KR20120075346A - Process for descaling a ferrite stainless steel strip using nitric acid free solutions - Google Patents

Abstract

PURPOSE: A method for cleansing ferrite stainless steel plate is provided to no need to apply an additional pickling process and to reduce a processing time of a mixed acid dipping process. CONSTITUTION: A pickling process comprises a neutral ream electrolysis process and a mixed acid dipping process for process for eliminating oxidation scales generated by a cold rolling a low chrome containing ferrite-based stainless of less than14%. The oxidation scales are eliminated by grinding with grinding brushes(5,6) including grinding particles. A grinding process is performed during or after or both during and after a neutral salt electrolysis process. The grinding particles are silicon carbide. A base material of the grinding brush is formed with the synthetic resins of polyamide. A diameter of brush hair is 0.8 to1.2mm.

Description

Pickling method of ferritic stainless steel sheet {PROCESS FOR DESCALING A FERRITE STAINLESS STEEL STRIP USING NITRIC ACID FREE SOLUTIONS}

The present invention relates to a pickling method for economically pickling a ferritic stainless steel cold rolled sheet requiring surface quality, and to a pickling method for removing the scale to the maximum before the mixed acid immersion step without applying an additional pickling process.

Stainless steel cold rolled steel sheet is usually subjected to a heat treatment process of 800 ~ 1150 ℃ in order to obtain mechanical properties after cold rolling, wherein the high temperature oxygen in the furnace reacts with the surface of the cold rolled steel sheet (SiO ₂ , (Cr, Fe) ₃ O ₄ ) is generated. However, this oxidation scale not only makes the product appearance poor, but also this oxidation scale serves as a starting point for corrosion of the steel plate surface, leading to a decrease in corrosion resistance.

Therefore, in order to obtain a beautiful surface quality and to improve corrosion resistance, oxidizing scales are removed, and physical descaling (brush, shot blasting), electrolytic descaling (sodium sulfate, sulfuric acid, nitrate electrolyte), chemical descaling (salt, Stainless steel is produced by combining various methods such as mixed acid), and this process is called a pickling process.

In this stainless steel pickling process, it has been pickled by using a mixture of 80 to 180 g / L nitric acid and 2 to 40 g / L hydrofluoric acid in order to form a beautiful surface quality and passivation film evenly to secure corrosion resistance. Among these, nitric acid lowers the pH in the pickling bath to increase the activity of hydrofluoric acid, and oxidizes the divalent iron ions dissolved on the surface of the steel sheet to trivalent so as to maintain an appropriate redox potential for pickling.

However, the use of such nitric acid produces NOx, an air emission control substance, and nitrate nitrogen (NO ₃ -N) from waste acid and washing water. Due to the limitation of NOx concentration in the discharge facility, the production cost increases significantly due to the additional installation and operation cost of the environmental pollution prevention facility in the pickling process.

In order to solve this problem, a conventional pickling solution is used to replace nitric acid with sulfuric acid hydrochloric acid in the pickling process, and the insufficient oxidizing power does not include nitric acid replaced by hydrogen peroxide, potassium permanganate, trivalent iron ions and air injection. Pickling methods have been developed.

Electrolysis using such a solution containing no nitric acid is carried out by an immersion process for a mixed acid solution containing no neutral salt electrolytic acid-nitric acid to remove the oxidation scale after cold rolling annealing of ferritic stainless steel. However, in this case, there was a problem that a pickling failure of the acid pickling occurred.

In general, the mixed acid immersion process for a solution that does not contain nitric acid is a process that minimizes damage to the base material than the mixed acid tank to which nitric acid is applied, and is a process for selectively removing only scale. Therefore, if the oxidation scale is not removed to the maximum degree in the pretreatment process, the treatment time of the mixed acid immersion process for the mixed acid solution containing no nitric acid becomes long, and a problem of poor pickling occurs.

Therefore, an additional electrolytic pickling process is required to improve the pickling performance. However, the introduction of additional electrolytic pickling process, there arises a problem that additional equipment and process costs are required accordingly.

In the mixed acid pickling method performed by dipping in an acid solution consisting of sulfuric acid, hydrofluoric acid, and hydrogen peroxide, the application of an additional pickling step is unnecessary by removing the scale as much as possible before immersion in the mixed acid after cold rolling annealing of the ferritic stainless steel. The present invention also provides a low cost pickling method capable of shortening the processing time of a mixed acid immersion step and a pickling apparatus to which the pickling method can be applied.

