Nippon Steel Sumitomo Metal Corporation

Nippon Steel Corporation

Hiroyuki Ban

Hiroyuki Kawata

Takeshi Yasui

Naoki Maruyama

Takuya Kuwayama

Kaoru Hiramatsu

Akinobu Murasato

Akinobu Minami

Kohei Ueda

Takayuki Kitazawa

A steel sheet has a predetermined chemical composition, and a surface exhibits an absorption peak at which a reflectance is not less than 50% nor more than 85% in a range of wave numbers of 1200 cmto 1300 cmby a Fourier transform-infrared spectroscopy analysis by a reflection absorption spectrometry method, and does not exhibit an absorption peak in a range of wave numbers of 1000 cmto 1100 cm, or exhibits an absorption peak at which a reflectance is 85% or more in the range of wave numbers of 1000 cmto 1100 cm, wherein Ni of 3 mg/mto 100 mg/madheres to the surface.

Steel sheet

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

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

Hiroyuki Ban