WO2015016538A1

WO2015016538A1 - Method for pickling high-chrome ferritic stainless steel

Info

Publication number: WO2015016538A1
Application number: PCT/KR2014/006812
Authority: WO
Inventors: 김진석; 이용헌; 김지훈
Original assignee: 주식회사 포스코
Priority date: 2013-07-31
Filing date: 2014-07-25
Publication date: 2015-02-05
Also published as: CN105431574B; CN105431574A; JP2016525632A; JP6105167B2; KR101461815B1

Abstract

The present invention relates to a method for pickling high-chrome ferritic stainless steel, and more specifically, provides a method for pickling a high-chrome ferritic stainless cold-rolled plate, the method comprising: an electrolytic pickling step of subjecting a high-chrome ferritic stainless cold-rolled plate containing 17-26 wt% of chrome to neutral salt electrolytic pickling and sulfuric acid electrolytic pickling; and a mixed acid soaking step of soaking the cold-rolled plate, which has been subjected to the electrolytic pickling step, in a mixed acid solution containing sulfuric acid and hydrofluoric acid, wherein, in the electrolytic pickling step, the amount of applied current is controlled depending on the annealing temperature, and in the mixed acid soaking step, the content of free HF contained in the mixed acid solution and the oxidation-reduction potential (ORP) are controlled depending on steel components.

Description

Pickling method of high chrome ferritic stainless steel

The present invention relates to a method for pickling high chromium ferritic stainless steel cold rolled steel sheets requiring high surface quality at high speed, and more specifically, to adjust the amount of applied current during electrolytic pickling according to the annealing temperature and the steel grade component system, The present invention relates to a method for effectively removing annealing scale present in the cold rolled steel sheet by pickling by adjusting the free hydrofluoric acid content and redox potential in the solution.

In general, ferritic stainless steels are classified into low chromium ferritic stainless steels and high chromium ferritic stainless steels depending on the chromium content. In general, a chromium content of 11 to 14% by weight is called low chromium ferritic stainless steel, and a chromium content of 17 to 26% by weight is called high chromium ferritic stainless steel.

Since the characteristics of the scale formed during the annealing heat treatment vary depending on the chromium content, the pickling method according to the chromium content should be performed differently. In general, in the case of low chromium ferrite steel, the scale is thickly formed during the annealing heat treatment, and in the case of the high chromium ferrite steel, the scale is thinner than the low chromium ferrite steel.

In addition, there is a difference in the heat treatment temperature during annealing according to the characteristics of the steel type, the higher the annealing temperature is formed, the thicker the scale, the higher the content of chromium component in the scale.

On the other hand, such a scale may not only reduce the appearance quality of the product, but also may cause corrosion from the oxidized scale to reduce the corrosion resistance, it is necessary to remove the scale formed on the surface through the pickling process.

Typically, in the manufacture of stainless steel cold rolled steel sheet, physical descaling, such as brush treatment or shotball blasting, sodium sulfate, sulfuric acid or Various methods, such as electrolytic descaling using a nitric acid electrolyte and the like, chemical descaling by a salt bath or mixed acid, etc., have been performed. This process is referred to as a 'pickling process'.

In this pickling process, one-step electrolytic pickling, such as electrolytic descaling, and two-step mixed acid immersion, such as chemical descaling, are distinguished.

Thus, the process of pickling stainless steel cold rolled steel sheet has conventionally been performed using a mixed acid solution containing 80 to 180 g / L nitric acid and 2 to 40 g / L hydrofluoric acid. However, the nitric acid contained in the mixed acid solution lowers the pH in the pickling bath to increase the activity of hydrofluoric acid, and serves to maintain a redox potential appropriate for pickling by oxidizing divalent iron ions dissolved in the surface of the steel sheet to trivalent. It generates NOx, an air emission control substance, and contains a large amount of nitrate nitrogen (NO ₃ -N) in waste acid and washing water.

Accordingly, it is required to additionally install and operate an environmental pollution prevention facility in the pickling process by restricting the total amount of nitrogen in the discharged water and limiting the concentration of NOx in the air discharge facility by strengthening environmental regulations at home and abroad. In the operation of such a facility, a lot of costs are generated, which leads to a significant increase in production costs.

Therefore, in order to solve this problem, a method of replacing nitric acid contained in the mixed acid solution used in the conventional pickling process with hydrochloric acid or sulfuric acid, etc. has been proposed. In this case, since the oxidative property of the mixed acid solution is weak, the dissolution rate on the scale is low. In the dissolution process, blackening may occur on the metal surface.

Thus, methods have been developed to compensate for the insufficient oxidative power by hydrogen peroxide, potassium permanganate, trivalent iron ions and air injection, so as to reach the strip surface potential with the best pickling properties, and especially on degradation, especially during decomposition. Hydrogen peroxide, which does not give it, is commonly used as an oxidizing agent.

In this case, however, it is not easy to find an appropriate dosage of the added oxidizing agent, and in some cases, it may result in unpickling or peroxidation, and it is effective for problems such as blackening on the surface of the steel sheet. There is a problem that can not cope with.

On the other hand, US Patent Publication No. 5908511 contains sulfuric acid, hydrofluoric acid, iron salts and hydrogen peroxide is regularly added, and the pickling is adjusted by adjusting the composition of the wetting agent, the brightening agent, the corrosion inhibitor, etc., the pickling solution is Fe (III) and redox accordingly A technique for managing the oxidation-reduction potential (ORP) through automatic control has been proposed.

In addition, the pickling solution of the US patent is commercialized as a CLEANOX352 product is the world's most widely used, the pickling method of the patent has been put to practical use in wire rods and hot rolled products.

