JPS5881991A - One surface zinc electroplated steel plate excellent in paint acceptance - Google Patents

Abstract

PURPOSE:To obtain a one surface zinc electroplated steel plate excellent in the acceptance for paint, by a method wherein a Zn or a Zn base alloy plated layer is applied to one surface of a steel plate and a plated layer of Fe or an Fe-Zn alloy containing Fe in a specific amount or more on the other surface thereof in a specific thickness. CONSTITUTION:A Zn or a Zn base alloy electroplated layer is applied to one surface of a steel plate and an electroplated layer of Fe or an Fe-zn alloy containing 10wt% or more Fe is further applied on the other surface thereof in a coating amount of 1mg/m<2> or more. If the coating amount of the plateing is 1mg/m<2> or less, post-finishing effect is not almost developed and the purpose can be sufficiently attained in an amount of 3mg/m<2> or less usually.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a single-sided zinc-based electroplated steel sheet having excellent coating compatibility.

Conventionally, single-sided zinc-based electroplated steel sheets have been used exclusively for automotive applications. In automotive applications, the front side of the car body is coated with a resin-based paint approximately 80 μm thick, and conventional zinc-based electroplated surfaces have poor compatibility with paint, so galvanizing is not applied. On the other hand, the back side of the car body often suffers corrosion that leads to pitting in the environment in which the car is used, especially in a cold and humid environment or in a marine climate. For this reason, single-sided zinc-based electroplated steel sheets have been used for the back side of vehicle bodies, in which only one side of the steel sheets is electroplated with zinc or a zinc-based alloy.

Conventionally, this single-sided zinc-based electroplated steel sheet has been subjected to anionic electrodeposition coating after zinc phosphate chemical conversion treatment in the automobile painting line, but in this case, the steel sheet side that is not subjected to zinc-based electroplating (hereinafter referred to as non-zinc It has been pointed out that the coating compatibility of the plated surface (referred to as the plated surface) is lower than that of cold-rolled steel sheet, which is also used as a material.

This is because in the zinc-based electroplating process, current is passed through only one side of the steel plate to perform single-sided plating.
A slight leakage current also flows through the non-galvanized surface, and metals such as zinc are electrodeposited. Although various measures have been taken to prevent this, it is difficult to completely eliminate electrodeposition of zinc and the like, and zinc ions from the plating solution may also adhere to non-zinc plated surfaces. This phenomenon is not only caused by galvanizing, but also by zinc plating, other additive metals,
For example, the same applies to alloy plating containing one or more of N1, Or%Co, Mo, Sn, etc.
In particular, under low current density conditions such as leakage current, there is a phenomenon in which zinc is preferentially electrodeposited, so that zinc is mainly electrodeposited on non-galvanized surfaces. It was thought that the decrease in paint compatibility was due to the electrodeposition of zinc, etc., so conventionally,
Post-treatments such as brushing, chromate treatment, and zinc phosphate spraying on the non-galvanized surface after plating have been used, but it has been difficult to improve the coating compatibility to the level of the cold-rolled steel sheet surface.

Recently, however, in order to further improve corrosion resistance, automotive painting lines have been gradually converted to a method that performs cationic electrodeposition coating after zinc-iron phosphate chemical conversion treatment, and even in this case, deterioration of coating compatibility has become a problem. Although this problem has been dealt with by post-processing such as brushing as mentioned above, it is not sufficient and an improvement is strongly desired.

The inventor of the present invention has investigated ways to improve the coating compatibility of single-sided zinc-based electroplated steel sheets, such as water-resistance adhesion and spot rust resistance, to a level higher than that of ordinary cold-rolled steel sheets. The other side of a conventional single-sided zinc-based electroplated steel sheet that has been electroplated with zinc or a zinc-based alloy,
In other words, when a zinc phosphate/iron-based chemical conversion treatment is applied to a non-galvanized surface, a very small amount, for example, 1! /m2 ~ I P / ~2 Zn is dissolved during chemical conversion treatment, but at that time, Zn ions are locally concentrated in a small amount of Zn film near the non-galvanized surface and in the boundary film between the chemical conversion film and hopite Z. n3 (P 04 ) 2・4
It was thought that H20 precipitated and formed a non-uniform phosphate film, which caused deterioration in water resistant adhesion and spot rust resistance. Also, if Zn remains after phosphate treatment,
It was thought that this would cause further deterioration of water resistant adhesion and the like.

