Method of descaling stainless steel and apparatus for same

Abstract: A method of descaling stainless steel of the present invention, comprises the step (a) of anode-­ electrolyzing stainless steel having scales produced thereon in an aqueous neutral salt solution, the step (b) of anode-electrolyzing or immersing the stainless steel in an aqueous alkaline solution, these steps (a) and (b) being performed in the order of either (a), (b) or (b), (a); and the step of cathode-electrolyzing the stainless steel treated in the two steps in an aqueous nitric acid solution or immersing it in an aqueous solution of a mixture containing nitric acid and fluoric acid. The descaled stainless steel obtained by this method exhibits excellent glossiness and smoothness.

Description: The present invention relates to the descaling of stainless steel, and particularly to a method and an apparatus for descaling which is suitable for removing at high speed oxide scales occurring in continuous annealing after cold rolling, as well as to descaled stainless steel obtained by the method. Cold-rolled stainless steel strips are generally subjected to annealing heat treatment for the purpose of changing work-hardened layers. Oxide scale occurring on the surface of stainless steel during this treatment cause significant deterioration in its value as a commercial product. Such oxide scales therefore must be removed. Descaling methods involve a method in which stainless steel is electrolyzed in a strong acid such as sulfuric acid and a method in which stainless steel is immersed in a molten alkaline salt. These methods involve problems with respect to the roughness of the surface, a long time required for descaling and difficulty in handling. In order to resolve the problems, there have been proposed a method in which stainless steel is electrolyzed in an aqueous neutral salt solution and then immersed in an aqueous solution of a mixture containing nitric acid and fluoric acid, as disclosed in Japanese Patent Examined Publication No. 38-12162, and a method in which stainless steel is electrolyzed in an aqueous neutral salt solution and then electrolyzed in a solution containing nitric acid ions, as disclosed in Japanese Patent Examined Publication No. 53-13173. In the above-mentioned prior art, attention is paid only to descaling of the surface of the stainless steel, and insufficient consideration is given to the glossiness and the smoothness of the surface of the stainless steel descaled or to the acceleration of descaling treatment and improvement in a high-temperature operation when effecting immersion in a molten alkaline salt. There are therefore difficult problems with respect to an increase in the speed of descaling and an improvement in the workability. It is an object of the present invention to provide a high-speed descaling method and apparatus which are not accompanied by high-temperature treatment and to provide descaled stainless steel which exhibits glossiness and good smoothness. The present invention provides a method comprising the step (a) of anode-electrolyzing stainless steel having scales thereon in an aqueous neutral salt solution, the step (b) of anode-electrolyzing or immersing stainless steel in an aqueous alkaline solution, the steps (a) and (by being performed in the order of either (a), (b) or (b), (a); and the step of cathode-electrolyzing the stainless steel, which had been treated in both steps, in an aqueous nitric acid solution or immersing it in an aqueous nitric-fluoric acid mixture. In order to perform the method, the present invention also provides an apparatus for continuously descaling the stainless steel as is set out in claim 10, more specifically comprising an electrolytic cell (A) containing an aqueous neutral salt solution and having a plurality of positive and negative electrodes, an electrolytic cell or immersion cell (B) containing an aqueous alkaline solution and having a plurality of positive and negative electrodes, both cells (A) and (B) being provided in the order of (A), (B) or (B), (A), and an electrolytic cell containing an aqueous nitric acid solution or an immersion cell containing an aqueous solution of a mixture of nitric acid and fluoric acid, which electrolytic cell has a plurality of positive and negative electrodes and both of which cells are provided behind the two cells (A) and (B). Regarding austenitic or ferritic stainless steel, there is used AISI 410, 430, 304, 316 or the like. Each of the electrolytic cells of the continuous descaling apparatus has electrodes which are insoluble electrodes disposed opposite to a stainless steel strip continuously moved at high speed. The steel strip can be moved at 