JPH0297663A - Production of steel sheet plated with zn-mg by vapor deposition - Google Patents

Abstract

PURPOSE:To provide a steel sheet plated with a Zn-Mg alloy by vapor deposition and having superior workability by heating a Zn-Mg alloy bath having a required compsn. and by depositing generated Zn-Mg mixed vapor on a travelling steel sheet in a uniformly mixed state satisfying a prescribed composition ratio. CONSTITUTION:A steel sheet 3 is plated with a Zn-Mg alloy by vapor deposition in a vapor deposition chamber 1 by exposure to Zn-Mg mixed vapor generated from an evaporation vessel 2. In this case, a Zn-Mg alloy bath whose compsn. corresponds to the desired compsn. of a formed plating layer is prepd. and heated in the vessel 2. The Zn-Mg mixed vapor is deposited on the travelling steel sheet 3 in a uniformly mixed state satisfying a prescribed composition ratio and a Zn-Mg alloy plated steel sheet having superior workability can be provided.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for manufacturing a Zn-=Mg alloy vapor-deposited steel sheet that can form a uniform alloy composition ratio in a plating layer without variation. It is.

[Prior Art] As a method for continuously producing Zn-Mg alloy coated steel sheets by vacuum evaporation, as shown in FIG. Zn within 12,
Z-n, M mixed with Mg or pre-alloyed
A method was adopted in which these metals were heated and evaporated under vacuum to be deposited on the running steel plate 3. However, this method is subject to uniform regulation based on the vapor pressure of each metal, so it is practically impossible to adjust the metal vapor composition.

Therefore, as shown in Fig. 8, evaporation tanks 2a and 2b into which Zn and Mg are charged separately are provided in the evaporation chamber 1, and the heating conditions of each evaporation tank are independently controlled to produce a mixed metal with a desired composition ratio. A method of forming steam and depositing it on the steel plate 3 was adopted.

However, according to research conducted by the present inventors, it was found that the Zn-Mg alloy vapor-deposited plating layer obtained by the latter method did not have a uniform composition when viewed in the layer direction. -10% (meaning of weight %, same below)
When manufacturing Mg alloy vapor-deposited steel sheets, temporarily,
As shown in FIG. 9(a) [or (b)], an evaporation tank 2a charged with Mg (Zn in FIG. 9(b)) is arranged upstream of the running steel plate 3, and Zn (Fig. 9 (
In b), it was found that when vacuum evaporation plating is performed with the evaporation tank 2b charged with Mgl placed, a evaporation plating layer with a cross-sectional state as shown in FIG. 9(C) [or (d)] is formed. Ta. Furthermore, when a Zn-35%Mg alloy evaporated steel sheet is manufactured by changing the plating component composition and using the evaporator arrangement as described above, FIG. 9(e) shows.

It turns out that it looks like (f).

As is clear from FIGS. 9(c), (d), (e), and (f), the composition in the plating layer changes in the thickness direction of the layer. For example, FIG. 9(c) 9 and (d) are composed of a pure Zn layer and a Znz Mg layer [the vertical relationship is reversed in (C) and (d)], whereas in FIGS. 9(e) and (f), a pure Mg layer layer, ZnMg layer, and Zn2Mg layer [in (e) and (f), the vertical relationship is reversed as in the previous example].

If the component composition in the plating layer changes in the thickness direction of the layer in this way, the workability, corrosion resistance, or weldability of the plated steel sheet will be adversely affected, as will be described later. Moreover, the relationship between the above changes is unstable as it changes depending on the spacing between the evaporators, as can be seen from the thickness changes of each composition. Zn-based alloy coated steel sheets are used for automobile exterior panels, interior panels, It is also widely used in household electrical appliances, etc., and these are processed into various shapes by press molding. Therefore, excellent workability is required, but as mentioned above, in Zn-Mg alloy coated steel sheets that have a multilayer structure in which the composition changes in the thickness direction, the hardness within the coating layer varies in the thickness direction. Because of this difference, there is a problem in that delamination phenomena such as flaking occur during molding and processability deteriorates.

Furthermore, in a case where a pure Mg layer exists on the base steel plate side as shown in FIG. 9(e), if there are pinholes in the plating layer, it will cause damage in a corrosive environment where chlorine ions, etc. are present.
The pure Mg layer at the bottom is preferentially corroded, and the accompanying H
3, the plating layer blisters and eventually peels off from the base steel sheet, which is an extremely undesirable phenomenon.

