JPH10219500A - Finish electrolytic pickling method for descaling of stainless steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a finish electrolytic pickling method for descaling of a stainless steel strip which is capable of easily executing a waste liquid treatment without the generation of materials to be to an environmental problem and is capable of cost effectively executing finish electrolytic pickling to the surface quality of excellent brightness without the remaining of scale on a par with the conventional method of executing the finish electrolytic treatment by using an aq. nitric acid soln. as an electrolyte. SOLUTION: An alternating current electrolysis is executed at a current density in a range of 3 to 8A/dm<2> while the electrolyte contg. sulfate of a molar ratio of 0.4 to 0.6 to a sulfuric acid concn. in an aq. sulfuric acid soln. having a sulfuric acid concn. of 0.25/L to 0.7mol/L is kept at a liquid temp. in a range of 40 to 60 deg.C and the concn. of the metal ions formed in the aq. soln. accompanying a descaling action is maintained at <=0.5 in the molar ratio of the iron ion concn. to the sulfuric acid concn.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention does not generate nitrate or hexavalent chromium ions, which are environmentally problematic, when descaling a stainless steel strip of ferrite series or the like in descaling, and facilitates waste liquid treatment. Can be done to
A method for finishing electrolytic pickling in descaling of stainless steel strip that can be economically electrolytically pickled to a surface quality excellent in gloss with no scale residue and equivalent to the conventional method of performing a finishing electrolytic treatment using a nitric acid aqueous solution as an electrolytic solution. Things.

[0002]

2. Description of the Related Art JIS G which can be said to be a typical alloy steel strip containing at least nickel and / or chromium.
Hot-rolled stainless steel strip products specified in 4304 “Hot-rolled stainless steel strip and steel strip” are generally prepared by subjecting a hot-rolled stainless steel strip to heat treatment including annealing, pickling or a treatment similar to this pickling. It is manufactured by passing through a series of lined annealing and pickling steps for application. A hot-rolled stainless steel sheet product specified in JIS G4304 “Hot-rolled stainless steel sheet and steel strip” is manufactured by shearing the hot-rolled stainless steel strip to a desired size.

Further, No. 2D and No. 2 specified in JIS G4305 "Cold rolled stainless steel sheet and steel strip".
B, No.3, No.4, BA and other cold-rolled stainless steel strip products with various surface finishes are made from the hot-rolled stainless steel strip produced through the above-mentioned annealing and pickling process, The sheet was repeatedly passed through the cold rolling process and the annealing and pickling process which were lined as necessary, and the line was formed as necessary to remove residual scale and ground flaws on the material surface between these processes. It is manufactured through a polishing process, a temper rolling process, and a refining process in which shearing, cutting, and the like are performed. Further, a cold-rolled stainless steel sheet product defined in JIS G4305 “Cold-rolled stainless steel sheet and steel strip” is manufactured by shearing the cold-rolled stainless steel strip to a desired size.

As described above, a stainless steel strip product is subjected to a hot rolling treatment, a heat treatment including annealing after the hot rolling, and a soft annealing treatment of a work hardened by cold rolling. The surface of the material after treatment varies to some extent,
A scale mainly composed of an oxide such as Fe or Cr is generated. Then, in order to obtain a final product having good surface quality, scale generated on the material surface has an adverse effect in each process, so it is necessary to completely remove the scale each time and then send it to each process. is necessary. For this reason, a descaling process is performed after each of the above processes.

As this descaling treatment, various descaling methods have been conventionally implemented or proposed, and sulfuric acid, nitric acid, hydrochloric acid,
2. Description of the Related Art An immersion pickling process of immersing in a hydrofluoric acid or a mixed acid chemical solution obtained by mixing them and performing descaling and uniform and appropriate passivation at the same time has been widely performed.

However, scale formed on the surface of a stainless steel strip material is generally dense and difficult to remove.
Since it is difficult to perform high-speed processing to improve productivity and perform complete descaling processing by a processing method using only the immersion pickling processing as described above, the stainless steel strip must be machined before the immersion pickling processing. Pretreatment, chemical pretreatment, or a combination of these pretreatments have been performed. The former mechanical pretreatment is to use a device such as shot blasting or a scale breaker prior to the immersion pickling process to generate cracks in the scale layer to promote the contact between the acid and the material in the pickling process. This is a process that facilitates descaling. Also, the latter chemical pretreatment method,
This is a process in which some components of the scale layer are denatured by chemical treatment such as anodic electrolysis in an aqueous solution using a molten alkali salt or sodium sulfate as an electrolyte to weaken the bonding force between the scale and the material.

After such a pretreatment, the above-mentioned immersion pickling treatment as a finish pickling treatment has been performed for the purpose of complete descaling, adjustment of surface gloss and passivation. In addition, when the electrolyte to be subjected to the finishing pickling treatment contains nitric acid, hydrochloric acid or hydrofluoric acid, the electrolytic pickling treatment has been performed to improve the descaling ability.
When sulfuric acid is used as the electrolyte, its descaling ability is extremely high, so that the immersion pickling treatment has been performed exclusively.

Further, in the finishing pickling treatment, a proper descaling can be performed, and a uniform and reproducible surface can be treated. The composition of the pickling solution is controlled. This is because hot-rolled stainless steel sheets and steel strip products (hereinafter, may be referred to as hot-rolled stainless steel products) and JIS G4304 as described above.
In producing a cold-rolled stainless steel sheet and a steel strip product (hereinafter, may be referred to as a cold-rolled stainless steel product) specified in IS G4305, in addition to complete descaling, uniformity of the surface gloss. And reproducibility are required, especially for cold rolled stainless steel products.
o. With 2B-finished products, the surface finish is greatly affected by the surface finish during pickling, and if the above pickling conditions are not appropriate, not only the desired surface finish cannot be obtained, but also the pickling cost and environmental pollutants. Therefore, it is necessary to properly control the pickling conditions such as the composition and temperature of the pickling solution.

