PROCESS FOR DESCALING COLD ROLLED AND ANNEALED STEEL

Abstract: A cold-rolled and annealed stainless steel strip is completely descaled in a short process period by electrolizing the strip with an aqueous solution containing ranges of x (g/l) = 50 to 270 (1) y (g/l) = (-0.01 x + 3.8) to (-0.05x + 21) (2), where x is concentration of nitric acid in g/l and y is concentration of chlorine in g/l.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a process for descaling cold-rolled and annealed stainless steel strip by electrolytic pickling, and more particularly to a continuous process for removal of scales on the surface thereof in a short time. Description of the Prior Art So far, known methods for continuously descaling the cold-rolled stainless steel strip, are for example, as a preparatory step, salt treatment in a molten alkali salt consisting essentially of NaOH and Na₂CO₃, or electrolytic treatment in a solution of neutral salt, such as Na₂SO₄ or NaNO₃, followed by, as main step, the immersion in an aqueous solution of sulfuric acid, nitric acid added hydrogen fluoride, or nitric acid, or electrolytic treatment in an aqueous solution of sulfuric acid or nitric acid. Among above methods disclosed in Japanese Laid-open Patent No.59-59900, it is general that suitable combinations of acids or electrolysis for descaling depend on how difficult it is to remove scale on steel and the difficulties depend on the kind of steel or annealing conditions of the steel to be descaled. Also in these complex processes, however, it takes a long time for the full descaling to be accomplished, and this is still a cause for limited efficiency in production of cold-rolled stainless steel strips. It is very inconvenient to regulate concentrations of many different salts and acids. Salt treatment is inevitably accompanied by a substantial supplement of salt carried away with the descaled steel strips. In an attempt to solve the above-mentioned problems, the inventors made previously a proposal (Japanese Laid-open Patent No. 049197/1987), which has enabled the descaling of steels previously comparatively difficult to do so, such as SUH409 obtained by annealing at 900°C or higher temperature on a CAL (continuous annealing line), under the condition of 5% of H₂ balance by N₂ and dew point of 20°C below zero. It,however, is of the so-called two-step-electrolysis system requiring two electrolytic cells and two electrolytic solutions and particularly consisting of electrolysis with a highly concentrated sulfuric acid solution of 900-1250 g/l, followed by with a nitric acid solution containing HCl, FeCl₃, NaCl, or the like. SUMMARY OF THE INVENTION It is an object of the invention to solve the above-mentioned defects or shortcomings involving the prior art, for instance, low productivity and difficulty of control of the process, and to provide a simplified and inexpensive process for descaling the cold-rolled and annealed stainless steel strip. For achieving the above-mentioned object, the invention has been accomplished on the basis of the discovery, as a result of study from different aspects, that electrolysis with an aqueous solution of nitric acid containing chloride, in which the respective concentrations of them are within certain concentration ranges, can accomplish the descaling SUH409 steel strip (obtainable by annealing at temperatures not lower than 900°C and difficult to be descaled) in a short time without preparatory treatment with salt. In this way, the invention is characterized by electrolysis with an aqueous solution of nitric acid containing at least one chloride selected from the group consisting of HCl, NaCl, and FeCl₃, in which the concentration of nitric acid "x (g/l)" and the concentration of chlorine "y (g/l)" are within the ranges fulfilling the following equations x (g/l) = 50 to 270 y (g/l) = (-0.01 x +3.8) to (-0.05x + 21) The invention is concerned with the composition of electrolytic solutions for descaling the cold-rolled and annealed steel strip, containing nitric acid as a major component and chloride as an additive, wherein respective concentrations of nitric acid and chlorine from chloride contained therein are within the ranges fulfilling the above-mentioned equations (1) and (2). BRIEF DESCRIPTION OF THE DRAWING FIG.1 is a graph illustrating the preferable range of descaling according to the present invention. PREFERRED EMBODIMENT OF THE INVENTION SUH409 steel strip obtained by cold-rolling and annealing at 900°C or higher temperature in the above- mentioned CAL can be descaled at a high speed by electrolysis with an aqueous solution of nitric acid containing chloride, in which their respective concentrations are within the range defined in equations (1) and (2), otherwise highly efficient descaling can not be accomplished at any high concentration. In the process according to the present invention, the concentration of nitric acid is defined to be in the range of from 50 to 270 g/l because the concentration when below 50 g/l or above 270 g/l, may result in a poor descaling ability. Owing to another difficulty encountered in the process that the production of NO x increases with increasing concentration of nitric acid, a preferable upper limit concontration of nitric acid is about 200 g/l. For obtaining an excellent descaled surface at a high efficiency, it is preferred for the lower limit of nitric acid concentration to be 100 g/l. Suitable chlorides to be added to nitric acid solution are HCl, NaCl, and FeCl₃, and these are effective when used solely or in combination of at