Nippon Steel Sumitomo Metal Corporation

Nippon Steel Corporation

Akinobu Minami

Naoki Maruyama

Hiroyuki Kawata

Akinobu Murasato

Takuya Kuwayama

Takeshi Yasui

Kaoru Hiramatsu

Hiroyuki Ban

Yuji Yamaguchi

Natsuko Sugiura

Shigeru Yonemura

Masafumi Azuma

Takamasa Suzuki

Takafumi Yokoyama

Chisato Wakabayashi

Hajime Hasegawa

Yusuke Tsunemi

Tatsuya Obuchi

Tsuyoshi Oki

Provided are a steel sheet having a predetermined chemical composition, and a steel microstructure comprising, by vol %, ferrite: 1 to 50%, ratio of nonrecrystallized ferrite in the ferrite: 0 to 50%, tempered martensite: 1% or more, retained austenite: 5% or more, fresh martensite: 0 to 10%, total of pearlite and cementite: 0 to 5%, and balance: bainite, and, when analyzing the surface by an EPMA, an area ratio of regions with an Al/Siratio of 0.2 or less is 50% or less, and a tensile strength is 980 MPa or more, and a method for producing the same.

Steel sheet and method for producing same

The present invention provides a high-strength steel sheet excellent in impact resistance. The high-strength steel sheet contains predetermined contents of C, Si, Mn, P, S, Al, Ti, N, and O, with the balance being iron and inevitable impurities, and has a steel sheet structure in which, in a &#x215b; thickness to &#x215c; thickness region across &#xbc; of a sheet thickness, 1 to 8% retained austenite is contained in volume fraction, an average aspect ratio of the retained austenite is 2.0 or less, an amount of solid-solution Mn in the retained austenite is 1.1 times an average amount of Mn or more, and TiN grains having a 0.5 &#x3bc;m average grain diameter or less are contained, and a density of AlN grains with a 1 &#x3bc;m grain diameter or more is 1.0 pieces/mmor less, wherein a maximum tensile strength is 900 MPa or more.

High-strength steel sheet excellent in impact resistance and manufacturing method thereof, and high-strength galvanized steel sheet and manufacturing method thereof

The present invention provides a high-strength steel sheet excellent in shape fixability. The high-strength steel sheet contains C, Si, Mn, P, S, Al, N, and O with predetermined contents, in which a retained austenite phase of 5 to 20% in volume fraction is contained, an amount of solid-solution C contained in the retained austenite phase is 0.80 to 1.00% in mass %, Wis 1.10 times or more W, Wis 1.10 times or more W, and when a frequency distribution is measured with respect to a sum of a ratio between Wand Wand a ratio between Wand W, a mode value of the frequency distribution is 1.95 to 2.05, and a kurtosis is 2.00 or more.

High-strength steel sheet and high-strength galvanized steel sheet excellent in shape fixability, and manufacturing method thereof

High strength steel sheet and high strength galvanized steel sheet which are excellent in shapeability which secure a tensile maximum strength 900 MPa or more high strength while obtaining excellent ductility and stretch flangeability, which sheets have predetermined compositions of ingredients, have steel sheet structures which contain volume fraction 1 to 20% of residual austenite phases, and which have martensite transformation points of the residual austenite phases of &#x2212;60&#xb0; C. or less.

High strength steel sheet and high strength galvanized steel sheet excellent in shapeability and methods of production of same

There is provided a high-strength hot-dip galvanized steel sheet and the like excellent in mechanical cutting property, which are capable of obtaining high ductility while ensuring high strength with maximum tensile strength of 900 MPa or more. The high-strength hot-dip galvanized steel sheet has a sheet thickness of 0.6 to 5.0 mm and has a plating layer on a surface of a steel sheet with component compositions being set in appropriate ranges, in which the steel sheet structure contains a 40 to 90% ferrite phase and a 3% or more retained austenite phase by volume fraction. In the retained austenite phase, a solid solution carbon amount is 0.70 to 1.00%, an average grain diameter is 2.0 &#x3bc;m or less, an average distance between grains is 0.1 to 5.0 &#x3bc;m, a thickness of a decarburized layer in a steel sheet surface layer portion is 0.01 to 10.0 &#x3bc;m, an average grain diameter of oxides contained in the steel, sheet surface layer portion is 30 to 120 nm and an average density thereof is 1.0&#xd7;10oxides/mor more, and moreover, a work hardening coefficient (n value) during a 3 to 7% plastic deformation is 0.080 or more on average.

