JPH05247699A - Neutral salt electrolyzing method for descaling from stainless steel strip and device therefor - Google Patents

Abstract

PURPOSE:To reduce the electrodeposition of molten metal in a bath at the electrolized part in the cathode of a stainless steel strip even in the case of an indirect conducting system, at the time of neutral salt electrolyzing treatment in which the removal of scales formed on the surface of a stainless steel strip is executed by annealing treatment, and to manufacture the stainless steel strip of excellent quantity from the economical viewpoint. CONSTITUTION:The sheet width and sheet passing rate of a stainless steel strip S passed through a neutral salt electrolyzing stage in which descaling from the stainless steel strip S is executed are respectively detected by a sheet width detector 3 and a sheet passing rate detector 2. By using a computing element 9 computing electrolyzing current value from both detected value and objective electric quantity for subjecting the stainless steel strip S to electrolyzing treatment, a DC power unit 7 is controlled to automatically regulate the electrolyzing current value.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to electrodeposition of molten metal in a bath at the cathodic electrolysis section of a stainless steel strip during the neutral salt electrolysis treatment for descaling the scale produced on the surface of the stainless steel strip by the annealing treatment. TECHNICAL FIELD The present invention relates to a neutral salt electrolytic treatment method for descaling a stainless steel strip, which is capable of producing a stainless steel strip that is economical and excellent in quality, and an apparatus for carrying out this method.

[0002]

2. Description of the Related Art As one of the annealing treatment methods performed on a cold rolled stainless steel strip, there is an atmospheric annealing method in which a fuel is burner-burned in an atmospheric atmosphere furnace to perform a direct flame heat treatment. In this atmospheric annealing treatment, metal oxide (hereinafter,
Descaling treatment is performed after the annealing treatment in order to remove this and produce a stainless steel strip of good quality. In general, the descaling process of the stainless steel strip is continuously performed in a series of lines together with the annealing process of the previous process.

As the descaling method, electrolytic treatment in an aqueous solution of sodium sulfate (hereinafter sometimes simply referred to as neutral salt electrolytic treatment) or immersion treatment in a mixed molten salt of caustic soda and sodium nitrate is performed. It is widely practiced to perform pretreatment such as (hereinafter, simply referred to as salt dipping treatment) and the like, followed by dipping in nitric hydrofluoric acid, electrolytic treatment in a nitric acid aqueous solution, or a combination thereof. Among these treatment methods, the neutral salt electrolytic treatment has been widely adopted in recent years since it was disclosed in Japanese Patent Publication No. 38-12162.

The features of this neutral salt electrolysis treatment are that it is easy to obtain a beautiful surface texture as compared with the salt dipping treatment, that the pH of the aqueous solution is close to neutral, and that it is easy to handle, and that the working environment is good. There are things like that. Further, the electrolysis in the neutral salt electrolysis treatment is an indirect electrolysis in which the anode and the cathode are alternately placed in a non-contact manner with respect to the passing direction of the stainless steel strip and the cathodic electrolysis and the anodic electrolysis are sequentially performed on the stainless steel strip. It is widely practiced. Conventionally, the action of this neutral salt electrolytic treatment is as disclosed in Japanese Patent Publication No. 38-12162.
It was thought that the stainless steel strip having a scale was subjected to the dissolution action of the scale during the anodic electrolysis, and was not particularly affected during the cathodic electrolysis treatment to generate hydrogen.
In JP-A-63-161194, metal ions such as Cr and Fe dissolved in the bath are reduced during cathodic electrolysis of stainless steel strips, and these are deposited on the surface of the stainless steel strips to form scale-like substances. Therefore, it is disclosed that the descaling is not sufficient even after the subsequent nitric hydrofluoric acid immersion treatment, and in order to prevent this precipitation, the current density during cathodic electrolysis is limited according to the concentration of dissolved metal in the bath. Is proposed. Further, JP-A-1-96398 discloses that the concentration of hexavalent chromium ions and the pH in a bath are limited.
-96399 and Japanese Patent Application Laid-Open No. 1-96400 disclose that an anode electrolysis part and a cathode electrolysis part are separated and that the conditions of each of the separated baths are limited. In JP-A-2-173300, reduction is performed in the bath. It is proposed to add a chemical agent to reduce and change hexavalent chromium ions. However, all of these methods have a drawback that they are not practical because they require facility modification and the adjustment of the current density during the electrolytic treatment increases the work load.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art and requires a cathodic electrolysis section of a stainless steel strip in spite of the indirect energization method without requiring major facility modification. It is an object of the present invention to carry out a neutral salt electrolysis treatment for descaling of a stainless steel strip while maintaining a proper electrolysis action of the neutral salt with less electrodeposition of the dissolved metal in the bath.