60 m/minute or more so that rolling, annealing and descaling can be continuously effected. In the descaling of the present invention, slight oxide scales occurring on the surface of the strip during annealing in a non-oxidizing atmosphere can be removed. The descaling of the invention is therefore particularly suitable for scales formed in an amount of 100 µg/cm² or less. The annealing atmosphere is preferably one of a combustion exhaust gas of a liquified natural gas, butane gas and etc. containing a slight oxidizing gas. Such scales can be substantially removed by practicing the method of the present invention using the apparatus therefor, whereby stainless steel having excellent glossiness and smoothness can be obtained at high speed in an easy treatment. The method of descaling stainless steel of the present invention comprises the steps of removing a chromium oxide layer formed on the outermost surface of the stainless steel, removing a chromium oxide layer containing manganese and iron, and removing iron oxide, these steps being successively performed by using appropriate solutions. The method of descaling stainless steel of the present invention also comprises the steps of dissolving to Cr₂O²⁻ ions a chromium oxide contained in scales formed on the surface of the stainless steel, dissolving to CrO₄² ⁻ ions the chromium oxide contained in the scales and dissolving to Fe²⁺ ions the iron oxide contained in the scales, these steps being successively performed by using appropriate solutions. The method of continuously producing a stainless steel strip at high speed of the present invention, comprises the steps of cold-rolling a descaled stainless steel strip after hot-rolling, annealing the strip by electrical heating in a non-oxidizing atmosphere, anode-electrolyzing the stainless steel strip in an aqueous neutral salt solution after cooling the annealed strip, anode-electrolyzing the strip in an aqueous alkaline solution, and cathode-electrolyzing the strip in an aqueous nitric acid solution, these steps being successively performed while the stainless steel is conveyed at high speed. An apparatus for continuously producing a stainless steel strip of the present invention according to claim 12, comprises a cold-rolling mill for cold-rolling a descaled stainless steel strip after hot-rolling, an annealing furnace for annealing the strip by electrically heating it in a non-oxidizing atmosphere after the cold-rolling, a cooling apparatus for cooling the strip after the annealing, and a descaling apparatus for descaling the strip after the cooling, the descaling apparatus comprising an electrolytic cell containing an aqueous neutral salt solution and having a plurality of positive and negative electrodes, an electrolytic cell containing an aqueous alkaline solution and having a plurality of positive and negative electrodes, and an electrolytic cell which is provided behind the two cells and which cell contains an aqueous nitric acid solution and which ell has a plurality of positive and negative electrodes. The scales occurring on the surface of stainless steel during the annealing treatment are composed of a spinel oxide. Normal annealing treatment (at 800°C or higher) causes an iron-chromium spinel oxide composed of FeCr₂O₃ containing Fe₃O₄. The electrolysis or immersion treatment of stainless steel having scales is effected for removing scales in each of the aqueous neutral salt solution, the aqueous alkaline solution and the aqueous nitric acid solution or aqueous solution of a mixture of nitric and fluoric acid, and has the following function: The electrolysis using the neutral salt has the function of mainly dissolving the chromium contained in the iron-chromium spinel oxide. Namely, the potential-pH diagram of a Cr-H₂O system (M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions (1966), Pergamon Press) shows that chromium is dissolved to Cr₂O₇²⁻ ions by anode polarization at +0.2 V or higher based on a saturated calomel electrode within the pH region of neutral to acid. Normal electrolysis using a neutral salt employs as an electrolytic solution an aqueous Na₂SO₄ solution which has the function of increasing the conductance of the electrolytic solution. Since electrolysis is generally effected within the pH region of neutral to weak acid, the scales are dissolved to Cr₂O₇²⁻ ions. The concentration of the electrolytic solution is preferably 10 to 30% by weight, more preferably 15 to 25% by weight. The temperature of the solution is 60 to 95°C and the current density thereof is not more than 10 A/dm² and preferably 1 to 10 A/dm². The electrolysis in an aqueous alkaline solution such as an aqueous NaOH, LiOH or KOH solution or the