JPS6393890A - Two-layer plated steel sheet - Google Patents

Abstract

PURPOSE:To obtain a low-coat plated steel sheet having superior adhesion and corrosion resistance, by forming a plated Zn or Zn alloy layer as a lower layer with a specified amount and an electroplated Al-Mn alloy layer as an upper layer with a specified amount on the surface of a steel sheet. CONSTITUTION:A plated Zn alloy layer as a lower layer is formed on one side or both sides of a steel sheet by 0.05-<1.0g/m<2> per one side. An electroplated Al-Mn alloy layer as an upper layer is formed on the lower layer by 1-50g/m<2> by electroplating in a molten salt. The proper amount of Mn in the Al-Mn alloy is 10-30wt%. Thus, a two-layer plated steel sheet having superior adhesion and corrosion resistance is obtd. at a low cost.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a 2N plated steel plate with an Al alloy stone surface, which can be manufactured industrially by electroplating, and has excellent adhesion and corrosion resistance.

(Prior art) Al and AI2 alloy plated steel sheets (hereinafter referred to as Al-based plated steel sheets) exhibit corrosion resistance superior to ordinary Zn-plated steel sheets, and are also highly effective in preventing oxidation of steel at high temperatures, making them economical. Used as a heat-resistant or corrosion-resistant material.

However, conventional Aq-based plating is mostly done by molten metal immersion plating, and the amount of plating deposited is 2 on one side.
Only 8Q plated steel sheets with a relatively thick thickness of 0 to 100 g/+vr were manufactured. In addition, since Al-based hot-dip plating is performed at a high temperature of 600°C or higher, it has a negative effect on the base material, and furthermore, a brittle Fe-Al alloy layer develops between the plating film and the W4 temporary base material, making it difficult to process the steel sheet. There was a serious drawback in that deterioration was inevitable.

On the other hand, it has been considered for some time that Al-based plating can be performed using an electroplating method that can be operated at low temperatures, but since Al cannot be deposited from an aqueous solution at all, the main component is aluminum chloride (AlC2a). Electroplating in a molten salt bath using a mixed chloride containing alkali chloride as a flux has been considered as a more practical Al-based electroplating method (for example, Japanese Patent Publications No. 45-29085 and No. 45-4).
8-38064).

In Al-based molten salt electroplating, the above-mentioned Japanese Patent Publication No. 4
As described in Japanese Patent No. 5-29085, if water gets mixed into the aluminum chloride molten salt plating bath, the bath will deteriorate severely due to hydrolysis of aluminum chloride, and a beautiful plated surface will not be obtained. It is important not to allow water to mix into the bath, as this can lead to serious problems such as extremely poor current efficiency. Therefore,
It is necessary to dry the steel plate thoroughly before charging it to the molten salt plating bath.

However, when a bare steel sheet is dried in the atmosphere, an oxide film is formed on the surface of the steel sheet even at a low temperature of about 80° C., and the adhesion of the plating film is reduced. Al-based plating has much lower sacrificial anticorrosion ability than Zn-based plating, so if adhesion decreases, protection of the steel plate will not be sufficient, especially at film defects such as end faces or uneven parts, and corrosion resistance will decrease. .

Furthermore, if the adhesion of the plating decreases, it will naturally have an adverse effect on the paintability.

In order to improve adhesion, it is possible to anodically melt the steel plate in a molten salt bath to remove the oxide film on the surface, but since molten salt is used, equipment costs are high and it is not practical. isn't it.

(Problems to be Solved by the Invention) Therefore, it is an object of the present invention to solve the problem of A by the molten salt electroplating method.
Even if an oxide film is formed on the steel plate surface by drying the steel plate in the atmosphere before plating,
An object of the present invention is to provide an Al-based plated steel sheet that can provide a plated film with excellent adhesion at low cost.

(Means for Solving the Problems) In order to achieve the above object, the present inventors focused on the pre-plating method and studied the improvement of adhesion by base plating treatment with various metals. As a result, mW of Zn or Z
The base plating is n-alloy, and the upper layer Al-based plating is Al
It has been found that by using -Mn alloy plating, an M-based plated steel sheet with particularly improved adhesion and corrosion resistance of the plating film can be obtained.