However, in the case of the pickling method of the above-described US patent, the production cost of the product is more than 20% higher than the conventional, and adopts a complex solution composition and management method, which is a problem, and most crucially, the pickling rate is 1.5 to It has a relatively slow speed of about 3 g / m ² · min and poses a problem that it is not suitable for high speed pickling lines in which mixed acid pickling should be completed within 10 to 100 seconds.

Thus, as an improved patent for the US patent, European Patent Publication No. 1040211 and US Patent Publication No. 2000-560982 have proposed a method of increasing the pickling rate by adding copper and chlorine ions to the pickling composition, but the method In this case, when the open circuit potential (OCP) formed on the surface of the ferritic stainless steel sheet is lower than 0.1 V, which is the redox potential of the copper ions, copper particles precipitate on the surface of the steel sheet during the pickling process to discolor the steel sheet. If the pickling solution contains more than a certain concentration of chlorine ions, pitting corrosion may occur.

The present invention makes it possible to provide a stainless steel sheet having no residual oxidation scale and excellent surface glossiness.

In addition, the present invention, when immersing the steel sheet in the mixed acid solution, it is possible to determine the appropriate amount of hydrogen peroxide to be added to the mixed acid solution according to the redox potential, so that the surface of the steel sheet is not pickled or over-acidified, or black phenomena occur To prevent it.

In addition, the present invention does not use nitric acid as a mixed acid solution used in the pickling process of the steel sheet, it is possible to reduce the burden of treatment for waste water and exhaust gas.

According to one embodiment of the present invention, an electrolytic pickling step of neutral salt electrolytic pickling and sulfuric acid electrolytic pickling of a high chromium ferritic stainless steel cold rolled steel sheet containing 17-26 wt% chromium annealed at an annealing temperature of 1030 ° C. or more and 1050 ° C. or less. ; And

It comprises a mixed acid immersion step of immersing the cold rolled steel sheet subjected to the electrolytic pickling in a mixed acid solution containing sulfuric acid and hydrofluoric acid,

The sum of the application of the electrolytic neutral salt during the pickling step pickling current and applied to the electrolytic pickling sulfate, the amount of current provides a pickling method of the high-chromium ferritic stainless cold-rolled steel sheet, which is adjusted to not more than 12A / dm ² or more 20A / dm ^2.

According to another embodiment of the present invention, an electrolytic pickling step of neutral salt electrolytic pickling and sulfuric acid electrolytic pickling of a high chromium ferritic stainless steel cold rolled steel sheet containing 17-26 wt% chromium annealed at an annealing temperature of 930 ° C. or more and less than 1030 ° C. ; And

In the electrolytic pickling step, the sum of the applied current amount of neutral salt electrolytic pickling and the applied current amount of sulfuric acid electrolytic pickling is adjusted to 30 A / dm ² or more and 50 A / dm ² or less, to provide a method for pickling a high chromium ferritic stainless steel cold rolled steel sheet.

The amount of applied current of the neutral salt electrolytic pickling may be 8 A / dm ² or more, and the amount of applied current of sulfuric acid electrolytic pickling may be 0 or 8 A / dm ² or more.

When the silicon (Si) content is 0.3% by weight or less or the molybdenum (Mo) content is 0.5% by weight or less with respect to the total weight of the cold rolled steel sheet,

The mixed acid solution used in the mixed acid immersion step includes 80 to 150 g / L sulfuric acid and 5 to 12 g / L free hydrofluoric acid, and an redox potential (ORP) may be 550 mV or more.

Hydrogen peroxide may be added to the mixed acid solution such that the redox potential of the mixed acid solution is maintained at 550 mV or more.

When the silicon (Si) content is more than 0.3% by weight or the molybdenum (Mo) content is more than 0.5% by weight relative to the total weight of the cold rolled steel sheet,

The mixed acid solution used in the mixed acid immersion step includes 80 to 150 g / L sulfuric acid and 20 to 30 g / L free hydrofluoric acid, and an redox potential (ORP) may be 320 mV or more.

Hydrogen peroxide may be added to maintain the redox potential of the mixed acid solution at 320 mV or more.

The neutral salt electrolytic pickling may be performed using an electrolyte solution containing sodium sulfate at a concentration of 100 to 250 g / L.

The temperature of the electrolyte used during the neutral salt electrolytic pickling may be 50 ~ 90 ℃.

The neutral salt electrolytic pickling may be performed for 24 to 100 seconds.

The sulfuric acid electrolytic pickling is an electrolyte solution comprising a metal sulfate formed by dissolving at least one metal selected from the group consisting of iron, chromium, nickel, copper, manganese and titanium in 50 to 150 g / l sulfuric acid or 50 to 150 g / l sulfuric acid. Can be performed using

The temperature of the electrolyte solution used in the sulfuric acid electrolytic pickling may be 30 ~ 60 ℃.

The sulfuric acid electrolytic pickling may be performed for 10 to 50 seconds.

The mixed acid immersion step may be performed for 25 to 90 seconds.

The surface potential of the cold rolled steel sheet during the mixed acid immersion step may be maintained in the range of -0.2 to 0V.

The present invention provides a high chromium ferritic stainless steel cold rolled steel sheet having no residual oxidation scale and excellent surface glossiness.

In addition, the present invention, when immersing the steel plate in the mixed acid solution, it is possible to determine the appropriate amount of hydrogen peroxide that is added to the mixed acid solution according to the redox potential, so that the surface of the steel sheet is not pickled or over-acidified, or black phenomena occur To prevent it.

1 is a graph showing the Cr / Fe content ratio according to the depth of the annealing scale of the 439 steel and 441 steel.

Figure 2 shows the applied current region of the electrolytic pickling neutral salt electrolytic cell and sulfuric acid electrolytic cell according to the annealing temperature.

