C] is the amount of C by mass %, [Si] is the amount of Si by mass %, and [Mn] is the amount of Mn by mass %, includes 40% to 95% ferrite and 5% to 60% martensite in area fraction, and optionally further includes 10% or less pearlite in area fraction, 5% or less retained austenite in volume fraction, and less than 40% bainite in area fraction. The total of the area fraction of ferrite and the area fraction of martensite is 60% or more, the hardness of martensite measured with a nanoindenter satisfies H2/H1&#x3c;1.10 and &#x3c3;HM&#x3c;20.

Cold-rolled steel sheet

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 heat-treated steel sheet member having a chemical including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn: 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: optional amounts of Ni, Cu, Mo, V, Ca, Al, Nb, and REM, with the balance: Fe and impurities. The steel sheet member has a microstructure comprising: mainly martensite; and retained austenite of which a volume ratio is 5.0% or higher, a number density of retained carbide in the steel sheet member having circle-equivalent diameters of 0.1 mm or larger is 4.0&#xd7;10/mmor lower, [(log f&#x2212;log f(0.02))/0.02&#x3c;20.0] is satisfied when mechanical properties are measured using a sheet specimen specified in ASTM E8, a tensile strength is 1.4 GPa or higher, and a total elongation is 8.0% or higher.

Heat-treated steel sheet member and method for producing the same

A steel sheet for heat treatment having a chemical composition including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn; 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: 0 to 1.0%; Ni: 0 to 2.0%; Cu: 0 to 1.0%; Mo: 0 to 1.0%; V: 0 to 1.0%; Ca: 0 to 0.01%; Al: 0 to 1.0%; Nb: 0 to 1.0%; REM: 0 to 0.1%; and the balance: Fe and impurities, wherein a maximum height roughness Rz on a surface of the steel sheet is 3.0 to 10.0 &#x3bc;m, and a number density of carbide being present in the steel sheet and having circle-equivalent diameters of 0.1 &#x3bc;m or larger is 8.0&#xd7;10/mmor lower.

Steel sheet for heat treatment

A heat-treated steel sheet member having a composition including, by mass %: C: 0.05 to 0.50%; Si: 0.50 to 5.0%; Mn: 1.5 to 4.0%; P: 0.05% or less; S: 0.05% or less; N: 0.01% or less; Ti: 0.01 to 0.10%; B: 0.0005 to 0.010%; Cr: 0 to 1.0%; Ni: 0 to 2.0%; Cu; 0 to 1.0%; Mo: 0 to 1.0%; V: 0 to 1.0%; Ca: 0 to 0.01%; Al: 0 to 1.0%; Nb: 0 to 1.0%; REM: 0 to 0.1%; and the balance: Fe and impurities. The steel sheet member has a microstructure comprising mainly martensite and retained austenite of which a volume ratio is 0.2 to 1.0%, a number density of retained carbide in the steel sheet member having circle-equivalent diameters of 0.1 mm or larger is 4.0&#xd7;10/mmor lower, a tensile strength is 1.4 GPa or higher, and a yield ratio is 0.65 or higher.

A hot-stamped steel according to the present invention has a predetermined chemical composition, satisfies (5&#xd7;[Si]+[Mn])/[C]&#x3e;10 when [C] is the amount of C by mass %, [Si] is the amount of Si by mass %, and [Mn] is the amount of Mn by mass %, includes 40% to 95% ferrite and 5% to 60% martensite in area fraction, and optionally further includes 10% or less pearlite in area fraction, 5% or less retained austenite in volume fraction, and less than 40% bainite in area fraction. The total of the area fraction of ferrite and the area fraction of martensite is 60% or more, the hardness of martensite measured with a nanoindenter satisfies H2/H1&#x3c;1.10 and &#x3c3;HM&#x3c;20, and TS&#xd7;&#x3bb; which is product of tensile strength TS and hole expansion ratio &#x3bb; is 50000 MPa&#xb7;% or more.