least two of them. They are preferred to be added within the range defined by the following equation y (g/l) = (-0.01 x +3.8) to (-0.05x + 21) wherein x is concentration of nitric acid and y is concentration of chlorine, because otherwise, whether exceeding or not reaching the range, poor descaling results. For obtaining an excellent descaled surface, it is more preferred to fulfill the following equation y (g/l) = (-0.01 x +3.8) to (-0.02x + 8.8). Suitable temperatures for the solution are within the range of 25°C (room temperature) to 80°C. There is a tendency with higher temperature to be higher in descaling efficiency and on the other hand to increase in production of NO x , and thus the preferable range is between 40 and 65°C. Descaling efficiency becomes higher with increasing current density and thus is accompanied by more advantageous results. Too large electric current densities lead to adverse results, for example, increased production of NO x and rough surface, it therefore is from 5 to 20 A/dm² that is preferable. Example SUH 409 and SUS430 steel strips obtained cold-rolled and annealed on CAL were sampled. The conditions under which these steel strips were annealed and the appearance of the scales are summarized in Table 1. The scale of SUH 409 was light yellow blue, and that of SUS430 was brown-yellow-green. Tables 2 and 3 gives the data involving electrolytic pickling in the process according to the invention (Example groups I and II), and those according to conventional processes (Comparative Example groups IA and IIA). The data include composition and temperature of electrolytic solution, electric current density, electrolysis time, and evaluation of descaling effect. In Fig.1, the data in Table 3 are plotted to depict the relation between the tendency of SUS430 steel to be descaled and contents of acids. The data of these steel strips involving electrolysis by conventional techniques including Na₂SO₄ electrolysis method are given in Table 4 (Comparative Example group IB) and Table 5 (Comparative Example group IIB). The results include electrolysis conditions and evaluation of descaling effects, etc. All electrolyses of which conditions are given in Tables 2, 3 (Fig. 1), 4 and 5 were carried out in a model pickling tank. Descaling effects in these Tables were obtained by evaluation with the naked eye in the comparison with a reference sample fully descaled, and indicated using four grade expressed by the symbols: Superior, ○ Good, Δ Incomplete, and x Inferior. As apparent from comparison between the results in Tables 2 (Example group I), and 3 (Example group II of which the results are plotted in Fig.1) obtained in the process according to the invention and those in Tables 4-1 {Comparative Example group IA), 4-2 (Comparative Example group IIA) and 5-1 (Comparative Example group IB] and 5-2 {Comparative Example group II9), the process according to the invention which regulates the concentrations of nitric acid and chlorine from chloride is obviously superior in descaling effect to the conventional technique. In addition the quantity of electricity per surface to be descaled was smaller in the process according to the invention than that by the conventional techniques. For example, when SUH409 was sampled, in the process according to the invention 20 A/dm² x 3.2 = 64 C/dm²; in a Comparative Example IB-47, (a) Na₂SO₄ electrolysis 10 A/dm² x 5 = 50 C/dm² (b) Nitric acid electrolysis 20 A/dm² x 5 = 100 C/dm² Total 150 coul/dm². Moreover this total quantity of electricity by the conventional technique resulted in insufficient descaling effect. Thus the conclusion can be made that the process according to the invention is obviously superior. Features of the present invention reside in the use of aqueous solutions of nitric acid containing chloride as an additive, as an electrolytic solution for descaling simply and at a high efficiency the cold-rolled and annealed steel strip, and in regulating its composition. From the view of these, it is a matter of course that the invention can be practiced either solely or in combination with conventional technique. The present invention can be applied to all types of stainless steels. As apparent from the above-mentioned Examples, advantages of the present invention are as follows: a) One electrolytic solution descaling can be practiced without needing salt-treatment, and this contributes to much simplification of the process. b) Descaling time is possible to be shortened, so that subjects to be descaled can pass at a high speed through electrolytic solution, with improved productivity. c) Reduced quantity of electricity per surface compared with prior art, and d) Substantially-reduced descaling cost resulting from the preceding a), b), c) and d).

Claims: 1. A process for descaling cold-rolled and annealed stainless steel strip comprising electrolizing the strip with an aqueous solution which has concentrations of nitric acid and chlorine (as chloride) within the ranges fulfilling the following equations x (g/l) = 50 to 270 y (g/l) = (-0.01 x + 3.8) to (-0.05x + 21) wherein x is concentration of nitric acid and y is concentration of chlorine. 2. A process for descaling cold-rolled and annealed stainless steel strip as claimed in claim 1 wherein said chlorine source is composed of at least one chloride selected from HCl, NaCl and FeCl₃.

Inventors: I, KANJI RESEARCH LABORATORIES OF, KAWASAKI, TATSUO RESEARCH LABORATORIES OF, YASUHARA, EIKO RESEARCH LABORATORIES OF
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: C25F1
IPC codes: C25F1