The present invention relates to a pickling method including a neutral salt electrolytic pickling process and a mixed acid immersion step including sulfuric acid, hydrofluoric acid, and hydrogen peroxide to remove an oxide scale generated by cold rolling annealing of 14% or less low chromium-containing ferritic stainless steel. And, before the blending step provides a pickling process comprising a grinding step of removing the oxidative scale by grinding with a grinding brush containing abrasive particles.

Preferably, the grinding process may be performed during, after, or both of the neutral salt electrolysis processes.

In addition, the abrasive particles may be silicon carbide, it is preferable that the particle size number # 100 to # 240 according to the KSL 6001 standard.

The grinding brush is a brush base material is made of a polyamide synthetic resin, it is preferable that the diameter of the brush hair is 0.8 to 1.2mm.

The neutral salt electrolysis process uses a 100 ~ 250g / L sodium sulfate solution at a temperature of 50 ~ 90 ℃ as an electrolyte, the current of 10 ~ 30A / dm ² so that the potential of the surface of the steel sheet can be charged in the order of +,-, + It can be carried out by applying to the surface of the cold rolled steel sheet for 5 to 55 seconds, the mixed acid immersion step is the initial composition of the mixed acid solution of sulfuric acid 70 ~ 200g / L, free hydrofluoric acid 0.5 ~ 10g / L and hydrogen peroxide concentration 7g / L or more It may be carried out by immersing the steel sheet in a mixed acid solution containing, and substantially no iron ions for 10 to 100 seconds.

By applying the method and apparatus of the present invention, in the pickling of ferritic stainless steel cold rolled steel sheet, the use of nitric acid eliminates the burden of treating wastewater and exhaust gas, and scales to the maximum before immersion in mixed acid. By removing it, mixed acid pickling can be performed within a short time, it is unnecessary to apply an additional pickling step, and the processing time of the mixed acid dipping step can be shortened.

1 is a process diagram schematically showing a pickling process according to the present invention.

The present invention relates to a pickling method including a neutral salt electrolytic pickling process and a mixed acid immersion step including sulfuric acid, hydrofluoric acid, and hydrogen peroxide to remove an oxide scale generated by cold rolling annealing of 14% or less low chromium-containing ferritic stainless steel. It provides a method and apparatus for pickling ferritic stainless steel cold rolled steel sheet, including a grinding step of removing an oxidative scale by grinding with a grinding brush including abrasive particles before the mixed acid dipping step.

In general, the mixed acid immersion process for the mixed acid solution containing no nitric acid is a process of selectively removing only the scale to minimize the damage to the base material than the mixed acid tank to which nitric acid is applied. The immersion time of the mixed acid immersion step for the mixed acid solution containing no nitric acid can be shortened. Accordingly, the present invention is to remove the oxidized scale to the maximum by physically grinding the stainless steel by applying a grinding brush containing abrasive particles during or after the neutral salt electrolytic pickling process, which is usually applied to the pickling process, before or after the mixed acid immersion process.

To this end, in the pickling apparatus for removing the oxidation scale generated by cold rolling annealing of the ferritic stainless steel including the neutral salt electrolytic bath and the mixed acid bath, the oxidation scale is removed by grinding with a grinding brush containing abrasive particles before the mixed acid bath. Install the grinding brush unit.

The grinding process performed using the grinding brush is a conventional brush that can replace the cleaning nylon brush mounted on the rear end of the neutral salt electrolyzer with a grinding brush, in order to install a new equipment or to change the process It can be installed at low cost without the need.

However, for effective grinding, such a grinding brush may be installed to be performed during or after the neutral salt electrolytic pickling process, and may be applied to all of them if necessary.

The grinding brush preferably includes abrasive particles for grinding. The abrasive particles may be particles such as SiC, Al ₂ O ₃ , ZrO, and the like, and the abrasive particle size may be those of # 100 to 240 in the KSL 6001 standard. In the case of using finer abrasive grains, there may be scale remaining on the surface of the steel sheet, and when it exceeds 240, damage to the surface of the base material is not preferable.

In addition, it is preferable that the grinding brush is made of a polyamide-based synthetic resin that is resistant to friction, such as a material of the brush base material, instead of the nylon used in the cleaning brush, and brush diameters of 0.8 to 1.2 mm may be used.

