JPH0534439B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an efficient dethrailing method for continuously removing oxidized scale from the surface of a cold-rolled stainless steel strip. [Prior art] Generally, when stainless steel cold-rolled steel strip is subjected to heat treatment such as annealing or quenching in an oxidizing atmosphere, oxide scale is formed on the surface of the steel strip, so descaling treatment is required to remove the oxide scale. It will be done. Pickling using sulfuric acid, hydrochloric acid, nitric-fluoric acid (a mixed acid of nitric acid and hydrofluoric acid), etc. is generally used for descaling, but the oxide scale that forms on cold-rolled stainless steel strips is dense and strong. Therefore, it is difficult to completely descale. Therefore, as a pretreatment method to facilitate pickling, immersion treatment in molten alkali salt (salt treatment) or
Electrolytic treatment in a neutral salt aqueous solution as shown in 12162 has been developed and put into practical use. [Problems to be solved by the invention] Electrolytic treatment in a neutral salt aqueous solution has the advantages of being easier to obtain a beautiful surface texture than salt treatment, and because the solution is neutral, the working environment is excellent. be. However, in order to obtain the salt treatment and its effect, which has excellent descaling ability, electrolysis requires a large amount of energy and a long electrolysis time, which requires a long electrolytic bath. There are drawbacks. The present invention aims to increase the descaling efficiency, reduce the required electrical energy, and shorten the electrolysis time without sacrificing the advantages of electrolytic treatment in a neutral salt aqueous solution. [Means for solving the problem] Generally, when descaling a steel plate, it is necessary to increase the threading speed in order to improve productivity. , it is necessary to perform the anodic reactions alternately. From this point of view, normally, as shown in the schematic diagram in FIG. An indirect electrolysis method is adopted in which the steel strips are arranged in the traveling direction of the steel strips 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 steel strip, and when it passes between the positive electrodes, a cathodic reaction occurs on the surface of the steel strip, and descaling occurs while undergoing both reactions alternately. Processing is being done. In the anodic reaction, Cr and Fe, which are the main metal elements constituting the oxide scale, are oxidized to hexavalent Cr ions and trivalent Fe ions, respectively, and are eluted into the solution, thereby progressing descaling. On the other hand, the cathode reaction inevitably occurs due to the indirect electrolysis method, but conventionally only the hydrogen gas generation reaction occurs, and although hydrogen gas bubbles can be expected to have a slight oxidized scale removal effect, it hardly contributes to descaling. It was thought that it would not. 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, in the neutral salt aqueous solution that is being industrially descaled, there are
Contains metal ions such as Cr and Fe, and in the cathode reaction in such a solution, in addition to hydrogen gas generation, metal ions such as Cr and Fe are reduced.
Precipitated on the steel strip surface, scale-like substances (Cr, Fe
It was discovered that a reaction occurs in which oxides such as oxides or hydrated oxides are attached. It has been found that such a reaction opposite to descaling in the cathode reaction is remarkable in the cathode reaction after undergoing the anodic reaction, and becomes remarkable when the Cr ion concentration exceeds a certain value. Therefore, in the conventional method, the descaling reaction in the anode reaction and the deposition reaction of scale-like substances in the cathodic reaction occur alternately, which requires a large amount of electrical energy and requires a long electrolysis time. I can say that I needed it. It was also revealed that most of the reduction and precipitation of metal ions such as Cr and Fe during the cathode reaction was caused by Cr ions. Based on the above-mentioned new knowledge, the present inventors conducted further studies and found that while conventionally the solution where the steel strip surface undergoes the cathodic reaction and the solution where the anode reaction occurs are the same solution, The hexavalent Cr ion concentration in the solution and the solution
It has been found that descaling ability can be greatly improved by limiting the PH value to a specific range. That is, in the present invention, when descaling a cold-rolled stainless steel strip by indirect electrolysis in a neutral salt aqueous solution, the aqueous salt aqueous solution (cathode reaction liquid) at a location where a cathode reaction is caused on the surface of the steel strip is prepared as follows. A stainless steel cooling method characterized in that the solution satisfies the condition A below, and the aqueous salt solution (anode reaction liquid) at the location where the anode reaction occurs on the surface of the steel strip satisfies the condition B below, and is decomposed. This is a method for descaling rolled steel strip. A: Hexavalent Cr ion concentration in the solution 0.1g/or less,
PH value of solution 1 or more and 6 or less B: Hexavalent Cr ion concentration in the solution 10g/or less,
PH value of the solution is 2 or more and 6 or less [Function] Figure 1 schematically shows an example of a method for separating the solution of the positive electrode 1 where the steel strip causes a cathodic reaction and the solution of the negative electrode 2 where the anode reaction occurs. . The reaction in which scale-like substances adhere to the steel strip 3 while the cathode reaction is occurring is caused by the hexavalent substance in the solution.