In addition, the present inventors 1. First patent application 1986-100153
In the above, a Zn or Zn alloy plating layer with an adhesion amount of 1 to 100 g/rrr and an adhesion fi of 1 to 20 g/rrr
We have proposed a steel material with excellent corrosion resistance that is coated with two layers of Al-Mn alloy layer. Electroplating of pure Al using a molten salt bath had a major drawback in that dendritic structures tended to develop in the plating film unless the current density was lowered, making it difficult to obtain a good plating film. in case of,
We focused on the fact that a high-quality plating film can be obtained at low cost by operating in a practical high current density range (2OA/dm" or higher) at a low temperature almost equivalent to that of 8Q plating, and as a result, it exhibits corrosion resistance that exceeds Al1 plating. However, in the two-layer plating disclosed in Japanese Patent Application No. 100153/1982, Zn or a Zn alloy was used to ensure corrosion resistance by utilizing the sacrificial anticorrosion ability of Zn. Plating is Ig/m
Applied with a coating weight of 2 or more. However, Z of such adhesion amount
When n-plating is applied as a lower layer, as will be shown later in the examples, the effect of improving adhesion is not sufficient (in particular, it was found that corrosion resistance after painting deteriorates. Corrosion-resistant plated steel sheets are coated with Since paints are often used as paints, it is very important that the paint has good corrosion resistance.

Here, the present invention provides plating coating amount o, os per one side.
g/m or more, 1.0 g/rrr ungrooved Zn or Zn
Lower layer consisting of alloy plating layer and plating coating weight 1-50g
This is a two-layer plated steel sheet with excellent adhesion and corrosion resistance, which has an upper layer consisting of an Al-Mn alloy electroplated layer of /% on one or both sides of the steel sheet.

(Function) In the two-layer plated steel sheet of the present invention, the lower layer has an adhesion of 10.0
Zn plating or Zn alloy plating of 5 g/m or more and less than 1.0 g/r+f (hereinafter referred to as Zn-based plating)
It is a layer. In the case of Zn alloy plating, N is the alloying element.
i, Mn, Fes Co, Cr, etc. are suitable;
The amount of the alloying element added is preferably about 5 to 30% by weight in the plating film. Zn-based plating adhesion amount is 0.0
If the amount is less than 5 g/=, the desired improvement in adhesion cannot be obtained, and if the amount of Zn-based plating is more than 1.0 g/7 g, the adhesion after painting will particularly deteriorate, and the adhesion after painting from the end surface will be reduced. In the present invention, corrosion is likely to progress.
Effects can be obtained with n-based plating, and it is also advantageous in terms of cost.

Note that since the lower layer Zn-based plating has such a thin basis weight, it is generally formed by an electric Zn plating method using a conventional method.

The upper layer is an Al-Mn alloy plating stone with a coating weight of 1 to 50 g/cd. If the adhesion amount is less than 1l g/m, the good corrosion resistance exhibited by Al-based plating cannot be obtained at all, and on the other hand, if the adhesion amount of the upper layer is 50 g/n? If it exceeds this value, the workability of the steel sheet will deteriorate significantly.

The upper layer Al-Mn alloy plating is a well-known Af2C23-
It can be carried out in the same manner as conventional molten salt electroplating using an electroplating bath in which Mn is also added as chloride to an XcQ (x is alkali metal) mixed molten salt bath. The -n content in the Q-Mn alloy is optimally about 10 to 30% by weight; outside this range, the appearance of the plating tends to deteriorate and the corrosion resistance also decreases somewhat. The decrease in corrosion resistance is particularly noticeable when the Mn content is low;
If the content is low, it will also have negative effects on plating operations, such as a decrease in current density. A particularly preferred Mn content is within the range of 15 to 25% by weight.

Further, alloying elements other than Q and Mn can also be added in small amounts as long as they do not adversely affect the performance of the plated steel sheet.

Examples of such other alloying elements include Fe, Cr, Ni
Examples include, but are not limited to, 5Ti, and two or more of such other alloying elements may be added.These elements may also be added as chlorides to the molten salt bath. It is preferable to do so.

Generally, the plating temperature for Q-Mn alloy plating using a mixed molten salt bath is about 170 to 200°C, and the current density is 100°C.
See No. 153.

In Al-Mn alloy electroplating using a molten salt bath, avoiding moisture contamination is particularly important for plating efficiency and maintaining plating quality.
Before applying Mn alloy electroplating, it is necessary to thoroughly wash the underlying Zn-based steel plate with water, and then heat and dry it to completely remove moisture.
Good adhesion is ensured by the base plating layer, so
This heat drying can be performed in the atmosphere, making it possible to easily carry out molten salt Al-based electroplating.

The base steel plate of the two-layer plated steel plate of the present invention is generally a cold-rolled steel plate, but a hot-rolled steel plate may also be used, or one that has been subjected to chemical conversion treatment in advance.The two-layer plated steel plate of the present invention includes:
The above two-layer plating may be provided on either one side or both sides of the steel plate.

In the case of double-sided coating, the lower and upper layers mentioned above have excellent adhesion, so they can be used as is as a corrosion-resistant steel plate, but depending on the purpose of use, they can be further coated, and the adhesion of the coating film is also good. , a coated steel plate with excellent coating corrosion resistance can be obtained. Note that, prior to painting, chromate treatment can be performed by a conventional method.

Hereinafter, the present invention will be explained in more detail with reference to Examples.