In addition, when a pure Mg layer exists on the outermost surface side as shown in Figure 9(f) (usually these plated steel sheets are used after coating), the Mg layer is present at the interface between the coating film and the plating layer. Active dissolution (anodic reaction) occurs, and H2 is generated accordingly.
(cathode reaction), which causes paint film blistering (anode blistering, cathode blistering) and paint film peeling, so it is practically impossible to apply it to automobile exterior panels, home appliances, etc. that are painted. .

[Problem to be solved by the invention] As stated above, Zn-Mg
When a steel plate coated with a vapor-deposited alloy is manufactured, the composition of the plating layer changes in the thickness direction, causing problems such as corrosion resistance during processing or after commercialization.

Therefore, in the present invention, a method for producing a Zn-Mg alloy vapor-deposited steel sheet was studied, which allows the composition to be uniform in the direction of the thickness of the plating layer.

[Means for solving the problems] The present invention that can solve the above problems is Zn-Mg
In manufacturing a Zn-Mg alloy coated steel sheet, a Zn-Mg alloy bath having a bath composition corresponding to the desired composition of the vapor-deposited plating layer is prepared, the mixed bath is heated, and a mixed vapor of Zn and Mg is mixed with the composition according to the composition. Zn is deposited on a running steel plate in a state where the ratio satisfies a predetermined ratio and is uniformly mixed;
The gist of this method is to evaporate Mg and Mg at a predetermined ratio from separate containers, and to uniformly mix these rising vapors up to the lower surface of the running steel plate and deposit them on the steel plate.

[Operations and Examples] Zn- such that the component composition ratio is uniform in the thickness direction of the plating layer.
The present inventors conducted various studies regarding means for obtaining a Mg-based alloy vapor-deposited steel sheet. As a result, Zn vapor and Mg
It has been found that if a state in which the steam is sufficiently mixed at a predetermined ratio is stably formed, the steam is deposited on the surface of the steel plate, and an alloy vapor-deposited plating layer with a stable composition ratio is formed.

Specifically, as shown in Figure 's1, an evaporation tank 2 is installed in the evaporation chamber 1.
is provided, and a raw material supply pipe 7 is connected to the lower side of the evaporator 2 for reasons described later. A Zn--Mg alloy having a predetermined composition ratio is stored in the evaporation tank 2, and vapor having a uniform composition is evaporated to fill the evaporation chamber 1. If the steel plate 3 is run in the direction of the arrow in this state, a Zn-Mg alloy having a predetermined composition ratio and a constant composition in the thickness direction is deposited on the steel plate 3. In FIG. 1, 8 is an exhaust pipe.

At this time, the composition of the Zn-Mg alloy bath in the evaporation tank 2 does not directly match the composition of the plating layer on the steel sheet, and according to the results of the studies conducted by the present inventors, the composition of the Zn-Mg alloy bath and the evaporation layer composition are different. ) A relationship as illustrated in Figure 2, No. 1.2.3 was obtained between the layer compositions. That is, the relationship between the Zn-Mg alloy bath composition and the resulting plating layer structure is dynamic. In No. 1, the bath composition is Zn-71%Mg, whereas the plating layer composition is Zn-64%Mg. , the composition of the plating layer is Zn
It is in a eutectic state of Mg phase and Mg phase, and has a -layer structure. In addition, in tooth 2, the bath composition was Zn-49%Mg.
On the other hand, the plating layer composition is Zn-33%Mg, and the plating layer has a single layer structure consisting of a ZnMg phase. Further, in positive case No. 3, the bath composition is Zn-34%Mg, whereas the plating layer composition is Zn-21%Mg, and the plating layer has a single layer structure consisting of a ZnzMg phase.