At present, in the case of ferritic stainless steel,
For example, No. 2 specified in JIS G4305
As a pickling solution for cold-rolled stainless steel products having a surface finish of B, a nitric acid aqueous solution, which is an oxidizing acid, is generally used in consideration of the ease of passivation treatment. Refers to those having the resulting surface. In the pickling step of electrolytically pickling by this aqueous nitric acid solution, in order to maintain appropriate pickling conditions,
If the pickling work is aged (metal ions are accumulated in the pickling solution and the pickling ability is reduced), the aged pickling solution is discarded and freshly prepared with new acid. The pickling solution is replaced with a new pickling solution, pickling is performed again with the new pickling solution, and a part of the pickling solution adheres to the steel strip in the process of passing the stainless steel strip and is taken out. When the pickling solution is reduced by taking out, an operation of adding the pickling solution is performed.

However, for the purpose of suppressing eutrophication in public waters, a discharge standard for nitrogen derived from nitric acid (hereinafter, sometimes simply referred to as nitrate nitrogen) is set, and the pickling solution to be discarded and the Since the waste liquid of the pickling solution becomes an environmental problem, it is necessary to perform a treatment for rendering these pickling solutions harmless. That is, the aged pickling solution has a high acid concentration, and the pickling solution taken out has a low acid concentration because it is diluted by washing water for cleaning the steel strip. Also, in the waste acid treatment step, alkali is added and neutralized to be rendered harmless and discharged to public waters. However, nitrate is discharged as it is because of its high solubility in water.

As a prior art, when attention is paid to a finishing pickling method using an aqueous solution containing no nitrate ion as an electrolytic solution, for example, pickling concerning the immersion method is described in the Stainless Steel Handbook, 3rd ed., Page 1133 (edited by the Stainless Steel Association). Although the composition of the solution is indicated, only nitric acid is indicated as the pickling solution for the electrolytic method, which has a high descaling ability and can be expected to improve the processing speed. Japanese Unexamined Patent Publication (Kokai) No. 62-60900 discloses a descaling method for a stainless steel material as an aqueous solution containing an appropriate amount of each of alkali ions, hydrogen ions, sulfate ions, and nitrate ions, or alkali ions, hydrogen ions, nitrate ions, or the like. A method of anodic electrolysis of a stainless steel material in an aqueous solution containing an appropriate amount of each of the halogen ions has been disclosed.However, since this method contains a considerable amount of nitrate ions, it can be a measure for reducing nitrate nitrogen. Absent. In addition, since many ionic species are contained, the management becomes complicated. Also, Japanese Unexamined Patent Publication No.
In order to enhance the electrolytic action of neutral salt electrolytic treatment or nitric acid electrolytic treatment, JP-A-59899 discloses an electrolytic pickling unit in which a cathode plate is disposed in a tank of a neutral salt aqueous solution or a nitric acid aqueous solution and an anode in a tank of a sulfuric acid aqueous solution. There has been proposed an electrolysis method in which a power supply unit provided with a plate is separated. That is, this method is to dispose a cathode plate in a tank of a neutral salt aqueous solution or a nitric acid aqueous solution and perform only anodic electrolysis on a stainless steel strip, thereby improving descaling action and avoiding electrolytic cathodic electrolysis. Although electrolytic deposition of dissolved metal in the atmosphere is suppressed, electrolytic pickling is performed with a neutral salt aqueous solution or nitric acid aqueous solution, so that nitrate ions and chromate ions that are environmentally problematic are discharged. There was a problem.

[0012]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned drawbacks of the prior art, and performs a finishing electrolytic pickling in descaling of a stainless steel strip such as a ferrite series.
Nitrate and hexavalent chromium, which have excellent descaling ability and excellent surface quality such as gloss, which are equivalent to the conventional method of performing a finishing electrolytic treatment using an aqueous nitric acid solution as an electrolytic solution, and which are environmentally problematic. An object of the present invention is to provide a finishing electrolytic pickling method for descaling a stainless steel strip which can easily perform waste liquid treatment without generating ions and can perform finish pickling economically.

[0013]

In order to solve this problem, the present inventor has proposed a nitric acid solution as a pickling solution in order to prevent nitrate ions, which cause eutrophication in public waters, from being contained in a waste liquid. Use a solution that does not contain ions, have descaling ability equivalent to nitric acid, is not harmful to stainless steel strip, has few environmental problems, is versatile on an industrial scale, and is cheaper As a result of studying a solution satisfying the following condition, the inventors focused on sulfuric acid. However,
When an aqueous solution of sulfuric acid using sulfuric acid alone is used as the electrolyte,
Since there was a problem that the descaling ability was too large to cause roughening of the stainless steel strip, various experiments were conducted to achieve both the opposing effects of having sufficient descaling ability and suppressing the roughening of the surface.

First, sodium sulfate and nitric acid which have been conventionally used as electrolytes, sulfuric acid, and an electrolytic solution obtained by adding various chemicals to sulfuric acid are incompatible with the descaling ability and suppression of skin roughness due to the addition of the chemicals. The effect was investigated.