The pH value of the solution is 6 when the Cr ion concentration is less than 0.1g/
In the following cases, it will be very slight and will be easily removed in the next anode reaction as shown in Figures 3 and 4, but if these conditions are exceeded, a large amount of scale-like substances will adhere and the next anode will be removed. Removal by reaction becomes difficult. Figure 3 shows the descaling situation when the hexavalent Cr ion concentration of the cathode reaction solution was changed under the conditions of Table 2 No. 7 of Examples described below, and the descaling index was 1, 2, 3, and 4 represent large scale remaining, medium scale remaining, small scale remaining, and completely descaled, respectively. Further, FIG. 4 similarly shows the descaling situation when the pH of the cathode reaction liquid was changed under the conditions shown in Table 2, No. 7 of Examples. If the concentration of hexavalent Cr ions in the solution at a location where an anodic reaction occurs on the steel strip is 10 g/ or more, or if the PH value of the solution exceeds 6,
As shown in FIG. 6, the descaling reaction in the anode reaction is suppressed, and the descaling ability is significantly reduced. Figure 5 shows the descaling situation when the concentration of hexavalent Cr ions in the anode reaction solution was changed under the conditions No. 7 in Table 2 of Examples described later. This figure also shows the descaling situation when the PH value of the anode reaction solution was changed under the conditions shown in Example Table 2 No. 7. In addition, if the PH value of the cathode reaction liquid in the steel strip becomes less than 1, uniform descaling becomes difficult.
If the pH value of the anode reaction solution is 2 or less, the beauty of the surface after descaling will be impaired. The reason for dividing the anode reaction liquid and the cathode reaction liquid is that, as mentioned above, the conditions for each solution are different in order to significantly improve the descaling ability. This is because the permissible range becomes extremely narrow, with a valence Cr ion concentration of 0.1 g/or less and a pH value of 2 to 6. In addition, there is no limit to the method for adjusting the concentration of hexavalent Cr ions in the solution to below the upper limit of the present invention.
For example, due to the increase in the amount of descaling, hexavalent Cr
There are two methods: draining a part of the solution with a high ion content and adding a new solution, or adding a reducing agent to reduce the trivalent content.
Methods such as reducing to Cr ions and removing precipitates 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 The value was approximately 10g/or more. Further, the pH value of the solution is preferably adjusted using sulfuric acid and sodium hydroxide. It is better to have a low concentration of metal ions other than Cr such as Fe, Ni, Mn, etc. in the neutral salt aqueous solution, but since the amount dissolved is small, the effect is minor compared to hexavalent Cr ions, so there is no particular limitation. . 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. Neutral salts include sulfuric acid, acetic acid, and hydrochloric acid.
Na salt and K salt can be used alone or in combination, but
Sodium sulfate is suitable from the viewpoint of economy and surface finish. The appropriate concentration and temperature of the neutral salt aqueous solution are 100 to 300 g/70 to 90°C. The appropriate current density is 2 to 15 A/dm ² 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 simply by electrolysis in a neutral salt aqueous solution, but if this is insufficient, complete descaling can be achieved by subsequent pickling treatment. It becomes possible to do so. Pickling after electrolytic treatment in a neutral salt aqueous solution is carried out in the same manner as conventional methods, i.e., ferritic and martensitic stainless steels are mainly immersed in nitric acid or nitric acid electrolysis, and austenitic stainless steels are mainly immersed in nitric acid and hydrofluoric acid is used. Immersion is applied. [Example] A plate with a thickness of 0.8 mm annealed in an oxidizing atmosphere
SUS410, SUS430, and SUS304 were subjected to electrolysis in a neutral salt aqueous solution under various conditions and subsequent pickling treatment using a descaling device, and the descaling status was observed. Regarding SUS410, we also investigated the case where the pickling treatment was omitted. The order of electrolysis in a neutral salt aqueous solution was assumed to be the electrode arrangement shown in FIG. 2 for the conventional method, and the electrode arrangement shown in FIG. 1 for the invention method and comparative method. All cathode reaction current densities are 12A/dm ² .
The anode reaction current density was kept constant at 6 A/dm ² in all cases. An aqueous solution of Na ₂ SO ₄ was used as the neutral salt aqueous solution, the concentration of the neutral salt was about 200 g/distance, the solution temperature was kept constant at 85° C., and H ₂ SO ₄ and NaOH were used to adjust the pH. Example 1 Table 1 shows the results obtained for SUS410. The No. 1 conventional method has a total electrolysis time of 36 seconds and a low amount of electricity.
Descaling was possible by electrolysis in a neutral salt aqueous solution at 144 coulombs/dm ² and nitric acid immersion treatment. On the other hand, No. 3, whose hexavalent Cr ion concentration and solution PH value are within the range of the present invention, requires pickling only with aqueous salt solution electrolysis with a total electrolysis time of 12 seconds and an electricity amount of 48 coulombs/ ^dm2 . It was possible to descale without having to do anything. Further, in No. 2 treated by the conventional method under the same total electrolysis time and electricity amount conditions as those of the present invention, a considerable amount of scale remained.

