Jfe Steel Corporation

Yoshimasa Himei

Yoshikazu Suzuki

Yoichi Makimizu

Gentaro Takeda

Hiroshi Hasegawa

Hideyuki Takahashi

Yoshimasa Funakawa

Noriaki Kohsaka

Takashi Ueno

Yoshitsugu Suzuki

Michitaka Sakurai

Hiromi Yoshitomi

Takeo Kikuchi

A continuous hot-dip galvanizing apparatus has a vertical annealing furnace, one or more hearth rolls, a hot-dip galvanizing apparatus, an alloying line, and humidified gas supply ports. When the steel sheet having a Si content of 0.2 mass % or more is conveyed inside the annealing furnace, the humidified gas supply ports positioned in a latter part of the soaking zone supply the humidified gas to the soaking zone and the at least one dry gas supply port supplies the dry gas to the soaking zone. When the steel sheet having a Si content of less than 0.2 mass % is conveyed inside the annealing furnace, the plurality of the humidified gas supply ports do not supply the humidified gas to the soaking zone and the at least one dry gas supply port supplies the dry gas to the soaking zone.

Continuous hot dip galvanizing apparatus

Provided is a method for manufacturing a high-strength galvanized steel sheet. Heating in a first half of oxidizing treatment is performed at a temperature of 400&#xb0; C. to 750&#xb0; C. in an atmosphere having a particular Oconcentration and a particular HO concentration, and heating in a second half of the oxidizing treatment is performed at a temperature of 600&#xb0; C. to 850&#xb0; C. in an atmosphere having a particular Oconcentration and a particular HO concentration. Subsequently, heating in a heating zone for reduction annealing is performed to a temperature of 650&#xb0; C. to 900&#xb0; C. at a particular heating rate in an atmosphere having a particular Hconcentration and a particular HO concentration with the balance being Nand inevitable impurities, and soaking in a soaking zone for the reduction annealing is performed in an atmosphere having a particular Hconcentration and a particular HO concentration with the balance being Nand inevitable impurities.

Method for manufacturing high-strength galvanized steel sheet

Provided is a method for producing a galvannealed steel sheet. When the steel sheet passing through the soaking zone is a type of steel containing 0.2 mass % or more of Si, both dry gas and humidified gas are supplied to the soaking zone, where the humidified gas is supplied only from the humidified gas supply port positioned in a latter part of the soaking zone among a plurality of humidified gas supply ports, where the latter part of the soaking zone is determined considering a sheet passing speed V and a target temperature T on the exit side of the soaking zone.

Method for producing galvannealed steel sheet, and continuous hot dip galvanizing apparatus

A high-strength galvanized steel sheet having improved post-work impact resistance, a method for producing the high-strength galvanized steel sheet, and a high-strength member produced using the steel sheet. The high-strength galvanized steel sheet includes a steel sheet having a microstructure including ferrite and carbide-free bainite, martensite and carbide-containing bainite, and retained austenite, the total area fraction of ferrite and carbide-free bainite being 0% to 55%, the total area fraction of martensite and carbide-containing bainite being 45% to 100%, and the area fraction of retained austenite being 0% to 5%. Additionally, a galvanizing layer is disposed on the steel sheet. The density of gaps that cut across the entire thickness of the galvanizing layer in a cross section of the galvanizing layer, which is taken in the thickness direction so as to be perpendicular to the rolling direction, is 10 gaps/mm or more.

High-strength galvanized steel sheet, high-strength member, and method for producing high-strength galvanized steel sheet

Provided are a steel sheet having a tensile strength of 950 MPa or more and good toughness and a method for manufacturing the same. The steel sheet has a specific composition and a metallographic structure containing: a ferrite area fraction of 30% or less (including 0%), a tempered martensite area fraction of 70% or more (including 100%), and a retained austenite area fraction of 4.5% or less (including 0%), wherein the average aspect ratio of an iron based carbide, precipitated in tempered martensite grains, having a grain size in the largest 10% is 3.5 or more.

Steel sheet and method for manufacturing the same

A high-strength steel sheet contains, in mass %, C: 0.07 to 0.14%, Si: 0.65 to 1.65%, Mn: 1.8 to 2.6%, P: 0.05% or less, S: 0.005% or less, Al: 0.08% or less, N: 0.0060% or less, Ti: 0.005 to 0.030%, B: 0.0002 to 0.0030%, and either or both of Cr: 0.01 to 0.40% and Mo: 0.01 to 0.50% and satisfies the expression (1); where: an average grain size of a ferrite phase is 1.5 &#x3bc;m or less; an area ratio of the ferrite phase is 2% or more and 15% or less; an area ratio of a tempered martensite phase is 75% or more and 96% or less; and a total length per unit area of an interface between an untempered martensite phase and the ferrite phase and an interface between the untempered martensite phase and the tempered martensite phase is 6.3&#xd7;10&#x3bc;m/mto 5.0&#xd7;10&#x3bc;m/m.

High-strength steel sheet and manufacturing method therefor

High strength steel sheets that have excellent ductility and low-temperature toughness and methods for producing such high strength steel sheets. A high strength steel sheet includes a composition containing, on a mass % basis, C: 0.05% to 0.30%, Si: 0.5% to 2.5%, Mn: 0.5% to 3.5%, P: 0.003% to 0.100%, S: 0.02% or less, Al: 0.010% to 1.5%, and N: 0.01% or less, the balance being Fe and unavoidable impurities, and a steel microstructure including a ferrite phase with an area fraction of 10% to 70%, a hard second phase with an area fraction of 30% to 90%, and a carbide that is at an interface between a ferrite phase and a hard second phase and that has an average equivalent-circle diameter of 200 nm or less.

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Tokyo, Japan

Yoshimasa Himei