FIG. 1 shows a commercially available SUS 430 bright annealed material having a thickness of 1.0 mm as a test piece, which was prepared using sodium sulphate, nitric acid,
In various electrolytes of sulfuric acid, nitric acid / sulfuric acid, sodium sulfate / sulfuric acid, ferric nitrate / sulfuric acid, and ferrous sulfate / sulfuric acid, the liquid temperature is 50
The graph shows the pickling loss and the glossiness of the surface when anodic electrolysis was performed for 13 seconds, cathodic electrolysis was performed for 26 seconds, and anodic electrolysis was performed for 13 seconds while sequentially switching the polarity at a temperature of 6 ° C. and a current density of 6 A / dm ² . The abscissa indicates the acid washing loss, and the ordinate indicates the gloss, indicating the relationship between the two. The gloss was measured with a gloss meter (Gardner mirror trigloss).
The glossiness of the untreated material was indicated by と 共 に, and the glossiness after pickling was also shown to compare the change in the glossiness of the surface. The suffix of the plot in the drawing indicates the concentration (mo
l / L), in the case of a single electrolyte, the concentration as it is, and in the case of a two-component electrolyte, the concentration (mol / L) corresponding to the display of electrolyte A / electrolyte B, respectively. ing.

The evaluation of each electrolyte will be described below. For sodium sulfate, the effect of its concentration on pickling loss and gloss is very small. That is,
The concentration of the aqueous sodium sulfate solution shown in FIG.
Although they were 0.125, 0.25, 0.5, and 1.0 mol / L, the decrease in gloss and the loss in pickling were small compared to the gloss of the test piece before the treatment. For nitric acid, as the concentration increases, the pickling loss increases and the gloss decreases, but the effect of the concentration is relatively small since the degree of change is small in any case. With respect to sulfuric acid, as the concentration of sulfuric acid increases, the increase in weight loss in pickling and the decrease in glossiness become remarkable. That is, in the case of electrolytic pickling using sulfuric acid as an electrolyte, when the sulfuric acid concentration is as low as 0.25 mol / L, the increase in the pickling weight loss and the decrease in the glossiness are low, but when the concentration becomes 0.5 mol / L or more, the increase in acid cleaning loss is low. The increase in pickling loss and the decrease in glossiness are extremely large. This quantitatively shows that the descaling ability by the electrolytic method using sulfuric acid as an electrolyte is high, but at the same time the surface gloss is reduced.

On the other hand, the concentration of sodium sulfate / sulfuric acid is 0.12 (mol / L) /0.25 (mol / L) and 0.25 (mol / L).
/0.5 (mol / L) aqueous solution reduces the glossiness by 0.5 mol / L
L concentration of nitric acid is the same as the case of electrolytic pickling as an electrolyte, and the pickling loss is larger than that of the case of electrolytic pickling with nitric acid as an electrolyte.
Even in an aqueous solution having a sulfuric acid concentration of 0.35 (mol / L) /0.7 (mol / L), the decrease in gloss is equivalent to the case of electrolytic pickling using nitric acid at a concentration of 2.0 mol / L as an electrolyte. Is larger than that in the case of electrolytic pickling, which indicates that it has a large descaling ability without reducing the glossiness in a wide concentration range. When the same experiment was performed using potassium sulfate instead of sodium sulfate, the same result as that of sodium sulfate was obtained.

Further, the ferrous sulfate concentration / sulfuric acid concentration of 0.72 (mol / L) /1.0 (mol / L) corresponding to the aged sulfuric acid aqueous solution due to the decrease in pickling ability due to accumulation of iron ions. In the aqueous solution, the increase in pickling weight loss and the decrease in glossiness were small, but as the concentration of ferrous sulfate decreased and approached a pure sulfuric acid aqueous solution, the increase in pickling weight loss and the decrease in glossiness increased. became. The ferric nitrate concentration / sulfuric acid concentration is 0.72
In the (mol / L) /1.0 (mol / L) aqueous solution, the increase in pickling loss and the decrease in gloss were small, but the nitric acid
As the iron concentration decreased and approached the pure sulfuric acid aqueous solution, the increase in the acidification loss and the decrease in the glossiness increased. Furthermore, the concentration of nitric acid / sulfuric acid is 0.1 to 0.8 (mol / L) /1.0 (mol
In the aqueous solution of (L), the pickling loss was very large, and as a result, the glossiness was greatly reduced. As the nitric acid concentration was increased, the pickling weight was significantly increased.

In the above experiment, the hexavalent chromium ion concentration in the electrolytic solution after the electrolytic treatment was examined. As a result, hexavalent chromium ion was detected when the electrolytic solution was sodium sulfate alone aqueous solution. Hexavalent chromium ions were not detected in other electrolyte compositions except for the above.

Next, since descaling means dissolving the material immediately below the scale and releasing the material including the upper scale, iron, chromium and nickel as the main components of the stainless steel strip are contained in the actual electrolyte. Large amounts of metal ions accumulate and adhere to the stainless steel strip, and the electrolyte is taken out or the concentration of the effective acid changes due to the consumption of sulfuric acid. The effect on descaling ability and glossiness was investigated.

FIG. 2 shows a SUS430 steel sheet, which has been scaled by annealing under conditions capable of obtaining appropriate material properties, used as a test piece, and used as an aqueous nitric acid solution or an aqueous iron ion / nitric acid solution (corresponding to an aged nitric acid aqueous solution). ), Sulfuric acid alone aqueous solution, sodium sulfate / sulfuric acid aqueous solution, iron ion / sulfuric acid aqueous solution (corresponding to aged sulfuric acid aqueous solution), iron ion / sodium sulfate / sulfuric acid aqueous solution (corresponding to aged sodium sulfate / sulfuric acid aqueous solution) FIG. 3 shows the loss of pickling, the glossiness of the surface, and the descaling state when an aqueous solution was prepared and subjected to electrolytic treatment under the same conditions as in the experiment shown in FIG.

