Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Genki Abukawa

Masafumi Azuma

Kunio Hayashi

Eisaku Sakurada

Katsuya Nakano

Yuya Suzuki

Shohei Yabu

Kazuo Hikida

Kengo Takeda

Riki Okamoto

Kaoru Kawasaki

Kenichiro Otsuka

Takeshi Toyoda

Takashi Yasutomi

Hiroyuki Tanaka

Hiroyuki Kawata

Daisuke Maeda

Yuji Yamaguchi

Yuri Toda

Hiroshi Shuto

Jun Ando

Takafumi Yokoyama

Akifumi Sakakibara

Naoki Inoue

Shingo Fujinaka

Kazumasa Tsutsui

Tetsu Asato

Shota Endo

Marina Mori

Kenta Morisawa

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, the microstructure includes, by volume percentage, 70% or more of martensite, tempered martensite, and bainite in total and 5% to 20% of residual austenite, in a surface layer region that is a range from a surface to a position at 1/10 of a sheet thickness, a sum of an average pole density of an orientation group consisting of {211}<111> to {111}<112> and a pole density of a crystal orientation of {110}<001> is 6.0 or less, a concentration of solid solution carbon in the residual austenite is 0.5 mass % or more, and a tensile strength is 980 MPa or more.

This hot-rolled steel sheet has a predetermined chemical composition and predetermined metallographic structure, a ratio between a maximum depth of a region where, on one surface, a rotation angle between a normal line of the one surface and a (011) pole near the normal line of the one surface is 5° or less and a maximum depth of a region where, on the other surface, a rotation angle between a normal line of the other surface and a (011) pole near the normal line of the other surface is 5° or less is 1.00 to 1.20, and a tensile strength is 1150 MPa or more.

Hot-rolled steel sheet and manufacturing method thereof

This high-strength steel sheet contains predetermined chemical components, in a range from a surface to 1/10 of a sheet thickness in a sheet thickness direction, an average pole density of an orientation group 1 that is an orientation group expressed by ϕ1=0° to 90°, Φ=50° to 60° and ϕ2=45° in terms of an Euler angle is 1.5 or less, an average pole density of an orientation group 2 that is an orientation group expressed by ϕ1=450 to 85°, Φ=85° to 90° and ϕ2=45° in terms of the Euler angle is 1.5 or more, an S value that indicates a degree of randomness of surface properties is 7.5 or less, an E value that indicates a degree of concentration of the surface properties is 0.04 or more, and a tensile strength is 590 MPa or more.

High-strength steel sheet

A high-strength steel sheet includes a predetermined chemical composition, a microstructure contains, by volume percentage, 95% or more of tempered martensite and bainite in total, and a remainder consists of ferrite and pearlite, the microstructure contains 5.0×10pieces/mm or more of, per unit volume, precipitate having an equivalent circle diameter of 5.0 nm or less and containing Ti, Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20 μm from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the surface is 0.85 or more, a tensile strength is 980 MPa or more, and a product of the tensile strength and elongation is 12,000 MPa×% or more.

High-strength steel sheet and method for manufacturing same

Adopted are a hot-rolled steel sheet having a predetermined chemical composition, in which a region, where a rotation angle between a normal line of a surface and a (011) pole near the normal line becomes 5° or less, is 0.150 or less from the surface in terms of a sheet thickness direction position standardized by a sheet thickness, and a region, where the rotation angle becomes 20° or more, is 0.250 or more from the surface in terms of the sheet thickness direction position standardized by the sheet thickness and a manufacturing method thereof.

This hot-rolled steel sheet has a predetermined chemical composition, a microstructure includes 80% or more of tempered martensite by a volume percentage and a remainder consisting of one or more of ferrite, pearlite, bainite, fresh martensite, and residual austenite, the tempered martensite includes 5×10pieces/mmor more of precipitates containing Ti and having an equivalent circle diameter of 5 nm or less per unit volume, in a surface layer region that is a range from a surface to a 1/10 position of a sheet thickness, a sum of an average pole density of a crystal orientation group consisting of {211}<111> to {111}<112> and a pole density in a crystal orientation of {110}<001> is 6.0 or less, and a tensile strength is 980 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

A hot-rolled steel sheet includes, as chemical composition, C, Si, Mn, and sol.Al. In the hot-rolled steel sheet, a sum of an average of pole densities in a crystal orientation group consisting of {211}<111> to {111}<112> and a pole density in a crystal orientation of {110}<001> is 0.5 to 6.0 in the surface region, and the tensile strength is 780 to 1370 MPa.

A high-strength steel sheet includes a predetermined chemical composition, a microstructure contains, by volume percentage, 80% or more of tempered martensite, and a remainder consists of ferrite and bainite, the microstructure contains 5.0×10pieces/mmor more of, per unit volume, precipitate containing Ti and having an equivalent circle diameter of 5.0 nm or less, Hvs/Hvc which is a ratio of an average hardness Hvs at a position of a depth of 20 μm from a surface to an average hardness Hvc at a position of 0.20 to 0.50 mm from the surface is 0.85 or more, and the tensile strength is 1,180 MPa or more.

A hot-rolled steel sheet includes, as chemical composition, C, Si, Mn, and sol.Al. In the hot-rolled steel sheet, an average of pole densities in crystal orientation group consisting of {110}<110> to {110}<001> in surface region is 0.5 to 3.0, a standard deviation of the pole densities in the crystal orientation group is 0.2 to 2.0, and the tensile strength is 780 to 1370 MPa.

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 blank material is formed from a steel sheet, a first quenching of the blank material is performed, and a second quenching of the blank material is performed after the first quenching. When the first quenching is performed, the blank material is heated to a first temperature of not lower than (Ac3 point&#x2014;50)&#xb0; C. nor higher than 1200&#xb0; C. at an average heating rate of 2&#xb0; C./sec or more, and the blank material is cooled from the first temperature to a second temperature of 250&#xb0; C. or lower. When the second quenching is performed, the blank material is heated from the second temperature to a third temperature of not lower than (Ac3 point&#x2014;50)&#xb0; C. nor higher than 1200&#xb0; C. at an average heating rate of 2&#xb0; C./sec or more, and the blank material is cooled from the third temperature to a fourth temperature of 250&#xb0; C. or lower. Forming of the blank material is performed in the first quenching or the second quenching or both of the above.

Hot stamped part and manufacturing method thereof

High strength steel sheet having a tensile strength of 800 MPa or more comprising a middle part in sheet thickness and a soft surface layer arranged at one side or both sides of the middle part in sheet thickness, wherein each soft surface layer has a thickness of more than 10 &#x3bc;M and 30% or less of the sheet thickness, the soft surface layer has an average Vickers hardness of more than 0.60 time and 0.90 time or less the average Vickers hardness of the sheet thickness 1/2 position, and the soft surface layer has a nano-hardness standard deviation of 0.8 or less is provided.

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Tokyo, Japan

Genki Abukawa