JPH0196398A - Method for electrolytically descaling cold rolled stainless steel strip with neutral salt - Google Patents

Abstract

PURPOSE:To effectively and continuously remove oxide scale on the surface of a steel sheet by successively causing cathode reaction and anode reaction on the surface of the steel sheet in an aq. neutral salt soln. having a specified concn. of sexivalent Cr ions and a specified pH. CONSTITUTION:A cold rolled stainless steel strip 3 is continuously passed through an aq. neutral salt soln. 4 having <=8g/l concn. of sexivalent Cr ions and 2-6pH in an electrolytic cell 5. Cathode reaction is first caused once or plural times on the surface of the strip 3 with only anodes 1 and then anode reaction is caused once or plural times with only cathodes 2. The efficiency of descaling of the strip 3 by indirect electrolysis is increased, electric energy required is reduced and the electrolysis time is shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an efficient descaling method for continuously removing oxidized scale from the surface of a cold-rolled stainless steel strip.

[Conventional technology]

Generally, when a cold rolled stainless steel strip is subjected to heat treatment such as annealing or hardening in an oxidizing atmosphere, oxide scale is formed on the surface of the copper strip, so a descaling treatment is performed to remove the oxide scale.

Pickling using l&acid, hydrochloric acid, nitric-fluoric acid (a mixed acid of nitric acid and hydrofluoric acid), etc. is generally used for descaling treatment, but the oxide scale that forms on cold-rolled stainless steel strips is dense. Because it is so strong, it is difficult to completely descale it. On the other hand, as a pretreatment method to facilitate pickling, molten alkali 1'! 1 immersion treatment (salt treatment) or electrolytic treatment in a neutral salt aqueous solution as shown in Japanese Patent Publication No. 38-12162 has been developed and put into practical use.

[The dark problem that the invention attempts to solve]

Electrolytic treatment in a neutral salt aqueous solution has the advantage that it is easier to obtain a beautiful surface quality compared to liquid treatment, and because the solution is neutral, it provides an excellent working environment. However, in order to obtain the same effect as salt treatment, which is superior in descaling, it requires a large amount of electrical energy for electrolysis, and requires a long electrolysis time, so long electrolysis is required. There are disadvantages such as the need for a tank.

The present invention has been made for the purpose of partially increasing the descaling efficiency, reducing the required electrical energy, and shortening the electrolysis time without sacrificing the advantages of electrolytic treatment in a neutral salt aqueous solution. be.

[Means for solving dark issues]

Generally, as shown in the schematic diagram of FIG. 2, electrolysis in a neutral salt aqueous solution 4 of a cold-rolled stainless steel strip is performed by placing an anode 1 and a cathode 2 in an electrolytic tank 5 with the stainless steel strip 3 sandwiched from above and below.
An indirect electrolysis method is adopted in which the steel plates are arranged in the direction of movement on the 1st floor and a DC voltage is applied between the two poles. When the stainless steel strip passes between the negative electrodes, an anodic reaction occurs on the surface of the copper strip, and when it passes between the positive electrodes, a cathodic reaction occurs on the surface of the copper strip, and descaling occurs as the stainless steel strip undergoes both reactions alternately. Processing is being done.

In the 7-node reaction, Cr and Fe, the main metal elements that make up the oxide scale, are oxidized to hexavalent Cr ions and trivalent Fe ions, respectively, and are eluted into the solution, which progresses descaling. .

On the other hand, the cathode reaction inevitably occurs due to the indirect electrolysis method, but conventionally only a hydrogen gas generation reaction occurs, and a slight oxidation scale removal effect by hydrogen gas bubbles can be expected.

It was thought that it would make little contribution to descaling.

The present inventors focused on the cathode reaction when electrolysis is carried out industrially in a neutral salt aqueous solution, and as a result of detailed investigation and research into the reaction behavior, we found that under the conventional electrolytic treatment conditions, An important discovery was made that the cathode reaction greatly inhibits descaling.

