JPH02228493A - Production of zinc alloy electroplated steel sheet - Google Patents

Abstract

PURPOSE:To regulate current density for plating, to rapidly remove gas and to obtain a plating film having a uniform desired compsn. by controlling the electrode areas of insoluble anodes in accordance with the flow rates of forward and counter flows of a plating soln. and the moving speed of the strip. CONSTITUTION:A plating soln. is allowed to flow between one insoluble anode and a strip in the same direction as the moving direction of the strip. The plating soln. is allowed to flow between another insoluble anode and the strip in the reverse direction to the moving direction of the strip. At this time, the electrode areas of the insoluble anodes on the forward and counter flow sides are controlled by setting shielding plates and the current density of each of the anodes is changed without changing the amt. of electric power impressed. A Zn alloy electroplated steel sheet with a plating film having a uniform desired compsn. is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a zinc-based alloy electroplated steel sheet.

[Prior art] Zinc-based alloy electroplated steel sheets such as 2n-Fe have various properties such as corrosion resistance, paintability, workability, and weldability, so they can be used as rust prevention for automobiles, home appliances, building materials, etc. Widely used as steel plate. The film composition of these alloy-plated steel sheets strongly depends on the mass transfer at the plating cathode interface, so maintaining the optimum alloy composition requires strict control of plating conditions. Fluctuations in the flow state near the strip interface cause variations in the resulting alloy composition, leading to performance deterioration.

On the other hand, in recent years, various plating apparatuses have been devised to improve plating productivity. For example, Tokuko Sho 61-220
40 and Japanese Patent Laid-Open No. 61-190094, an electrolytic cell using a fluid cushion is proposed. This device creates static pressure between the electrode and the strip, giving it a force to restore the strip to the center between the electrodes, and has the feature that it can shorten the distance between the electrodes.

Furthermore, in Japanese Patent Publication No. 62-15638, in a vertical plating cell, the plating solution is supplied between the strip and the anode from above the electrode using gravity; In order to complete the plating solution in between, it is essential to use an insoluble anode, but it has the advantage that the pass line is stable, the distance between the electrodes can be shortened, and the non-plated surface does not come into contact with the plating solution during one-sided plating. have

Japanese Patent Publication No. 61-22040 and Japanese Patent Publication No. 61-22040
In Publication No. 90094, since the jet of plating solution is discharged from the center of the electrode, there is a flow in the same direction as the strip traveling direction (hereinafter referred to as "forward flow") and a flow in the opposite direction (r).
(hereinafter referred to as "counterflow"). That is, a forward flow and a counterflow coexist in one electrolytic cell. In addition, the special public corporation Showa 6
The same applies to the publication No. 2-15638. In other words, since gravity is used, flow only exists from the top to the bottom. Furthermore, since it is a vertical plating apparatus, there is a down bath going from top to bottom and an up bath going from bottom to top. Therefore, a forward flow and a counterflow exist in one electrolytic cell.

It is difficult to manufacture alloy-plated steel sheets using the above-mentioned plating equipment in which forward flow and counterflow flow coexist, and this is particularly noticeable when the alloy composition fluctuates depending on the flow rate, such as zinc-based alloy plating. In order to manufacture alloy-plated steel sheets that are flow rate dependent using these plating devices, it is necessary to freely vary the flow rates on the upstream and counterflow sides as the strip movement speed changes.

However, in the techniques disclosed in Japanese Patent Publication No. 61-22040 and Japanese Patent Application Laid-Open No. 61-19'0094, static pressure is formed between the electrode and the strip, and the strip is restored to the center between the electrodes. The main purpose of this method is to shorten the distance between the electrodes, and because the flow velocity on the forward and counterflow sides cannot be freely varied, the manufacturing range is limited. There is.

In addition, in Japanese Patent Publication No. 15638/1983, the flow rate of the plating solution between the anode and the strip is obtained using 1 gravity, so the flow rate is determined by the distance between the electrodes.

