JPH0241599B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to an electrolytic pickling method for stainless steel cold rolled and annealed steel strips, and more particularly to an electrolytic pickling method for removing surface scale after annealing of stainless steel cold rolled steel strips in a short time. Used in the field of manufacturing cold rolled steel strips. [Prior Art] A stainless steel strip after cold rolling is annealed mainly for the purpose of removing rolling strain and improving various properties. Conventional annealing methods include so-called bright annealing in a reducing atmosphere such as a hydrogen atmosphere or (hydrogen + nitrogen) atmosphere, and continuous annealing in an oxidizing combustion gas atmosphere such as propane gas or coke oven gas. However, in recent years, continuous annealing lines (hereinafter referred to as CAL) for ordinary steel have come into use to increase productivity. Among these annealing methods, when using combustion gas and when using CAL, an oxide film, that is, scale, is formed on the surface because the annealing is performed in an oxidizing atmosphere. These scales are especially suitable for CAL
The scale caused by molding is generally called temper color and is very thin, but if the oxide film remains on it, it has poor corrosion resistance and mold wear during press work etc. becomes a serious problem, so it must be removed. be. For this purpose, pickling treatment or grinding with a belt grinder is usually performed. Although the scale is thin, it is very dense and strong and difficult to remove, making the pickling process complicated. Conventionally, continuous descaling methods for cold-rolled stainless steel after continuous annealing include salt treatment by immersion in molten alkali salts mainly composed of NaOH and _NaCO3 ;
Alternatively, Cr ⁺³ can be removed by pretreatment such as electrolytic treatment in a neutral salt solution such as Na ₂ SO ₄ or Na ₂ NO ₃ .
After changing to Cr ⁺⁶ , it is immersed in an aqueous solution of sulfuric acid, nitric-fluoric acid (nitric acid + hydrofluoric acid), nitric acid, or electrolytically treated in an aqueous sulfuric acid solution or an aqueous nitric acid solution. It had been. Unexamined Japanese Patent Publication 1983-
59900, Japanese Patent Publication No. 38-12162, etc., these immersion or electrolytic methods are generally used depending on the difficulty of descaling caused by differences in steel type and annealing conditions. However, even in these cases, it still takes a long time to completely descale, which is a cause of inhibiting the production efficiency of cold-rolled stainless steel strips. In order to increase the line speed while maintaining descaling performance, it is necessary to increase the length of various immersion tanks and electrolytic tanks in order to ensure the immersion time in molten alkali salts and various acids, electrolysis time, etc. as described above. This requires a large amount of capital investment and a large indoor space to install it, which in reality has been dealt with by low threading speeds. In addition, since many types of salts and acids are used, controlling their concentrations is a heavy burden. As described above, both conventional methods of descaling cold-rolled and annealed stainless steel strips have had problems. [Problems to be Solved by the Invention] The purpose of the present invention is to solve the above-mentioned problems of the prior art in descaling stainless steel cold-rolled and annealed steel strips, that is, low productivity and complicated process control. The purpose of the present invention is to provide a highly efficient descaling method using a liquid. [Means and effects for solving the problems] The gist of the present invention is as follows.
That is, in an electrolytic pickling method for cold-rolled stainless steel and annealed steel strip, which is electrolytically treated using nitric acid as an electrolyte,
This is an electrolytic pickling method for cold-rolled and annealed stainless steel strip, characterized in that an aqueous solution containing a fluorine compound added to nitric acid is used as the electrolytic solution. Generally, stainless steel is subjected to passivation treatment with nitric acid in the final step of pickling, in order to impart high corrosion resistance as a material function. This passivation treatment is achieved by immersion in nitric acid or electrolysis with nitric acid, and this treatment is an essential step in ensuring the functionality of stainless steel. In order to passivate stainless steel, immersion in oxidizing acid is effective, and nitric acid is used for practicality on an industrial scale. As an electrolyte, the larger the dissociation constant in nitric acid, the more effective it is, but nitrates have a small descaling effect, and chlorides cannot achieve uniform descaling. In order to perform high-efficiency descaling with a single solution, the present inventors conducted extensive experiments on electrolytic conditions using nitric acid. Since we found that the addition of a fluorine compound is effective, we conducted further research based on this knowledge and completed the present invention. When electrolytically pickling cold-rolled stainless steel and annealed steel strips using nitric acid as the electrolyte, the current density and electrolysis time may vary slightly depending on the annealing conditions of the cold-rolled stainless steel sheets, but it is possible to pickle them in a hydrogen or (hydrogen + nitrogen) atmosphere. It has been found that plates annealed in a low oxidizing atmosphere and plates annealed in an oxidizing atmosphere can be descaled in a relatively short time and reliably when an aqueous nitric acid solution containing a fluorine compound according to the present invention is used as an electrolyte. did. There is no difference in descaling performance no matter what kind of fluorine compound is added, but from the viewpoint of industrial use, KF, NH ₄ F, NH ₄ HF ₂ ,
It is preferably selected from (NH ₄ ) ₃ FeF ₆ and HF. In this case, the amount of fluorine compound added is 0.3 in terms of fluorine equivalent.
-5% range. The reason for this is that even if the amount of fluorine compound added is very small, the descaling efficiency is improved by increasing the electrolytic current density and the temperature of the electrolytic bath, but when the amount is less than 0.3%, the current density is 50 A/d
m ² or more, even if the temperature is raised to 60°C or more, descaling is not only difficult, but also a peroxidation phenomenon occurs due to the increase in current density and temperature, and defects such as yellowing of the surface appear, so the lower limit is set at 0.3%. And so. Furthermore, if the amount added exceeds 5%, even if the temperature of the pickling bath is room temperature, the reaction heat will cause overpickling and roughen the surface of the stainless steel, so the upper limit is limited to 5% in terms of fluorine equivalent. Should. However, if the nitric acid concentration in this case is less than 5%, the effect of adding a fluorine compound is small, and if it exceeds 20%, descaling reaction and NOx production were observed, so the upper limit was set at 20%, and the range of 5 to 20% was set. Limited. [Example] Various stainless steel strips after cold rolling were continuously annealed in the actual continuous annealing line CAL or annealing pickling line (hereinafter referred to as APL) as test materials, and the annealing atmosphere was After changing the annealing conditions such as temperature and generating scale on the surface of each sample material, electrolytic pickling was performed using the electrolyte composition according to the requirements of the present invention and requirements outside the present invention, and the scale removal status was evaluated. A comparative test was conducted to compare. As a sample material
Three types of steel, JIS standard SUH409, SUS430 and SUS304, were used, and the pickling line was bypassed in APL. The annealing conditions and the state of scale generated for each sample material are shown in Table 1.

