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+T)×(20+2×log.sub.10(t))≥8000  (1)

40≤T≤160  (2) where T: heating temperature (° C.) of the coated steel sheet, and t: holding time (hr) at the heating temperature T.

A coated steel material including: a base steel, and a coating layer containing a Zn—Al—Mg alloy layer disposed on a surface of the base steel, wherein the coating layer has a predetermined chemical composition, and, in a backscattered electron image of the Zn—Al—Mg alloy layer that is obtained at a time of observing the surface of the Zn—Al—Mg alloy layer after polishing to ½ of the layer thickness, under a scanning electron microscope at a magnification of 100×, Al crystals are present, and the average value of the cumulative circumferential length of the Al crystals is 88 to 195 mm/mm.sup.2.

The present invention provides a method for manufacturing a hot-dip galvanized steel sheet, comprising performing a heat treatment step, a hot dip plating step, and an alloying treatment step on a steel sheet having high Si and Mn content. The heat treatment step comprises a first heating phase and a soaking phase; a first heating atmosphere of the first heating stage contains 0.01-0.5% of O.sub.2 by volume, and the balance is N.sub.2 and inevitable impurities; a soaking atmosphere of the soaking stage contains 0.5% or more of H.sub.2 by volume, and the balance is N.sub.2 and inevitable impurities; dew points of the first heating atmosphere and the soaking atmosphere are controlled to be greater than or equal to −20° C. The present invention further provides a hot-dip galvanized steel sheet and a vehicle component. According to the present invention, by controlling the heat treatment atmospheres, the enrichment of Si and Mn alloy elements on the surface of the steel sheet, an interface between iron oxide and a base steel sheet, and an interface between reduced iron and the base steel sheet is suppressed, so that when a steel sheet having high Si and Mn content is selected as a base material for the hot-dip galvanized steel sheet, after undergoing the alloying treatment step, the hot-dip galvanized steel sheet can have both a plating layer of sufficient Fe content and a high elongation at break rate.

Equipment for the continuous hot dip-coating of a metal strip 9 including an annealing furnace, a tank 2 containing a liquid metal bath 3, a snout connecting the annealing furnace and tank 2, through which the metal strip 9 runs in a protective atmosphere and the lower part of the snout, the sabot 5, is at least partly immersed in the liquid metal bath 3 in order to define with the surface of the bath, and inside this snout, a liquid seal 6, an overflow 7 not connected to the snout, the overflow 7 including at least one tray 8, placed in the vicinity of the strip 9 when entering the liquid metal bath 3 and encompassed by liquid seal 6.

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.

The invention relates to a method for removing residues present on a metal strip at the outlet of a cooling stage of a continuous line, the residues being formed during a cooling of said metal strip by a non-oxidizing liquid solution for the metal strip and a stripping liquid solution for the oxides present on the surface of the strip, or by a mixture of this liquid solution and a gas. The method according to the invention is characterized in that it comprises a step of reducing the residues by hydrogen.

Provided are an austenite-based molten aluminum-plated steel sheet comprising: a steel plate which contains, by weight %, 0.3 to 0.9% of C, 12 to 25% of Mn, 0.5 to 2.5% of Si, 0.3 to 3.0% of Al, 0.01 to 0.5% of Ti, 0.05 to 0.5% of V, 0.01-0.5% of Mo, 0.01-0.2% of Sn, 0.001-0.1% of Co, and 0.001-0.1% of W, the remainder being Fe and unavoidable impurities; and a molten aluminum-based plated layer formed on a surface of the steel plate, and a method for producing the same.

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.

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.

A method comprises: annealing a steel sheet by conveying the steel sheet through a heating zone, a soaking zone, and a cooling zone in the stated order in an annealing furnace; and then applying a hot-dip galvanized coating onto the steel sheet discharged from the cooling zone. Reducing or non-oxidizing humidified gas and reducing or non-oxidizing dry gas are supplied into the soaking zone. A CO gas concentration is measured using a CO gas concentration meter provided in an exhaust portion for gas in the soaking zone. A decarburized layer thickness of the steel sheet is calculated from the measured CO gas concentration. At least one of a flow rate and a dew point of the humidified gas is controlled so that the calculated decarburized layer thickness is less than or equal to a predetermined thickness.