In other words, the neutral salt aqueous solution that is industrially descaled contains metal ions such as Cr and Fe eluted in the anode reaction, and the cathode reaction in such a solution causes a hydrogen gas generation reaction. In addition, a reaction occurs in which metal ions such as Cr and Fe are reduced and deposited on the surface of the copper strip, and scale-like substances (possibly oxides of Cr and Fe or water-use oxides) are attached. I found out.

Therefore, in the conventional method, the descaling reaction in the anode reaction and the precipitation and adhesion reaction of scale-like substances in the cathodic reaction occur alternately, so the electrical energy required for descaling is large and the electrolysis time is also long. It can be said that it was necessary.

It has also been found that most of the reduction and precipitation of metal ions such as Cr and Fe in the cathode reaction is caused by Cr ions.

Therefore, the present inventors researched methods to reduce the descaling inhibition reaction in the cathode reaction, and found that the specific electrolysis order, the limitation of the amount of hexavalent Cr ions in the neutral salt aqueous solution, and the pH value of the neutral salt aqueous solution. It has been found that by combining this with limitation, the descaling inhibition reaction in the cathode reaction can be significantly reduced.

The present invention is constructed based on the above-mentioned newly obtained knowledge.

That is, when descaling a cold-rolled stainless steel strip by indirect electrolysis in a neutral salt aqueous solution, first a cathodic reaction is caused on the surface of the copper strip one or more times. The electrolytic sequence is such that only the nod reaction occurs once or multiple times, and hexavalent Cr in the neutral salt aqueous solution is used.
The ion concentration is 8g/state or less, and the pH &
This is a method for descaling a cold-rolled stainless steel strip, characterized in that j is 2 or more and 76 or less.

[Effect]

What is the reason for the electrolysis order as in the present invention?

■ If the cathode reaction occurs in a state where the surface is covered with a dense oxide scale, metal ions mainly consisting of Cr are reduced, and scale-like substances are precipitated and attached on the oxide scale, the primary anode It is relatively easily removed along with oxide scale by reaction.

■ After undergoing the anode reaction, a considerable amount of the oxide scale has been removed, and when the cathode reaction is performed with a part of the base iron under the oxide scale exposed, if scale-like substances are precipitated on the surface of the base metal, the next anode is applied. Reaction makes removal very difficult.

This is based on new knowledge.

In other words, an important point of the present invention is not to allow the cathode reaction to occur after the anode reaction.
An example of an electrode arrangement realizing the electrolysis sequence of the present invention is schematically shown in FIG.

The dimensions and number of the negative electrode 2 and the positive electrode 1 are not particularly limited in the present invention. They should be set appropriately by considering the production scale of the descaling equipment, target steel type, strip speed, electrolytic circuit resistance, etc. In short, for the stainless steel strip 3 with oxidized scale, It is important to first cause a cathodic reaction, then to cause an anodic reaction, and not to cause a cathodic reaction after the anodic reaction.

The reason for limiting the hexavalent Cr ion concentration in the neutral salt aqueous solution 4 to 8 g/l or less is that if it exceeds 8 g/l, even if the electrolysis sequence of the present invention is adopted, the precipitation of scale-like substances in the cathode reaction will be excessive. This is because, as a result of this and the suppression of the descaling reaction in the anode reaction, the descaling ability is significantly reduced as shown in FIG.

FIG. 3 shows the conditions of N'o and 7 in Table 2 of Examples described below.
This figure shows the descaling situation when the hexavalent Cr ion concentration is changed. Vertical axis descaling index 1.2, 3
．． 4 is an index representing large remaining scale, medium remaining scale, small remaining scale, and complete descaling, respectively.

Although it is better to have a low concentration of metal ions other than Cr in a neutral salt aqueous solution, such as Fe, Ni, and Mn ions, they have little dissolution history, so the influence is minor compared to hexavalent Cr ions, so it is especially limited. I don't.