Therefore, in order to obtain a plating layer with the desired composition ratio, it is necessary to adjust the alloy composition of the Zn-Mg bath to a composition ratio that will yield the desired plating layer composition (by learning), and then use steam with a uniform composition (by learning). and obtain a plating layer with the desired alloy composition on the steel plate. At this time, in the Zn-Mg alloy bath, Zn has a higher vapor pressure than Mg, and therefore Zn evaporates more easily.
As the g concentration increases, it becomes impossible to maintain the bath composition as described above. Therefore, each metal (especially Zn) is
Alternatively, the Zn-Mg alloy is melted and the molten raw metal is fed through the feed pipe 7 so as to have the desired bath composition.There are two methods of feeding the raw material to each evaporation tank: a solid feeding method and a feeding method in a molten state. However, in industrial manufacturing processes, it is preferable that raw materials are supplied continuously, and it is easier to supply raw materials in a molten state than to continuously supply raw materials in a solid state. This is because raw materials in solid form (granules,
Block-shaped.

In order to continuously supply materials (sheets, wires, etc.) from the atmosphere to a vacuum evaporation tank, it must be supplied through a differential pressure chamber, which not only complicates the structure of the evaporation equipment but also requires equipment. The cost will also be higher. The method of continuously supplying the raw material in the molten state to the evaporation tank is easier than the above-mentioned method of supplying the raw material in the solid state, and since various methods have already been put into practical use, the method of the present invention also has the method of continuously supplying the raw material in the molten state to the evaporation tank. The raw material is fed to the evaporation tank according to a conventional method.

Phase diagrams of pure Zn, pure Mg, and Zn-Mg alloys of various compositions are shown in FIG. According to this, the melting point of pure Zn is 419°C, the melting point of pure Mg is 650°C, and various Zn-M
Since the melting point of g-alloy is relatively low at 340 to 600°C, each metal was separately prepared in a melting chamber (not shown).
It is preferable in terms of operation to melt the liquid and continuously supply it to the evaporation tank through the guide pipe 7. However, Zn, Mg, Zn-Mg
When an alloy is heated and melted in the atmosphere, it is easily oxidized and becomes dross, which not only causes a decrease in yield, but also Mg and Zn-Mg alloys with a high Mg content are oxidized violently, so in some cases can catch fire, creating operational safety problems. Therefore, when heating and melting these raw materials outside the deposition chamber, the molten bath should be sealed with an inert gas such as N2 gas to prevent oxidation, or the surface of the molten bath should be covered with sulfur-based flux, etc. Measures such as shielding the surface from the atmosphere are necessary.

Also, the vapor composition in the evaporation chamber can be adjusted to a predetermined ratio by the following method. That is, as shown in FIG. 3 or 4 Z
Separate evaporation tanks 2a for n and Mg.

The evaporated ZnJis and Mg vapors are introduced into a mixing duct 5 through ducts 4a and 4b provided in each evaporation tank, and uniformly mixed while passing through the mixing duct 5, and deposited on the steel plate. 3 is a schematic diagram showing a case where the evaporation tanks 2a and 2b are provided inside the vapor deposition chamber 1, and FIG. 4 is a schematic diagram showing a case where the evaporation tanks 2a and 2b are provided outside the vapor deposition chamber 1.

When heating the evaporation tank to obtain Zn and/or Mg vapor in each of the above vapor deposition methods, the evaporation tank 2 (or 2a) is heated as shown in FIG.

It is preferable to provide a heater 6 not only around 2b) but also above the surface of the raw material bath for heating, because it is preferable that the Zn or Mg vapor be generated from the bath surface, and for this purpose the outermost surface of the bath should be This is because it is preferable to keep it at the highest temperature. If the temperature at the bottom of the bath is higher than that at the bath surface and locally heated, a splash phenomenon may occur, making it difficult to keep the amount of evaporation stable. Evaporation tank 2 with one Mg
When evaporating from
The n or Mg vapor is directly deposited on the steel plate, causing poor appearance of the plated surface and causing problems in terms of products. The heating of the ducts 4a and 4b will be described later.

Heating means for the evaporation tank include various electrical resistance heating methods such as nichrome wire heaters, Kanthal heaters, molybdenum heaters, and ceramic heaters, as well as electronic j! (EB) Heating method or high frequency heating method can be used, but Zn
Since Mg and Mg have a high equilibrium vapor pressure under vacuum, a sufficient amount of evaporation can be obtained by electric resistance heating without using electron beam heating or high-frequency heating. A Zn-Mg alloy vapor-deposited steel sheet can be obtained at a plating rate that is satisfactory.

Further, the electric resistance heating method is preferable from the viewpoint of the initial cost of the heating source, running cost, durable life, maintenance, and structural problems of the vapor deposition chamber.