High-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet excellent in mechanical cutting property, and manufacturing method thereof

A base steel sheet has a hot-dip galvanized layer formed on a surface thereof, in which, in a steel sheet structure in a range of &#x215b; thickness to &#x215c; thickness centered around &#xbc; thickness of a sheet thickness from a surface, a volume fraction of a retained austenite phase is 5% or less, and a total volume fraction of phases of bainite, bainitic ferrite, fresh martensite, and tempered martensite is 40% or more, an average effective crystal grain diameter is 5.0 &#x3bc;m or less, a maximum effective crystal grain diameter is 20 &#x3bc;m or less, and a decarburized layer with a thickness of 0.01 &#x3bc;m to 10.0 &#x3bc;m is formed on a surface layer portion, in which a density of oxides dispersed in the decarburized layer is 1.0&#xd7;10to 1.0&#xd7;10oxides/m, and an average grain diameter of the oxides is 500 nm or less.

High-strength hot-dip galvanized steel sheet excellent in impact resistance property and manufacturing method thereof, and high-strength alloyed hot-dip galvanized steel sheet and manufacturing method thereof

High strength steel sheet which secures tensile maximum strength 900 MPa or more high strength while having excellent shapeability, which high strength steel sheet which is excellent in shapeability characterized by having a predetermined composition of ingredients, by the steel sheet structure including a ferrite phase and martensite phase, by the ratio of Cu particles incoherent with the bcc iron being 15% or more with respect to the Cu particles as a whole, by a density of Cu particles in the ferrite phase being 1.0&#xd7;10/mor more, and by an average particle size of Cu particles in the ferrite phase being 2.0 nm or more.

Provided are: a galvannealed steel sheet, improved in the adhesiveness of a plated layer with a base steel sheet, as a galvannealed steel sheet, prepared by using a high-strength steel sheet as a base material; and a method for producing the galvannealed steel sheet. A galvannealed layer is formed on the base steel sheet including a high-strength steel having a predetermined composition. The average amount of Fe in the galvannealed layer is 8.0 to 12.0%. The absolute value of a difference (&#x394;Fe) between the amount of Fe in the vicinity of an interface with the base steel sheet and the amount of Fe in the vicinity of the external surface of the plated layer in the plated layer is 0.0 to 3.0%.

Galvannealed layer and steel sheet comprising the same, and method for producing the same

The present invention provides a high-strength galvanized steel sheet with maximum tensile strength of 900 MPa or more. The high-strength galvanized steel sheet has an alloyed galvanized layer formed on a surface of a base steel sheet containing predetermined amounts of C, Si, Mn, P, S, Al, N, O with a balance being constituted of iron and inevitable impurities, in which in a structure of the base steel sheet, retained austenite is limited to 8% or less in volume fraction, kurtosis K* of the hardness distribution between 2% hardness and 98% hardness is &#x2212;0.30 or less, a ratio between Vickers hardness of surface layer of the base steel sheet and Vickers hardness of &#xbc; thickness of the base steel sheet is 0.35 to 0.70, and a content of iron in the alloyed galvanized layer is 8 to 12% in mass %.

High-strength galvanized steel sheet excellent in bendability and manufacturing method thereof

Steel contains each of C, Si, Mn, P, S, Al, N, O, at a range from &#x215b; thickness centered around a &#xbc; sheet thickness from a surface to &#x215c; thickness centered around the &#xbc; sheet thickness from the surface at a base steel sheet, a structure of the base steel sheet contains, in volume fraction, 3% or more of a retained austenite phase, 50% or less of a ferrite phase, and 40% or more of a hard phase, average dislocation density is 5&#xd7;10/mor more, solid-solution C amount contained in the retained austenite phase is in mass % 0.70 to 1.00%, X-ray random intensity ratio of FCC iron in an texture of the retained austenite phase is 3.0 or less, ratio between a grain diameter relative to a rolling direction and a grain diameter relative to a sheet width direction of the retained austenite phase is 0.75 to 1.33.

High-strength hot-dip galvanized steel sheet, high-strength alloyed hot-dip galvanized steel sheet excellent in bake hardenability, and manufacturing method thereof

A hot-dip galvanizing layer or an alloyed hot dip galvanizing layer is formed on the surface of a base steel sheet in which in volume fraction, 40 to 90% of a ferrite phase and 5% or less of a retained austenite phase are contained, and a ratio of non-recrystallized ferrite to the entire ferrite phase is 50% or less in volume fraction, and further a grain diameter ratio being a value of, of crystal grains in the ferrite phase, an average grain diameter in the rolling direction divided by an average grain diameter in the sheet width direction is 0.75 to 1.33, a length ratio being a value of, of hard structures dispersed in island shapes, an average length in the rolling direction divided by an average length in the sheet width direction is 0.75 to 1.33, and an average aspect ratio of inclusions is 5.0 or less.

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Tokyo, Japan

Akinobu Minami