For an aqueous solution of nitric acid alone, the nitric acid concentration is 0.1
The pickling weight loss gradually increased with the increase in the range of 5 to 4.0 mol / L, but at low concentrations, scale residue was observed overall, and even when the concentration was extremely increased, the scale was partially reduced. Remains were noted. The iron ion / nitric acid aqueous solution, that is, the nitric acid aqueous solution in which iron ions were accumulated and aged, showed almost the same results as in the case of the new solution with respect to the loss in pickling, glossiness and scale residue. With respect to the aqueous solution of sulfuric acid alone, as the sulfuric acid concentration increased in the range of 0.25 to 2.0 mol / L, the amount of pickling loss increased remarkably and no scale residue was observed, but the glossiness became remarkable. . In particular, sulfuric acid concentration is 0.
When the concentration was as low as 25 mol / L, high gloss was obtained without residual scale, but it was noted that when the sulfuric acid concentration was 0.5 mol / L or more, the glossiness was reduced. In the case of an iron ion / sulfuric acid aqueous solution, that is, an aqueous solution of sulfuric acid in which iron ions are accumulated and aged, when the sulfuric acid concentration is as high as 1.0 mol / L or more, the surface of the test piece is partially remarkably roughened to 0.25 mol / L.
When it was low, the scale residue was partially observed. When the sulfuric acid concentration was 0.5 mol / L, high gloss was exhibited without any scale residue.

The aqueous solution of sodium sulfate / sulfuric acid has a sulfuric acid concentration of 0.5 mol /
The acid washing loss is somewhat suppressed by L, but there is no scale residue. Furthermore, not only when the sulfuric acid concentration is 0.25 mol / L, but also
Even at l / L, high gloss is maintained. For iron ion / sodium sulfate / sulfuric acid aqueous solution, that is, sodium sulfate / sulfuric acid aqueous solution in which iron ions are accumulated and aged, pickling loss is almost equal to that of sodium sulfate / sulfuric acid aqueous solution (new solution) and glossiness is sodium sulfate. / Sulfuric acid aqueous solution (new solution) shows a slight decrease,
In the case of 1.0 mol / L or more, skin roughness was partially observed. Then, when the sulfuric acid concentration at which no roughening of the skin was observed was determined, it was found that the sulfuric acid concentration should be 0.7 mol / L or less and the iron ion concentration should be in the molar ratio of up to 0.5 of the sulfuric acid concentration.

From the above experimental results, by using an aqueous solution obtained by adding sodium sulfate to sulfuric acid as the electrolytic solution, sufficient descaling ability can be obtained in a considerably wide range of sulfuric acid concentration of 0.25 mol / L to 0.7 mol / L. It has been found that stainless steel can be electrolytically pickled in a state having excellent gloss and excellent gloss.

Therefore, the effects of the descaling capability on the polarity of the stainless steel strip and the effect on the surface gloss during the electrolytic treatment using an aqueous solution obtained by adding sodium sulfate to sulfuric acid as the electrolytic solution were investigated. FIG. 3 shows an aqueous solution of sodium sulfate / sulfuric acid aqueous solution and an aqueous solution of iron ion / sodium sulfate / sulfuric acid aqueous solution.
3 shows the loss of pickling, the surface glossiness, and the descaling state of the test piece when the test was carried out at / dm ² and different electrolysis times of 26 seconds and 52 seconds.

In the case of sodium sulphate / sulfuric acid aqueous solution (new solution), in the case of only anodic electrolysis in which the polarity of the test piece was the anode, some scale remained in a short electrolysis time of 26 seconds, but high descaling was observed. Was carried out, in the case of only cathodic electrolysis with the polarity of the test piece as the cathode, although a slight pickling loss was observed, the scale remained,
Since the color tone that was blue before the electrolytic treatment changed to yellow after the electrolytic treatment, if the color tone of the scale is caused by the interference color, it is considered that the scale thickness was reduced. That is, it is considered that part of the oxide in the crystal grain boundary was also dissolved and removed, and the number of scale holes was increased. Therefore, when this sodium sulfate / sulfuric acid aqueous solution (new solution) is used as the electrolytic solution, descaling is performed in both the anodic electrolysis and the cathodic electrolysis, and the anode electrode and the cathode electrode are alternately provided in the electrolytic cell. By arranging and performing the alternating electrolysis, it is considered that a synergistic effect of the anodic electrolysis and the cathodic electrolysis is added, and a high descaling action can be performed.

It has been found that the aged iron ion / sodium sulfate / sulfuric acid aqueous solution of sodium sulfate / sulfuric acid aqueous solution has almost the same descaling ability as the new sodium sulfate / sulfuric acid aqueous solution. A similar experiment was performed using potassium sulfate instead of sodium sulfate, and the same result as in the case of sodium sulfate was obtained.

From the above experimental results, it was found that a sulfate / sulfuric acid aqueous solution in which a sulfate made of sodium sulfate or potassium sulfate was added to an aqueous sulfuric acid solution was preferable. In order to perform the finishing electrolytic pickling in descaling of the stainless steel strip, a sulfate having a molar ratio to the sulfuric acid concentration of 0.4 to 0.6 is added to a sulfuric acid aqueous solution having a sulfuric acid concentration of 0.25 mol / L or more and 0.7 mol / L or less. While containing the electrolytic solution, while maintaining the solution temperature in the range of 40 to 60 ° C. and the concentration of metal ions generated in the aqueous solution with descaling action in a molar ratio to the sulfuric acid concentration of 0.5 or less in iron ion concentration, The present inventors have found that it is necessary to perform alternating electrolysis at a current density in the range of ³ to 8 A / dm ² .

That is, when the concentration of sulfuric acid is less than 0.25 mol / L, it is very complicated to maintain the concentration in response to the aging of the electrolyte or the removal of the electrolyte by adhering to the material. The reason for this is that if the amount exceeds 0.7 mol / L, the skin becomes partially rough as described above.