The reason why the upper limit of the pH value of the neutral salt aqueous solution was set at 6 is that when the pH value exceeds 6, the descaling ability decreases significantly as shown in Figure 4 for the same reason as when the hexavalent Cr6+ ion concentration exceeds 8 g/liter. This is to do so.

FIG. 4 shows the descaling situation when the pH value of the solution was changed under the conditions of N007 in Table 2 of Examples described below. The lower limit of the pH value is set to 2 because if it is less than 2, the smoothness of the surface of ferritic and martensitic stainless steel in particular deteriorates and the beauty of the surface after descaling is lost.

There is no limit to the method of adjusting the concentration of hexavalent Cr ions in the solution to below the upper limit of the present invention, but for example, by discharging a part of the solution in which the amount of hexavalent Cr ions has increased due to an increase in the amount of descaling. A method of introducing a new solution, a method of introducing a reducing agent to reduce the amount to trivalent Cr ions, and removing a precipitate can be applied. In addition, in conventional electrolysis in a neutral salt aqueous solution, the influence of the amount of hexavalent Cr ions in the solution was not known, so there is no known value, but according to measurements by the present inventors, in a steady state It showed a value of approximately 1 Og/1 or more.

Further, the pH value of the solution is preferably adjusted using sulfuric acid or sodium hydroxide.

In electrolytic treatment in a neutral salt aqueous solution, the above-mentioned electrolysis order, concentration of hexavalent Cr ions in the aqueous solution, and limitation of the pH value of the solution greatly reduce the descaling inhibitory effect in the cathodic reaction and reduce the descaling effect in the anode reaction. It has the effect of promoting scale reactions, greatly increasing the ability of descaling enemies.

Regarding other conditions such as the type of neutral salt, the concentration and temperature of the neutral salt aqueous solution, and the current density, conventional conditions are also applied to the present invention.

As the neutral salt, Na salts and K salts such as sulfuric acid, nitric acid, and hydrochloric acid can be used alone or in combination, but sodium sulfate is suitable from the viewpoint of economy and surface finish. The concentration and temperature of the neutral aqueous solution are 100 to 300 g, respectively.
/ 70790°C is appropriate.

Appropriate current densities are 2 to 15 A/drn' for both the anode reaction current density and the cathode reaction current density.

In the case of stainless steel, which has relatively good descaling properties, it is possible to descale it by electrolysis alone in a neutral salt aqueous solution, but if this is insufficient, it can be completely removed by subsequent pickling treatment. It becomes possible to descale.

Pickling after electrolytic treatment in a neutral salt aqueous solution is carried out in the same way as conventional methods. For ferritic and martensitic stainless steels, nitric acid immersion or nitric acid electrolysis is mainly applied, and for austenitic stainless steels, Nitrofluoric acid immersion is mainly applied.

〔Example〕

5U plate with a thickness of 0.8 mm annealed in an oxidizing atmosphere
For S410, 5US430, and SUS 304, electrolysis in a neutral salt aqueous solution under various conditions and subsequent pickling treatment were performed using a descaling experimental device, and the descaling status was observed.

Regarding 5US410, a case where the pickling treatment was omitted was also investigated.

The order of electrolysis in a neutral salt aqueous solution is the second order for the conventional example.
The electrode arrangement shown in the figure is used, and the first
Each experiment was carried out assuming the electrode arrangement shown in the figure. All cathode reaction current densities are 12A/drr! ', and the anode reaction current density was all set to 6 A/drn'-constant.

As the neutral salt aqueous solution, an aqueous solution of Na2SO4 is used,
The concentration of neutral salt is approximately 200 g/1. Solution temperature is 85°C
The pH was kept constant and H2SO4 and NaOH were used to adjust the pH.

Example-1 Table 1 shows the results obtained for SUS410.

In the conventional No. 1 using the electrode arrangement shown in Figure 2, the total electrolysis time was 36 seconds, the amount of electricity was 144 coulombs/d, and descaling was achieved through neutral salt aqueous electrolysis and nitric acid attack treatment.Φ In contrast, N00 of the example in which the hexavalent Cr ion concentration and the pH value of the solution are within the range of IJ1 of the present invention in the electrolysis sequence of the present invention.
3 has a total electrolysis time of 12 seconds and an amount of electricity of 48 coulombs/dm.
Descaling was possible using only neutral salt aqueous electrolysis without the need for pickling.

