Nippon Steel Sumitomo Metal Corporation

Nippon Steel Corporation

Yuuki Kanzawa

Hiroshi Shuto

Tatsuo Yokoi

Masafumi Azuma

Akifumi Sakakibara

Nobuhiro Fujita

Akihiro Uenishi

Natsuko Sugiura

Takeshi Toyoda

Ken Kimura

Mitsuru Yoshida

Nobusato Kojima

Shinya Saitoh

Daiki Kamada

Ryohta Niiya

Kohhei Kamiya

Mayuko Kikuzuki

A hot-rolled steel sheet according to an aspect of the present invention has a predetermined chemical composition, in which, in a location at a depth of &#xbc; of a sheet thickness from a surface, an area ratio of ferrite is 10% to 55%, a total area ratio of bainite and martensite is 45% to 90%, a total area ratio of the ferrite, the bainite, and the martensite is 90% or more, an average crystal grain size is 12.0 &#x3bc;m or less, in a texture measured in a sheet thickness central portion, a maximum pole density of orientation groups of {100} &#x3c;011&#x3e;, {211} &#x3c;011&#x3e;, {311} &#x3c;011&#x3e;, {110} &#x3c;011&#x3e;, and {332} &#x3c;113&#x3e; is 8.0 or less, a total of pole densities of {211} &#x3c;011&#x3e; and {332} &#x3c;113&#x3e; is 10.0 or less, and a tensile strength is 950 MPa or more.

Hot-rolled steel sheet and method for manufacturing same

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.

Hot-rolled steel sheet and manufacturing method thereof

There is provided a steel sheet with strain induced transformation type composite structure having a predetermined chemical composition in which a microstructure at the &#xbc; thickness of the steel sheet includes, by area ratio, 50% to 85% of a polygonal ferrite, 3% to 10% of a residual austenite, 5% to 47% of bainite and 1% or less of a fresh martensite and a tempered martensite in total and satisfies the equation of 0.01&#x3c;Ex.C/fsd&#x2264;0.015, the microstructure includes 1&#xd7;10pieces/cmor more of precipitates containing TiC, an average grain diameter of the residual austenite 1.0 &#x3bc;m to 5.0 &#x3bc;m at an equivalent circle diameter, an average of closest distances of the austenite is 3.0 &#x3bc;m to 10.0 &#x3bc;m, and an average diameter of the precipitates is 3 nm or less.

Steel sheet with strain induced transformation type composite structure and method of manufacturing same

This high strength hot rolled steel sheet has a predetermined chemical composition, in which the structure of the high strength hot rolled steel sheet contains martensite in an area ratio of 20% or more and 60% or less and ferrite in an area ratio of 40% or more, and the total area ratio of the martensite and the ferrite is 90% or more, the average grain size of the martensite is 5.0 &#x3bc;m or more and 50 &#x3bc;m or less, the ratio of the hardness of the martensite to the hardness of the ferrite is 0.6 or more and 1.6 or less, and the tensile strength of the high strength hot rolled steel sheet is 980 MPa or more.

High strength hot rolled steel sheet and manufacturing method thereof

Provided is a high-strength hot-rolled steel sheet consisting of, in mass %, C: 0.01% to 0.2%, Si: 0% to 2.5%, Mn: 0% to 4.0%, Al: 0% to 2.0%, N: 0% to 0.01%, Cu: 0% to 2.0%, Ni: 0% to 2.0%, Mo: 0% to 1.0%, V: 0% to 0.3%, Cr: 0% to 2.0%, Mg: 0% to 0.01%, Ca: 0% to 0.01%, REM: 0% to 0.1%, B: 0% to 0.01%, P: less than or equal to 0.10%, S: less than or equal to 0.03%, O: less than or equal to 0.01%, one or both of Ti and Nb: 0.01% to 0.30% in total, and the balance being Fe and inevitable impurities. The steel sheet has a structure in which a total volume fraction of tempered martensite or lower bainite is 90% or more, a dislocation density thereof is greater than or equal to 5&#xd7;10(1/m) and less than or equal to 1&#xd7;10(1/m) and 1&#xd7;10(numbers/mm) or more iron-based carbides are included therein.

High-strength hot-rolled steel sheet having excellent baking hardenability and low temperature toughness with maximum tensile strength of 980 MPa or more

There are provided a high-strength hot-rolled steel sheet securing low-temperature toughness and having excellent stretch flangeability by controlling a structural fraction and a hardness difference among structures, and a manufacturing method thereof. A hot-rolled steel sheet contains: C: 0.01 to 0.2%; Si: 0.001 to 2.5% or less; Mn: 0.10 to 4.0% or less; P: 0.10% or less; S: less than 0.03%; Al: 0.001 to 2.0%; N: less than 0.01%; Ti: (0.005+48/14[N]+48/32[S]) % or more and 0.3% or less; Nb: 0 to 0.06%; Cu: 0 to 1.2%; Ni: 0 to 0.6%; Mo: 0 to 1%; V: 0 to 0.2%; Cr: 0 to 2%; Mg: 0 to 0.01%; Ca: 0 to 0.01%; REM: 0 to 0.1%; and B: 0 to 0.002%, and has: an texture in which, at a central portion of a sheet thickness located between &#x215c; to &#x215d; thickness positions of the sheet thickness from a surface of the steel sheet, an average value of X-ray random intensity ratios of a group of {100}&#x3c;011&#x3e; to {223}&#x3c;110&#x3e; orientations of a sheet plane is 6.5 or less and an X-ray random intensity ratio of a {332}&#x3c;113&#x3e; crystal orientation is 5.0 or less; and a microstructure in which a total area ratio of tempered martensite, martensite and lower bainite is more than 85%, and an average crystal grain diameter is 12.0 &#x3bc;m or less.

A hot-rolled steel sheet has a composition containing: 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; and O: 0.01% or less, and one kind or a total of two kinds of Ti and Nb for 0.01 to 0.30%. An average effective crystal grain diameter at a sheet thickness &#xbc; part is 10 &#x3bc;m or less, and an average effective crystal grain diameter at a part of a range of 50 &#x3bc;m from a surface is 6 &#x3bc;m or less. A structure of the steel sheet is tempered martensite or lower bainite, and a volume fraction thereof is 90% or more as a total.

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Tokyo, Japan

Yuuki Kanzawa