In addition, Zn-Mg is deposited by the vacuum evaporation method as described above.
When manufacturing alloy plated steel sheets, evaporated Z
n and Mg vapors adhere to the walls of the deposition chamber and the inner wall of the vapor mixing duct, making it necessary to frequently perform maintenance on the equipment, including the interior of the deposition chamber, and each time the vacuum state is broken and the operation is stopped. This sometimes caused problems.

Therefore, in order to solve this problem, the present inventors installed a heater 6 around the vapor deposition chamber (not shown) or around the duct to heat the wall surface of the vapor deposition chamber or the wall surface of the duct, and by changing the heating temperature, The presence or absence of vapor adhesion was investigated. The results are shown in Table 's1. As is clear from Table 1, in order to prevent the vapors of evaporated Zn and Mg from adhering, the temperature of the wall surface of the deposition chamber or the inner wall surface of the duct must be kept at a temperature of 650°C or higher, preferably 700°C or higher. It was found that it was necessary to maintain the temperature.

At this time, the steel plate enters the vapor deposition chamber in a state that has been preheated to 200 to 250 tons. The increase in plate temperature due to radiant heat from the vapor deposition chamber is very small compared to the above factors (this is clear from calculations and experience). Therefore, the deposition chamber is 65
Even if the temperature is 0 to 700℃, the steel plate passing through it will contain Zn.
- Mg alloy is deposited and plated.

In addition, when the degree of vacuum is sufficiently high as in vacuum evaporation,
If the wall surface is even slightly hotter than the vapor temperature of Zn and Mg,
Steam does not stick to the walls. The temperature of the mixed vapor of Zn and Mg is thought to be around 650 (actually around 600 from experience).
If the wall surface is heated to a temperature higher than that, neither Zn nor Mg will adhere. In other words, if the wall surface of the vapor deposition chamber and the inner wall surface of the duct are heated to the above temperature, there will be no need for frequent maintenance of the apparatus including the vapor deposition chamber.

[Effects of the Invention] Since the present invention is configured as described above, the plating layer in the Zn-Mg alloy vapor-deposited steel sheet obtained by the method of the present invention has a constant composition in the thickness direction. Therefore, according to the method of the present invention, it is possible to provide a Zn-Mg alloy vapor-deposited steel sheet with excellent workability and corrosion resistance.

[Brief explanation of the drawing]

Figures 1, 3 and 4 show Zn-M according to the method of the present invention.
A schematic diagram showing a method for manufacturing a g-based alloy vapor-deposited steel sheet,
Figure 2 is a diagram showing the relationship between alloy bath composition and vapor deposited plating layer structure, Figure 5 is a diagram showing the bath surface heating method, and Figure 6 is a diagram showing the relationship between alloy bath composition and vapor deposited plating layer structure.
Figures 7 and 8 show the relationship between Mg alloy (containing Zns+O%, Mg-0%) composition and melting point.
FIG. 9 is a diagram showing a method for producing a Mg-based alloy vapor-deposited steel sheet, and FIG. 9 is a diagram showing a conventional plating method and the structure of the resulting plating layer. 1... Evaporation chamber 2.2a, 2b... Evaporation tank 3... Steel plate 4a, 4b-Duct 5...
Mixing duct 7...raw material supply pipe

Claims (2)

[Claims] (1) When manufacturing a Zn-Mg alloy vapor-deposited steel sheet, a Zn-Mg alloy bath having a bath composition corresponding to the desired vapor-deposited plating layer composition is prepared, the mixed bath is heated, and the Zn-Mg alloy bath is heated.
A method for producing a Zn--Mg alloy vapor-deposited steel sheet, which comprises depositing a mixed vapor of Mg and Mg on a running steel sheet in a state where the composition ratio thereof satisfies a predetermined ratio and is uniformly mixed. (2) In manufacturing Zn-Mg alloy vapor-deposited steel sheets, Zn and Mg are evaporated from separate containers at a predetermined ratio, and the Zn vapor and Mg vapor are uniformly mixed until they reach the bottom surface of the running steel sheet, and then the Zn and Mg vapors are mixed onto the steel sheet. Z characterized by being vapor-deposited on
A method for producing an n-Mg alloy vapor-deposited steel sheet.