In an aqueous solution having a low concentration of sulfuric acid and a high concentration of sodium sulfate, descaling action is reduced and hexavalent chromium ions are detected in the aqueous solution. In an aqueous solution having a high concentration of sulfuric acid and a low concentration of sodium sulfate, the gloss is reduced. However, since the descaling ability was improved and hexavalent chromium ions were not detected in the aqueous solution, a further appropriate addition amount of sulfate was examined, and the molar ratio to the sulfuric acid concentration was 0.4 to 0.6.
I was able to find out.

Further, when the stainless steel strip is electrolytically pickled, the descaling ability is reduced because metal ions (mainly iron ions) are accumulated in the electrolytic solution due to the reaction with the electrolytic solution. It has been confirmed in various experiments that the increase in the current density and the electrolysis time can cope with this. However, when the iron ion concentration is 0.5 mol / L or more, the current density and the electrolysis time are increased to improve the descaling ability. Since the skin is likely to be partially roughened, the upper limit of the sulfuric acid concentration needs to be 0.5 mol / L.

The reason for setting the liquid temperature to 40 to 60 ° C. is that the liquid temperature is 40 ° C.
If it is lower, sufficient descaling capacity cannot be obtained,
Also, if the temperature exceeds 60 ° C., the skin becomes easily rough,
The reason for setting the current density to 3 to 8 A / dm ² is that if the current density is lower than 3 A / dm ² , a sufficient descaling effect cannot be obtained, and 8 A / d 2
This is because, if it is higher than m ^{2, the} amount of gas generated at each electrode is significantly increased rather than that the descaling action is improved, and electric energy does not efficiently contribute to the descaling action. The electrolysis time is appropriately set so as to obtain a desired descaling capacity according to the size of the electrolytic pickling tank, the composition of the electrolytic solution, the state of the scale of the stainless steel strip to be subjected to the final electrolytic pickling, and the like. good.

As described above, the sulfuric acid concentration is 0.25 mol / L or more.
In a sulfuric acid aqueous solution of 0.7 mol / L or less, an electrolytic solution containing a sulfate having a molar ratio of 0.4 to 0.6 with respect to the sulfuric acid concentration is adjusted to a solution temperature within a range of 40 to 60 ° C. and an aqueous solution with descaling action. Alternating electrolysis at a current density in the range of ³ to 8 A / dm ² while maintaining the concentration of the generated metal ions in the molar ratio of the iron ion concentration to the sulfuric acid concentration of 0.5 or less, thereby forming the ferritic stainless steel strip. Investigating that it is possible to perform finish electrolytic pickling that has the same or better descaling ability as compared to the conventional electrolytic pickling method using an aqueous solution of nitric acid as an electrolytic solution, and that does not cause a decrease in gloss. The present invention has been completed. However, when the alternating electrolysis is performed with the anode electrolysis time and the cathode electrolysis time being in a ratio of (1 to 4): 1, the finish electrolysis is more reliably performed in a state in which no scale remains and excellent gloss is obtained. Can be pickled When replenishing the electrolytic solution in the electrolytic solution using sodium sulfate as the sulfate, a replenisher prepared by mixing sulfuric acid and sodium hydroxide at a ratio to obtain the concentrations of sulfuric acid and sodium sulfate to be replenished is added. Since the effective sulfuric acid concentration and sulfate concentration in the solution can be easily adjusted, the maintenance of the electrolytic solution can be easily performed, and since this finishing electrolytic pickling method has excellent descaling ability, The present inventors have also completed the present invention by investigating that a stainless steel strip annealed after being cold-rolled can be subjected to a finish electrolytic pickling treatment without pretreatment.

[0034]

BEST MODE FOR CARRYING OUT THE INVENTION A method for finishing electrolytic pickling in descaling a stainless steel strip according to the present invention will be described below in detail with reference to FIG. FIG. 4 is an explanatory view showing an electrolytic pickling apparatus in a series of lined annealing electrolytic pickling apparatuses including an annealing furnace, an electrolytic pickling apparatus, and a plate feeding apparatus.

In the drawings, reference numerals 1 to 3 denote anode electrodes 1a, respectively.
And a cathode electrode 1b are alternately arranged, and a predetermined electrolytic solution is built in the electrolytic pickling tank. At least the finishing electrolytic pickling is performed. The tank and, if necessary, the electrolytic pickling tank, which is pretreated with an electrolytic solution containing sulfuric acid as an electrolyte, requires that the material be resistant to sulfuric acid. It is preferable to use a rubber lining that has been subjected to acid resistance or a tank whose inside is covered with acid resistant brick.

The electrodes 1a and 1b disposed in the electrolytic pickling tanks 1, 2 and 3 only need to withstand a sulfuric acid aqueous solution, and the material of the anode electrode 1a has been generally used conventionally. A lead-based material or a material coated mainly with a metal oxide such as platinum, rubidium or iridium is preferably used, and the material of the cathode electrode 1b is a high acid resistance as high as that of the anode electrode 1a since cathode corrosion protection acts. It is not necessary to have titanium, SUS
304 and the like are preferably used.

Although not shown, the temperature of the electrolyte is usually maintained at a predetermined temperature higher than the normal temperature (the temperature of the electrolyte in the electrolytic pickling tank used for carrying out the method of the present invention is 40 to 60 ° C.). Therefore, a device for heating is provided, and it is only necessary to appropriately select live steam blowing or indirect heating using a heat exchange device.

4 is a DC power supply for electrolysis, 5 is a deflector roll, 6 is a dipping roll, 7 is a brush roll, 8 is a backup roll of the brush roll 7, 9 is a pickling liquid circulation pump, and 10 is a pickling liquid circulation. A pipe 11 is a reserve tank for circulating pickling liquid.