(Example) Thickness 0.8mm, width! After cleaning the surface of a cold-rolled steel plate with a length of 400 fins by degreasing and pickling, it was electrolytically plated with Zn or Zn alloy at various coating weights under the following plating conditions.

Plating bath composition made by 1 sum and 2 kumi: Zn5Oa 400 g/lN
a1SQs 75 g/e pH: 1.8 Bath temperature: 60°C Current density: 5 A/dm” Zn alloy plating is also performed by adding alloying element components to a sulfate bath so that the alloying element content in the plating film reaches a predetermined weight. The plating conditions were the same as above, except that the amount was added in such a manner that the plating conditions were as follows.

Next, after thoroughly washing this Zn-plated steel sheet with water,
It was dried in a dryer at 0° C. for 5 minutes to completely remove moisture, and immediately subjected to Al-Mn alloy plating treatment in a mixed molten salt bath under the following conditions.

ToQ-1n 2-layer gold plating ' Plating bath composition: AlCQl 74.1% by weight NaCQ
14.1 wt'J% to C211.8 wt% Bath temperature: 200°C Current density: 40 A/da'' Regarding Mn, Mn CQ was added to the molten salt bath.

The obtained two-layer plated steel sheet was tested for plating adhesion, plating appearance, corrosion resistance, and paint corrosion resistance. The results,
Table 1 below shows the amount of Zn deposited on the lower layer, as well as the composition and deposition l of the upper layer.

For comparison, an Al-Mn single-layer plated steel sheet without Zn plating, or a two-layer plated steel sheet whose Zn coating amount or Al-Mn alloy coating weight was outside the range of the present invention was similarly prepared and tested. The results are also shown in Table 1.

(Continued from the top () C3 + Table continued) 1), 2) are all in g/po3) Plating adhesion test: DuPont impact test after two-layer plating (ball head diameter 2 inches, energy 0. Evaluated by tape peeling after 8 kgf-+*) and judged according to the following criteria: 20 No peeling, Δ
Slight peeling, × Significant ♀11 peeling.

4) Plating appearance: Visually evaluated based on the following criteria: ○ Good, △ Slightly poor, X poor.

5) Corrosion resistance test: A cut cross section of an as-plated sample was sealed with resin and a salt spray test (J I 5-Z2371'') was performed, and the number of durability days until red rust appeared was expressed.

6) Painted Corrosion Resistant Plating W4 After temporary chromate treatment, a 2-coat polyester coating was applied (total film thickness 20 μm), and the above salt spray test was conducted for 240 hours with the cut section exposed. Evaluation is based on the peeling status between the upper layer plating and the lower layer plating.
The case where the 1-separation phenomenon was clearly observed is marked as ×, and the case where it was not observed is marked as ○. - is an example that is not applicable because it is a single layer plating.

7) Remarks: *mark means an example of the present invention.

As can be seen from the results in Table 1 above, all of the two-layer plated steel sheets of the present invention marked with this mark have good plating adhesion and corrosion resistance both unpainted and after painting. However, the Mn content of the Q-Mn alloy is 10 weight 9
When the Mn content is less than 3, the appearance of the plating and the corrosion resistance of the unpainted state deteriorate, and even if the Mn content exceeds 30% by weight, the appearance of the plating tends to deteriorate.

On the other hand, if the Zn base plating is omitted, sufficient adhesion of the plating cannot be obtained, and the amount of Zn plating deposited is 1.0 g.
/rt? Even if it exceeds the above, the adhesion between the upper layer and the lower layer decreases, and the corrosion resistance of the coating decreases. Also, if the upper layer is pure M, corrosion resistance,
The corrosion resistance of both coatings decreases, and the same result occurs when the coating weight of the upper layer is less than Ig/m.

(Effect of the invention) 21' of the invention! As mentioned above, when the steel plate is heated and dried in the atmosphere to remove water, which is essential during Al plating using a mixed molten salt bath, the Zn-based base plating loses a fine 1[t] of Due to its presence, it is possible to provide an Al-based plating film with excellent adhesion, and the surface layer is
By using l-Mn alloy plating, a high-quality Al-based plating film can be obtained at a current density within a practical range, and the corrosion resistance is further improved compared to pure Al plating. As a result, a plated steel sheet with excellent adhesion and corrosion resistance can be obtained at low cost and in an efficient electroplating operation.

Claims (2)

[Claims] (1) A lower layer consisting of a Zn or Zn alloy plating layer with a coating weight of 0.05 g/m^2 or more and less than 1.0 g/m^2 per side, and an Al- layer with a coating weight of 1 to 50 g/m^2 per side. A two-layer plated steel sheet with excellent adhesion and corrosion resistance, which has an upper layer consisting of a Mn alloy electroplated layer on one or both sides of the steel sheet. (2) The Mn content of the upper layer Al-Mn alloy is 10 to
The two-layer plated steel sheet according to claim 1, characterized in that the content is 30% by weight.