Figure 3 shows the concentration of free hydrofluoric acid contained in the mixed acid solution used in the mixed acid immersion step and the redox potential region of the mixed acid solution according to the content of silicon and molybdenum contained in the cold rolled steel sheet.

Figure 4 (a) to (d) shows a photograph of the surface of the stainless steel after pickling in Comparative Example 1, Example 1, Comparative Example 8 and Comparative Example 9.

5A to 5C show photographs of the surface of stainless steel after pickling in Comparative Examples 16, 6, and 28. FIG.

6 (a) and 6 (b) show photographs of the surface of stainless steel after pickling in Comparative Examples 29 and 31. FIG.

Figure 7 (a) and (b) is a photograph showing the surface of the stainless steel after pickling in Comparative Example 32 and Example 16.

(A) and (b) of FIG. 8 are photographs of the surface of stainless steel after pickling in Comparative Examples 33 and 19. FIG.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the present invention. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

The present invention relates to a method for pickling to remove an oxidation scale present on the surface of a high chromium ferritic stainless steel cold rolled steel sheet containing 17 to 26% by weight chromium based on the total weight of the cold rolled steel sheet.

More specifically, the method for pickling a high chromium ferritic stainless steel cold rolled steel sheet provided by the present invention includes an electrolytic pickling step of neutral salt electrolytic pickling and sulfuric acid electrolytic pickling of the high chromium ferritic stainless steel cold rolled steel sheet and a cold rolled steel sheet subjected to the electrolytic pickling. To include a mixed acid immersion step immersed in a mixed acid solution containing sulfuric acid and hydrofluoric acid, the amount of applied current is adjusted according to the annealing temperature during the electrolytic pickling step, free hydrofluoric acid contained in the mixed acid solution according to the steel species component during the mixed acid immersion step It is characterized by adjusting the content of (Free HF) and the redox potential (ORP).

In the pickling method of the present invention, the high chromium ferritic stainless steel cold rolled steel sheet may be subjected to a neutral salt electrolytic pickling and sulfuric acid electrolytic pickling in an electrolytic pickling process.

At this time, the neutral salt electrolytic pickling may be performed using an electrolyte solution containing sodium sulfate of 100 ~ 250g / L concentration as an electrolyte, specifically after immersing the cold rolled steel sheet in the electrolyte solution having a temperature of 50 ~ 90 ℃ In order that the potential of the surface of the steel sheet can be charged at least once in the order of +,-, +, a current of 8 to 30 A / dm ² may be applied for 24 to 100 seconds.

In addition, the sulfuric acid electrolytic pickling is a sulfuric acid of 50 ~ 150g / l or a metal sulfate formed by the electrochemical dissolution of at least one metal selected from the group consisting of iron, chromium, nickel, copper, manganese and titanium in the sulfuric acid of the concentration. It can be carried out using an electrolyte containing an electrolyte, specifically, after immersing the cold rolled steel sheet in the electrolyte having a temperature of 30 ~ 60 ℃, the potential of the steel plate surface at least once in the order of +,-, + It can be carried out by applying a current of 0 ~ 30A / dm ² density for 10 to 50 seconds to be charged over.

On the other hand, in the case of high chromium ferritic stainless steel, the content of chromium is high in the scale generated during annealing, in particular, the higher the annealing temperature, the higher the chromium content. At this time, the higher the chromium content contained in the annealing scale, the easier the dissolution of the oxide during neutral salt electrolytic pickling. For example, when the annealing temperature is 1030 ° C. or higher, the concentration of the chromium is severely concentrated, and the neutral salt and sulfuric acid electrolytic pickling is performed. Under high application current conditions, there is a risk of overacid washing due to surface melting.

Therefore, according to one embodiment of the present invention, when electrolytic pickling the cold-rolled steel sheet annealed at a relatively high temperature of 1030 ℃ or more, 1050 ℃ or less, the sum of the applied current amount of neutral salt electrolytic pickling and the applied current amount of sulfuric acid electrolytic pickling If less than 12A / dm ² , fine pickling may occur, and if it exceeds 20A / dm ² , surface erosion may occur due to over pickling. Therefore, for effective pickling, the amount of applied current of neutral salt electrolytic pickling and sulfuric acid electrolytic pickling the sum is preferably adjusted to 20A / dm ² or less than 12A / dm ^2.

On the other hand, according to another embodiment of the present invention, 930 ℃ When electrolytic pickling of the cold rolled steel sheet annealed at a relatively low temperature of less than 1030 ℃, if the sum of the applied current amount of neutral salt electrolytic pickling and the applied current amount of sulfuric acid electrolytic pickling is less than 30A / dm ² , fine pickling may occur, If it exceeds 50A / dm ² , surface erosion may occur due to over pickling. Therefore, for effective electrolytic pickling, the sum of the applied current of neutral salt electrolytic pickling and the applied current of sulfuric acid electrolytic pickling is 30 A / dm ² or more and 50 A / dm ² or less It is preferable to adjust so that.

However, in the present invention, the amount of applied current of neutral salt electrolytic pickling is 8 A / dm ² or more, and the amount of applied current of sulfuric acid electrolytic pickling is 0 or 8 A / dm ² or more regardless of the annealing temperature during electrolytic pickling.

Specifically, when the amount of applied current during neutral salt electrolytic pickling is less than 8 A / dm ² , the surface potential for dissolving the annealing scale is not formed, and the effect of the neutral salt electrolytic pickling may be insignificant. It is preferable that the amount of applied current is 8 A / dm ² or more.

In addition, when the amount of applied current is greater than 0 and less than 8 A / dm ² during electrolytic pickling of sulfuric acid, a problem may occur in that the surface is uneven and rough after pickling. / dm ² or 8A / dm ² or more is preferable.