Therefore, this non-galvanized surface is coated with an ultra-thin layer of iron or iron-zinc alloy plating mainly composed of iron, such as 1! /m"～3
When coated with ~2 plating layers, zinc phosphate
When an iron-based chemical conversion treatment is applied, this iron or iron-zinc coating dissolves together with the previously deposited zinc, but because the iron concentration near the surface film is relatively high, a uniform phosphor with good paint compatibility is formed. Fohilite, Zn2Pa(P-0,)2@4H2
0 forms and circles.

Furthermore, even if the iron-zinc film remains after the phosphate treatment, the result is that the water-resistant adhesion and spot rust resistance are improved more than those of the cold-rolled steel sheet.

The present invention has been completed based on the above knowledge, that is,
The present invention has an electroplated layer of zinc or an alloy mainly composed of zinc on one side of a steel sheet, and an electroplated layer of iron or an iron-zinc alloy containing 10% by weight or more of iron on the other side.
This invention relates to a single-sided zinc-based electroplated steel sheet with excellent coating compatibility, which has an i characteristic of having a diameter of m2 or more. ~ The present invention is applicable to both anionic electrodeposition coating systems and cationic electrodeposition coating systems.

The iron content in the plating layer is at least 10% by weight or more, particularly preferably 40% by weight or more in the case of a cationic electrodeposition coating system. If the iron content is low, the formation rate of phosphohilite is insufficient, and if the adhering zinc on the base remains after phosphate treatment, the waterproof adhesion etc. of each electrodeposition coating system will deteriorate. If the coating amount is less than 1 llp/m2, the effect of post-finishing will hardly be seen, so the coating amount should be 1 y/m2.
It is necessary to have 2 or more. Even if the amount of coating is large, there is no problem in performance, but it is not economical, and usually 3 ji / m
``The following is sufficient to achieve the objective.However, depending on the objective of significantly improving the spot rust resistance performance of the non-galvanized surface compared to cold rolled steel sheets, the objective may be easily achieved by increasing the coating amount even thicker. I can do it.

After the plated steel sheet of the present invention is subjected to normal zinc-based electroplating on one side, it is washed with water, and then electroplated on the other side, that is, the non-galvanized side, under the following conditions in the next plating bath. After that, wash with water or hot water, and after drying, add brushing if necessary.

Plating condition reference example 1 (for Example 6) (1) Bath composition Fe80. -7H201009/It ZnSO,”7H2026t (NH,)2so, 20 px, so,
201/It ammonium citrate 6
o 1 (2) Current density 80A/dm2 (3) Temperature 45°C Plating condition reference example 2 (for Example 2) (1) Bath composition Fe80. *7H2024BjF/fiZ n S O
,*7 H2O40#(NH,)2So, 1
18 #Kax 10'# Citric acid α5r (2) Current density 25 A / 6m2 (3) Temperature
Other plating compositions were similarly adjusted by varying the Fe++/zn++ weight ratio in the bath and the current density. Note that the amount of adhesion was adjusted by the plating time.

Table 1 below shows the coating compatibility when the iron-zinc composition and adhesion amount (coating amount) in the plating layer on the non-galvanized surface were changed.

Performance evaluation was carried out as follows: (1) Water resistance adhesion test After phosphate treatment, anionic electrodeposition coating or cationic electrodeposition coating was applied with a thickness of 20 μm, intermediate coating, and top coating to obtain a total coating film of 8.
0 to 90 μm (all dry film thickness), and after immersing this coated plate in warm water at 40°C for 10 days, immediately draw 100 squares that reach the base steel plate of 2 mm square, and peel it off with cellophane tape. It is expressed as the number of squares divided by the total number of squares.

(2) Spot rust resistance test Same as water resistance adhesion test, total film thickness 80 to 90 μm
0.6 mm above the paint film using an arrowhead with a cone angle of 90°
A scratch was made in the area around the scratch, and a cycle of 1 day of salt spray test in accordance with J.Eng. The maximum length between the tips of 7 scratches or thread rusts was defined as the rust diameter, and the average rust diameter of the 75 scratches was shown in mm.

As detailed above, according to the present invention, a single-sided zinc-based electroplated steel sheet with excellent coating compatibility was obtained.

Claims (1)

[Claims] 1. One side of the steel plate has zinc or an alloy electroplated layer mainly composed of zinc, and the other side has iron or 10% by weight of iron.
The iron-zinc alloy electroplated layer containing 1ql/m
A single-sided zinc-based electroplated steel sheet with excellent coating compatibility, characterized by having 2 or more.