Hot-stamped steel, cold-rolled steel sheet and method for producing hot-stamped steel

This high-strength cold-rolled steel sheet contains, in mass %, C: 0.02% to 0.20%; Si: 0.001% to 2.5%; Mn: 0.01% to 4.0%; P: 0.001% to 0.15%; S: 0.0005% to 0.03%; Al: 0.001% to 2.0%; N: 0.0005% to 0.01%; and O: 0.0005% to 0.01%; in which Si+Al is limited to less than 1.0%, and a balance being composed of iron and inevitable impurities, in which an area ratio of bainite in a metal structure is 95% or more, at a sheet thickness center portion being a range of &#x215d; to &#x215c; in sheet thickness from the surface of the steel sheet, an average value of pole densities of the {100}&#x3c;011&#x3e; to {223}&#x3c;110&#x3e; orientation group is 4.0 or less, and a pole density of the {332}&#x3c;113&#x3e; crystal orientation is 5.0 or less, and a mean volume diameter of crystal grains in the metal structure is 7 &#x3bc;m or less.

High-strength cold-rolled steel sheet having excellent local deformability

This high-strength hot-rolled steel sheet having excellent local deformability contains, in mass %, C: 0.07% to 0.20%; Si: 0.001% to 2.5%; Mn: 0.01% to 4.0%; P: 0.001% to 0.15%; S: 0.0005% to 0.03%; Al: 0.001% to 2.0%; N: 0.0005% to 0.01%; and O: 0.0005% to 0.01%; and a balance being composed of iron and inevitable impurities, in which an area ratio of bainite in a metal structure is 95% or more, at a sheet thickness center portion being a range of &#x215d; to &#x215c; in sheet thickness from the surface of the steel sheet, an average value of pole densities of the {100}&#x3c;011&#x3e; to {223}&#x3c;110&#x3e; orientation group is 4.0 or less, and a pole density of the {332}&#x3c;113&#x3e; crystal orientation is 5.0 or less, and a mean volume diameter of crystal grains in the metal structure is 10 &#x3bc;m or less.

High-strength hot-rolled steel sheet having excellent local deformability and manufacturing method thereof

The present invention provides a high-strength steel sheet including: C: 0.03 to 0.25 mass %, Si: 0.1 to 2.0 mass %, Mn: 0.5 to 3.0 mass %, P: not more than 0.05 mass %, T.O: not more than 0.0050 mass %, S: 0.0001 to 0.01 mass %, N: 0.0005 to 0.01 mass %, acid-soluble Al: more than 0.01 mass %, Ca: 0.0005 to 0.0050 mass %, and a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, with a balance including iron and inevitable impurities, in which the steel sheet contains a chemical component on a basis of mass that satisfies 0.7&#x3c;100&#xd7;([Ce]+[La]+[Nd]+[Pr])/[acid-soluble Al]&#x2266;70 and 0.2&#x2266;([Ce]+[La]+[Nd]+[Pr])/[S]&#x2266;10, the steel sheet contains compound inclusion including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at least one element of O and S, and a second inclusion phase having a component different from that of the first inclusion phase and containing at least one element of Mn, Si, and Al, the compound inclusion forms a spherical compound inclusion having an equivalent circle diameter in the range of 0.5 &#x3bc;m to 5 &#x3bc;m, and a ratio of the number of the spherical compound inclusion relative to the number of all inclusions having the equivalent circle diameter in the range of 0.5 &#x3bc;m to 5 &#x3bc;m is 30% or more.

High-strength steel sheet exhibiting excellent stretch-flange formability and bending workability, and method of producing molten steel for the high-strength steel sheet

Manufacturing method of a high-strength cold-rolled steel sheet having excellent local deformability

A high-strength steel sheet includes, by mass %, C: 0.03% to 0.30%, Si: 0.08% to 2.1%, Mn: 0.5% to 4.0%, P: 0.05% or less, S: 0.0001% to 0.1%, N: 0.01% or less, acid-soluble Al: more than 0.004% and less than or equal to 2.0%, acid-soluble Ti: 0.0001% to 0.20%, at least one selected from Ce and La: 0.001% to 0.04% in total, and a balance of iron and inevitable impurities, in which [Ce], [La], [acid-soluble Al], and [S] satisfy 0.02&#x2266;([Ce]+[La])/[acid-soluble Al]&#x3c;0.25, and 0.4&#x2266;([Ce]+[La])/[S]&#x2266;50 in a case in which the mass percentages of Ce, La, acid-soluble Al, and S are defined to be [Ce], [La], [acid-soluble Al], and [S], respectively, and a microstructure includes 1% to 50% of martensite in terms of an area ratio.

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Tokyo, Japan

Yoshihiro Suwa