JPH0452284A - High corrosion-resistant two-layer plated steel sheet and its production - Google Patents

Abstract

PURPOSE:To give excellent corrosion resistance to a steel sheet by providing an alloy hot-dip galvanized layer comprising specified wt.% of Fe and Al and the balance Zn on the steel sheet and then providing a Mg-Al alloy plating layer containing a specified amt. of Mg thereon to a specified deposition amt. CONSTITUTION:On the surface of a steel sheet, an alloy hot-dip galvanized layer comprising 7-20wt.% Fe, 0. 05-0. 5wt.% Al, and the balance Zn and inevitable impurities is formed as the first layer. Then, as the second layer, a Mg-Al alloy plating layer containing 0.1-80wt.% Mg is formed to 0.1-30g/m<2> buildup amt. By this method, the obtd. plated steel sheet has good workability and surface properties.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a two-layer plated steel sheet having high corrosion resistance suitable for use in building materials, home appliances, and automobiles, and a method for manufacturing the same.

(Prior Art) In recent years, the amount of surface-treated steel sheets used in industrial fields such as architecture and automobiles has increased, and the required performance has become increasingly diverse and strict.

Taking rust-proof steel sheets for automobiles as an example, in North America and other countries where snow-melting salt is sprayed in the winter, surface-treated steel sheets with high rust-proofing properties are required. For some time now, Zn, Ni-Zn alloys, Fe
- Rust-proof steel sheets electroplated with Zn alloy or the like are widely used. However, recently, rust-proof steel plates used for car bodies are required to have a rust-proof ability that can withstand corrosion for a long period of time, such as a 10-year warranty on perforation resistance. Does not have rusting power,
These anti-corrosion steel plates are generally used with a coating weight of 20 to 30g, but if these steel plates are to meet the requirements for a 10-year guarantee on perforation, the coating weight must be more than doubled. It is said that there must be. However, as the manufacturing cost of electroplated steel sheet increases significantly as the amount of coating increases, it must be said that it is extremely difficult to apply it from a cost perspective. This refers to a phenomenon in which corrosion of a steel plate progresses from areas where the coating is insufficient, resulting in pitting corrosion, and in some cases penetrating corrosion.

For this reason, in recent years, two methods have been used to improve the corrosion resistance of rust-proof steel sheets. One is a method of placing a chromate film and a thin resin film on a zinc or zinc alloy coated steel sheet to make an organic coated steel sheet, and the other is a hot-dip galvanized steel sheet manufactured by a hot-dip galvanizing process, or an alloying treatment of this. This is a method of increasing the amount of coating on alloyed hot-dip galvanized steel sheets. In particular, the latter method of thickening hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets is advantageous because not only are these steel sheets themselves relatively inexpensive, but the increase in manufacturing costs associated with an increase in coating weight is also relatively small. , which is advantageous in terms of cost. In the case of hot-dip galvanized steel sheets, the amount used for automobiles is small from the viewpoint of weldability, but in the case of alloyed hot-dip galvanized steel sheets, coating coverage of about 45 g/m'' has traditionally been considerable. Therefore, the coating weight of this alloyed hot-dip galvanized steel sheet was thickened to about 60 g/m'',
Although we are trying to meet the requirements for a 10-year warranty on perforation resistance, alloyed hot-dip galvanized steel sheets have traditionally been considered to be a material with many problems in press formability, and in fact, they are prone to flaking and powdering during forming. There is a phenomenon in which the plating film peels off from the steel sheet.

This peeling becomes more noticeable as the amount of coating increases, so thickening alloyed hot-dip galvanized steel sheets will cause more trouble during the forming process than before, and will also reduce spot weldability. become.

For this reason, various methods have been proposed to improve the corrosion resistance of alloyed hot-dip galvanized steel sheets without particularly increasing the coating thickness. JP-A-52-131934 discloses a method of plating with a hot-dip galvanizing bath containing 3 to 22% Al. Although the method described in JP-A-63-48945 has higher corrosion resistance than conventional alloyed hot-dip galvanized steel sheets, the corrosion resistance is not satisfactory. Furthermore, when a large amount of Mg is added to the hot-dip galvanizing bath, the workability of the plating film decreases. Japanese Patent Publication No. 52-131
In the method described in Publication No. 934, the surface of the plating film after alloying becomes rough and cannot withstand practical use.If plating is performed in a hot-dip galvanizing bath containing such a large amount of Al and alloying treatment is performed, alloying will occur. The progression of the disease occurs very unevenly,
The surface of the plating film after alloying treatment exhibits significant unevenness.