like has the function of dissolving the chromium contained in the scales. It is found that, in this case, the electrolytic potential is obtained by anode polarization at a noble potential of a about -0.35 V or more based on a saturated calomel electrode at pH 13 to 14. In other words, the chromium oxide can be effectively removed by being dissolved to CrO₄²⁻ at a potential significantly lower than that of the above-mentioned electrolysis using a neutral salt. The concentration of an alkali metal hydroxide is preferably 30 to 50 wt% for electrolysis and 50 to 70 wt% for immersion. The temperature of the solution is in a range of 60 to 95° and the current density thereof is not more than 10 A/dm² and preferably 1-10 A/dm². The electrolysis in an aqueous nitric acid solution has the function of dissolving iron in the scales. In this case, the electrolysis is effected by using as a cathode the stainless steel. Fe included in the spinel oxide scales contain divalent and trivalent iron. Although the divalent iron dissolves in a normal aqueous acid solution, the dissolution speed of the trivalent iron is very low. However, practical dissolution speed can be obtained by reducing trivalent iron to divalent iron. The cathode electrolysis effected in an aqueous nitric acid solution supplies electrons to the stainless steel so as to reduce trivalent iron to divalent iron and, at the same time, remove iron as Fe²⁺ ions by dissolving it in nitric acid, as shown by the following reaction formula: Fe³⁺ (oxide) + e⁻ → Fe²⁺ (ion) The temperature of the solution is 40 to 80°C, and the current density thereof is not more than 10 A/dm² and preferably 1 to 10 A/dm². The concentration of nitric acid is preferably 5 to 20 wt%, and the concentrations of nitric acid and fluoric acid in the mixture thereof are preferably 10 to 20 wt% and 1 to 5 wt%, respectively. The spinel oxide scales occurring on the stainless steel can be removed by the above-described three types of electrolysis with high levels of efficiency and workability and at high speed. In the combination of the three types of electrolysis, even if the order of the electrolysis in an aqueous neutral salt solution and the electrolysis in an aqueous nitric acid solution is changed, the same effect can be obtained. It is effective to perform the electrolysis in an aqueous nitric acid solution in the final step after the chromium oxide, which cannot be easily removed, has been removed. Since the present invention is not accompanied by high-temperature treatment such as conventional treatment with a molten alkaline salt, the workability is significantly improved. The electrolysis in an aqueous neutral salt solution and the electrolysis in an aqueous nitric acid solution involves a problem with respect to its speed of dissolution of the scales which problem is caused by a slightly low degree of efficiency of the electrolysis in the aqueous neutral salt solution. However, this problem can be improved by alkali electrolysis with a high degree of efficiency, resulting in an increase in the speed of removal of scales. In the method of descaling stainless steel of the present invention, when the amount of the scales is small, the anode electrolysis in an aqueous alkaline solution enables the chromium oxide to be dissolved simply by immersing the stainless steel in the aqueous solution, without electrolysis. Even if immersion in an aqueous solution of a mixture of nitric acid and fluoric acid is effected in place of the electrolysis in the aqueous nitric acid solution, the same descaling effect is obtained. The present invention has the effect of rapidly descaling stainless steel without using a molten salt bath at a high temperature, which molten salt bath cannot be easily handled. The present invention also has the effect of obtaining a beautiful stainless steel plate of high quality having good surface conditions which cannot be obtained by conventional electrolysis. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a drawing showing both means and steps in the descaling process in an embodiment of the present invention; and Fig. 2 is a potential-pH drawing of a Cr-H₂O system which is provided for explaining the function of the present invention. