TECHNICAL FIELD

The present disclosure relates to a method of manufacturing a surface-treated zinc-nickel alloy-electroplated steel sheet.

BACKGROUND ART

A cold-rolled material, plated with a Pb—Sn alloy (Terne metal) containing tin and lead, was mainly used for automobile fuel tank steel sheets until the 1980s, when corrosion resistivity and formability were considered important. This is because Pb—Sn plated layers not only form a protective film on their own to have excellent corrosion resistivity for protecting a Fe base iron but also have excellent ductility and lubricating properties, which facilitate deep drawing processing.

From the 1990s, however, an issue of reducing environmentally hazardous substances was raised nationwide, and efforts to research and develop lead (Pb)-free plating have been continuously made. In this regard, various alloy systems such as Al—Si, Sn—Zn, Zn—Ni, and the like, have newly emerged as plated steel sheets for fuel tanks.

In particular, Zn—Ni alloy-electroplated steel sheets contain about 11 wt % of Ni in a plating layer, resulting in a solid plating layer and a higher melting point as compared to a pure Zn-plated steel sheet. Besides, weldability with a low current may be feasible compared to pure Zn, and corrosion resistivity is excellent.

Meanwhile, in the prior art, a post-treatment based on trivalent chromium (Cr3+) or hexavalent chromium (Cr6+), which is treated as a type of a hazardous substance, is applied to secure more improved corrosion resistivity and fuel resistance of the Zn—Ni alloy electroplated steel sheet.

In the present disclosure, a method of manufacturing a surface-treated Zn—Ni alloy-electroplated steel sheet employing an eco-friendly alkaline electrolytic solution excluding any harmful substances and having improved corrosion resistivity and paintability by electrolytic etching a Zn—Ni alloy-electroplated steel sheet in a specific range of electrical parameters to form a certain roughness has been suggested.

DISCLOSURE

Technical Problem

The present disclosure is to provide a method of manufacturing a surface-treated Zn—Ni alloy-electroplated steel sheet with excellent corrosion resistivity and paintability, treated in an eco-friendly alkaline electrolytic solution free of harmful substances such as lead and chromium.

Technical Solution

According to an aspect of the present disclosure, a manufacturing method of a surface-treated Zn—Ni alloy electroplated steel sheet includes preparing a Zn—Ni alloy electroplated steel sheet comprising a steel sheet and a Zn—Ni alloy-plated layer formed on the steel sheet, in which a content of Ni in the Zn—Ni alloy-plated layer is 5 wt % to 20 wt % (S1); preparing an alkaline electrolytic solution in which 4 g/L to 250 g/L of potassium hydroxide (KOH), sodium hydroxide (NaOH), or both thereof is added to distilled water (S2); inside the alkaline electrolytic solution, obtaining a surface-treated electroplated steel sheet by placing the Zn—Ni alloy electroplated steel sheet as an anode and installing another metal sheet as a cathode, and applying 2 V to 10 V of an alternating or direct current to conduct electrolytic etching such that a 3-point average value of an arithmetic average roughness (Ra) of a surface of the Zn—Ni alloy electroplated steel sheet reaches 200 nm to 400 nm (S3).

In S2of preparing the alkaline electrolytic solution, 60 g/L to 250 g/L of KOH or NaOH may be added.

Further, the 3-point average value of the arithmetic average roughness (Ra) may be 200 nm to 250 nm.

After S3of obtaining the surface-treated electroplated steel sheet, a 3-point average value of a root-mean-square roughness (Rq) of the surface of the surface-treated Zn—Ni alloy-electroplated steel sheet may be 290 nm to 600 nm.

In addition, a 3-point average value of a maximum roughness (Rmax) of the surface of the surface-treated Zn—Ni alloy-electroplated steel sheet after S3of obtaining the surface-treated electroplated steel sheet may be 2900 nm to 5000 nm.

Advantageous Effects

According to the present disclosure, a surface-treated Zn—Ni alloy electroplated steel sheet having excellent corrosion resistivity and paintability can be manufactured by applying electricity in an eco-friendly alkaline electrolytic solution free of any hazardous substances such as lead and chromium. In this case, a surface roughness can be controlled through changes in a current density, an application time, and the electrolytic solution, thereby increasing utilization as a steel sheet for automobile fuel tanks.

Various advantages and beneficial effects of the present disclosure are not limited to the foregoing, it will be readily understood in the course of describing the specific embodiments of the present disclosure.

