JPH0765213B2 - Molten salt electroplating method for steel materials - Google Patents

Description

TECHNICAL FIELD The present invention relates to a molten salt electroplating method for steel materials, and particularly to a molten salt electroplating method for Al having excellent plating weldability.

(Prior Art) Conventionally, for example, molten salt aluminum electrolytic plating has been known as a plating method for steel materials, but it has hardly been put to practical use. This is because the steel material (steel strip, steel wire, etc.) had to be previously treated and activated by an appropriate means before electrolytic plating in the AlCl ₃ mixed molten salt.
In particular, it is difficult to completely remove the surface oxide film, and development of an efficient means therefor is desired. On the other hand, the molten salt electroplating bath easily deteriorates when water is mixed in, and the water management in the plating line must be strictly controlled together with the management of the plating atmosphere.

Conventionally, the surface of a steel material to be plated has been surface-activated by a wet method such as pickling and then water washing. Therefore, it is necessary to provide a drying step to dry the steel material prior to plating. However, high temperatures in the atmosphere (eg 180
If heated and dried at (.degree. C.), the plating adhesion of the molten salt electrolytic plating to be performed next will be significantly reduced.

In the past, when performing wet salt pretreatment and performing molten salt electrolytic plating on steel material with a water film in this way, the drying and preheating conditions of the steel material have not been sufficiently clarified, and plating adhesion It was inevitable that something bad was done.

(Problems to be Solved by the Invention) Therefore, the inventors of the present invention have made various studies on the cause of such a decrease in the plating weldability, and as a result, an oxide film is formed again on the surface of the steel material during heating and drying of the steel material. I knew that it was because I would end up.

In order to prevent the formation of such an oxide film, it is necessary to perform the drying at a low temperature and raise the temperature by preheating in an inert gas. On the other hand, however, the oxide film that has once been formed must be removed by some means.

By the way, as an activation site to remove such oxide film
The anodic electrolytic treatment in a molten salt bath of AlCl ₃ has been proposed by the present applicant. The steel material to be treated is used as an anode to promote the dissolution of the surface.

Therefore, the degree of oxidation of the steel surface changes depending on the drying conditions after washing with water, and the adsorption of oxygen on the steel surface also occurs.Therefore, the anodic electrolysis conditions must be changed accordingly, which complicates the troublesome operation. It was being done. In particular, in the case of plating continuous materials such as steel strips and steel wires, fluctuations in the line speed accompany changes in the drying and preheating conditions and long-term fluctuations, as well as the time required for all processes, including the coating weight. The treatment was a problem that could not be dealt with substantially.

Further, when excessive oxidation occurs, even if the amount of current is increased to remove such oxides by anodic electrolysis, a huge amount of electricity is required for anodic electrolysis at that time, so In addition to the remarkable deterioration of the properties, the contamination of the electrolytic solution with the eluted Fe ions becomes remarkable.

Thus, the object of the present invention is to carry out electroplating using a molten salt, particularly Al-based alloy plating, continuously with a steel strip or steel wire.
It is to provide a means for stabilizing the quality of a plating film and extending the bath life when it is carried out in an AlCl ₃ mixed salt at about 150 to 220 ° C.

(Means for Solving the Problems) Thus, the inventors of the present invention have variously studied drying / preheating conditions in the drying and / or preheating process in order to achieve such an object. Knowing that there are conditions, the present invention has been completed.

Here, the gist of the present invention is a method of continuously performing a molten salt electrolytic Al plating of a steel strip or a steel wire, including a pre-washing step, a water washing step, a drying step, a preheating step, and a molten salt bath. In the molten salt electroplating method consisting of the anodic dissolution activation step of, and the molten salt electroplating step, the surface temperature of the steel strip or steel wire in the drying step performed in the atmosphere is set to a temperature within the range of 30 to 100 ° C. Suppress, over 100 ℃ ~ 250 ℃
Is a molten salt electrolytic Al plating method, characterized in that the preheating step within the range is carried out in an inert gas atmosphere.

Here, the inert gas atmosphere can prevent the steel material from being oxidized.
N ₂ Ar gas, the higher the purity, the better, but considering that it is continuous plating, the content of oxygen and moisture is usually
It should be 0.1% or less. Further, the "prewashing step" refers to a pretreatment step such as so-called degreasing, pickling (including electrolysis), and water washing. According to a preferred embodiment of the present invention, the water washing step is 40 to 100 ° C.
In this case, the drying is fast and the steel surface oxidation can be minimized.

In the present invention, Al plating includes not only Al plating but also Al alloys such as Al-Mn and Al-Ti alloys.

(Operation) The present invention will now be described in further detail with reference to the accompanying drawings.

FIG. 1 of the accompanying drawings is a schematic explanatory view of a molten salt electrolytic Al plating apparatus for steel products for carrying out the method according to the present invention.