Since this method is uniquely determined by the surface resistance of the electrode, the physical properties of the plating solution, etc., it has the disadvantage that, like the invention described above, the flow velocity on the forward flow side and the counterflow side cannot be freely varied. Furthermore, since this device requires the use of an insoluble anode, the gas generated from the anode must be quickly removed, but since the direction of the jet and the direction of gas rise are different, the anode and the strip are different. Gas stagnates in that area, and current stops flowing in that area due to its high electrical resistance, resulting in poor plating, and an increase in local current density in the surrounding area, resulting in uneven alloy composition and discoloration of the plating. In particular, on the upstream side, the moving direction of the strip is also opposite to the rising direction of the electrolytic bubbles, so even the cathode gas from the strip stays, resulting in poor quality in the high line speed range.

[Problems to be Solved by the Invention] As an electroplating apparatus that solves the above-mentioned problems, the apparatus shown in FIGS. 1 to 5, in which forward flow and counterflow are mixed, is used. FIG. 1 shows a horizontal plating device, and FIGS. 2 to 5 show a vertical plating device.

In the plating apparatus shown in FIGS. 1 to 5, since the j@ flow and the counter flow coexist, the distance between the conductor roll 5 and the insoluble anode 4.4' can be shortened, so that the metal of the strip 3 can be reduced. It has the advantage of reducing voltage loss due to resistance. Therefore, it is desired to establish a method for manufacturing zinc-based alloy coated steel sheets in a plating device that uses both forward flow and counter flow, which is suitable for such energy saving.

An object of the present invention is to provide a method for producing a zinc-based alloy electroplated steel sheet having a uniform film having a desired alloy composition in the above-mentioned plating apparatus that uses a mixture of forward flow and counter flow.

[Means for Solving the Problems] The inventors have conducted extensive research to solve the above-mentioned problems. As a result, we obtained the findings described below.6 That is, the plating current density was adjusted by controlling the electrode area of the insoluble anode according to the flow rate of the forward flow and countercurrent of the plating solution and the moving speed of the strip. This achieves rapid gas removal and provides a plated film that is uniform and has the desired alloy composition.

This invention was made based on the above-mentioned knowledge, and a plurality of insoluble anodes are arranged in a plating bath to which a plating solution is supplied, and the strips are continuously moved, and one of the insoluble anodes and the strip between the insoluble anode and the strip, the plating solution is flowed in the same direction as the moving direction of the strip, and between the other insoluble anode and the strip, the plating solution is flowed in the direction opposite to the moving direction of the strip, Thus, the method for producing a zinc-based alloy electroplated steel sheet by plating the surface of the strip is characterized in that the plating current density is adjusted by controlling the electrode area of each of the insoluble anodes.

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a sectional view of a horizontal plating apparatus showing an embodiment of the manufacturing method of the present invention. In Fig. 1, 1 is a horizontal plating device, 2 is a plating tank, 3 is a strip of material to be plated, 4.4', 4a, 4a' are insoluble anodes,
5 is a conductor roll, 6 is a back unroll, 7 is a plating solution, 8 is a dam roll, 9 is a support roll, and 1o is a jet header. In the drawing the arrow indicates the direction of movement of the strip 3. As shown in FIG. 1, insoluble anodes 4 and 4', 4a and 48 are provided oppositely on the upper and lower sides of the strip 3. strip 3
moves between insoluble anodes 4 and 4' and between 4a and 4a' in the direction shown by the arrow. On the upstream side of the support roll 9 provided in the center of the plating tank 2, there are insoluble anodes 4, 4 on the inlet side.
a first jet header 10a for discharging plating solution to ';
10a is a second plate on the downstream side of the support roll 9 for discharging the plating solution to the insoluble anodes 4a and 4a' on the outlet side.
Jet header 10b.

10b are provided respectively. The strip 3 moves in the direction of the arrow and the insoluble anodes 4, 4' and 4a,
4a' and the strip 3, the rudder 10a, 1. A counterflow opposite to the moving direction of the strip 3 flows from Oa to the second jet header 10.
A forward flow in the same direction as the moving direction of the strip 3 is discharged from b and 10b, respectively.

The insoluble anodes 4.4', 4a, 4a' have an electrode area (hereinafter referred to as "electrode area") of the insoluble anode between the insoluble anodes 4.4', 4a, 4a' and the strip 3.
A shielding plate 12 is provided to change the

In the present invention, the reason why we decided to change the current density on the forward and counterflow sides by controlling the electrode areas on the forward and counterflow sides without changing the amount of electricity input to the forward and counterflow sides is as follows. I will explain.

