JPH11269694A - Production of electrogalvanized steel strip having excellent surface appearance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a plated steel strip which is free of unequalness and patterns and has a good surface skin by cleaning the steel strip by setting a final electrode polarity at minus in an electrolytic cleaning stage and executing annealing in a hydrogen-contg. nitrogen atmosphere, then sending the steel strip to an electrogalvanizing vessel. SOLUTION: The steel strip is electrolytically cleaned by setting the final electrode polarity of the electrolytic cleaning of the steel strip at minus and after the steel strip is annealed in the hydrogen-contg. nitrogen atmosphere, the steel strip is subjected to electrogalvanizing. The uniformly oxidized films formed on the surfaces of the electrolytically cleaned steel strip act as a kind of barrier layers. Even if there are the seizure of a rolling mill lubricant, the elements deposited from the atmosphere gas, the surface thickening of easily oxidative elements, etc., at the time of annealing, the oxidation reaction of the steel strip surfaces is made gentle and uniform without the local acceleration of the oxidation reaction. If such steel strip is fed into the electroplating vessel and is subjected to electroplating, the current density distribution is made uniform and the deposition amt. of the plating metal is made constant over the entire area of the steel strip surfaces.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an electro-galvanized steel strip having excellent surface appearance without surface defects such as unevenness and pattern.

[0002]

2. Description of the Related Art In many cases, electro-galvanized steel sheets are used unpainted in applications such as automobile parts and home electric appliances, and require a much better surface skin than hot-dip galvanized steel sheets. The causes of unevenness and patterns that deteriorate the surface skin are:
It is classified into those derived from the electroplating equipment and those derived from the surface properties of the original plate. The cause derived from the electroplating apparatus can be eliminated by improving the electroplating apparatus.
However, the cause derived from the surface properties of the original plating sheet is difficult to identify because the original plating sheet is manufactured through various manufacturing steps, and even if it can be identified, the present situation does not lead to a drastic solution. Moreover, some of them cannot be confirmed at the stage of the original plate, but are first confirmed after the electro-Zn plating.

Therefore, in Japanese Patent Application Laid-Open No. 9-202992, the surface of the steel strip for Zn precipitation is homogenized by forming a nickel plating layer on the surface of the steel strip prior to electroplating. In JP-A-9-20991, a surface of a steel strip to be electroplated is refined by applying a sulfur compound such as thiourea or alkylthiourea to a degreased steel strip surface and annealing it.

[0004]

However, the nickel pre-plating requires the installation of a dedicated facility in the electroplating line, and consumes expensive nickel, resulting in an increase in manufacturing cost. Further, when a plating bath is added to a pickling bath before plating, equipment for controlling the concentration of a plating solution is required. Even with the method of applying sulfur compounds,
The consumption of chemicals increases the production cost, and requires special coating equipment and equipment for controlling the concentration in the wash water.

[0005]

SUMMARY OF THE INVENTION The present invention has been devised in order to solve such a problem. The present invention provides a steel strip surface having a constant property in an electrolytic cleaning step prior to annealing.
It is an object of the present invention to produce an electro-galvanized steel strip having a good surface without irregularity, pattern, or the like without incurring new equipment or additional steps. In the production method of the present invention, in order to achieve the object, the steel strip is maintained at a noble potential in the final stage of electrolytic cleaning, electrolytically cleaned, then annealed in a hydrogen-containing nitrogen atmosphere, and then electroplated with zinc. It is characterized by.

[0006]

The cause of defects such as unevenness and pattern on the surface of the electro-Zn plating layer is due to an annealing process performed for material adjustment prior to electro-Zn plating. The original plate for plating is usually prepared through an annealing process of electrolytic cleaning → annealing after cold rolling, but in the annealing step, rolling oil residues are seized on the steel strip surface, and elements are precipitated from the atmospheric gas on the steel strip surface. There are reactions. In addition, a reaction occurs in which easily oxidizable elements such as Si and Mn diffuse from the inside of the steel strip to the surface of the steel strip and concentrate. Rolling oil residue, precipitated elements, surface concentrated layer, etc.
In the plating step, the precipitation state of Zn deposited on the surface of the steel strip is partially changed, which causes the surface skin to be uneven.

The present inventors have conducted various investigations and studies on methods for suppressing the occurrence of defects on the premise that a cause of the occurrence of defects will occur in the annealing step. As a result, they have found that the occurrence of defects varies greatly depending on the polarity of the final electrode in the electrolytic cleaning step prior to annealing. In the electrolytic cleaning step, a plurality of electrolytic cells are arranged in series, and electrolytic cleaning is performed while alternately maintaining the steel strip at positive and negative potentials. In this case, the effect of the polarity of the final electrolytic electrode on the occurrence of defects is as follows. Inferred.

[0008] When the polarity of the electrolytic final electrode is positive, the steel strip passing through the electrolytic cleaning layer becomes negative. Hydrogen bubbles are generated on the steel strip surface maintained at a lower potential, and the reducing power of hydrogen activates the steel strip surface. The activated surface causes a non-uniform oxide film to form on the steel strip surface when the electrolytically cleaned steel strip is rinsed. The uneven oxide film
Although not detected by observing the surface of the steel strip after annealing, it grows in the subsequent annealing step and is brought into the electroplating tank. As a result, the pickling reaction and the electrolytic deposition reaction on the surface of the steel strip become non-uniform, and defects such as unevenness and bubble pattern are generated in the formed electro-Zn plating layer. On the other hand, if the polarity of the final electrode at the time of electrolytic cleaning is made negative, the steel strip surface maintained at a noble potential is electrolytically cleaned. In this state, oxygen bubbles are generated from the steel strip surface. The generated oxygen gas forms a uniform oxide film on the surface of the steel strip. In the steel strip having this surface state, the oxide film does not grow unevenly even if it is rinsed with water.