[0006]

Means for Solving the Problems As a result of intensive research to solve the above problems, the present inventor has found that in neutral salt electrolysis treatment, dissolution of a stainless steel strip in a cathodic electrolysis section in a bath in an indirect current electrolysis apparatus is dissolved in a bath. The electrodeposition of metal was investigated, and the following conclusions were obtained from the results shown in Table 1. That is, hard lead (lead antimony alloy) was used as an anode in a sodium sulfate aqueous solution having a simulated adjusted dissolved metal concentration (here, chromic acid was added as hexavalent chromium), and a platinum wire was immersed as a cathode. Connect a direct current power supply between these electrodes and set the current density to 5A.
The amount of metal deposited on the platinum wire serving as the cathode was measured by varying the electrolysis time under a constant / dm ² and the effect of the electrolysis quantity on the electrodeposition was investigated. The pH of the aqueous solution was adjusted with caustic soda to investigate the effect of pH. From the results of this investigation, the causal relationship between hexavalent chromium concentration and pH and electrodeposition could not be clarified, but electrodeposition did not occur under each condition when the electrolysis time was as short as 23 seconds.

[0007]

[Table 1]

In the neutral salt electrolytic treatment apparatus of the applicant of the present invention, the total time for the stainless steel strip to undergo the cathodic electrolytic treatment is 4 to 20 seconds, which is inevitable in view of the facility conditions. The determined anode electrolysis time is 12 to 60 seconds. In addition, the anode current density that determines the target scale dissolution action of the neutral salt electrolytic treatment is generally 2 to 6 A / dm.
It is ² , and our company basically adopts this current density, and the typical steel grades are SUS410, SUS43.
0, SUS304, depending on the annealing conditions, the amount of electricity, which is approximately the product of current density and electrolysis time, is 50 to 100 C / dm ²
If so, it is confirmed that the descaling effect of the neutral salt electrolysis treatment can be obtained, and the descaling is performed even if the current density is changed according to the above-mentioned anodic electrolysis time of 12 to 60 seconds so as to maintain this amount of electricity. It was also confirmed that there was no change in the effect. From the above results, in the investigation results shown in Table 1 above, the electric quantity was 115 C / dm ² or less corresponding to the electrolysis time of 23 seconds at the steel strip cathode current density of 5 A / dm ² which was the condition under which the electrodeposition phenomenon did not occur. By doing so, it was clarified that there is no electrodeposition of the molten metal in the bath on the surface of the stainless steel strip and the required descaling effect can be obtained.

Generally, the electrolytic action is determined by the amount of electricity, which is the product of the current density applied to the material to be treated and the electrolysis time. Therefore, if the optimal amount of electricity can be determined, the current density or the electrolysis time is adjusted to maintain it. All you have to do is do it. Generally, in the annealing pickling process of a stainless steel strip, the strip running speed is changed according to the strip thickness and strip width of the strip running. That is, when both the plate thickness and the plate width are large, the plate is passed at a low speed to obtain a predetermined material temperature, and when the plate thickness and the plate width are both small, the plate is passed at a high speed. This is because the amount of heat energy that can be supplied by the annealing furnace is limited, so that the amount of heat sufficient to heat a stainless steel strip having a certain volume to a certain temperature is almost the same. On the other hand, since the pickling step is continuous with the annealing step, the strip is passed at the same strip speed as the annealing step. Therefore, if there is an electrolytic treatment device such as a neutral salt electrolysis device in the pickling step, the plate passing speed, that is, the electrolysis time, will change depending on the plate thickness and the plate width. Conventionally, mainly from the viewpoint of energy saving, the electrolytic current value was manually changed according to the plate width so as to maintain at least the target current density. Adjustment was not possible due to work load issues. That is, in the industrial electrolytic treatment, it was a labor-intensive task to adjust the current density or the amount of electricity each time under the circumstances where the width of the stainless steel strip or the strip running speed was changed.

In view of this, the present inventor has tried to suppress the electrodeposition of the molten metal in the bath at the cathodic electrolysis portion of the stainless steel strip in the neutral salt electrolytic treatment for descaling of the indirect energization type stainless steel strip. The present invention has been made by discovering that at least the object of the present invention can be achieved if the amount of electricity is constant during the salt electrolysis treatment. That is, for detecting the plate width and the plate passing speed of the stainless steel strip which is passed through the neutral salt electrolytic treatment step of descaling the stainless steel strip, both detection values and the electrolytic treatment of the stainless steel strip are performed. It has been clarified that the object of the present invention can be achieved by automatically adjusting the electrolysis current value by using a calculator that calculates the electrolysis current value from the target electricity amount.