For example, a Zn-Fa alloy electroplated steel sheet will be described as an example. In the Zn-Fe alloy plating formed on the surface of the strip, the performance of the plating film depends on the Fe content in the film, and the optimum Fe content is said to be 10 to 20 wt.%. When the Fe content is less than 10 wt.%, it has properties similar to Zn, and the effect of containing Fe is weak.

Blisters occur quickly and there is no improvement in corrosion resistance after painting compared to Zn plating. On the other hand, the Fe content is 20wt,
If the Fe content exceeds 10 to 2%, sacrificial corrosion resistance deteriorates and corrosion resistance such as red rust resistance deteriorates.
It is preferably within the range of 0 wt.%. In order to limit the Fe content within this range, plating conditions must be strictly controlled.

On the other hand, the alloy composition of this Zn-Fe alloy plating strongly depends on plating conditions such as flow rate, that is, mass transfer at the cathode interface.
e content decreases.

Since the jet direction with respect to the strip movement direction is different between the forward flow side and the counterflow side, the strip reference relative velocity (forward flow side: the absolute value of the value obtained by subtracting the strip movement speed from the plating solution flow rate, and the counterflow side: the plating solution flow rate) It is difficult to manufacture the above-mentioned zinc-based alloy-plated steel sheet in which the flow rate depends on the alloy composition.

In other words, on the downstream side, as the strip moving speed increases, the relative velocity decreases, becomes zero when it becomes equal to the jet velocity, and then increases in the opposite direction.In other words, the Fe content in the film increases by - The value reaches its maximum value when the relative velocity is zero, and then decreases.

On the other hand, on the counter-current side, the relative velocity always increases as the strip moving speed increases, and at this time the Fe content always decreases. Therefore, in a plating apparatus in which forward flow and counterflow flow coexist, as the strip moving speed increases, the difference in Fe content between the forward flow side and the counterflow side increases from zero.

It reaches a maximum when the relative velocity on the downstream side is zero, and then decreases.

In addition, when the current application time per electrode is short (when the strip movement speed is fast or the electrode length is short), the diffusion layer at the cathode is in the growth stage and is thin. The supply of Zn, which is under rate-limiting conditions, progresses and the Fe content in the film decreases.6 Furthermore, since the mass transfer is sufficiently enhanced in this region, the flow rate dependence of the alloy composition (1) and the current application time dependence (1) decrease. . Therefore, as the strip moving speed increases, the difference in Fe content between the upstream side and the countercurrent side becomes smaller (when the relative speed on the upstream side is zero).

Furthermore, in vertical plating equipment, it is necessary to consider the rising speed of electrolytic gas generated from the anode and cathode.
Particularly in the case of downward flow opposing the upward flow of gas, degassing performance decreases, and fluctuations in alloy composition occur due to the increase in local current.

As described above, it is difficult to produce alloy plating with a uniform composition in a plating apparatus that uses both forward and counterflow, and requires new alloy composition control parameters. That is current density. That is, the problem is solved by lowering the current density under conditions where the Fe content increases, such as when the relative speed is low, and increasing the current density under conditions where the Fe content decreases. Here, a structure is used in which the current density on the forward flow side is reduced, and a structure in which the current density on the counterflow side is increased. At this time, the amount of electricity input is not controlled because it causes a decrease in productivity, but is controlled by changing the electrode area.

Next, a method for controlling the electrode area will be specifically explained.

In the horizontal plating apparatus shown in FIG. 1, electrodes (insoluble anodes 4a, 4a') larger than necessary are installed in advance, and a shielding plate 12 is provided between the strip 3 and the insoluble anodes 4a, 4a'. , the electrode is shielded by a shielding plate 12 to control the electrode area.

In the vertical plating apparatus shown in FIG. 2, an electrode (insoluble anode 4b) larger than necessary is installed in advance, a shielding plate 12a is provided between the strip 3 and the insoluble anode 4b, and the electrode is connected by the shielding plate 12a. Shield and control electrode area.