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the attached drawings. (Embodiment 1) Fig. 1 shows an apparatus in Embodiment 1 of the method of descaling a stainless steel strip of the present invention. A stainless steel strip 1 which was cold-rolled by a 10-stage cold-rolling mill 15, annealed in a continuous annealing furnace 16 and then cooled by a cooling apparatus 17 and which had scales occurring on its surface is introduced into an electrolytic cell 2 containing an aqueous neutral salt solution through a looper 18. The electrolytic aqueous neutral salt solution cell 2 was filled with an aqueous solution of 20% Na₂SO₄ at pH 6. In the electrolytic cell 2, a positive voltage was applied to the stainless steel strip 1 from a pair of upper and lower positive electrodes 3, and a pair of counter electrodes 3' on both sides of the positive electrodes 3 served as negative electrodes so that a current flowed from the stainless steel strip 1 to the counter electrodes 3' through the aqueous Na₂SO₄ solution. The chromium in the scales was dissolved to Cr₂O₇²⁻ ions with the flow of the current. The stainless steel strip 1 was then placed in a water washing bath 4 so that Na₂SO₄ remaining on the surface thereof was washed away with water. The strip 1 was then introduced into a wringer roll 5 by which the washing water was squeezed out and then into an electrolytic cell 6 containing an aqueous alkaline solution. The electrolytic aqueous alkaline solution cell 6 was filled with an aqueous solution of 40% NaOH, and a positive voltage was applied to the stainless steel strip 1 from a pair of upper and lower positive electrodes 7 so that a current flowed to a pair of upper and lower counter electrodes 7' through the aqueous NaOH solution. At this time, the current flowing caused the chromium oxide in the scales to be dissolved and removed as CrO₄²⁻. The chromium oxide was removed from the surface of the stainless steel strip 1 to leave iron oxides thereon. The stainless steel strip 1 was then introduced into a water-washing bath 8 in which the NaOH remaining on the surface was removed by washing with water, and then into a wringer roll 9 by which the washing water was squeezed out. The stainless steel strip 1 was then introduced into an electrolytic aqueous nitric acid cell 10. The electrolytic aqueous nitric acid cell 10 was filled with an aqueous solution of 10% nitric acid, and current flowed to the stainless steel strip 1 through a pair of upper and lower positive electrodes 11, which were provided on both of the right and left sides of a pair of central counter electrodes 11′ serving as negative electrodes. An insoluble electrode such as a titanium-palladium coated plate, a titanium-platinum coated plate or the like was used as each of the positive and negative electrodes 11 and 11′ for the purpose of preventing the electrode from being consumed by dissolution in the aqueous nitric acid. These electrodes may be provided in a portion of the entire width of the steel strip 1 or over the entire width thereof. In this embodiment, the electrodes did not contact with the steel strip, but the electrodes may be brought into contact with the steel strip. However, the former case is preferable. Since the stainless steel was subjected to cathode electrolysis, Fe(III) contained in the scales was reduced to Fe(II) to be dissolved to e²⁺ in the aqueous solution, as described above. The scales composed of the iron-chromium spinel oxides which occurred on the stainless steel strip are removed by the above-described three types of electrolysis with a high level of efficiency and at a high speed. The stainless steel strip 1 was then introduced into a water-washing bath 12 in which the remaining HNO₃ was removed by water washing. As can be seen from Table 1, in the embodiment of the present invention, the scales were completely removed, and the surface of the stainless steel strip 1 exhibited a smooth, glossy and beautiful mirror surface after the scales has been removed. On the contrary, in the comparative example shown in Table 1 which employed a conventional method, the scales were incompletely removed, or the stainless surface became cloudy and rough after the scales had been removed. In Embodiment 1, the stainless steel strip 1 discharged from the electrolytic aqueous nitric acid solution cell 10 was introduced into a water-washing bath 12 in which the HNO₃ remaining on the surface was removed, then into a wringer roll 13 by which the washing water was squeezed out, dried by a dryer 14 and then sent to the next step. In the electrolysis in this embodiment, it is a matter of course that the scales can be easily removed by increasing the temperature of each of the electrolytic cells. Table 1 shows the results of descaling of a stainless steel in Embodiment 1 and of descaling by conventional methods (aqueous neutral salt solution electrolysis + aqueous nitric acid solution electolysis, aqueous neutral salt solution electrolysis + aqueous nitric acid-fluoric acid mixture immersion) in Comparative Examples 1 and 2. The stainless steel used was a ferritic AISI 430 0.5-mm thick plate. The conditions of the electrolysis were