(Problems to be Solved by the Invention) The RB of the present invention is intended to improve the corrosion resistance of an alloyed hot-dip galvanized steel sheet without increasing the coating weight.

Specifically, we will use a two-layer plated steel plate that has excellent hole resistance, blister resistance, workability, and surface quality after painting, with a coating weight of about 40 to 45 g/m'' for alloyed hot-dip galvanized #4 plate. The purpose is to provide a manufacturing method thereof.

(Means for Solving the Problems) The above-mentioned Mg and Al improve corrosion resistance. this is,
Mg or Al in the plating film becomes concentrated on the plating surface layer during the alloying process after hot-dip plating, and in a corrosive environment, each
To form stable corrosion products of l103 or MgO,
It is presumed that this suppresses the progress of corrosion. but,
If a hot-dip galvanized steel sheet is produced using a hot-dip galvanizing bath containing a large amount of Al as in the past, the surface properties of the plating film after alloying will be significantly deteriorated. When used to manufacture alloyed hot-dip galvanized steel sheets, the result is a plating film with poor workability.

However, the present inventors plated a steel sheet in a normal hot-dip galvanizing bath that has a relatively low Alfi degree and does not contain Mg, and after alloying it, on top of this alloyed hot-dip galvanized steel sheet, Al by ion plating method or other methods? It has been found that when coated with an alloy of 1 g, the surface quality is not impaired and the alloyed hot-dip galvanized steel sheet has a thin coating, but has sufficient corrosion resistance and excellent workability.

The gist of the present invention is (1) and (2) below.

(1) 7 to 20% by weight of Fe as the first layer on the steel plate surface
, has an alloyed hot-dip galvanized layer consisting of 0.05 to 0.5% by weight of Al, the remainder being Zn and unavoidable impurities, and has an Mg content of 0.1 to 80% by weight as the second layer.
A highly corrosion-resistant two-layer plated steel sheet having a g-Al alloy plating layer and having an adhesion amount of the Mg-Al alloy plating layer of 0.1 to 30 g/m''.

(2) 0.05 to 0.3 wt% A of the steel plate after pretreatment
After plating with a hot-dip galvanizing bath containing Zn and the rest consisting of Zn and unavoidable impurities and alloying, forming a Mg-Al alloy plating layer on the alloyed hot-dip galvanizing layer by ion plating. The method for producing a highly corrosion-resistant two-layer plated steel sheet as described in (1) above, which is characterized by:

(effect) The present invention will be explained in detail below.

First, the reason why the composition of each film and the amount of deposited second layer of the two-layer plated steel sheet of the first invention of the present application are limited as described above will be explained.

Fe in the first alloyed hot-dip galvanized layer is Fe that flows from the steel sheet base through mutual diffusion between the steel sheet and the plating layer during the alloying treatment after hot-dip plating. If this Fe content is less than 7% by weight, weldability and blister resistance after painting will decrease, and if it exceeds 20% by weight, in addition to markedly decreasing pitting resistance, powdering resistance will also decrease, and press processing Sometimes, the plating film tends to peel off.

Similarly, A contained in the first alloyed hot-dip galvanized layer
1 is Al that is added to the hot-dip galvanizing bath in order to suppress the generation of dross during hot-dip plating. Lowering the Al content in the film to less than 0.05% by weight means extremely reducing the amount of Al in the molten zinc bath, which in turn increases the reaction between the steel plate and the molten metal, increasing the amount of dross generated. Not only will operability deteriorate, but the plating film after alloying will have poor workability. In order to improve the corrosion resistance of the plating film, it is better to include a large amount of Al, but if the Al content in the film exceeds 0.5% by weight, the alloying rate will be significantly reduced, operability will be reduced, and the The surface of the film becomes more uneven and the paintability deteriorates. Therefore, A in the film
The upper limit of l is preferably 0.5% by weight.

The second Mg-Al alloy plating layer is coated with Al20. A stable corrosion product of MgO and MgO is formed to suppress corrosion of the first alloyed hot-dip galvanized layer.

