Abstract: It is an object to provide a process for manufacturing galvannealed steel sheets exhibiting excellent anti-powdering property when press formed, and uniform frictional properties in a coil. A steel strip is galvanized in a bath having a low aluminum content after entering it at a high temperature as defined in relation to the aluminum content of the both, so that the formation of a .zeta. phase may be promoted. Then, the strip is heated for alloying in a high-frequency induction heating furnace so as to have a temperature not exceeding 495.degree. C. when leaving the furnace to yield a plated steel strip having a coating containing a uniformly distributed .zeta. phase. After such heat treatment and cooling, the strip can be plated with an iron or iron-alloy top coating having an iron content of at least 50% and a coating weight of at least 1 g/m.sup.2 to achieve an improved press formability.

Abstract: A surface treated steel sheet comprises: a steel sheet containing C, Si, Mn, P, S, Ni, Ti, Cu and B; a diffused alloy layer containing Fe, Ni, and P; and a zinc coating layer formed on the diffused alloy layer.A method for producing a surface treated steel sheet comprises the steps of: preparing a steel sheet; coating the steel sheet with a Ni-P coating layer; heat-treating the steel sheet coated with the Ni-P coating layer; and forming a zinc coating layer on the diffused alloy layer.

Abstract: It is an object to provide a process for manufacturing galvannealed steel sheets having high anti-powdering property as required when they are press formed, and stabilized frictional properties in a coil. Steel sheets are plated in a bath having a low aluminum content, while they have, when entering the bath, a low temperature as defined in relation to the aluminum content of the bath, so that an alloying reaction may be prevented. Then, the sheets are heated for alloying in a high-frequency induction heating furnace so that the sheets leaving the furnace may have a temperature of from over 495.degree. C. to 520.degree. C. to yield galvannealed consisting mainly of a .delta..sub.1 phase. An iron or iron-alloy top coating having an appropriate iron content can be applied onto the plated steel surface to improve its paintability.

Abstract: A method for manufacturing an iron-zinc alloy plated steel sheet having two plating layers and excellent in electropaintability and press-formability, which comprises the steps of: applying an alloying treatment under a temperature of from 420.degree. to 520.degree. C. to a zinc dip-plated steel sheet to form on the surface of the steel sheet an alloying-treated iron-zinc alloy dip-plating layer having on the surface thereof fine irregularities comprising numerous fine concavities and numerous fine convexities; then applying a temper rolling treatment at a reduction rate of from 0.3 to 1.

Abstract: A method for manufacturing an iron-zinc alloy plated steel sheet having two plating layers and excellent in electropaintability and press-formability, which comprises the steps of: passing a steel sheet through a zinc dip-plating bath to form a zinc dip-plating layer on at least one surface of the steel sheet; then, heating the steel sheet, on which the zinc dip-plating layer has been formed, to alloy the zinc dip-plating layer and the surface portion of the steel sheet, so as to form, on the surface thereof, an alloying-treated iron-zinc alloy dip-plating layer as a lower layer having a plating weight of from 30 to 120 g/m.sup.

Abstract: An electroplated steel sheet having a plurality of coatings, excellent in workability, corrosion resistance and water-resistant paint adhesivity, which comprises:an iron-chromium-zinc alloy coating as a lower layer, having a coating weight of from 0.1 to 40 g/m.sup.2 per side, formed on the surface of a steel sheet, consisting essentially of:iron: from 3 to under 15 wt. %,chromium: from 0.1 to 1 wt. %, andthe balance being zinc and incidental impurities;another iron-chromium-zinc alloy coating as an intermediate layer, having a coating weight of from 20 to 59.9 g/m.sup.2 per side, formed on the iron-chromium-zinc alloy coating as the lower layer, consisting essentially of:iron: from 10 to 40 wt. %,chromium: from over 1 to under 30 wt.