as follows: Aqueous neutral salt solution electrolysis: anode electrolysis, current density of 6A/dm² Aqueous alkaline solution electrolysis: anode electrolysis, current density of 3A/dm² Aqueous nitric acid solution electrolysis: cathode electrolysis, current density of 2A/dm² In this embodiment, as a result of the above-mentioned electrolysis performed while the above-described AISI430 steel strip was moved at a speed of 100 m/minute, the same results as those shown in Table 1 were obtained. In addition, when AISI304 stainless steel was descaled by using immersion in a nitric acid-fluoric acid mixture in place of the aqueous nitric acid solution in the final step, it was confirmed that the scaling was able to be effectively performed. It is also possible to alternately perform anode electrolysis and cathode electrolysis over a given length of the steel strip in the aqueous neutral salt solution electrolysis and the aqeuous nitric acid solution electrolysis. The descaled steel strip may be rinsed, bright-annealed and then coiled through a bridal roll, as occasion demands. The annealing furnace 16 may be such a system as heating the stainless steel strip through the Joule heat generated directly by direct electrical power in a non-oxidizing atmosphere such as N₂ gas or the like. Alternatively, the annealing may be effected by heating of a combustion exhaust gas of liquified natural gas, butane gas and etc. The above-mentioned descaling method enabled a continuous production process comprising the steps of cold-rolling, annealing and descaling and thus enables treatment at the above-described speed of 100 m/minutes. Thus, this embodiment enables the treatment at a speed higher than 60 m/minute which is the speed of conventional methods. The direct heating by electric power was effected by causing high electrical current to flow through a given length of the steel strip between turn rollers 20 and 21. The annealing was effected at a temperature of 850 to 1150°C for about 3 minutes or less. After annealing, the steel strip was forced to be cooled to room temperature by the flowing of gas at high speed along the steel strip from a non-oxidizing gas flow apparatus 22. At the anode electrolysis step, there occurred cathode electrolysis at the portion of the central positive electrode 3 so that the reducing reaction of the scale was generated, while anode electrolysis occurred at both sides of the negative electrodes 3′ shown in Fig. 1 so that oxidation reaction was generated with the steel strip being dissolved to bring about the descaling thereof. Thus, the anode electrolysis step means that an area in which the anode electrolysis occurs is larger than another area in which the cathode electrolysis occurs. The cathode electrolysis step is reverse to the case of the anode electrolysis step with respect of the number of pieces of the positive and negative electrodes, that is, the area of the occurrence of cathode electrolysis is larger than the area of the occurrence of anode electrolysis at the cathode electrolysis step. Each of the electrodes is provided so that each electrode covers the whole width of the steel strip. Thus, a pair of front and rear side electrodes may be one or a plurality of divided electrode portions provided with respect to the whole width of the steel strip. (Embodiment 2) Fig. 2 shows the potential-pH drawing (25°C) of a Cr-H₂O system. Embodiment 2 employs a method of descaling in which the order of the electrolytic aqueous neutral salt solution cell 2 and the electrolytic aqueous alkaline solution cell 6 in the process of the descaling method of Embodiment 1 was reversed. That is, a stainless steel strip was first placed in the electrolytic aqueous alkaline solution cell in which the strip was electrolyzed in the aqueous alkaline solution, while a positive voltage being applied to the strip. The stainless strip was then introduced into the electrolytic aqueous neutral salt cell in which the strip was electrolyzed in the aqueous neutral salt solution while a positive voltage being applied to the stainless steel strip. The stainless steel strip was then electrolyzed in the aqueous nitric acid solution cell, while a negative voltage was applied to the strip. Water washing effected between these stages of the electrolysis, water washing after the aqueous nitric acid solution electrolysis and the hydro-extraction were the same as those employed in Embodiment 1. This method enabled the attainment of the stainless steel strip having a surface with no scale, smoothness and glossiness. Table 2 shows the conditions and results of the treatment. Table 2 also shows the other embodiments and the results thereof in Embodiments 2 to 7.

