JPH0689474B2 - Steel plate with two layers - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a two-layer plated steel sheet having an Al alloy layer on the surface, which is industrially producible by an electroplating method and has excellent adhesion and corrosion resistance.

(Prior Art) Al and Al alloy-plated steel sheets (hereinafter referred to as Al-based plated steel sheets) are more economical than ordinary Zn-plated steel sheets, and economically effective because they have a large effect of preventing oxidation of steel at high temperatures. Used as a heat resistant or corrosion resistant material.

However, most conventional Al-based plating is performed by the molten metal immersion plating method, and the coating amount is 20 ~
Only an Al-plated steel sheet with a relatively thick basis weight of 100 g / m ² was manufactured. In addition, since Al-based hot dipping is performed at a high temperature of 600 ° C or higher, the base metal is adversely affected, and a brittle Fe-Al alloy layer develops between the plating film and the steel sheet substrate, resulting in workability of the steel sheet. There was a serious drawback that deterioration was inevitable.

On the other hand, it has been considered for some time to perform Al-based plating by an electroplating method that can be operated at low temperature, but since aluminum cannot be electrodeposited from an aqueous solution at all, aluminum chloride (AlCl ₃ ) is mainly used. Electroplating in a molten salt bath with a mixed chloride containing alkali chloride as a flux as a component has been considered as a more practical Al-based electroplating method (eg, Japanese Patent Publication Nos. 45-29085 and 48- (See Japanese Patent No. 38064).

In Al-based molten salt electroplating, the above-mentioned Japanese Patent Publication No. 45-
As described in Japanese Patent No. 29085, when water is mixed into a molten salt plating bath of aluminum chloride, the bath is severely deteriorated due to hydrolysis of aluminum chloride, so that not only a beautiful plated surface cannot be obtained but also current It is important not to mix water in the bath, as it has a great adverse effect such as extremely deteriorated efficiency. Therefore, it is necessary to sufficiently dry the steel sheet before charging it into the molten salt plating layer.

However, when a bare steel sheet is dried in the air, an oxide film is formed on the surface of the steel sheet even if it is dried at a low temperature of about 80 ° C, and the adhesion of the plating film is reduced. Since the Al-based plating has a much lower sacrificial anticorrosion ability than the Zn-based plating, if the adhesion is lowered, the steel sheet is not sufficiently protected particularly at a film defect portion such as an end face or a scratched portion, and the corrosion resistance is reduced. Further, if the adhesion of the plating is reduced, the paintability is naturally adversely affected.

Deterioration of coating corrosion resistance is inevitable due to deterioration of corrosion resistance after coating.

In order to improve the adhesion, a method of anodic dissolution of the steel sheet in a molten salt bath to remove the oxide film on the surface is possible, but since molten salt is used, equipment costs are high, and it is not practical. Absent.

(Problems to be Solved by the Invention) Therefore, the object of the present invention is to provide Al by a molten salt electroplating method.
Al-based plating, which is excellent in adhesion and therefore also excellent in coating corrosion resistance, even if an oxide film is formed on the surface of the steel plate by drying the steel plate in the atmosphere before plating It is to provide a steel plate.

(Means for Solving Problems) In order to achieve the above-mentioned object, the present inventors have focused on the pre-plating method and studied the improvement of adhesion by the undercoating treatment with various metals. As a result, a small amount of Zn or Zn alloy is underplated, and the Al-based plating of the upper layer is an Al-Mn alloy plating, whereby an Al-based plated steel sheet with particularly improved adhesion and corrosion resistance of the plating film can be obtained. I found out.

The present inventors have found that in Japanese Patent Application Sho 60-100153 previously deposited amount and Zn or Zn alloy plating layer 1~100g / m ^2, Al-Mn alloy layer of adhesion amount from 1 to 20 g / m ² We proposed a steel material with excellent corrosion resistance, which was plated with two layers. Electroless plating of pure Al in a molten salt bath has a major drawback that a dendritic structure is likely to develop in the plating film unless a current density is lowered, and a good plating film cannot be obtained. In this case, a high-quality plating film can be obtained at low cost by operating in a practical high current density range (20 A / dm ² or more) at a low temperature almost equal to that of Al plating, and as a result, it shows corrosion resistance superior to that of Al plating. The above proposal was made with this in mind. However, in the two-layer plating of Japanese Patent Application No. 60-100153, Zn or Zn alloy plating is applied at an adhesion amount of 1 g / m ² or more in order to ensure corrosion resistance by utilizing the sacrificial corrosion prevention ability of Zn. To be done. However, it has been found that, when the Zn plating having such an adhesion amount is applied as the lower layer, the effect of improving the adhesiveness is not sufficient, and the corrosion resistance after coating is particularly deteriorated, as will be shown in Examples later. Corrosion-resistant plated steel sheets are often used after being coated, so that it is very important that the coating corrosion resistance is good.