[0011]

The details of the neutral salt electrolytic treatment method and apparatus for descaling the stainless steel strip according to the present invention will be described below with reference to FIG. FIG. 1 is a schematic explanatory diagram illustrating the configuration of a neutral salt electrolytic treatment apparatus for descaling a stainless steel strip according to the present invention. In the figure, S is an annealed stainless steel strip (the annealing device other than the neutral salt electrolysis device is not directly related to the present invention, so the description thereof is omitted), and the bridle roll 1 is passed through a deflector roll to be neutral. It is introduced into the electrolytic bath 5 of the salt electrolysis tank 4 through the dipping roll 8 on the entry side, and in the neutral salt electrolysis tank 4, the indirect electrolysis of anodic electrolysis and cathodic electrolysis is sequentially performed by the cathode electrode plate 6a and the anode electrode plate 6b. It is led out from the electrolytic bath 5 of the neutral salt electrolytic bath 4 through the dipping roll 8 on the outlet side, and is introduced into the nitric hydrofluoric acid mixed acid bath or the nitric acid electrolytic bath 10 through the deflector roll and is subjected to pickling treatment. It is. At this time, the strip width of the stainless steel strip S is detected by the strip width detection device 3 before passing through the neutral salt electrolytic bath 4. As the plate width detecting device 3, a device using a light source installed on the lower surface of a generally used stainless steel strip S in the plate width direction and a light receiving light source (CdS element, photodiode, etc.) installed on the upper surface, A linear sensor camera (CCD) can be used. Further, the strip speed of the stainless steel strip S is also detected by the strip speed detecting device 2. The plate passing speed detecting device 2 may be calculated based on the output of a pulse generator that detects the number of rotations of a rotating roll attached to the bridle roll 1, or the Doppler effect of ultrasonic waves or laser light may be calculated. The one used may be used. Thus, the detected value of the plate width detected by the plate width detecting device 3 and the detected value of the plate passing speed detected by the plate passing speed detecting device 2 are input to the calculator 9 having a changeable electric quantity setting function. Then, the current of the DC power supply device 7 is controlled so that the electrolytic current value supplied to the anode electrode plate 6a and the cathode electrode plate 6b is maintained at the calculated electrolytic current value according to the following equation.

L = (Q × W × P × 100) / (P × 60
/ V) where L: electrolytic current value (A) Q: quantity of electricity (C / dm ² = A · sec / dm ² ) W: plate width (m) P: cathode electrode length (m) V: through Plate speed (m / min)

In the above formula, the current density D (A / dm ² ) is the electrolytic current value L (A) and is the total area of the cathode projection on the stainless steel strip.
Since it is divided by (dm ² ), it is expressed by D = L / (10W × 10P) = L / (W × P × 100), and the electrolysis time T during which the stainless steel strip passes through the cathode T
Since (sec) is the cathode electrode length P (m) divided by the plate speed (m / min), it is expressed as T = P / (V / 60), and the quantity of electricity Q (C / dm ² ) is Since it is a product of D (A / dm ² ) and electrolysis time T (sec), it is expressed by Q = D × T, and can be derived from these three equations.

Even if the quantity of electricity is set to 115 C / dm ² , it is assumed that the current density is 115 A / in one second for the anode electrolysis time.
Under the condition of dm ² , the descaling action of neutral salt electrolysis is difficult to obtain. This is because it relates to the electrophoresis of various ions in the bath. The investigation by the present inventor has confirmed that at least the anode electrolysis time needs to be 4 seconds in addition to the current density in order to obtain a proper descaling effect of the neutral salt electrolysis treatment. Further, as in the method of the present invention, a method of automatically adjusting the electrolytic current value so that the target amount of electricity for electrolytically treating a steel strip is constant is another electrolytic pickling method such as sulfuric acid electrolysis or nitric acid electrolysis. Although it can be used to set the current value, in these electrolytic pickling methods, the soluble electrode typified by ordinary high silicon cast iron is often used as the anode electrode, and the electrode area gradually decreases when the soluble electrode is used. Therefore, the current density and the electrolysis time change, and it is difficult to apply a constant amount of electricity, which is difficult to adopt. In this case, if the insoluble electrode is adopted, it is possible to carry out the same method as the method of the present invention, and it is considered that the electrodeposition of the dissolved metal in the bath at the cathode portion of the stainless steel strip can be suppressed.

