Jfe Steel Corporation

Mai Aoyama

Yoshiyasu Kawasaki

Yoshimasa Funakawa

Yoshitsugu Suzuki

Yoichi Makimizu

Yuki Toji

Hiroshi Hasegawa

Hiroshi Matsuda

Katsutoshi Takashima

Satoshi Maeda

Hideyuki Kimura

Katsuya Hoshino

Shunsuke Yamamoto

Tomomi Kanazawa

Yusuke Okumura

Kazuki Endo

Yusuke Fushiwaki

Noriaki Kohsaka

Masaki Koba

Ei Okumura

Hideyuki Takahashi

Minoru Tanaka

Takashi Kawano

Kenji Kawamura

Takako Yamashita

Gentaro Takeda

Koichiro Fujita

Kazuki Endoh

To provide an Fe-based electroplated steel sheet that not only has excellent chemical convertibility or excellent coating appearance when subjected to hot-dip galvanizing, but also has excellent resistance to cracking in resistance welding. Disclosed is an Fe-based plated steel sheet including: a cold-rolled steel sheet having a chemical composition containing Si in an amount of 0.1 mass % or more and 3.0 mass % or less; and an Fe-based electroplating layer formed on one or both surfaces of the cold-rolled steel sheet with a coating weight per surface of 1.0 g/mor more, in which a thickness of an internal oxidation layer is 2.00 μm or less, and an average value of C concentration in a range of 10 μm to 20 μm in a thickness direction from the surface of the Fe-based electroplating layer is 0.10 mass % or less.

Fe-based electroplated steel sheet and galvannealed steel sheet, and methods of producing same

A high strength steel sheet and method for manufacturing the same. The steel sheet has a specified chemical composition and a specified microstructure. A value calculated by dividing the average Mn content in retained austenite by the average Mn content in ferrite is 1.5 or more, a value calculated by dividing the sum of the area fraction of as-quenched martensite having a circle-equivalent grain size of 3 μm or more and the area fraction of retained austenite having a circle-equivalent grain size of 3 μm or more by the sum of the area fraction of all the as-quenched martensite and the area fraction of all the retained austenite is less than 0.4, and a value calculated by dividing the area fraction of retained austenite grains adjacent to three or more ferrite grains having different crystal orientations by the area fraction of all the retained austenite is less than 0.6.

High strength steel sheet and method for manufacturing the same

Provided is a galvanized steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if crystal orientations of an Fe-based electroplating layer and a cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. The galvanized steel sheet has a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % to 3.0 mass %; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface of more than 20.0 g/m, and a galvanized layer formed on the Fe-based electroplating layer, where crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50 % at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.

Galvanized steel sheet, electrodeposition-coated steel sheet, automotive part, method of producing electrodeposition-coated steel sheet, and method of producing galvanized steel sheet

Provided is an alloyed galvanized steel sheet with excellent resistance to cracking in resistance welding at a welded portion, even if crystal orientations of an Fe-based electroplating layer and a cold-rolled steel sheet are integrated at a high ratio at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. The alloyed galvanized steel sheet has a Si-containing cold-rolled steel sheet containing Si in an amount of 0.1 mass % to 3.0 mass %; an Fe-based electroplating layer formed on at least one surface of the Si-containing cold-rolled steel sheet with a coating weight per surface exceeding 20.0 g/m, and an alloyed galvanized layer formed on the Fe-based electroplating layer, where crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio of more than 50% at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.

Alloyed galvanized steel sheet, electrodeposition-coated steel sheet, automotive part, method of producing electrodeposition-coated steel sheet, and method of producing alloyed galvanized steel sheet

A high-strength galvanized steel sheet and a method for producing the steel sheet. The steel sheet has a composition that includes C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn: 2.00% to 10.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, N: 0.0005% to 0.0100%, Ti: 0.005% to 0.200%, on a mass basis, and Fe and inevitable impurities. Additionally, the steel sheet has concentration of solute Mn at a depth of 5 &#x3bc;m or less from a surface of the steel sheet that is 1.50% by mass or less, and a value obtained by dividing the average mass percentage of Mn in retained austenite by the average mass percentage of Mn in ferrite is 2.0 or more.

Method for manufacturing high-strength galvanized steel sheet and high-strength galvanized steel sheet

A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80% and tempered martensite being greater than 5% and 20% or less in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 &#x3bc;m or less; and the retained austenite being 3 &#x3bc;m or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

High-strength steel sheet and method for manufacturing same

A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80%, martensite being 5% to 35%, and tempered martensite being 0% to 5% in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 &#x3bc;m or less; and the retained austenite being 3 &#x3bc;m or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

A method for producing a high-strength hot-dip galvannealed steel sheet, in which a high-strength steel sheet is used as a base material, includes a rolling step (x) of rolling a hot-dip galvannealed steel sheet with a coating layer having an Fe concentration of 8% to 17% by mass, and a heat treatment step (y) of heating the coated steel sheet which has been subjected to the rolling step (x) under the conditions satisfying the following formulae (1) and (2):(273+)&#xd7;(20+2&#xd7;log())&#x2265;8000&#x2003;&#x2003;(1)40&#x2264;&#x2264;160&#x2003;&#x2003;(2)