However, when the applied current amount of sulfuric acid electrolytic pickling is 0A / dm ² , when the annealing scale dissolves largely in neutral salt electrolytic pickling, which is a preliminary step of sulfuric acid electrolytic pickling, deposition proceeds without application of current to prevent damage to the base metal. It means to be.

In addition, in the present invention, with respect to the upper limit of the applied current amount of the neutral salt electrolytic pickling and sulfuric acid electrolytic pickling, the value is determined by the sum of the applied current amounts of the two electrolytic pickling, but is not particularly limited, but it is economical such as operating cost In consideration of the fact that, for example, the upper limit of the amount of applied current is preferably 30 A / dm ² or less.

FIG. 2 is a diagram illustrating an applied current region of a neutral salt electrolytic cell and a sulfuric acid electrolytic cell that are electrolytic pickled according to an annealing temperature in the present invention. In FIG. 2, an annealing temperature of a ferritic stainless steel containing 17 to 26% by weight of chromium is 1030. The applied current range of the neutral salt electrolytic cell and sulfuric acid electrolytic cell which can be pickled in the region of -1050 ° C is shown, and the area II shows the applied current range of the neutral salt electrolytic cell and sulfuric acid electrolytic cell which can be pickled at annealing temperature of 930 ° C or more and less than 1030 ° C.

While the oxide layer of chromium and iron present on the surface of the high chromium ferritic cold rolled steel sheet is removed during the electrolytic pickling in the present invention, the silicon oxide layer may remain. Therefore, in the pickling method of the present invention, the silicon oxide remaining on the steel sheet by performing the mixed acid immersion step of immersing the cold rolled steel sheet subjected to the electrolytic pickling in the electrolytic pickling step in a mixed acid solution containing sulfuric acid and hydrofluoric acid. The layer of can be removed.

Hydrofluoric acid (HF) contained in the mixed acid solution used in the mixed acid immersion step is dissociated in the solution as shown in the following formula (1), and hydrogen ions (H ⁺⁾ provided by dissociating sulfuric acid as shown in the following formula (2) The equilibrium state is changed by the concentration of c, i.e. acidity.

^{^{HF ↔ H + + F - (}} 1)

_{_{_{^{H 2 SO 4 ↔ HSO 4 -}}}} + H + ↔ SO 4 2- + 2H + (2)

However, in the case of hydrofluoric acid, it has a pickling force in a free hydrofluoric acid (Free HF) state, and the free hydrofluoric acid penetrates the interface between the silicon oxide and the base material to dissolve silicon oxide and iron (Fe), and then dissolves silicon. Ions and iron ions are combined in the form of FeF _x ^(3-x) , H ₂ SiF ₆ and the like to remove them from the steel sheet surface.

Therefore, in the present invention, in the mixed acid immersion step, it is preferable to use a mixed acid solution having a concentration of 5 to 30 g / L of free hydrofluoric acid. When the concentration of free hydrofluoric acid is less than 5 g / L, the concentration of free hydrofluoric acid is small, so that there is a lack of solubility in the silicon oxide layer may cause the problem of fine pickling on the surface of the steel sheet, when the concentration exceeds 30 g / L, The rate of erosion of the base material may increase, resulting in a rough surface of the steel sheet after pickling.

As described above, the hydrofluoric acid contained in the mixed acid solution provides pickling force capable of removing the silicon oxide layer on the surface of the steel sheet, but it is necessary to maintain the effective free hydrofluoric acid concentration in the mixed acid solution above a certain acidity. Therefore, in the present invention, in order not to dissociate the hydrofluoric acid contained in the mixed acid solution, it is preferable that the mixed acid solution contains sulfuric acid at a predetermined concentration or more together with the hydrofluoric acid.

At this time, a suitable sulfuric acid concentration is preferably 80 to 150 g / l, and when the concentration of sulfuric acid is less than 80 g / l, the effective free hydrofluoric acid concentration is not maintained, so that dissociation of hydrofluoric acid occurs and the pickling force is weakened. If the concentration is more than 150g / L may cause problems such as heat generation during the sulfuric acid dilution operation is difficult to operate.

On the other hand, in the oxidation scale of ferritic stainless steel, silicon oxide exists in both the grain surface of the ferritic crystal and the grain boundary between grain and grain, and the silicon oxide of the grain boundary exists in a deeper position inside the base material. In the case of austenitic stainless steels, the corrosion resistance of crystals is high and erosion is preferentially from the grain boundaries, whereas ferritic stainless steels have low corrosion resistance, so there is no difference in the erosion rate between the grains and the grains. It is completely dissolved without selectivity. Therefore, in order to remove all the silicon oxide, a considerable part of base metal needs to be dissolved.

At this time, Fe ²⁺ is eluted from the base material, and the eluted Fe ²⁺ is oxidized to Fe ³⁺ when reacted with an oxidizing agent such as hydrogen peroxide, and then combined with free hydrofluoric acid to form a FeF _x ^(3-x) complex. Can be removed from the steel plate surface. The reaction may be expressed as in the following formulas (3) to (6), it is possible to increase the pickling rate only when such a process proceeds smoothly.

^{^{^{Fe 0 → Fe 2+ + 2e -}}} (3)

^{_{_{Fe 2+ + H 2 O 2 →}}} Fe 3+ + OH + OH -? (4)

Fe ³⁺ + 3HF → FeF ₃ + 3H ⁺ (5)

^{^{^{Cu 2+ + 2e - → Cu 0}}} (6)

At this time, when the concentration of hydrogen peroxide in the mixed acid pickling solution is insufficient, the reaction of Formula (4) does not occur, the Fe ²⁺ concentration on the surface of the steel sheet is locally increased, the reaction in the left direction of the formula (3) is predominant. In this case, as shown in Equation (6), Cu, which is present as an additive or an impurity in Fe and stainless steel, is reprecipitated on the surface of the steel sheet, and a blackening phenomenon occurs in which the steel sheet surface turns black. Therefore, it is desirable to maintain the content of hydrogen peroxide at a constant level at all times so that the reaction of Formula (4) can be sufficiently made.