[0009] The effect of the polarity of the electrolytic final electrode becomes more pronounced in the subsequent annealing step. In the annealing step, usually 1 to
The steel strip is heated to 600 to 800 ° C in a reducing atmosphere of 3% by volume H ₂ -N ₂ and a dew point of -20 to 0 ° C. If an uneven oxide film is formed on the surface of the steel strip to be annealed,
The oxidation condition of the steel strip surface varies depending on the non-uniform oxide film, and the progress of the oxidation reaction varies between the easily oxidized portion and the hardly oxidized portion, and the non-uniformity of the oxide film is further promoted. When a steel strip on which a non-uniform oxide film is formed and grown is electroplated, there is a difference in pretreatment and pickling properties, and the surface resistance is partially different. The amount fluctuates. As a result, it is presumed that a bubble pattern or the like is generated on the surface of the electroplated steel strip.

On the other hand, in a steel strip electrolytically cleaned with the final electrode polarity being negative, a uniform oxide film formed on the surface of the steel strip acts as a kind of barrier layer. The oxidation reaction on the steel strip surface is made slow and uniform without local promotion of the oxidation reaction even if there is a surface concentration of a precipitation element or an oxidizable element from the surface. When this steel strip is fed into an electroplating tank and electroplated, the current density distribution is made uniform, and the amount of plating metal deposited is constant over the entire surface of the steel strip. From these facts, it is presumed that unevenness, pattern, and the like hardly occur in the steel strip electrolytically cleaned with the final electrode polarity being negative.
In fact, this inference is supported in the examples described below.

[0011]

[Example] Cold rolled low carbon steel with a thickness of 1.0 mm is continuously used
Before annealing in the annealing equipment, 70% NaOH 2% aqueous solution
And a current density of 4 A / dm^Two For electrolytic cleaning. electrolytic
In the cleaning process, two electrolytic cells are arranged in series, and
The polarity of the electrode is negative. Electrolytically cleaned steel strip
Was rinsed with water and introduced into an annealing furnace, where 3% by volume H _Two −
N_Two To 700 ° C in a reducing atmosphere with a dew point of -20 to 0 ° C
Heated. Steel strip after annealing to continuous electric Zn plating line
Conveyed and pre-picked with a 30 g / l sulfuric acid aqueous solution at a temperature of 40 ° C
After that, a pH 1 sulfuric acid bath (ZnSO_Four 300 g / l, N
a _Two SO_Four 100 g / l, H_Two SO_Four 30g / l)
And current density of 100 A / dm^Two Was electroplated with Zn.
The electrolysis condition is that the basis weight is 20 g / m.^Two Set to be
did.

[0012] The surface of the obtained electro-galvanized steel strip was visually observed, and the state of pattern generation on the surface of the electro-galvanized layer was examined. Visual observation did not detect any appearance defects such as patterns. When the relationship between the generation of the bubble pattern and the line speed was investigated, as shown in FIG. 1, at a line speed exceeding 50 m / min, the electrolytic Zn-plated steel strip had a healthy surface skin with almost no bubble pattern detected. . In this case, only a slight foam pattern was detected for the first time when the electroplating was performed under conditions where the line speed was intentionally reduced, the current density was increased, and the effect of the electrolytic reaction was likely to occur.

On the other hand, when the steel strip electrolytically cleaned by making the final electrode polarity positive in the electrolytic cleaning step is similarly annealed and electroplated with zinc, as shown in FIG. 2, 1.5 A / dm ²
At current densities higher than, a large amount of bubble patterns were detected on the surface of the electro-Zn plating layer regardless of the line speed. As is clear from this comparison, the steel strip surface is homogenized by making the final electrode polarity negative during electrolytic cleaning, and the steel strip surface is homogenized, and the electrolytic deposition reaction during electroplating becomes uniform over the entire surface. It turns out to be.

[0014]

As described above, according to the present invention, the steel strip is electrolytically cleaned with the final electrode polarity being negative in the electrolytic cleaning step, whereby the oxidation reaction on the steel strip surface is locally localized in the subsequent annealing step. The steel strip whose surface has been homogenized has been sent to the electroplating tank while suppressing fluctuations. Therefore, in the electroplating step, the electrolytic deposition reaction of the plating metal is made uniform over the entire surface of the steel strip, and an electro-zinc-plated steel strip having a good surface without unevenness or pattern is produced. This method does not require extra steps such as the application of sulfur compounds and nickel pre-plating, and is a simple operation that only sets the final electrode polarity to negative during electrolytic cleaning. High quality electro-galvanized steel strip is produced at low cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the effects of line speed and electrolytic current density on the generation of bubble patterns when a steel strip electrolytically cleaned with a negative electrode polarity at the time of electrolytic cleaning is subjected to electrolytic Zn plating.

FIG. 2 is a graph showing the effects of the line speed and the electrolytic current density on the generation of a bubble pattern when a steel strip electrolytically cleaned by making the final electrode polarity positive during electrolytic cleaning is electro-Zn plated.

Claims (1)

[Claims] 1. In a final stage of electrolytic cleaning, a steel strip is maintained at a noble potential and electrolytically cleaned, then annealed in a hydrogen-containing nitrogen atmosphere, and then subjected to electro-Zn plating. A method for producing an electro-galvanized steel strip.