Here, the present invention, a coating amount of 0.05 g / m ² or more per one surface, a lower layer consisting of Zn or Zn alloy plating layer of less than 1.0 g / m ^2, and a coating amount of 1-50 g / m ² of Al-Mn. It has an upper layer consisting of an alloy electroplated layer on one or both sides of a steel sheet, and has excellent adhesion, corrosion resistance and coating corrosion resistance.
It is a layer-plated steel sheet.

(Function) In the two-layer plated steel sheet of the present invention, the lower layer has an adhesion amount of 0.05 g /
It is a Zn plating layer or a Zn alloy plating layer (hereinafter referred to as Zn-based plating) layer of m ² or more and less than 1.0 g / m ² . In the case of Zn alloy plating, Ni, Mn, Fe, Co, Cr, etc. are suitable as the alloying element, and the addition amount of the alloying element is 5 in the plating film.
About 30% by weight is suitable. Zn-based plating coverage is 0.05
If it is less than g / m ² , the desired improvement in adhesion cannot be obtained.
Further, when the amount of Zn-based plating adhered is 1.0 g / m ² or more, the adhesion after coating is particularly reduced, and corrosion from the end face after coating is likely to proceed. In the present invention, the effect can be obtained with such a small amount of Zn-based plating, which is also advantageous in terms of cost. The lower layer Zn
Since the system plating has such a thin weight, it is generally formed by an electric Zn plating method by a conventional method.

The upper layer is an Al-Mn alloy plating layer having a deposition amount of 1 to 50 g / m ² . When the adhesion amount is less than 1 g / m ² , good corrosion resistance of Al-based plating cannot be obtained at all, while when the adhesion amount of the upper layer exceeds 50 g / m ² , the workability of the steel sheet is significantly deteriorated.

The Al-Mn alloy plating of the upper layer is carried out by using the conventional AlCl ₃ -XCl (X is an alkali metal) mixed molten salt bath in which an Mn is also added as a chloride to the known molten AlCl ₃ -XCl mixed electroplating bath. It can be implemented similarly. Mn in Al-Mn alloy
The optimum content is about 10 to 30% by weight. Outside this range, the plating appearance tends to deteriorate and the corrosion resistance also decreases slightly. The decrease in corrosion resistance is remarkable especially when the Mn content is small, and when the Mn content is small, the plating operation is adversely affected by the decrease in current density. A particularly preferred Mn content is in the range of 15 to 25% by weight. Further, alloy elements other than Al and Mn can be added in small amounts as long as they do not adversely affect the performance of the plated steel sheet. Examples of such other alloy elements include, but are not limited to, Fe, Cr, Ni, Ti and the like, and two or more kinds of such other alloy elements may be added. These elements are also preferably added as chlorides to the molten salt bath.

Generally, the plating temperature of Al-Mn alloy plating with a mixed molten salt bath is about 170 to 200 ° C, and the current density is 20 to 80 A / dm ²
It is a degree. For details of the molten salt bath Al-Mn alloy electroplating, refer to the above-mentioned Japanese Patent Application No. 60-100153.

Al-Mn alloy electroplating in a molten salt bath is particularly important for maintaining plating efficiency and plating quality in order to avoid the inclusion of water, so as described above, before performing the upper layer Al-Mn alloy electroplating, It is necessary to thoroughly wash the underlying Zn-plated steel sheet with water, and then heat-dry it to completely remove water. In the present invention, since good adhesion is ensured by the Zn-based underplating layer, this heat drying can be performed in the atmosphere, and molten salt Al-based electroplating can be easily performed.

The base steel sheet of the two-layer plated steel sheet of the present invention is generally a cold rolled steel sheet, but a hot rolled steel sheet can also be used and may be one that has been previously subjected to chemical conversion treatment. The two-layer plated steel sheet according to the present invention may have the above-mentioned two-layer plating provided on one side or both sides of the steel sheet. In the case of double-sided coating, the amount of adhesion of the lower layer and the amount of adhesion of the upper layer are for one side.