High-strength hot-dip galvannealed steel sheet and method for producing same

A high-strength steel includes a steel structure with: in area fraction, 60.0% to less than 90.0% of ferrite, 0% to less than 5.0% of unrecrystallized ferrite, 2.0% to 25.0% of martensite, 0% to 5.0% of carbide, and 0% to 3.0% of bainite; in volume fraction, more than 7.0% of retained austenite; in a cross-sectional view of 100 &#x3bc;m&#xd7;100 &#x3bc;m, a value obtained by dividing number of retained austenite that are not adjacent to retained austenite whose crystal orientations are different by a total number of retained austenite being less than 0.80, an average crystal grain size of the ferrite being 6.0 &#x3bc;m or less, an average crystal grain size of the retained austenite being 3.0 &#x3bc;m or less, and a value obtained by dividing, by mass %, an average content of Mn in the retained austenite by an average content of Mn in steel being 1.50 or more.

High-strength steel sheet and production method thereof

Provided are a cold-rolled steel sheet which can preferably be used for manufacturing a high-strength galvanized steel sheet and methods for manufacturing the steel sheets. The cold-rolled steel sheet has a specified chemical composition, in which the Mn concentration in a surface layer of the steel sheet satisfies relational expression (1) and relational expression (2) below. 8&#x2264;(C/C)&#xd7;Mn . . . (1) (C/C)&#xd7;Mn&#x2264;2.5 . . . (2) where C: maximum Mn concentration in a region within 0.5 &#x3bc;m of the surface of a steel sheet in the thickness direction; C: average Mn concentration in a region from a position located 5 &#x3bc;m from a surface of a steel sheet in the thickness direction to a position located 5 &#x3bc;m from an opposite surface in the thickness direction; C: minimum Mn concentration in a region from 0.5 &#x3bc;m to 5 &#x3bc;m from the surface of a steel sheet in the thickness direction; and Mn: Mn content (mass %).

Cold-rolled steel sheet, coated steel sheet, method for manufacturing cold-rolled steel sheet, and method for manufacturing coated steel sheet

A method for manufacturing a high-strength galvanized steel sheet. The method includes a first heating step of holding the steel sheet in a temperature range of 750&#xb0; C. to 880&#xb0; C. for 20 s to 600 s in an atmosphere having an Hconcentration of 0.05% to 25.0% by volume and a dew point of &#x2212;45&#xb0; C. to &#x2212;10&#xb0; C., a cooling step, a rolling step of rolling the steel sheet with a rolling reduction of 0.3% to 2.0%, a pickling step of pickling the steel sheet with a pickling weight loss of 0.02 gram/mto 5 gram/min terms of Fe, a second heating step of holding the steel sheet in a temperature range of 720&#xb0; C. to 860&#xb0; C. for 20 sec. to 300 sec. in an atmosphere having an Hconcentration of 0.05% to 25.0% by volume and a dew point of &#x2212;10&#xb0; C. or lower, and a galvanizing step.

Method for manufacturing high-strength galvanized steel sheet

A method for manufacturing a high-strength galvanized steel sheet includes performing hot rolling, cold rolling, first annealing, pickling, and second annealing. The first annealing is performed to obtain a steel sheet having a steel microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio. The second annealing includes heating to an annealing temperature of 750&#xb0; C. or higher and 850&#xb0; C. or lower, holding at the annealing temperature for 10 seconds or more and 500 seconds or less, cooling at an average cooling rate of 1&#xb0; C./s or more and 15&#xb0; C./s or less, performing a galvanizing treatment, and cooling to a temperature of 150&#xb0; C. or lower at an average cooling rate of 5&#xb0; C./s or more and 100&#xb0; C./s or less.

A high strength galvanized steel sheet that has a tensile strength (TS) of 780 MPa or higher and excellent bendability, fatigue resistance, and surface appearance quality and a method for producing the high strength galvanized steel sheet. The steel sheet includes, at a position &#xbd; of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction.

High strength galvanized steel sheet and production method therefor

A method for producing high-strength galvannealed steel sheets having excellent workability and fatigue resistance. The method comprises subjecting a steel sheet to an oxidation treatment, the oxidation treatment including heating the steel sheet in an upstream stage at a temperature in the range of 400&#xb0; C. to 750&#xb0; C. in an atmosphere having an Oconcentration of not less than 1000 ppm by volume and a HO concentration of not less than 1000 ppm by volume and a downstream stage at a temperature in the range of 600&#xb0; C. to 850&#xb0; C. in an atmosphere having an Oconcentration of less than 1000 ppm by volume and a HO concentration of not less than 1000 ppm by volume. The method further comprises subjecting the steel sheet to reducing annealing, hot-dip galvanizing treatment and alloying treatment.

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Fukuyama, Japan

Mai Aoyama