In the vertical plating apparatus shown in FIG. 3, an electrode (insoluble anode 4c) larger than necessary is installed in advance on the downstream side, and the level of the plating solution 7 is moved up and down to control the electrode area.

In the vertical plating apparatus shown in FIG. 4, divided preliminary electrodes 44 are provided facing both sides of the strip 3 on each of the forward flow side and the countercurrent side, and a plurality of switches 15 are provided for changing the electrode area of the divided preliminary electrodes 44. The electrode area is controlled by switching ON or OFF. 10c is a jet header.

In the vertical plating apparatus shown in FIG. 5, a plurality of jet headers 1 which can be opened and closed by valves 14 are attached to each of the insoluble anodes 4d provided on opposite sides of the strip 3 on the upstream side.
0d, and by opening one ladder 10d to the jet of the insoluble anodes 4d and 4d on both sides, the position of the ladder 10d to the jet can be changed up and down, thereby controlling the electrode area.

As described above, by adjusting the current density by controlling the electrode area, Z
The plating film of the n-Fe alloy plated steel sheet has an Fa content of 1
It became possible to manufacture within the permissible range of 0 to 20vt,%.

Although the above description has been made using Zn-Fe alloy plating as an example, the present invention can be widely applied to general zinc-based alloy plating such as Zn-Ni.

[Examples] Next, the present invention will be explained in more detail with reference to Examples.

Using the horizontal plating apparatus 1 shown in Fig. 1, Zn-F
E-alloy plated steel sheets were manufactured by controlling the electrode area, changing the electrode length, and changing the strip movement speed, and plating a medium 1200nf1 strip independently on the forward flow side and the counterflow side. . Note that only the insoluble anode on the upstream side was controlled. and,
The Fe content in the film of the obtained Zn-Fe alloy plated steel sheet was measured and shown in Table 1. As a comparative example, on the downstream side,
Table 2 shows the results produced using an insoluble anode that was not controlled on either side of the counterflow. Then, it was investigated whether an alloy-plated steel sheet having a uniform plating film within an allowable range was manufactured. Fe content is 10-20tit,
Since the range of % is an appropriate range, the difference in Fe content between the upstream side and the counterflow side is at least Lout.

% or less, the difference in Fe content is shown.

Note that the plating conditions were as follows.

(1) Plating bath composition First bath ferrous sulfate: 325g/Q, zinc sulfate: 175g/ffi, sodium sulfate: 60 g/f2゜Second bath ferrous sulfate: 275g/Ill, zinc sulfate: 225g/ R5 Sodium sulfate: 60 g/Ω, (2) pl (= 1.5 As is clear from Tables 1 and 2, the plated steel sheet manufactured by the manufacturing method of the present invention has Fe The difference in Fe content was small and the film was uniform within the allowable range.On the other hand, the plated steel sheets manufactured by the manufacturing method other than the present invention had a large difference in Fe content between forward flow and counter flow. Ta.

[Effects of the Invention] As explained above, according to the present invention, uniform

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a horizontal plating apparatus showing one embodiment of the manufacturing method of the present invention, and FIGS. 2 to 5 are cross-sectional views of a vertical plating apparatus showing other embodiments. DESCRIPTION OF SYMBOLS 1... Horizontal plating device, 2... Plating tank, 3... Strip 4, 4', 4a, 4a' 4b, 4c, 4d-Insoluble anode, 5... Conductor roll, 6... Backup roll, 7... Plating solution, 8... Dam roll, 9... Support roll, 10.10a, 10b, 10c, 10cl... Jet header, 11... Vertical plating tank, 12.12a. ...Shielding plate, 13...Bottom roll, 14...Valve, 15...Switch, 44...Divided preliminary electrode.

Claims (1)

[Claims] 1. A plurality of insoluble anodes are arranged in a plating tank to which a plating solution is supplied, and a strip is continuously moved, and the plating solution is placed between one of the insoluble anodes and the strip while the strip is being moved. and the plating solution is flowed between the other insoluble anode and the strip in a direction opposite to the moving direction of the strip, thereby plating the surface of the strip. A method for producing an electroplated steel sheet using a zinc-based alloy, characterized in that the plating current density is adjusted by controlling the electrode area of each of the insoluble anodes.