Since the two-layer plated steel sheet of the present invention is excellent in adhesion and corrosion resistance, it can be used as it is as a corrosion resistant steel sheet, but it can be further coated depending on the purpose of use and the adhesion of the coating film is good, A coated steel sheet with excellent coating corrosion resistance can be obtained. Incidentally, prior to coating, a chromate treatment can be applied by a conventional method.

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

(Example) On a cold rolled steel sheet having a thickness of 0.8 mm, a width of 100 mm and a length of 400 mm, degreasing,
After cleaning the surface by pickling, electroless Zn or Zn alloy plating was performed with various amounts of adhesion under the following plating conditions.

Electrical Zn plating conditions Plating bath composition: ZnSO ₄ 400g / l Na ₂ SO ₄ 75g / l pH: 1.8 Bath temperature: 60 ° C Current density: 5A / dm ² Zn plating is also performed by plating alloy element components in a sulfate bath. The plating conditions were the same as above, except that the alloying element content in the coating was such that the content of the alloying element was a predetermined weight percent.

Next, after thoroughly washing this Zn-plated steel sheet with water,
Was completely dried to remove water, and immediately subjected to Al-Mn alloy plating treatment in a mixed molten salt bath under the following conditions.

Al-Mn alloy electroplating conditions Plating bath composition: AlCl ₃ 74.1 wt% NaCl 14.1 wt% K Cl 11.8 wt% Bath temperature: 200 ° C Current density: 40 A / dm ^{2 For} Mn, the Mn content in the plating film is prescribed. Wt% of
MnCl ₂ was added to the molten salt bath so that

The obtained two-layer plated steel sheet was tested for plating adhesion, plating appearance, corrosion resistance and coating corrosion resistance. The results are shown in the following Table 1 together with the lower layer Zn deposition amount and the upper layer plating composition and deposition amount.

For comparison, an Al-Mn-based single-layer plated steel sheet in which Zn plating was omitted, or a two-layer plated steel sheet having a Zn deposition amount or an Al-Mn alloy plating deposition amount outside the scope of the present invention was prepared in the same manner and tested. The results obtained are also shown in Table 1.

As can be seen from the results in Table 1 above, the double-layer plated steel sheets of the present invention marked with * all have good plating adhesion and uncoated and post-painted corrosion resistance. However, if the Mn content of the Al-Mn alloy is less than 10% by weight, the plating appearance and unpainted corrosion resistance tend to deteriorate, and if the Mn content exceeds 30% by weight, the plating appearance tends to deteriorate.

On the other hand, if the Zn undercoating is omitted, sufficient adhesion of the plating cannot be obtained, and even if the Zn plating adhesion amount exceeds 1.0 g / m ² , the adhesion between the upper layer and the lower layer will decrease and the coating corrosion resistance will be reduced. descend. Further, when the upper layer is pure Al, both the corrosion resistance and the coating corrosion resistance are low, and the same result is obtained when the upper layer adhesion amount is less than 1 g / m ² .

(Effect of the Invention) As described above, the two-layer plated steel sheet of the present invention has a trace amount when the steel sheet is heated and dried in the atmosphere for removing water which is indispensable for Al-based plating in the mixed molten salt bath. The presence of the Zn-based undercoating makes it possible to provide an Al-based plating film with excellent adhesion, and by using an Al-Mn alloy plating for the surface layer, it is possible to obtain a high-quality Al with a current density within the practical range.
A system-based plating film can be obtained, and the corrosion resistance is further improved as compared with pure Al-based plating. As a result, a plated steel sheet having excellent adhesion, corrosion resistance, and coating corrosion resistance can be obtained at low cost by an efficient electroplating operation.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Uchida 1-3-3 Nishi-Nagasumotodori, Amagasaki City, Hyogo Prefecture Sumitomo Metal Industries, Ltd. Research Institute (56) Reference Japanese Patent Publication No. 43-17883 B1)

Claims (2)

[Claims] 1. A coating weight of 0.05 g / m ² or more per side, 1.
A lower layer consisting of a Zn or Zn alloy plating layer of less than 0 g / m ² and an upper layer consisting of an Al-Mn alloy electroplating layer having a plating adhesion amount of 1 to 50 g / m ² are provided on one side or both sides of the steel sheet, Two-layer plated steel sheet with excellent adhesion, corrosion resistance, and coating corrosion resistance. 2. The Mn content of the upper layer Al—Mn alloy is 10 to 30.
The two-layer plated steel sheet according to claim 1, wherein the two-layer plated steel sheet is characterized in that the content is% by weight.