In the figure, the steel strip 2 from the recoiler 1 is passed through a degreasing tank 3, a water washing tank 4, and a pickling tank 5 that perform a pre-cleaning step, and a water washing tank 6 that performs a water washing step, and then a drying step of a drying chamber 7. It is sent continuously. In this drying chamber 7, first, heating gas,
For example, the steel strip 2 is heated by being blown with heated air,
To be dried. Next, the steel strip 2 is subjected to a preheating process, heated and dried in an inert atmosphere 9 which is shielded from the outside by a seal roll 8, and then fed into the anode electrolytic cell 10 in the same atmosphere, for example, AlCl ₃ molten salt. Anodic electrolysis is performed in the electrolytic bath. This is an activation process. Sign
Reference numeral 11 indicates an anode electrolytic conductor roll, and reference numeral 12 indicates a sink roll.

The steel strip 2 thus pretreated exits from the inert gas atmosphere through the compartment rolls 13, enters the molten salt electrolytic plating tank 14, and is subjected to predetermined plating.

It should be noted that molten salt electroplating, for example, molten salt electroplating of aluminum itself is already known and well understood by those skilled in the art, so further reference is omitted for the sake of simplicity.

Here, according to the present invention, the steel material surface temperature in the drying step performed in the atmosphere is controlled to a temperature in the range of 30 to 100 ° C, and the preheating step in the range of more than 100 ° C to 250 ° C is inactive. It is done in a gas atmosphere.

If the temperature of the steel material surface in the drying step in the air is 100 ° C. or lower, the oxidation of the steel material surface does not substantially proceed at the normal steel material conveying speed, and it is not necessary to adjust the atmosphere. On the other hand, heating is performed in an inert gas atmosphere as a preheating step in a temperature range of more than 100 ° C. where surface oxidation is promoted.

In other words, considering that the molten salt electroplating according to the present invention is performed continuously, the anodic electrolysis must be completed within about 2 seconds, but the temperature range of 100 ° C. or lower
This is the temperature at which an oxide layer having a thickness that can be removed within 2 seconds by anodic electrolysis under normal conditions is formed, and beyond this, surface oxidation proceeds so much that it cannot be removed by anodic electrolysis within 2 seconds. On the other hand, if the temperature is lower than 30 ° C, it is considered that the drying does not proceed substantially, so the lower limit temperature is set to 30 ° C.

Further, by performing the washing step at 40 ~ 100 ℃,
It is intended to prevent the oxidation of the surface of the steel material as much as possible and to accelerate the drying by using high temperature washing water. At this time, the lower limit temperature is set to 40 ° C., when the line speed is set to 100 m / min, the lower limit temperature that can be dried in 10 m of the drying step, that is, within 3 seconds is about 40 ° C. Therefore, if it is lower than this, it takes longer time to dry.

That is, it takes time for an oxide film to be formed at a low heating temperature in the atmosphere of 100 ° C or lower, and it is not a problem at normal steel material conveying speed, but if it exceeds 100 ° C, the oxide film is easily oxidized. As described above, since the film is formed, the heating conditions,
Especially the atmosphere is adjusted.

In the case of the illustrated example, the maximum temperature of the steel strip 2 immediately before leaving the drying chamber 7 and entering the preheating process is considered to be the upper limit temperature of the drying process, and it is adjusted to 30 to 100 ° C, while in the preheating process 100
Heating above ℃ is performed in an inert atmosphere.

Thus, according to the present invention, the formation of an oxide film is minimized, and therefore the anodization for surface activation may be carried out as necessary and minimum, so that the continuous treatment is efficiently carried out.

Next, the present invention will be further described with reference to examples.

Example 1 A flow cell (made of SUS316L) dedicated to molten salt was prepared, an Al plate having a purity of 99.8% was installed as an anode in the flow channel, and SPCD grade 0.8 mm thick mild steel plate was used under the conditions shown in Table 1, The present invention was carried out by electroplating in a molten salt.

The pretreatment of the raw steel sheet was performed by electrolytically cleaning the steel sheet sample in a 5% sodium orthosilicate solution as an anode at 10 A / dm ² for 15 S, and then
It was washed with water, then pickled with a 10% HCl aqueous solution for 20 seconds, and washed again with water. The steel sheet thus obtained is dried for 30 seconds in a drier (wind speed 10 m / sec) whose temperature is set at 40 to 160 ° C., and after completion of the drying, immediately switched to N ₂ gas, cooled and taken out, It was placed in a flow cell. This steel sheet was used as an anode to perform anodic electrolysis at 20 A / dm ² for 1 to 12 seconds, and then the anode was switched to perform molten salt electroplating in the same plating tank.