On the other hand, when pickling is performed by immersing the cold rolled steel sheet in a mixed acid solution as described above, it is possible to remove some residual silicon oxide layer by dissolving the base material, the higher the content of molybdenum (Mo) in the steel species It can be high to slow the dissolution rate during pickling, and the higher the content of silicon (Si), the thicker the layer of silicon oxide is formed, it is difficult for the mixed acid solution to dissolve the base material.

Therefore, in the pickling method of the present invention, the content and redox of the free hydrofluoric acid (Free HF) contained in the mixed acid solution according to the content of silicon (Si) and molybdenum (Mo) contained in the cold rolled steel sheet, Pickling can be effectively performed by adjusting the potential ORP.

That is, in the case of high chromium ferritic stainless steel, when the silicon (Si) content is 0.3% by weight or less or the molybdenum (Mo) content is 0.5% by weight or less, most of the annealing scale is removed in the electrolytic pickling step, and the silicon remaining in a trace amount Since only the oxides need to be removed, it is desirable to minimize the erosion of the base material by lowering the concentration of free hydrofluoric acid.

Therefore, in the present invention, when the silicon content is 0.3% by weight or less or the molybdenum content is 0.5% by weight or less, the mixed acid solution preferably includes 80-150 g / L sulfuric acid and 5-12 g / L free hydrofluoric acid. In order to prevent blackening, it is preferable to add hydrogen peroxide such that the redox potential (ORP) of the mixed acid solution is maintained at 550 mV or more.

On the other hand, if the silicon (Si) content in the high chromium ferritic stainless steel exceeds 0.3% by weight, a large amount of silicon oxide may be present on the surface of the steel sheet before the immersion step in the mixed acid solution after electrolytic pickling, in order to effectively remove Preference is given to using high concentrations of free hydrofluoric acid.

In addition, when the content of molybdenum (Mo) in the high chromium ferritic stainless steel exceeds 0.5% by weight, the rate of dissolution of the base material by the mixed acid solution is significantly increased due to the high corrosion resistance due to the high content of molybdenum contained in the steel sheet Since it may fall, it is preferable to use what contains a high concentration of free hydrofluoric acid as a mixed acid solution.

Therefore, in the present invention, when the silicon content is more than 0.3% by weight or the molybdenum content is more than 0.5% by weight, the mixed acid solution includes sulfuric acid at a concentration of 80 to 150 g / L and free hydrofluoric acid at a concentration of 20 to 30 g / L. In order to prevent blackening, it is preferable to add hydrogen peroxide such that the redox potential (ORP) of the mixed acid solution is maintained at 320 mV or more.

However, in the pickling method of the present invention, the redox potential range of the mixed acid solution does not have to be particularly limited in its upper limit regardless of the silicon and molybdenum content contained in the stainless steel, The metal content is very high, and in such a situation, if the redox potential is kept high, the excessive amount of fruit may be excessive, which may be an economical problem. For example, the upper limit is preferably maintained at 600 mV or less.

Figure 3 shows the concentration of free hydrofluoric acid contained in the mixed acid solution used in the mixed acid immersion step and the redox potential region of the mixed acid solution according to the content of silicon and molybdenum contained in the cold rolled steel sheet in the present invention.

Specifically, region III of FIG. 3 shows free hydrofluoric acid concentration which can be pickled when immersed in a mixed acid solution after electrolytic pickling in a stainless steel component having a Si content of 0.3 wt% or less or a Mo content of 0.5 wt% or less in a high chromium ferritic stainless steel. Is 5-12 g / L, and the redox potential range of the solution is 550 mV or more.

In addition, the region IV of FIG. 3 is capable of pickling when immersed in a mixed acid solution after electrolytic pickling with a stainless steel component system having a Si content of more than 0.3 wt% or a Mo content of more than 0.5 wt% in a high chromium ferritic stainless steel. Free hydrofluoric acid concentration is 20 ~ 30g / L, the redox potential range of the solution is more than 320mV.

In addition, in the present invention, the surface potential of the cold rolled steel sheet during the mixed acid immersion step is preferably maintained in the range of -0.2 to 0V. In case of stainless steel, the dissolution rate and pattern are determined according to the surface potential. At a low surface potential of less than -0.2 V, the base metal is mainly dissolved as Fe ²⁺ and reacts with SO ₄ ^2- to form FeSO ₄ compound on the surface. Black phenomena may form on the surface, in which case the specific orientation of the grains dissolves severely when the base material is dissolved on the surface, and the surface becomes very rough, and Fe ³⁺ , which can be removed only at a high surface potential, is not removed. As a result, the tax and tax may be mixed. On the other hand, if the surface potential exceeds 0 V, the potential becomes too high before the surface dissolves, leaving some undesirable portions to be dissolved and removed, and thus the pickling rate can be very slow. In order to maintain the surface potential of, the overconsumption is very high, which can be economically problematic.

In the present invention, the execution time of the mixed acid immersion step is not particularly limited and may vary depending on operating conditions. However, according to the pickling method of the present invention, it may be performed within a short time of 25 to 90 seconds.

Hereinafter, the present invention will be described in more detail with reference to specific examples. The following examples are merely examples to help understanding of the present invention, but the scope of the present invention is not limited thereto.