By providing such a grinding brush during or after the neutral salt electrolytic pickling process, the grinding can be suppressed even if the acid pickling process is not immersed in the mixed pickling process for a long time, thus reducing the pickling process time. have.

The oxidation scale removal may be performed by rotating the grinding brush, wherein the rotation speed of the grinding brush is preferably in the range of 800 to 1000 rpm when including the abrasive particles described above.

In the present invention, in the pickling process, the neutral salt electrolytic pickling process may be performed as generally performed, but is not limited thereto. For example, 100 to 250 g of a sodium sulfate solution at a temperature of 50 to 90 ° C. Using / L as the electrolyte, it is possible to carry out by applying a current of 10 ~ 30A / dm 2 to the cold rolled steel sheet surface for 5 to 55 seconds so that the potential of the steel sheet surface can be charged in the order of +,-, +. Furthermore, the mixed acid immersion process may also be carried out as generally performed, but is not limited thereto. For example, the mixed acid solution may include 70-200 g / L sulfuric acid and 0.5-10 g / L free hydrofluoric acid. It can be performed by immersing in the pickling composition whose initial hydrogen peroxide concentration in a mixed acid solution containing no ion is 7 g / L or more for 10 to 100 seconds.

Hereinafter, an Example is given and this invention is demonstrated more concretely. However, the present invention is not intended to be limited to the following examples.

[Example]

Comparative Example 1

After the cold-rolled annealing of the ferritic stainless steel sheet containing 14% or less of chromium, the pickling process was performed using a pickling process equipment composed of a neutral salt electrolyzer 1, a neutral salt electrolyzer 1, a mixed acid bath 1 and a mixed acid bath 2.

Specific process conditions are as follows.

200 g / L of sodium sulfate solution at 80 ° C is applied to a neutral salt electrolyzer containing electrolyte for 10 seconds to apply electrolysis of 10A / dm 2 for 20 seconds so that the potential of the steel plate surface can be charged in the order of +,-, +. It was done. Thereafter, the mixture was immersed and pickled for 25 seconds in the mixed acid bath in which the mixed acid pickling composition having a temperature of 120 g / L sulfuric acid, 5 g / L free hydrofluoric acid, and 10 g / L hydrogen peroxide was loaded.

The pickling of the steel plate pickled through the same process as described above is shown in Table 1 below.

Example 1

After the cold-rolled annealing of the ferritic stainless steel sheet containing 14% or less of chromium, the pickling process was performed using a pickling process equipment composed of a neutral salt electrolyzer 1, a neutral salt electrolyzer 1, a mixed acid bath 1 and a mixed acid bath 2. A grinding brush unit 1 made of a polyamide brush base material was installed between the neutral salt electrolyzers 1 and 2. The grinding brush unit has a brush hair diameter of 0.8-1.2mm, the brush includes an abrasive having a particle size of the particle number # 100-240 according to the KSL 6001 standard made of SiC material, the brush rotational speed is 800-1000rpm It set and polished. Specific conditions are as shown in Table 1.

Specific process conditions in the neutral salt electrolytic bath and mixed acid bath were carried out under the same conditions as in Comparative Example 1.

The pickling of the steel plate pickled through the same process as described above is shown in Table 1 below.

Example 2

Other conditions were carried out in the same manner as in Example 1 except that the facilities for the pickling process were performed in the order of the neutral salt electrolyzer 1, the neutral salt electrolyzer 2, the grinding brush 2, the mixed acid tank 1 and the mixed acid tank 2.

Specific process conditions in the neutral salt electrolytic bath and mixed acid bath were carried out under the same conditions as in Comparative Example 1.

The pickling of the steel plate pickled through the same process as described above is shown in Table 1 below.

Example 3

Other conditions were carried out in the same manner as in Example 1 except that the equipment for the pickling process was performed in the order of the neutral salt electrolyzer 1, the grinding brush 1, the neutral salt electrolyzer 2, the grinding brush 2, the mixed acid tank 1 and the mixed acid tank 2.

Specific process conditions in the neutral salt electrolytic bath and mixed acid bath were carried out under the same conditions as in Comparative Example 1.

The pickling of the steel plate pickled through the same process as described above is shown in Table 1 below.