Claims: A method of descaling stainless steel, comprising the steps of: anode - electrolyzing stainless steel in an aqueous neutral salt solution, anode - electrolyzing stainless steel in an aqueous alkaline solution or immersing said stainless steel in an aqueous alkaline solution; and cathode - electrolyzing said stainless steel, which had been treated in said former two steps, in an aqueous nitric acid solution or immersing said stainless steel in an aqueous nitric acid-fluoric acid mixture. The method of claim 1, characterized in that said anode - electrolyzing in an aqueous neutral salt solution is done before or after the treatment in the aqueous alkaline solution. The method of claim 2, characterized in that said aqueous neutral salt solution has a concentration of 10 to 30% by weight and more preferably of 15 to 25% by weight. The method of claim 1, characterized in that said aqueous alkaline solution contains NaOH, LiOH or KOH. The method of claim 4, characterized in that the concentration of an alkali metal hydroxide is 30 to 50% by weight for electrolysis and 50 to 70% by weight for immersion. The method of claim 1, 2 or 5, characterized in that the temperature of the solutions is 40 to 80°C. The method of claim 1, characterized in that the concentration of said nitric acid solution is 5 to 20% by weight for electrolysis and 10 to 20% by weight of nitric acid and 1 to 5% by weight of fluoric acid for immersion. The method of any of the preceding claims, characterized in that the current density is not more than 10 A/dm², and preferably 1 to 10 A/dm². The method of any of the preceding claims, characterized in that said immersing is done instead of electrolyzing if the amount of scales formed on said stainless steel strip is small. A method of continuously producing a stainless steel strip at a high speed, comprising in turn the steps of: cold-rolling a stainless steel material, which had been hot-rolled and then descaled, to thereby produce a cold-rolled strip; annealing said strip by electrical heating in a non-oxidizing atmosphere after said cold-rolling; cooling and then performing the steps of any of claims 1 to 9. An apparatus for continuously descaling stainless steel, comprising an electrolytic aqueous neutral salt solution cell (2) having a plurality of positive and negative electrodes (3, 3'), an electrolytic aqueous alkaline solution cell (6) having a plurality of positive and negative electrodes (7, 7') or an aqueous alkaline solution cell for immersion, and an electrolytic aqueous nitric acid solution cell (10) which is provided behind said former two cells (2, 6) and which has a plurality of positive and negative electrodes (11, 11') and/or an aqueous nitric acid fluoric acid mixture cell for immersion which is provided behind said former two cells (2, 6), further comprising means for continuously supplying stainless steel (1) for immersion into said cells (2, 6, 10), which include roll means for guiding a band of stainless steel (1) consecutively through each of said cells (2, 6, 10) in the vicinity of said electrodes (3, 3', 7, 7', 11, 11'). The apparatus of claim 11, comprising an annealing furnace for annealing said strip by electrically heating in a non-oxidizing atmosphere after said cold-rolling; cooling means for cooling after said annealing; and descaling means for descaling after said cooling, characterized in that said descaling means comprises an electrolytic aqueous neutral salt solution cell (2) having a plurality of positive and negative electrodes (3, 3'), an electrolytic aqueous alkaline solution cell (6) having a plurality of positive and negative electrodes (7, 7'), an electrolytic aqueous nitric acid solution cell (10) which is provided behind said former two electrolytic cells (2, 6) and which has a plurality of positive and negative electrodes (11, 11').

Inventors: ITOH MASAHIKO, IZUMIYA MASAKIYO, MIDORIKAWA HEIHATIRO, YAMAGUCHI TERUO, ITOH, MASAHIKO, IZUMIYA, MASAKIYO, MIDORIKAWA, HEIHATIRO, YAMAGUCHI, TERUO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: C25F1
IPC codes: C25F7, C25F1, C23G1, B21B45