In order to carry out the method of the present invention with such an apparatus, an electrolytic pickling tank 3 which is usually provided at the most downstream side and performs a final electrolytic pickling is placed in a sulfuric acid aqueous solution having a predetermined concentration and a predetermined concentration. A bath containing an electrolyte containing sulfate is prepared. As described above, the concentration of sulfuric acid is 0.25 mol / L or more.
0.7 mol / L or less, and the sulfate concentration is 0.4 to 0.6 in a molar ratio to the sulfuric acid concentration. This is because the sulfuric acid concentration
When the concentration is less than 0.25 mol / L, it becomes very complicated to maintain and control the concentration in response to the aging of the electrolytic solution or when the electrolytic solution adheres to the steel strip and is taken out, and when the concentration is 0.7 mol / L or more. This is because the descaling ability is high, and significant skin roughness occurs partially. The sulfate concentration is the molar ratio of sulfuric acid to sulfuric acid.
If it is lower than 0.4, it becomes easy to partially leave souker when metal ions in the electrolytic solution increase due to electrolytic pickling, and if it is higher than 0.6, hexavalent chromium ions, which are environmentally problematic, are contained in the electrolytic solution. This is because it easily occurs.

As the sulfate, sodium sulfate or potassium sulfate can be used. However, since potassium sulfate is expensive, it is economical to use sodium sulfate.

At this time, the annealed stainless steel strip S
The so-called pretreatment is carried out by passing plates through the electrolytic pickling tanks 1 and 2 provided upstream of the electrolytic pickling tank 3 for performing the finishing electrolytic pickling. In the tanks 1 and 2, a neutral salt electrolyte using an aqueous solution of sodium sulfate, an acidic electrolyte using an aqueous solution of sulfuric acid, or an alkaline electrolyte using an aqueous solution of sodium hydroxide, as in the conventional pretreatment. However, since the electrolytic pickling with the electrolytic solution used in the method of the present invention is very powerful, the generation of hexavalent chromium ions, which are environmentally problematic, can be generated not only in the final electrolytic pickling but also in the pretreatment. An electrolytic solution containing a predetermined concentration of a sulfate in a predetermined concentration of a sulfuric acid aqueous solution similar to that of the electrolytic pickling tank 3 may be built so as not to occur even in the washing.

Then, the stainless steel strip S was subjected to current density while maintaining the electrolytic solution containing a predetermined concentration of sulfate in the above-mentioned predetermined concentration of sulfuric acid aqueous solution in the electrolytic pickling tank 3 while maintaining the solution temperature within the range of 40 to 60 ° C. Performs alternating electrolysis in the range of ³ to 8 A / dm ² . Here, the alternating electrolysis is a process in which the anodic electrolysis and the cathodic electrolysis are repeated while the polarity of the stainless steel strip to be electrolyzed is alternately sequentially changed in the passing direction. At this time, the anode electrolysis time and the cathode electrolysis time are (1)
-4): 1 is preferable because sufficient descaling can be performed more reliably and the durability of the electrode can be ensured.

If the liquid temperature is lower than 40 ° C., a sufficient descaling ability cannot be obtained, and if the liquid temperature is higher than 60 ° C., the descaling ability becomes high and the stainless steel strip is likely to be roughened. ~ 60 ° C and current density of 3A / dm ²
If it is less than 10, sufficient descaling ability cannot be obtained, and 8
If it is higher than A / dm ^{2, the} amount of gas generated at each electrode will only increase remarkably and unnecessary electric energy will be consumed in addition to the descaling operation, which is uneconomical. Then, as the electrolytic pickling is performed, the metal ions are accumulated in the electrolytic solution and the amount thereof increases, but since the metal ions are mainly iron ions, the iron ion concentration is 0.5 or less in molar ratio to the sulfuric acid concentration. To maintain. This is because if the molar ratio of iron ion concentration to sulfuric acid concentration exceeds 0.5 as metal ions accumulated in the electrolytic solution, sufficient descaling ability cannot be obtained.

In addition, the concentration of the electrolyte is maintained in response to the aging of the electrolyte or the removal of the electrolyte attached to the steel strip, and the electrolyte is replenished in the electrolyte using sodium sulfate as the sulfate. At this time, by adding a replenisher prepared by mixing sulfuric acid and sodium hydroxide at a ratio to obtain the concentrations of sulfuric acid and sodium sulfate to be replenished, it is possible to easily adjust the effective sulfuric acid concentration and sulfate concentration in the electrolyte solution. This is preferable because the maintenance of the electrolyte can be easily performed.

The electrolytic pickling apparatus shown in FIG. 4 is provided with three electrolytic pickling tanks 1, 2 and 3 as described above. The electrolytic pickling tank 3 is an electrolytic pickling tank in which a finish electrolytic pickling is performed. When the electrolytic pickling tank 2 can be performed with a sufficient descaling capability depending on the electrolytic solution conditions such as the composition, the current density, and the electrolytic time, the electrolytic pickling tank 2 is used as the electrolytic pickling tank in which the finishing electrolytic pickling is performed. Substantially two electrolytic pickling tanks perform the pretreatment to the final electrolytic pickling.

[0046]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the present invention is a method for finishing electrolytic pickling of a stainless steel strip such as a ferritic stainless steel strip, according to JIS G430.
4. Stainless steel products having a surface finish specified in "Hot rolled stainless steel sheets and strips" and JIS G
No.2 of 4305 "Cold rolled stainless steel sheet and strip"
Since it is a method for obtaining a stainless steel product having a surface finish specified in the B finish, a conventional finishing electrolytic pickling method using a nitric acid aqueous solution which has been conventionally performed to obtain the stainless steel product is firstly performed. Shown as a comparative example,
Next, an example according to the method of the present invention will be described, and the surface properties of the stainless steel strip obtained by the example which is the method of the present invention with respect to the surface properties of the stainless steel strip obtained by the comparative example which is the conventional method will be evaluated.

