Method for manufacturing iron-zinc alloy plated steel sheet having two plating layers and excellent in electropaintability and pressformability

摘要

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 DEG to 520 DEG 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 1.5 to the alloying-treated steel sheet, to level the numerous fine convexities on the surface of the alloying-treated iron-zinc alloy dip-plating layer, so as to form on the surface of the steel sheet the alloying-treated iron-zinc alloy dip-plating layer as a lower layer having the numerous fine concavities on the surface thereof; and then applying an iron-zinc alloy electroplating treatment to the temper rolling treated steel sheet to form, on the alloying-treated iron-zinc alloy dip-plating layer as the lower layer, an iron-zinc alloy electroplating layer as an upper layer. The above-mentioned iron-zinc alloy electroplating treatment is carried out with an electric current density of from 50 to 150 A/dm 2 in an acidic electroplating bath containing iron ions and zinc ions, and having a pH value of from 1.0 to 4.0 and a temperature of from 40 DEG to 70 DEG C.