[0015]

[Examples] Tables 2 to 4 show the results of carrying out the method of the present invention on stainless steel strips SUS410, SUS430, and SUS304, which are typical steel grades in actual lines, together with comparative examples. Annealing conditions (furnace temperature, residual oxygen concentration, etc.) and strip running speed were not changed from normal conditions, descaling treatment was performed only by neutral salt electrolysis as pretreatment, and final treatment was nitric acid electrolysis for steel types SUS410 and SUS430. As the treatment, SUS304 was subjected to a nitric hydrofluoric acid immersion treatment. In carrying out the method of the present invention, the conditions of the nitric acid electrolysis treatment and the nitric hydrofluoric acid dipping treatment as the final treatment were changed so as to reduce the action thereof. From the results of Tables 2 to 4, it can be seen that the descaling treatment is facilitated by carrying out the method of the present invention.

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

[Table 4]

[0019]

INDUSTRIAL APPLICABILITY As described above in detail, the method and apparatus for neutral salt electrolytic treatment for descaling of stainless steel strip according to the present invention have various effects as listed below. The target value is great. (1) In order to avoid the electrodeposition of molten metal in the bath at the cathode part of the stainless steel strip and to maintain the set amount of electrolytic electricity that can give the descaling action that is the original neutral salt electrolysis treatment, Since it is possible to automatically change the electrolysis current value according to the strip width and strip passing speed, it is possible to improve the quality and the electrolysis power especially for thick stainless steel strips, where electrolysis time tends to be long. The cost can be reduced easily. (2) Since the adjustment of the electrolytic current value is automated, the labor required for the adjustment is reduced. (3) Since the electrodeposition of the dissolved metal in the bath at the cathode part of the stainless steel strip in the neutral salt electrolytic treatment apparatus was reduced,
Subsequent descaling by nitric hydrofluoric acid treatment became easy and the amount of acid used was reduced. As a result, the manufacturing cost and the waste acid treatment cost were reduced. This reduction in the amount of waste acid generated is preferable in terms of the global environment. (4) It could be easily implemented without major modification to the conventional neutral salt electrolytic treatment apparatus.

[Brief description of drawings]

FIG. 1 is a schematic explanatory diagram illustrating the configuration of a neutral salt electrolytic treatment apparatus for descaling a stainless steel strip according to the present invention.

[Explanation of symbols]

 S Stainless steel strip 1 Bridle roll 2 Plate speed detection device 3 Plate width detection device 4 Neutral salt electrolysis tank 5 Electrolysis bath 6a Cathode electrode plate 6b Anode electrode plate 7 DC power supply device 8 Immersion roll 9 Computing unit 10 Nitrofluoric acid mixed acid Bath or nitric acid electrolysis bath

Claims (3)

[Claims] 1. A strip width and a strip speed of a stainless steel strip to be stripped in a neutral salt electrolytic treatment step for descaling a stainless steel strip are respectively detected, and both detected values and the stainless strip are electrolyzed. A neutral salt electrolytic treatment method for stainless steel strip descaling, which comprises automatically adjusting an electrolytic current value using a calculator that calculates an electrolytic current value from a target amount of electricity for processing. 2. The neutral salt electrolytic treatment method for descaling a stainless steel strip according to claim 1, wherein the target amount of electricity is 50 to 115 C / dm ² . 3. The neutralized salt electrolytic cell is introduced by introducing the annealed stainless steel strip (S) into the electrolytic bath (5) of the neutral salt electrolytic cell (4).
In a neutral salt electrolysis treatment device for sequentially performing indirect electrolysis of cathode electrolysis and anodic electrolysis with an anode electrode plate (6a) and a cathode electrode plate (6b) in (4), before passing through the neutral salt electrolysis tank (4). A plate width detection device (3) for detecting the plate width of the stainless steel strip (S) and a plate passing speed detection device for detecting the plate passing speed of the stainless steel strip (S)
(2) is provided, and the detection value of the strip width detected by the strip width detection device (3) and the detection value of the strip passing speed detected by the strip passing speed detection device (2) are input. The electrolytic current value supplied to the anode electrode plate (6a) and the cathode electrode plate (6b) by the calculator (9) having the variable electric quantity setting function can be maintained at the electrolytic current value calculated by the following formula. DC power supply to
A neutral salt electrolytic treatment apparatus for descaling a stainless steel strip, which is characterized in that the current of (7) is controlled. L = (Q × W × P × 100) / (P × 60 / V) where L: electrolytic current value (A) Q: quantity of electricity (C / dm ² = A · sec / dm ² ) W: strip material Plate width (m) P: Cathode electrode length (m) V: Plate material speed (m / min)