In the following examples, 441 steel and 439 steel were used in the following examples, and the expression of the 441 steel and 439 steel indicates the content ratio of Cr / Fe component in the annealing scale, and the 441 steel was 1050 ° C. and the 439 steel was 970 ° C. heat-treated. The composition ratio of Cr / Fe content in the annealing scale is 8.5 in 441 steel and 4.3 in 439 steel.

In both steel grades, the chromium content is about 17 wt% based on the total weight, but the Cr / Fe ratio in the annealing scale is different due to the difference in annealing temperature due to the characteristics of the steel grade.

On the other hand, neutral salt electrolysis dissolves Cr ₂ O ₃ Cr ³⁺ oxide with Cr ⁶⁺ (Cr ₂ O ₄ ^2- ) ions, but Fe-based oxides are difficult to dissolve, resulting in the content of chromium in the scale. The higher the value, the better the scale removal. Therefore, both the 439 and 441 steels have high chromium content in the annealing scale, which is advantageous for neutral salt electrolytic pickling, but the 441 steel having a high Cr / Fe ratio may be more advantageous.

Furthermore, FIG. 1 is a graph showing the Cr / Fe ratios of the annealing scales of the 439 steel and the 441 steel, and the 439 steel and the 441 steel have a tendency to increase Cr / Fe due to the increase in the chromium content from the outer layer to the inner layer. Once fully entered into the base material, it can be seen that it converges to the Cr / Fe content ratio of the base material.

[Examples 1 to 5 and Comparative Examples 1 to 15]

Electrolytic pickling was carried out by adding a current density as shown in Table 1 below to a salt acid electrolyzer containing sodium sulfate as an electrolyte for 441 steel having an annealing temperature of 1050 ° C, and adding a current density as shown in Table 1 below to a sulfuric acid bath containing sulfuric acid as an electrolyte. Was then immersed in a mixed acid solution at 40 ° C. containing 80 g / L sulfuric acid and 8 g / L free hydrofluoric acid for 30 seconds. However, the residual hydrogen peroxide concentration was added to 3g / L to maintain the redox potential of the solution above 550mV.

After the pickling, the results are shown in Table 1 below, and when the scale does not remain on the surface of the steel sheet, it is expressed as ○, and when the scale remains, it is evaluated as unpickled and represented by ×. In addition, the case where the surface erosion by the superacid wash is represented by x.

In addition, the photograph of the surface of the stainless steel after pickling in Comparative Example 1, Example 1, Comparative Example 8, and Comparative Example 9 is shown to (a)-(d) of FIG.

Table 1

division	Neutral Salt Cell Current Density (A / dm ² )	Sulfuric acid electrolytic cell current density (A / dm ² )	synthesis	Pickling	Remarks
Comparative Example 1	6	0	6	x	Misan tax
Comparative Example 2	6	4	10	x	Misan tax
Comparative Example 3	6	8	14	x	Misan tax
Comparative Example 4	6	10	16	x	Misan tax
Comparative Example 5	6	20	26	x	Misan tax
Comparative Example 6	8	0	8	x	Misan tax
Comparative Example 7	8	4	12	x	Heterogeneous acid tax
Example 1	8	8	16	O	normal
Example 2	8	10	18	O	normal
Comparative Example 8	8	20	28	x	Taxation
Example 3	12	0	12	O	normal
Comparative Example 9	12	4	16	x	Heterogeneous acid tax
Example 4	12	8	20	O	normal
Comparative Example 10	12	10	22	x	Taxation
Comparative Example 11	16	20	36	x	Taxation
Example 5	16	0	16	O	normal
Comparative Example 12	16	4	20	x	Uneven pickling
Comparative Example 13	16	8	24	x	Taxation
Comparative Example 14	16	10	26	x	Taxation
Comparative Example 15	16	20	36	x	Taxation

(* 'Sum' in Table 1 means the sum of the current density of neutral salt electrolytic pickling and the current density of sulfuric acid electrolytic pickling)

As can be seen from Table 1, the neutral salt electrolytic cell current is 8A / dm ² or more, sulfuric acid electrolytic cell current is 0 or 8A / dm ² or more, as in Examples 1 to 5, the sum of the two electrolytic cell applied current amount is 12 A / dm ^2. If more than 20 a / dm ² or less can be seen a normal acid washing.

However, when the neutral salt electrolyzer current is less than 8 A / dm ² as in Comparative Examples 1 to 5, the scale dissolution amount is insufficient in the neutral salt electrolyzer, and the acid pickling occurs as in Comparative Examples 7, 9 and 12, and the neutral salt electrolyzer current is 8 A / dm ^2. Even if the current value of sulfuric acid electrolytic pickling is more than 0 and less than 8 A / dm ² it can be seen that non-uniform pickling occurred.

In addition, as in Comparative Examples 8, 10, 11, 13, 14, and 15, when the sum of the neutral salt electrolytic cell current and the sulfuric acid electrolytic cell current exceeds 20 A / dm ² , it can be seen that over-acidity occurs.

Figure 4 (a) to (d) is a photograph showing the surface of the stainless steel after pickling in Comparative Example 1, Example 1, Comparative Example 8 and Comparative Example 9, (a) is not pickled because the amount of applied current is insufficient It can be seen that there is a residual scale, and (b) can see the normal pickling surface according to Example 1. In addition, (c) it can be seen that the surface is not good due to the surface erosion occurs due to the pickling, (d) can also be seen that the surface is not good due to uneven pickling.

If the embodiment of the scale components in the chromium component ratio is high through the electrolytic pickling during pickling it is possible in a relatively low electrolytic current of 12A / dm ² or more 20A / dm ² or less the sum of the applied two electrolytic cell of the neutral salt electrolytic bath and the sulfuric acid electrolytic cell amperage It can be seen that.