Comparative Example 1 In the electrolytic pickling apparatus shown in FIG.
A 1.5 mol / L aqueous solution of sodium sulfate is bathed in the electrolytic pickling tank 1 to maintain the liquid temperature at 80 ° C., and the electrolytic effective length is 15
A 1.0 mol / L aqueous solution of sulfuric acid is built in the electrolytic pickling tank 2 to maintain the liquid temperature at 50 ° C., and the electrolytic pickling tank 3 having an effective electrolytic length of 30 m has a concentration of 1.0 mol / L. A cold rolled stainless steel strip S (steel type: SUS430, sheet thickness: 1.0 mm) annealed by an annealing furnace installed on the upstream side of the electrolytic pickling apparatus, maintaining a solution temperature of 50 ° C. by bathing a nitric acid aqueous solution. mm and a plate width of 1300 mm) are passed through the electrolytic pickling tanks 1 and 2 sequentially at a speed of 50 m / min to perform pretreatment, and then passed through the electrolytic pickling tank 3 to finish electrolysis. Pickling was performed. At this time, each electrolytic pickling tank 1,
In 2 and 3, the anode electrode and the cathode electrode were alternately arranged, and the anode current density of the stainless steel strip was 6 A / d.
m ² , and alternate electrolysis was performed with the ratio of the anode electrolysis time to the cathode electrolysis time being 3: 1. As a result of observing the surface of the stainless steel strip S thus electrolytically washed, it was found that there was no scale remaining and the gloss was high.
Then, when the stainless steel strip was passed at a passing speed of 70 m / min, scale residue was observed.

Comparative Example 2 In Comparative Example 1, a 1.5 mol / L aqueous solution of sodium sulfate was bathed in the electrolytic pickling tank 1 to maintain the liquid temperature at 80 ° C.
In the electrolytic pickling tank 2, a 10.0 mol / L aqueous solution of sodium hydroxide is bathed to maintain the liquid temperature at 80 ° C., and in the electrolytic pickling tank 3, a 1.0 mol / L aqueous nitric acid solution is bathed. The solution temperature was maintained at 50 ° C., and electrolytic pickling was performed in the same manner as in Comparative Example 1 except that the plates were sequentially passed through these electrolytic pickling tanks 1, 2, and 3 at a speed of 30 m / min. As a result, although the sheet passing speed was as low as 30 m / min and the processing capacity was reduced, as in Comparative Example 1, no scale remained and the gloss was high. Then, when the sheet was passed at the same sheet passing speed of 50 m / min as in Comparative Example 1, scale residue was observed.

Example 1 In the electrolytic pickling apparatus shown in FIG.
A 1.5 mol / L aqueous solution of sodium sulfate is bathed in the electrolytic pickling tank 1 to maintain the liquid temperature at 80 ° C., and the effective electrolytic length is 15 m.
A 1.0 mol / L sulfuric acid aqueous solution is built in the electrolytic pickling tank 2 and the liquid temperature is maintained at 50 ° C., and the 0.5 mol / L sulfuric acid and the concentration are stored in the electrolytic pickling tank 3 having an effective electrolytic length of 30 m. A cold rolled stainless steel strip annealed by an annealing furnace installed on the upstream side of this electrolytic pickling apparatus by maintaining a solution temperature of 50 ° C. by bathing a mixed aqueous solution with 0.25 mol / L sodium sulfate. S (Steel type: SUS)
430, board thickness: 1.0mm, board width: 1300mm), 70m / mi each
After passing through the electrolytic pickling tanks 1 and 2 in sequence at a speed of n to perform pretreatment, the plate was passed through the electrolytic pickling tank 3 to perform finish electrolytic pickling. At this time, in each of the electrolytic pickling tanks 1, 2, and 3, anode electrodes and cathode electrodes are alternately arranged,
The anode current density of the stainless steel strip was 6 A / dm ² , and the ratio of the anode electrolysis time to the cathode electrolysis time was 3: 1.
Alternating electrolysis was performed as the ratio of As a result of observing the surface of the stainless steel strip thus electrolytically pickled, there was no scale remaining and the gloss was equal to or higher than that of Comparative Example 1.

Example 2 In Example 1, the concentration of 0.5 mol was added to the electrolytic pickling tanks 1, 2, and 3.
/ L sulfuric acid and sodium sulfate having a concentration of 0.25 mol / L in a bath to maintain the liquid temperature at 50 ° C., and sequentially pass through these electrolytic pickling tanks 1, 2, and 3 at a speed of 120 m / min. Electrolytic pickling was performed in the same manner as in Example 1 except that the plate was plated. As a result, there was no residual scale as in Example 1, and the gloss was equal to or higher than that of Comparative Example 1.

Example 3 In Example 1, the concentration of 0.5 mol / L was added to the electrolytic pickling tanks 1 and 2.
A mixed aqueous solution of sulfuric acid and sodium sulfate having a concentration of 0.25 mol / L is bathed and maintained at a liquid temperature of 50 ° C., only water is bathed in the electrolytic pickling tank 3, and only the electrolytic pickling tanks 1 and 2 are prepared. The alternating current electrolysis is performed by setting the anode current density of the stainless steel strip to 6 A / dm ² and the ratio of the anode electrolysis time to the cathode electrolysis time at a ratio of 3: 1. The electrolytic pickling was performed in the same manner as in Example 1 except that the cleaning was performed. As a result, there was no residual scale as in Example 1, and the gloss was equal to or higher than that of Comparative Example 1.

Example 4 In Example 1, the ratio of the anodic electrolysis time to the cathodic electrolysis time in each of the electrolytic pickling tanks 1, 2, and 3 was 3:
Electrolytic pickling was performed in the same manner as in Example 1 except that the ratio was changed from 1 to 1: 1. As a result, similar to Example 1, no scale remained and the gloss was equal to or higher than that of Comparative Example 1.