[Table] As is clear from Table 1, the scale of SUH409 and SUS430 continuously annealed by CAL is
The color is pale yellow, or in the latter case, purple, a so-called temper color. On the other hand, continuous annealing with APL
The scales of SUH409, SUS430, and SUS304 are shiny deep blue in the former, and dull yellowish brown in the latter.The thickness of these scales is compared to the scale of the steel plate annealed by CAL mentioned above. Quite thick. The electrolytic solution composition, temperature, current density, and experimental results when each test material with different scale states shown in Table 1 were electrolytically pickled under the requirements of the present invention and conditions outside the present invention are shown in the second, third, and third tables, respectively. As shown in Tables 4, 5, and 6. The invention examples and comparative examples in Tables 2 to 6 are all based on model pickling tanks. Furthermore, the electrolytic solution compositions shown in these tables are all weight %, and F equivalent % is also written. Furthermore, the experimental results show the descaling determination for different electrolysis times as described. However, the judgment is the result of visual judgment determined by comparison with a completely descaled representative sample, and the symbols in each table have the meanings shown in Table 7.

[Table] As is clear from the experimental results of Invention Examples 1 to 5 and Comparative Examples 1 to 5 shown in Tables 2 to 6, 5 to 20
The present invention example using an electrolytic solution in which 0.3 to 5% fluorine compound is added in F equivalent to a nitric acid aqueous solution has a higher desorption rate than the comparative example using an aqueous solution of nitric acid alone or an electrolytic solution in which chloride or nitrate is added to nitric acid. This shows that the scale performance is clearly superior. It should be noted here that the descaling time of SUS430 and SUS304 annealed in an oxidizing atmosphere by propane gas combustion shown in Tables 5 and 6 is comparable to that of conventional methods such as JP-A-59-59900. This means that it has been significantly shortened. Moreover, these steel strips are cold-rolled stainless steel strips annealed on the same line as the experimental material shown in JP-A-59-59900, and have sufficiently grown scales. It is. That is, JP-A-59-59900
For example, in the case of SUS430, the conventional method is to perform a pretreatment of immersion in a molten alkali salt, then immerse it in an aqueous sulfuric acid solution, and then electrolyze it in an aqueous nitric acid solution.
Although descaling time requires at least 45 seconds, descaling can be achieved in 30 seconds or less using one-liquid electrolysis according to the method of the present invention. In the case of SUS304, JP-A-59
The conventional method using -59900 required a four-stage pickling process, and the entire process took about 30 seconds, but the method of the present invention enabled descaling in 20 seconds.

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〔Effect of the invention〕

Conventional descaling methods in the continuous annealing or annealing and pickling process of cold-rolled stainless steel strip using CAL or APL have required long-term treatment with multiple pickling processes, but the electrolytic pickling method according to the present invention By abolishing the conventional method of using a simple aqueous solution of nitric acid as an electrolyte and using an aqueous solution of nitric acid with a fluorine compound added as the electrolyte, we were able to achieve the following great effects. (b) Since descaling with one liquid became possible,
The process was greatly simplified. (b) Uniform and complete descaling became possible. (c) Descaling time can be reduced compared to conventional methods,
High-speed threading is now possible for CAL materials with relatively thin scales, making it possible to improve productivity. (d) As a result of (a), (b), and (c), descaling costs can be significantly reduced. The present invention proposes electrolyzing using an aqueous solution of nitric acid with a fluorine compound added to it during passivation treatment with nitric acid in the final pickling process of stainless steel cold-rolled and annealed steel strips. It is obvious that it can be applied not only in combination with conventional pickling methods, but also in combination with conventional pickling methods. The requirements for descaling by electrolysis are as follows:
Although electrolyte temperature, current density, electrolysis time, etc. are of course related, these requirements do not need to be particularly limited because they depend on the scale thickness caused by differences in steel type and annealing conditions. The descaling method according to the present invention is applicable to all types of stainless steel.

Claims (1)

[Scope of Claims] 1. A method for electrolytic pickling of cold-rolled or annealed stainless steel strip in which nitric acid is used as an electrolytic solution for electrolytic treatment, characterized in that an aqueous solution of nitric acid and a fluorine compound added is used as the electrolytic solution. Electrolytic pickling method for rolled and annealed steel strip. 2 The fluorine compounds include KF, NH ₄ F, NH ₄ HF ₂ ,
The method for electrolytic pickling of cold-rolled and annealed stainless steel strip according to claim 1, which is any one selected from (NH ₄ ) ₃ FeF ₆ and HF. 3. Electrolytic acid for cold rolled and annealed stainless steel strip according to claim 1 or 2, wherein the nitric acid concentration is 5 to 20% and the fluorine compound concentration is 0.3 to 5% per fluorine ion. Washing method.