[Examples 6 to 12 and Comparative Examples 16 to 28]

Electrolytic pickling was carried out by adding a current density as shown in Table 2 to the acidic salt bath containing sodium sulfate as an electrolyte in 439 steel having an annealing temperature of 970 ° C, and adding a current density as shown in Table 1 below to a sulfuric acid bath containing sulfuric acid as an electrolyte. Was then immersed in a mixed acid solution at 40 ° C. containing 80 g / L sulfuric acid and 8 g / L free hydrofluoric acid for 30 seconds. However, the residual hydrogen peroxide concentration was added to 3g / L to maintain the redox potential of the solution above 550mV.

After the pickling, the results are shown in Table 2 below. If no scale remained on the surface of the steel sheet, the result was expressed as ○. In addition, the case where the surface erosion by the superacid wash is represented by x.

In addition, photographs of the surface of the stainless steel after pickling in Comparative Example 16, Example 6, and Comparative Example 28 are shown in Figs.

TABLE 2

division	Neutral Salt Cell Current Density (A / dm ² )	Sulfuric acid electrolytic cell current density (A / dm ² )	synthesis	Pickling	Remarks
Comparative Example 16	6	0	6	x	Misan tax
Comparative Example 17	6	8	14	x	Misan tax
Comparative Example 18	6	12	18	x	Misan tax
Comparative Example 19	6	16	22	x	Misan tax
Example 6	6	24	30	O	normal
Comparative Example 20	8	0	8	x	Misan tax
Comparative Example 21	8	8	16	x	Misan tax
Comparative Example 22	8	12	20	x	Misan tax
Comparative Example 23	8	16	24	x	Misan tax
Example 7	8	24	32	O	normal
Comparative Example 24	16	0	16	x	Misan tax
Comparative Example 25	16	8	24	x	Misan tax
Comparative Example 26	16	12	28	x	Misan tax
Example 8	16	16	32	O	normal
Example 9	16	24	40	O	normal
Comparative Example 27	24	0	24	x	Misan tax
Example 10	24	12	36	O	normal
Example 11	24	20	44	O	normal
Example 12	24	26	50	O	normal
Comparative Example 28	24	30	54	x	Taxation

(* 'Sum' in Table 2 means the sum of the current density of neutral salt electrolytic pickling and the current density of sulfuric acid electrolytic pickling)

As can be seen from Table 2, as in Examples 6 to 12, the neutral salt electrolyzer current is 8 A / dm ² or more, and the sum of the two electrolytic cell applied currents of the neutral salt electrolyzer and the sulfuric acid electrolyzer is 30 A / dm ² or more and 50 A / dm ² or less The case can be seen from the normal pickling surface.

However, when the neutral salt electrolyser current is less than 8 A / dm ² as in Comparative Examples 16 to 19, the scale dissolution amount is insufficient in the neutral salt electrolyzer, and the neutral salt electrolyzer current is 8 A / dm ² or more as in Comparative Examples 20 to 27. Even if the sum of the neutral salt electrolytic cell current and the sulfate electrolytic cell current is less than 30A / dm ² it can be seen that the pickling occurs. In addition, as in Comparative Example 28, it can be seen that when the sum of the applied amounts of the two electrolyzers of the neutral salt electrolyzer and the sulfuric acid electrolyzer exceeds 50 A / dm ² , peracid washing occurs.

5 (a) to (c) is a photograph showing the surface of the stainless steel after pickling in Comparative Example 16, Example 6 and Comparative Example 28, (a) is a small amount of the applied current in Comparative Example 16 It can be seen that there is a residual scale, (b) can see the normal pickling surface according to Example 6, (c) is the surface erosion occurs due to the surface pickling in Comparative Example 28, the surface is not good You can see that.

Although not shown in this embodiment, even if the sum of the applied currents of the neutral salt electrolyzer and the sulfuric acid electrolyzer is within the range of the pickling, the case where the applied current of the sulfuric acid electrolyzer is less than 8 A / dm ² is a nonuniform pickling surface as shown in FIG. It is not preferable to represent.

If the chromium component ratio scale components through the above-described embodiment is relatively low is the electrolytic pickling hour neutral salt electrolytic bath, and two electrolytic cell applied current sum is 30A / dm ² or more very high applied current of 50 A / dm ² or less for the sulfuric acid electrolytic bath It can be seen that it is necessary.

[Examples 13 to 21 and Comparative Examples 29 to 33]

After the electrolytic pickling was carried out by applying the zone III and the zone IV in FIG. 3 according to the annealing temperature, the steel grades listed in Table 3 were used, and then 40% containing sulfuric acid at a concentration of 80 g / L and free hydrofluoric acid at the concentrations shown in Table 3 below. It was immersed for 30 second in the mixed acid solution of ° C. However, hydrogen peroxide was added to the mixed acid solution to maintain the redox potential (ORP) above the value shown in Table 3 below.

After the pickling, the results are shown in Table 3 below. If no scale remains on the surface of the steel sheet, the result is indicated by o. In addition, the case where the surface erosion by the superacid wash is represented by x.

In addition, the surface pictures of the stainless steel after pickling in Comparative Examples 29 and 31 are shown in FIGS. 6A and 6B, and the surface pictures of the stainless steel after pickling in Comparative Examples 32 and 16 are shown in FIG. A) and (b) are shown, and the surface photographs of the stainless steel after pickling in Comparative Examples 33 and 19 are shown in FIGS. 8A and 8B.