Further, in No. 2 (comparative example), which was treated by the above-mentioned conventional method under the same total electrolysis time and electricity amount conditions as the method of the present invention, a considerable amount of scale remained.

Example-2 Table 2 shows the results obtained for SUS430.

Conventional example No. 4 could be descaled by electrolysis in a neutral salt aqueous solution with a total electrolysis time of 43.2 seconds, an amount of electricity of 172.8 coulombs, and a nitric acid immersion treatment.

On the other hand, in Examples No. 6, No. 7, and No. 008, in which the concentration of hexavalent Cr ions and the pH value of the solution were variously changed within the range of the present invention in the electrolysis order of Examples, the total electrolysis time was 16.
Descaling was possible within 8 seconds with neutral salt aqueous electrolysis and nitric acid immersion treatment within 67.2 corons/dm' of electricity.

On the other hand, in Comparative Example No. 065, which was processed by the conventional method under the same total electrolysis time and electricity amount conditions as Example No. 016, a considerable amount of scale remained.

Comparative example No. 0 in which the pH value of the solution was below the lower limit of the present invention.
In No. 99, no scale remained, but the surface gloss after descaling was inferior to that of the method of the present invention and the conventional method. Example 3 The results obtained for SUS 304 are shown in Table 3.

Conventional example No. 10 has a total electrolysis time of 28.8 seconds and an amount of electricity of 11.5-2 coulombs/drr? Descaling was possible with neutral salt aqueous electrolysis and nitric-fluoric acid immersion.

On the other hand, Example No. 12 in which the concentration of hexavalent Cr ions and the pH value of the solution were within the range of the present invention in the electrolysis sequence of the present invention.
The total electrolysis time is 12 seconds, and the amount of electricity is 48 coulombs/dm'.
Descaling was possible with neutral salt aqueous electrolysis and nitric-fluoric acid immersion.

Further, in Comparative Example No. 11, which was treated by the conventional method under the same conditions of the total electrolysis time and amount of electricity as those of the present invention, scale remained.

〔Effect of the invention〕

By applying the descaling method of the stainless steel cold rolled steel strip of the present invention, the descaling process (greater 'J8Ci shrinkage between H + j) is more economical than the conventional method due to a significant reduction in productivity due to a significant reduction in china clay and electrical energy consumption. It becomes possible to obtain the effect.

Further, the descaling method of the present invention is based on ferrite.

It also has the versatility of being applicable to any of martensitic, austenitic, and two-phase stainless steels.

[Brief explanation of the drawing]

Fig. 1 is a diagram schematically showing an example of an apparatus embodying the neutral salt aqueous solution electrolysis method of the present invention, and Fig. 2 is a diagram schematically showing a typical example of a conventional neutral salt aqueous solution electrolysis treatment apparatus. Figure 3 is a graph showing the influence of the hexavalent Cr ion concentration in a neutral salt aqueous solution when descaling is performed using the electrolysis sequence of the present invention, and Figure 4 is a graph showing the influence of the concentration of hexavalent Cr ions in a neutral salt aqueous solution. It is a graph showing the influence of the pH value of a salt aqueous solution. l...Positive electrode 2...Cathode electrode 3...Stainless steel cold rolled steel strip 4...Neutral salt aqueous solution 5...Electrolytic cell

Claims (1)

[Claims] 1. When descaling a cold rolled stainless steel strip by indirect electrolysis in a neutral salt aqueous solution, the reaction on the surface of the steel strip during the electrolysis is first terminated by a cathodic reaction, then an anode reaction, and the neutral salt aqueous solution is A method for descaling a cold-rolled stainless steel strip, characterized in that the concentration of hexavalent Cr ions is 8 g/l or less and the pH value is 2 to 6.