Abstract: A high-strength hot-dip galvanized steel sheet that even on the premise of ordinary CGL heat cycle, has a low yield stress and excels in resistance to natural aging and baking hardenability without reliance on the use of expensive Mo; and a process for producing the same. The constituent composition thereof comprises 0.01 to less than 0.08% C, 0.2% or less Si, more than 1.0 to 1.8% Mn, 0.10% or less P, 0.03% or less S, 0.1% or less Al, 0.008% or less N and more than 0.5% Cr so that the relationship 1.95?Mn(mass %)+1.3Cr(mass %)?2.8 is satisfied and comprising the balance iron and unavoidable impurities. The structure thereof has a ferrite phase and a martensite phase of 2 to 15% area ratio, and the cumulative area ratio of pearlite phase and/or bainite phase is 1.0% or less. In the production of this hot-dip galvanized steel sheet, the temperature and cooling rate are controlled during the annealing/plating operation subsequent to cold rolling.

Abstract: A galvannealed steel sheet is manufactured by preparing a hot-dip galvanized steel sheet, alloying the steel sheet, and controlling time and temperature for alloying thereof depending on Si and Al contents. The hot-dip galvanized steel sheet contains 0.05 to 0.30% C, 0.01 to 2.0% Si, 0.08 to 3.0% Mn, 0.003 to 0.1% P, 0 to 0.07% S, 0.01 to 2.5% Al, 0 to 0.007% N, by mass, and the balance being Fe and inevitable impurities. Alloying time and temperature are controlled by the formula [Si+Al?1.5×10?7×t0.75×(T?465)3+0.117], where t is the total time (sec) of holding the sheet at 465° C. or more on alloying the coating layer thereon, and T is the average temperature (° C.) of the sheet during the total time t (sec) of holding the steel sheet at 465° C. or more on alloying the coating layer thereon.

Abstract: The invention provides a galvannealed steel sheet which has an oxide layer having 10 nm or larger thickness on the plateau of the coating layer flattened by temper rolling. With the use of the galvannealed steel sheet, no powdering occurs during press-forming, and stable and excellent sliding performance is attained. By selecting the area percentage of the plateau of the flattened coating layer to a range from 20 to 80%, making the coating layer single layer of &dgr;1 phase, and letting &zgr; phase exist in the &dgr;1 phase, further improved sliding performance and anti-powdering property are obtained.

Abstract: The invention provides a galvannealed steel sheet which has an oxide layer having 10 nm or larger thickness on the plateau of the coating layer flattened by temper rolling. With the use of the galvannealed steel sheet, no powdering occurs during press-forming, and stable and excellent sliding performance is attained. By selecting the area percentage of the plateau of the flattened coating layer to a range from 20 to 80%, making the coating layer single layer of &dgr;1 phase, and letting &zgr; phase exist in the &dgr;1 phase, further improved sliding performance and anti-powdering property are obtained.

Abstract: A zinciferous coated steel sheet comprises a steel sheet, a zinciferous coating layer formed on the steel sheet, a Fe--Ni--Zn--O film formed on the zinciferous coating layer, and an oxide layer formed on a surface portion of the Fe--Ni--Zn--O film. The method comprises providing an electrolyte of an acidic sulfate aqueous solution, carrying out an electrolysis treatment in the electrolyte under a current density ranging from 1 to 150 A/dm.sup.2, and carrying out an oxidation treatment to a surface of the zinciferous coated steel sheet.

Abstract: A zinciferous plated steel sheet which comprises a steel sheet, at least one zinciferous plating layer formed on at least one surface of the steel sheet, and an Fe--Ni--O film as an uppermost layer formed on the zinciferous plating layer. The total quantity of metallic elements in the Fe--Ni--O film is within a range of from 10 to 1,500 mg/m.sup.2, and the oxygen content in the Fe--Ni--O film is within a range of from 0.5 to under 30 wt. %. The ratio of the iron content (g/l) relative to the total quantity of the iron content (g/l) and the nickel content (g/l) in the Fe--Ni--O film is preferably within a range of from over 0 to under 1.0.

Abstract: A method for manufacturing a zinciferous plated steel sheet, comprises: forming a zinciferous plating layer on a steel sheet; and forming an Fe--Ni--O film on the zinciferous plating layer. The Fe--Ni--O film is formed by carrying out electrolysis with the steel sheet as a cathode in an aqueous solution, dipping the steel sheet in an aqueous solution, or spraying a mist on a surface of the zinciferous plating layer.

Abstract: An alloying-treated iron-zinc alloy dip-plated steel sheet excellent in press-formability, having, on the surface thereof, numerous fine concavities which satisfy the following conditions: (1) that the number of fine concavities having a depth of at least 2 .mu.m is within a range of from 200 to 8,200 per mm.sup.2 of the plating layer, and (2) that the total opening area per unit area of the fine concavities in the plating layer is within a range of from 10 to 70% of the unit area. The above-mentioned plated steel sheet is manufactured by subjecting a cold-rolled steel sheet to a zinc dip-plating treatment in a zinc dip-plating bath having an aluminum content of from 0.05 to 0.30 wt. %, in which the temperature region causing an initial reaction for forming an iron-aluminum layer is limited within a range of from 500.degree. to 600.degree. C., an alloying treatment in which an alloying treatment temperature is limited within a range of from 480.degree. to 600.degree. C., and a temper-rolling treatment.

Abstract: An inventive method may perform a plating the surface of the metals sheet without using a molten plating metal. This method successively melts a supplied solidus plating metal in close proximity of the passing metal sheet and adheres the molten plating metal as a plating film to the surface of the metal sheet, where the metal sheet passes upwardly and the plating metal is supplied through an upwardly-directed nozzle disposed near the passing sheet, and when or immediately before the plating metal is supplied from the nozzle, it is molten by a heat melting means. The molten plating metal forms a pool at a corner defined between the surface of the passing sheet and the tip of the nozzle, and the molten metal of the pool forms a plating adheres to the sheet surface and forms the plating film.