The plated sample was washed with water and dried, and then the adhesion of the plated film was evaluated by a DuPont impact test (ball head diameter 1/2 inch, potential energy 0.8 kgf-m). The results are summarized in FIG.

As is clear from the data shown in the figure, when the drying temperature, that is, the steel sheet surface temperature exceeds 100 ° C., good plating adhesion cannot be obtained by anodizing within 2 seconds in any case. That is, it is not necessary to adjust the atmosphere below 100 ° C., but when heating above 100 ° C., it is necessary to heat in an inert atmosphere.

When air at room temperature (20 ° C) was blown, the surface temperature of the steel plate was less than 30 ° C, the steel sheet did not dry sufficiently, and it took a long time at normal line speed. According to the present invention, the steel sheet surface temperature is at least 30 ° C or higher.

Example 2 Under the same conditions as in Example 1, the final washing with water was carried out at 40 to 100 ° C.
Was washed with hot water, blown with heated air at 40 ° C. and dried, and the adhesion of the plating film was evaluated under the same conditions as in Example 1. When washed with hot water at a temperature of less than 40 ° C, the drying was slow and the plating was uneven, but at 40 to 100 ° C the appearance was good and the adhesion of the plating film was good.

Example 3 Using the continuous molten salt electrolytic plating apparatus shown in FIG. 1, the same plating bath conditions as in Example 1 were used. Molten salt anode electrolytic baths were AlCl ₃ 52 mol%, NaCl 48 mol% and 200 ° C. Molten salt was used. Passing is 10 cold-rolled steel sheets with a thickness of 0.6 mm and a width of 200 mm.
It was driven at m / min. Under this condition, the temperature of the steel plate at the outlet seal roll of the drying chamber 7 was set to 95 ° C., and the inert atmosphere 9 was halved with N ₂ atmosphere (O ₂ concentration, 0.08%, H ₂ O concentration 0.01%) and the atmosphere. ,
The steel plate surface temperature in this zone was changed from 95 to 300 ° C by comparing in an atmosphere mixed with N ₂ 1/2 (O ₂ 10%, H ₂ O concentration 0.1%). The anodic electrolysis was conducted in the anodic electrolysis tank for 2 seconds (only the down pass was energized) and 4 seconds (both the down pass and the up pass were energized), and the current density was 10 A / dm ² , 20 A / dm ^2.
2 conditions. The evaluation of adhesion is the same as in Example 1.

The results are shown in FIG. In addition, in FIG. 3, the amount of anodic electrolysis is shown by the amount of anodic electrolysis.

Anode electrolytic energization amount (C / dm ² ) = current density (A / dm ² ) × energization time (S).

As is clear from Fig. 3, 100 in an inert gas atmosphere.
Adhesion can be secured by anodic electrolysis at 20 C / dm ^{2 at} ~ 250 ° C.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an apparatus for carrying out the method according to the present invention; FIG. 2 shows the relationship between the steel plate surface temperature in the drying step and the required treatment time in the anodic electrolytic treatment depending on the plating adhesion. The summarized graphs; and FIG. 3 are graphs summarizing the relationship between the preheating plate temperature and the anodic electrolytic energization amount during the N ₂ atmosphere preheating by the plating adhesion. 1: Recoiler, 2: Steel strip 3: Degreasing tank, 4: Washing tank 5: Pickling tank, 6: Washing tank 7: Drying room, 8: Seal roll 10: Anode electrolysis tank

Front page continuation (72) Inventor Toshio Nakamori 1-3, Nishi-Nagasumoto-dori, Amagasaki-shi, Hyogo Inside Sumitomo Metal Industries, Ltd. Research Institute (72) Hirohisa Seto 1-chome, Nishi-Nagasu-motodori, Amagasaki-shi, Hyogo No. 3 Sumitomo Metal Industries, Ltd. Technical Research Institute (72) Inventor Toshio Taguchi 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries Ltd. Hiroshima Research Institute (72) Inventor Junkichi Yoneda, Hiroshima City, Hiroshima Prefecture 4-6-22 Kannon Shinmachi, Nishi-ku Mitsubishi Heavy Industries Hiroshima Works

Claims (2)

[Claims] 1. A steel strip or steel wire is continuously molten salt electrolytic Al.
A method of plating, which includes a pre-washing step, a water washing step, a drying step, a preheating step, an anodic dissolution activation step in a molten salt bath,
And in the molten salt electroplating method comprising a molten salt electroplating step, the surface temperature of the steel strip or steel wire in the drying step performed in the atmosphere is suppressed to a temperature in the range of 30 to 100 ° C, and more than 100 ° C ~ Molten salt electrolysis, characterized in that the preheating step within the range of 250 ° C is performed in an inert gas atmosphere.
Al plating method. 2. The method according to claim 1, wherein the washing step is performed at 40 to 100 ° C.