If the Mg in this Mg-Al alloy plating layer is less than 0.1% by weight, the amount of MgO formed will be insufficient, and 80% by weight
If it exceeds Al, the amount of uzos formed will be insufficient.
The synergistic effect of 21g and 21g does not appear, and the improvement in corrosion resistance is small.
The second layer? If the amount of the 1g-^1 alloy plating layer is small, sufficient corrosion resistance will not be obtained, and even if the layer is plated too thick, the effect of improving corrosion resistance will be saturated, so the amount of adhesion should be 0.1 to 30.
When used after processing such as press molding, powdering resistance decreases if the plating layer is thick, so it is desirable that the coating amount be 10 g/s or less.

The two-layer plated steel sheet of the first invention of the present application can be manufactured by the method of the second invention of the present application. That is, a steel plate pretreated by a general method such as the Sendzimer method or the flux method is
．． Mg-
The Al alloy plating layer is formed by ion plating.

In the manufacturing method of the second invention, the Al concentration in the hot dip galvanizing bath is 0.05% by weight or more.
This is because if it is less than % by weight, the amount of dross generated will increase, and the workability of the film will be reduced as well as the operability will be reduced. On the other hand, when the Al concentration in the hot-dip galvanizing bath exceeds 0.3% by weight, the amount of Al in the film exceeds 0.5% by weight, and the surface of the film after alloying becomes noticeably uneven. Al in the hot-dip galvanizing bath has a tendency to concentrate in the film if its content is 1.0% by weight or less.

Therefore, if more than 0.3% by weight of Al is added to the hot-dip galvanizing bath, the amount of Al in the coating will be more than 0.5% by weight.

The alloying treatment after hot-dip plating may be performed in an alloying furnace within the plating line, or may be performed in an off-line batch furnace. In an alloying furnace, alloying is preferably carried out at a material temperature of 500 to 600°C. In a batch furnace, alloying can be carried out at a low material temperature of 350 to 380°C. In this alloying process, 7 to 20% by weight of Fe is introduced into the hot-dip galvanized layer.

After alloying, a Mg-Al alloy containing PIg of 0.1 to 80% by weight is applied to the alloyed hot-dip galvanized steel sheet using an ion plating method in an amount of 0.1 to 30 g/■2. For example, a deposition crucible charged with Mg-Al alloy may be placed in a vacuum chamber, and the MgO content may be adjusted to 0.1 to 80% by weight by evaporation using an electron beam gun. A thin deposition crucible charged with Mg and a deposition crucible charged with Al are placed in a vacuum chamber, and the irradiation of the electron beam gun is controlled separately for Al and Mg, so that the amount of PIg is 0.
The vapor deposition may be adjusted to 1 to 80% by weight.
At this time, it is preferable that the degree of vacuum in the vacuum chamber is higher than 10-'torr.

Note that the two-layer plated steel sheet of the first invention of the present application can be manufactured by a method other than the manufacturing method of the second invention.

That is, a Mg-Al alloy plating layer is formed by hot-dip plating or thermal spraying on an alloyed hot-dip galvanized steel sheet that has been plated and alloyed in the same hot-dip galvanizing bath as described above. Alternatively, it can be obtained by spraying Mg-Al alloy powder while the galvanized layer is in a molten state after hot-dip galvanizing, and then performing alloying treatment.

(Example) C: 0.002% by weight, Si: 0.01% by weight,
Mn: 0.25% by weight, P: 0.011% by weight
, S: 0.012% by weight, Sol, Al: 0.
0.025 wt%, Ti: 0.024 wt%.
Looms X 250m- from full hard material (steel)
After cutting out multiple test materials, washing these test materials with a solvent, electrolytically degreasing them in a NagCOs + NaOH aqueous solution, and using a hot-dip plating simulator, 25% Hz +
Reduction annealing was performed at a temperature of 840° C. for 30 seconds in an Nt atmosphere, and hot-dip plating was performed in a plating bath having a predetermined bath composition.

After hot-dip plating, the amount of adhesion was adjusted and a Mg-Al alloy layer was formed on the galvanized layer.

The Mg-Al alloy layer consists of 25% of the steel plate after hot-dip galvanizing.
After alloying at a temperature of 500° C. in a Hz + N x atmosphere, it was formed by ion plating, hot-dip plating, and thermal spraying. Alternatively, the hot-dip galvanized layer was formed by spraying Mg-Al alloy powder while it was in a molten state, and then alloying it at a temperature of 500'C in a 25% Hz+Nz atmosphere.

