Jfe Steel Corporation

Mai Miyata

Yoshitsugu Suzuki

Yasunobu Nagataki

Yoshiyasu Kawasaki

Yoichi Makimizu

Minoru Tanaka

Shinjiro Kaneko

Tatsuya Nakagaito

Kohei Hasegawa

Junichiro Hirasawa

Naoto Yoshimi

Hirokazu Sugihara

Hideki Nagano

Shinji Otsuka

Provided is a hot-rolled steel sheet having a composition containing 0.04 mass percent to 0.20 mass percent C, 0.7 mass percent to 2.3 mass percent Si, 0.8 mass percent to 2.8 mass percent Mn, 0.1 mass percent or less P, 0.01 mass percent or less S, 0.1 mass percent or less Al, and 0.008 mass percent or less N, the remainder being Fe and inevitable impurities. Internal oxides containing one or more selected from the group consisting of Si, Mn, and Fe are present at grain boundaries and in grains in a base metal. The internal oxides present at the grain boundaries in the base metal are located within 5 &#x3bc;m from the surface of the base metal. The difference between the depths at which the internal oxides are formed in the cross direction of the steel sheet is 2 &#x3bc;m or less.

Hot-rolled steel sheet for high-strength galvanized steel sheet or high-strength galvannealed steel sheet and method for manufacturing the same (as amended)

A method for manufacturing a galvanized steel sheet includes heating a base steel sheet in a heating zone such that the surface of the base steel sheet is heated at a temperature of 600&#xb0; C. or higher and 790&#xb0; C. or lower while a furnace temperature T&#xb0; C. in the heating zone of an annealing furnace is controlled based on the water vapor partial pressure Pof air fed into the heating zone, the base steel sheet having a chemical composition consisting of, by mass %, C: 0.05% or more and 0.25% or less, Si: 0.1% or more and 3.0% or less, Mn: 0.5% or more and 3.0% or less, P: 0.001% or more and 0.10% or less, Al: 0.01% or more and 3.00% or less, S: 0.200% or less, and the balance being Fe and inevitable impurities, heating the base steel sheet in the heating zone such that the surface of the base steel sheet is heated at a temperature of 630&#xb0; C. or higher and 850&#xb0; C. or lower in an atmosphere containing hydrogen gas having a partial pressure Pof 1000 Pa or more and 50000 Pa or less, water vapor gas having a partial pressure Pof 610 Pa or less, and the balance being Nand inevitable impurities, and galvanizing the base steel sheet.

Method for manufacturing galvanized steel sheet

A hot dip galvanized steel sheet includes a steel sheet, a hot dip galvanizing layer, and an intermetallic compound, in which the steel sheet has a chemical composition containing, by mass %, C: 0.03% or more and 0.70% or less, Si: 0.10% or less, Mn: 0.5% or more and 0.9% or less, P: 0.020% or more and 0.050% or less, S: 0.010% or less, Nb: 0.010% or more and 0.035% or less, N: 0.005% or less, Al: 0.10% or less, and the balance being Fe and inevitable impurities, in which the hot dip galvanizing layer contains 0.3% or more and 0.6% or less of Al, and in which the intermetallic compound contains 0.12 gmor more and 0.22 gmor less of Al and FeAlhaving an average grain diameter of 1 &#x3bc;m or less.

Hot dip galvanized steel sheet

A method of manufacturing a high strength galvanized steel sheet has a first heating step including heating to 400&#xb0; C. to 750&#xb0; C. in an atmosphere containing O: 0.1 to 20 percent and HO: 1 to 50 percent and heating to 600&#xb0; C. to 850&#xb0; C. in an atmosphere containing O: 0.01 to less than 0.1 percent and HO: 1 to 20 percent is applied to a steel sheet, a second heating step includes holding the steel sheet in an atmosphere containing H: 1 to 50 percent and having a dew point of 0&#xb0; C. or lower at 750&#xb0; C. to 900&#xb0; C. for 15 to 600 s, cooling to a temperature of 450&#xb0; C. to 550&#xb0; C., and holding is performed at that temperature for 10 to 200 s, and a galvanization treatment is applied.

Method for manufacturing high strength galvanized steel sheet having excellent stability of mechanical properties, formability, and coating appearance

A high-strength cold-rolled steel sheet having high chemical convertibility and a tensile strength of 590 MPa or more and a method for producing such a steel sheet are provided. The steel sheet contains, in terms of percent by mass, C: 0.05 to 0.3%, Si: 0.6 to 3.0%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.01 to 1%, N: 0.01% or less, and the balance being Fe and unavoidable impurities. The coverage ratio of reduced iron on a steel sheet surface is 40% or more. In order to produce such a steel sheet, an oxidation treatment is performed after cold rolling. Subsequently, annealing is conducted in a furnace in a 1 to 10 vol % H+balance Ngas atmosphere with a dew point of &#x2212;25&#xb0; C. or less.

High-strength cold-rolled steel sheet and method for producing the same

A galvannealed steel sheet having high corrosion resistance after painting includes, on a mass percent basis, C: 0.05% to 0.15%, Si: 0.1% to 1.0%, Mn: 0.5% to 2.7%, Al: &#x2266;1.00%, P: &#x2266;0.025%, S: &#x2266;0.025%, and Cr: 0% to 0.8% as chemical components, the remainder being Fe and incidental impurities. The galvannealed steel sheet includes a galvannealed layer in an amount of 20 to 120 g/mper side, a percentage of exposed zinc metal of the galvannealed layer surface being 20% or more and less than 80%, and an amount of internal oxidation in a region disposed not more than 5 &#x3bc;m from the surface of the steel sheet is 0.02 g/mor more and 0.1 g/mor less: Si&#x2266;0.68%, Cr&#x2266;&#x2212;1.25Si+0.85, and Si&#x3e;0.68%, Cr=0, wherein Si and Cr denote respective contents (mass %) of Si and Cr.

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Fukuyama, Japan

Mai Miyata