Grinding 1 Grinding 2 Pickling degree Granularity Brush diameter Rotation speed Granularity Brush diameter Rotation speed Comparative Example 1 - - - - - - Misan tax Example 1-1 100 0.8 800 - - - Pickling Examples 1-2 100 0.8 1000 - - - Pickling Example 1-3 240 1.2 800 - - - Misan tax Example 1-4 240 1.2 1000 - - - Misan tax Example 2-1 - - - 100 0.8 800 Misan tax Example 2-2 - - - 100 0.8 1000 Pickling Example 2-3 - - - 240 1.2 800 Misan tax Examples 2-4 - - - 240 1.2 1000 Pickling Example 3-1 240 1.2 800 100 0.8 1000 Taxation Example 3-2 240 1.2 1000 100 0.8 800 Taxation Example 3-3 240 1.2 1000 240 1.2 800 Pickling Example 3-4 100 0.8 1000 240 1.2 1000 Taxation Example 3-5 240 1.2 800 240 1.2 1000 Pickling Examples 3-6 100 0.8 1000 100 0.8 1000 Taxation Example 3-7 100 0.8 800 240 1.2 800 Taxation Examples 3-8 100 0.8 800 100 0.8 800 Taxation

As can be seen from Table 1, in the case of the comparative example, the electrolytic pickling ability was insufficient, and the residual scale after pickling by mixed acid immersion was observed, and the result of the micro pickling was shown.

On the other hand, in the case of Example 1, the brush diameter is 1.2mm, when the particle size using SiC of particle size # 100 according to the KSL 6001 standard, and grinding at a rotational speed of 800 to 1000rpm, sufficient pickling was performed without pickling. In the case of finer brushes, the residual scale was observed due to the scale remaining.

In the case of Example 2, it can be seen that results similar to those of Example 1 are obtained. However, in Example 2-1, the residual scale was prepared, and if the polishing was performed during neutral salt electrolysis, the base material was eluted under the scale in the electrolytic pickling process after polishing, but the polishing was not performed after the neutral salt electrolytic pickling. Therefore, it can be confirmed that the case of Example 1 is more advantageous than Example 2.

Furthermore, in Example 3, when using a fine brush as in Examples 3-3 and 3-5, there is no residual scale and a beautiful surface can be secured, but in other cases, an over-aciding occurs and polishing It was confirmed that the flaw remained strong and the surface gloss was somewhat deteriorated.

1. Neutral Salt Electrolyzer 2. Neutral Salt Electrolyzer
3. Mixed acid tank 4. Mixed acid tank
5. Grinding Brush Unit 1 6. Grinding Brush Unit 2
7. Strip

Claims (7)

A pickling process including a neutral salt electrolysis process and a mixed acid immersion process for removing an oxidized scale generated by cold rolling annealing of a low chromium-containing ferritic stainless steel of 14% or less, comprising abrasive particles prior to the mixed acid immersion process. A pickling process comprising a grinding step of removing an oxide scale by grinding with a grinding brush.
The pickling process according to claim 1, wherein the grinding step is performed during, after, or both of the neutral salt electrolysis processes.
The pickling process according to claim 1, wherein the abrasive particles are silicon carbide.
The pickling process according to claim 3, wherein the abrasive particles have particle size numbers # 100 to # 240 according to KSL 6001 standard.
The pickling process according to claim 1, wherein the grinding brush is a brush base material made of polyamide-based synthetic resin, and the diameter of the brush hair is 0.8 to 1.2 mm.
The neutral salt electrolysis process according to any one of claims 1 to 5, wherein 100 to 250 g / L of a sodium sulfate solution at a temperature of 50 to 90 ° C is used as an electrolyte, and the potential of the steel sheet surface is +,-, +. Pickling process characterized in that is performed by applying a current of 10 ~ 30A / dm 2 to the surface of the cold rolled steel sheet for 5 to 55 seconds to be charged in order.
The mixed acid immersion step according to any one of claims 1 to 5, wherein the mixed acid immersion step includes an initial composition of a mixed acid solution of 70-200 g / L sulfuric acid, 0.5-10 g / L free hydrofluoric acid, and a concentration of 7.0 g / L hydrogen peroxide. Pickling process characterized in that it is carried out by immersing in a mixed acid solution containing substantially no iron ions for 10 to 100 seconds.

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