In the ion plating method, a crucible in which Al and PIg are separately inserted is placed in a vacuum chamber maintained at a vacuum level of 10-' torr, the output of the electron beam gun is set to 30-'
By adjusting the distribution of irradiation to the crucible of l and Mg,
Covered with an Al alloy layer. In the hot-dip plating method, the season is 30 ~
Using a Mg-Al alloy plating bath containing 70% Mg
- coated with an Al alloy layer. In the thermal spraying method, a gas-type linear thermal spraying machine is used, and the pressures of acetylene, oxygen, and air are set at 1.5 kg/cm'', 3 kg/cm'', and 4 k, respectively.
g/cm” and then adjust Mg to 0.1 to 80% by weight.
A Mg-Al alloy wire rod adjusted to the above was sent out from the center of the nozzle and thermally sprayed to coat the Mg-^1 alloy layer. Mg-Al
The alloy powder is sprayed at 400"C in the carrier gas for 50 minutes.
kg/cm” of N2, particle size 5-20 μ-0 Mg
Mg-Al alloy powder containing 0.1 to 80% by weight of powder was sprayed onto the hot-dip galvanized surface.

For each two-layer plated steel sheet produced in this way,
Powdering resistance, corrosion resistance and appearance were investigated. These results are shown in Table 1 along with the film compositions and adhesion amounts of the first and second layers.

Powdering resistance is 6 from each two-layer plated copper plate.
A coupon of 0-ugly φ was punched out and subjected to a cylindrical drawing test with a punch diameter of 3° and a die shoulder radius of 3R.The outer cylindrical portion was peeled off with tape, and the degree of peeling was visually evaluated.

The evaluation was performed on a four-level scale, with O being good, ○ being fair, Δ being slightly poor, and × being poor.

The appearance was evaluated by visually observing the surface of each two-layer plated copper plate after phosphate chemical conversion treatment and cationic electrolyte R7I coating under the following conditions. O means good, × means bad.

[Phosphate chemical conversion treatment conditions]

Using a dipping type phosphate chemical treatment solution for automobiles (manufactured by Nippon Bariki Rising Co., Ltd.: PBL3080), the solution temperature was 43.
Treat at ℃ for 120 seconds.

[Painting conditions]

A regular cationic electrodeposition paint for automobiles (manufactured by Nippon Paint Co., Ltd.: U-80) was applied at a voltage of 200μ and an electrodeposition time of 3 minutes.
Paint baking is carried out at 180°C for 30 minutes. The coating thickness is 20
It's μ-rin.

Corrosion resistance was determined by scratching a two-layer plated steel plate that had been subjected to phosphate chemical conversion treatment and cationic electrodeposition under the above conditions, immersing it in a 5% NaCl solution for 1 hour, and then immersing it in a 60°
A dry/wet cycle test was conducted for 200 days in which the process of drying with C hot air for 1 hour was repeated for 200 days, and the maximum corrosion depth of the steel plate at the scratched portion was measured and evaluated.

(Hereinafter, blank spaces) Table 1 shows that the two-layer plated steel sheets of the invention examples (lt16 to lt37) are all improved in corrosion resistance, workability, and surface texture. On the other hand, the two-layer plated steel sheet of Comparative Example (Nllll-Nu 15), which falls outside the range specified by the present invention, is inferior in any of the above characteristics. In addition, Nu3
7 is a conventional alloyed hot-dip galvanized steel sheet which does not have an Mg-Al alloy plating layer and has a thick coating weight of 60 g/at'', but this is inferior in powdering resistance and corrosion resistance.

(Effects of the Invention) As shown in the examples, the double-layer coated steel sheet of the present invention has excellent corrosion resistance and has excellent workability even though it has a lighter coating weight than the conventional galvannealed steel sheet. And the surface quality is also good.

Claims (2)

[Claims] (1) 7 to 20% by weight of Fe as the first layer on the surface of the steel plate
, has an alloyed hot-dip galvanized layer consisting of 0.05 to 0.5% by weight of Al, the remainder being Zn and unavoidable impurities, and has an Mg content of 0.1 to 80% by weight as the second layer.
A highly corrosion-resistant two-layer plated steel sheet having a g-Al alloy plating layer and having an adhesion amount of the Mg-Al alloy plating layer of 0.1 to 30 g/m^2. (2) 0.05 to 0.3 wt% A of the steel plate after pretreatment
After plating with a hot-dip galvanizing bath containing Zn and the rest consisting of Zn and unavoidable impurities and alloying, forming a Mg-Al alloy plating layer on the alloyed hot-dip galvanizing layer by ion plating. The method for producing a highly corrosion-resistant two-layer plated steel sheet according to claim (1).