[0053]

As described in detail above, the method of electrolytic electrolytic pickling in descaling stainless steel strip according to the present invention has the following excellent effects, and is of industrial value. It is very big.

A surface gloss equal to or higher than that of the conventional nitric acid electrolysis method is obtained, and its descaling ability is superior to that of the nitric acid electrolysis method. A stainless steel strip having a desired surface property is prepared by using an aqueous solution containing nitric acid. You can get without doing. Therefore, it is not necessary to use nitric acid, which is an environmental problem, and since sulfuric acid and sodium sulfate are used at a lower price than nitric acid, JIS G4304 is used.
"Hot rolled stainless steel sheet and strip" and JIS G43
05 "Cold-rolled stainless steel sheet and steel strip" No. 2B finish can reduce the manufacturing cost of stainless steel products having the surface finish specified, and not only facilitate waste liquid treatment, but also save the environment Nutrition problems are enormous, with improved and industrial and social contributions.

As compared with the conventional neutral salt electrolysis method using sodium sulfate as an electrolytic solution, not only the descaling action and luster at the same level or higher can be obtained, but also in the neutral salt electrolysis method, Since hexavalent chromium ions that have been generated are not generated in the method of the present invention, equipment construction costs for detoxification, proportional costs and fixed costs required for the operation thereof can be reduced. Therefore, not only can the waste liquid treatment be performed easily and economically, but also the environmental problems are greatly improved, which contributes industrially and socially.

The conventional pickling apparatus for descaling performs various processes such as neutral salt electrolysis, sulfuric acid electrolysis, alkali electrolysis, and nitric acid electrolysis from the pretreatment to the finish pickling in various combinations. However, when the stainless steel strip to be subjected to finish electrolytic pickling is a stainless steel strip annealed after being cold-rolled, the electrolytic solution using the electrolytic solution of the method of the present invention without special pretreatment is used. Since this can be dealt with by increasing the length of the tank, it is possible to easily maintain and control the electrolytic solution conditions and the electrolytic conditions, and to reduce the cost required for managing the chemical solution.

In order to manufacture a chromium-based stainless steel strip represented by SUS430, a series of lined annealing electrolytic pickling apparatuses equipped with a conventional annealing furnace, electrolytic pickling apparatus and plate feeder are used for annealing. Since the processing capacity of the electrolytic pickling apparatus was inferior to the processing capacity of the furnace, the line speed was determined by the processing capacity of this electrolytic pickling apparatus. By carrying out the method of the present invention, the line speed can be increased, the processing capacity of the chromium-based stainless steel strip in a series of lined annealing pickling apparatuses is improved, and the productivity is extremely excellent.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing the effects of electrolytic solution on the descaling ability and surface gloss of a SUS430 light-annealed material using various electrolytic solutions by glossiness and acid washing loss.

FIG. 2 shows that the SUS430 steel strip is annealed to produce scale, and the annealed material is subjected to electrolytic pickling while changing the concentration of various electrolytes, to improve the descaling ability and surface gloss accompanying the change in the composition and concentration of the electrolyte. It is a figure which shows the influence which it gives by glossiness and pickling loss.

FIG. 3 is a graph showing the relationship between the concentration of the electrolytic solution used in the method of the present invention and the concentration of the electrolytic solution used in the method of the present invention. It is a figure which shows the influence which has on a scaling ability and surface gloss by glossiness and pickling loss.

FIG. 4 is a view showing an electrolytic pickling apparatus in a series of lined annealing electrolytic pickling apparatuses including an annealing furnace, an electrolytic pickling apparatus, and a plate feeding apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electrolytic pickling tank 1a Anode electrode 1b Cathode electrode 2 Electrolytic pickling tank 3 Electrolytic pickling tank 4 DC power supply for electrolysis 5 Deflector roll 6 Immersion roll 7 Brush roll 8 Backup roll 9 Pickling liquid circulation pump 10 Pickling liquid circulation Piping 11 Reserve tank for pickling liquid circulation S Stainless steel strip

Claims (5)

[Claims] 1. A sulfuric acid concentration of 0.25 mol / L or more and 0.7 mol / L.
In the following sulfuric acid aqueous solution, the molar ratio to the sulfuric acid concentration is 0.
An electrolytic solution containing a sulfate of 4 to 0.6 is prepared by changing the concentration of metal ions generated in an aqueous solution with descaling to a solution temperature in the range of 40 to 60 ° C. by an iron ion concentration and a molar ratio to the sulfuric acid concentration. The current density is 3 to 8 A while maintaining the current density at 0.5 or less.
A method for finishing electrolytic pickling in descaling a stainless steel strip, characterized in that alternating electrolysis is performed in the range of / dm ² . 2. The method of finishing electrolytic pickling in descaling a stainless steel strip according to claim 1, wherein the alternating electrolysis is performed with the anodic electrolysis time and the cathodic electrolysis time in a ratio of (1 to 4): 1. 3. The method of finishing electrolytic pickling in descaling a stainless steel strip according to claim 1, wherein sodium sulfate or potassium sulfate is used as the sulfate. 4. A replenisher prepared by mixing sulfuric acid and sodium hydroxide at a ratio to obtain the concentrations of sulfuric acid and sodium sulfate to be replenished when replenishing the electrolytic solution in the electrolytic solution using sodium sulfate as a sulfate. Claim 3
3. A method for finishing electrolytic pickling in descaling of a stainless steel strip as described in 1. above. 5. The descaling of a stainless steel strip according to any one of claims 1 to 4, wherein the stainless steel strip annealed after cold rolling is subjected to finish electrolytic pickling without pretreatment. Finish electrolytic pickling method.