【table】

[Table] Example 2 The results obtained for SUS430 are shown in Table 2. Conventional method No. 4 has a total electrolysis time of 43.2 seconds and an electric
Descaling was possible by electrolysis in a neutral salt aqueous solution at 172.8 coulombs/dm ² and nitric acid immersion treatment. On the other hand, No. 6, No. 7, in which the hexavalent Cr ion concentration and the pH value of the solution were variously changed within the scope of the present invention,
In all cases, No. 8 could be descaled by neutral salt aqueous electrolysis and nitric acid immersion treatment within a total electrolysis time of 16.8 seconds and an electricity amount of 67.2 coulombs/dm ² . On the other hand, in No. 5, which was treated by the conventional method under the same conditions of the total electrolysis time and the amount of electricity as the method No. 8 of the present invention, a considerable amount of scale remained. Comparative method No. 9, in which the pH value of the solution was below the lower limit of the present invention, did not leave scale residue, but the surface gloss after descaling was inferior to that of the present method and the conventional method.

【table】

[Table] Example 3 Table 3 shows the results obtained for SUS304. Conventional method No. 10 has a total electrolysis time of 28.8 seconds and an amount of electricity.
The second scale was possible by electrolysis in a neutral salt aqueous solution at 115.2 coulombs/dm ² and immersion in nitrofluoric acid. On the other hand, No. 12, in which the hexavalent Cr ion concentration and the pH value of the solution were within the range of the present invention, had a total electrolysis time of 12
Descaling was possible by electrolysis in a neutral salt aqueous solution and immersion in nitric-fluoric acid with an electrical charge of 48 coulombs/ ^dm2 . Further, in No. 11, which was treated by the conventional method under the same conditions of total electrolysis time and amount of electricity as those of the present invention, scale remained.

【table】

〔Effect of the invention〕

By applying the method for descaling cold rolled stainless steel strips of the present invention, it is possible to obtain economic effects by significantly improving productivity by significantly shortening the descaling treatment time and by significantly reducing the electrical energy used. becomes possible. In addition, the descaling method of the present invention uses ferrite-based,
It is also versatile in that it can be applied to any mantensite, austenite, or two-phase stainless steel.

[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. Figures 3 and 4 are graphs showing the effects on descaling when changing the hexavalent Cr ion concentration and PH value of the cathode reaction solution.
Figures 5 and 6 show the hexavalence of the anode reaction solution.
It is a graph showing the influence on descaling when changing Cr ion concentration and PH value. 1... Positive electrode, 2... Negative electrode, 3... Cold rolled stainless 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 solution of the electrolysis is divided into a cathode reaction solution and an anode reaction solution, and the cathode reaction that follows the anode reaction The hexavalent Cr ion concentration in the liquid is 0.1 g/or less and the PH value is 1 to 6, and the hexavalent Cr ion concentration in the anode reaction liquid is 10 g/or less and the PH value is 2 to 6. Neutral salt electrolytic descaling method for cold rolled stainless steel strip.