TABLE 3

division	Steel grade	Ingredient	Electrolytic pickling area	HF content (g / L)	ORP (mV)	Pickling	Remarks
Comparative Example 29	439	Cr-17 wt%	Area Ⅲ	5	400	X	Misan tax
Example 13	439	Cr-17 wt%	Area Ⅲ	5	550	O	normal
Comparative Example 30	439	Cr-17 wt%	Area Ⅲ	8	500	X	Misan tax
Example 14	439	Cr-17 wt%	Area Ⅲ	8	550	O	normal
Example 15	439	Cr-17 wt%	Area Ⅲ	12	600	O	normal
Comparative Example 31	439	Cr-17 wt%	Area Ⅲ	25	550	X	Taxation
Comparative Example 32	430J1L	Cr-19 wt% Si-0.5 wt%	Area IV	12	550	X	Misan tax
Example 16	430J1L	Cr-19 wt% Si-0.5 wt%	Area IV	20	320	O	normal
Example 17	430J1L	Cr-19 wt% Si-0.5 wt%	Area IV	25	340	O	normal
Example 18	430J1L	Cr-19 wt% Si-0.5 wt%	Area IV	30	370	O	normal
Comparative Example 33	436L	Cr-17 wt% Mo-1.0 wt%	Area IV	12	550	X	Misan tax
Example 19	436L	Cr-17 wt% Mo-1.0 wt%	Area IV	20	320	O	normal
Example 20	436L	Cr-17 wt% Mo-1.0 wt%	Area IV	25	340	O	normal
Example 21	436L	Cr-17 wt% Mo-1.0 wt%	Area IV	30	370	O	normal

(However, in Table 3, the 439 steel grade does not include molybdenum (Mo), and a silicon (Si) content of 0.3 wt% or less was used.)

As can be seen from Table 3, when pickling 439 steel containing 17 wt% chromium (Comparative Examples 29 to 31 and Examples 13 to 15), the redox potential was 550 mV or more, and the free hydrofluoric acid (HF) concentration was It can be seen that it is pickled normally in the range of 5 to 12 g / L. However, when the redox potential is less than 550 mV as in Comparative Example 29, it can be seen that the acid pickling occurs, and as in Comparative Example 31, when the free hydrofluoric acid concentration exceeds 12 g / L, it can be seen that the superacid is generated. . Redox potential of more than 550mV means that the concentration of hydrogen peroxide remaining during pickling should be maintained above a certain level. If hydrogen peroxide remains in solution, the redox potential is usually measured at 550 ~ 600mV.

On the other hand, even in steels containing 17% by weight of chromium, 430J1L and 436L steels in which Si content exceeds 0.3% by weight in the steel grade component system or Mo content exceeds 0.5% by weight, have free hydrofluoric acid (HF) concentration. Although pickling is possible at 20 to 30 g / L, as in Comparative Examples 32 and 33, when using a mixed acid solution containing a low concentration of hydrofluoric acid, it can be seen that pickling occurs. At this time, the redox potential can be pickled if it is 320 mV or more, and even if there is no residual hydrogen peroxide, it is possible if the ratio of Fe ³⁺ ions and Fe ²⁺ ions (Fe ³⁺ / Fe ²⁺ ) in the metal ions in the solution is 1 or more. In general, a constant amount of hydrogen peroxide must be injected to maintain the redox potential above 320 mV.

Figure 6 (a) and (b) is a photograph showing the surface of the stainless steel after pickling in Comparative Examples 29 and 31, (a) is the free hydrofluoric acid concentration and redox potential of the mixed acid solution in Comparative Example 29 It can be seen that the pickling occurs on the surface according to the low all, (b) can be seen that in the case of applying the concentration of free hydrofluoric acid in Comparative Example 31 exceeds the range of the present invention, the super-acid is generated, the surface erosion occurred .

In addition, (a) and (b) of Figure 7 shows a photograph of the surface of the stainless steel after pickling in Comparative Example 32 and Example 16, (a) has a low free hydrofluoric acid concentration in Comparative Example 32, so that It can be seen that, and (b) can see the shape of the normal pickling surface according to Example 16.

In addition, (a) and (b) of Figure 8 shows a photograph of the surface of the stainless steel after pickling in Comparative Example 33 and Example 19, (a) has a low free hydrofluoric acid concentration in Comparative Example 33, so that It can be seen that, and (b) can see the shape of the normal pickling surface according to Example 19.

Through the above examples, it can be seen that the pickling properties are different depending on the elements included in the steel grade, thereby affecting the free hydrofluoric acid concentration and the redox potential of the solution.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and changes can be made without departing from the technical spirit of the present invention described in the claims. It will be obvious to those of ordinary skill in the field.

Claims

An electrolytic pickling step of neutral salt electrolytic pickling and sulfuric acid electrolytic pickling of a high chromium ferritic stainless steel cold rolled steel sheet containing 17 to 26 wt% chromium annealed at an annealing temperature of 1030 ° C. or higher and 1050 ° C. or lower; And

The sum of the application of the neutral salt electrolytic pickling during the pickling step and the amount of current applied to the electrolytic pickling in sulfuric acid pickling method is the amount of current of the high-chromium ferritic stainless cold-rolled steel sheet, which is adjusted to not more than 12A / dm ² or more 20A / dm ^2.

An electrolytic pickling step of neutral salt electrolytic pickling and sulfuric acid electrolytic pickling of a high chromium ferritic stainless steel cold rolled steel sheet containing 17-26 wt% chromium annealed at an annealing temperature of 930 ° C. or higher and lower than 1030 ° C .; And

In the electrolytic pickling step, the sum of the applied current amount of neutral salt electrolytic pickling and the applied current amount of sulfuric acid electrolytic pickling is adjusted to 30 A / dm ² or more and 50 A / dm ² or less, the pickling method of the high chromium ferritic stainless steel cold rolled steel sheet.

The pickling of the high chrome ferritic stainless steel cold rolled steel sheet according to claim 1 or 2, wherein the applied current amount of the neutral salt electrolytic pickling is 8 A / dm ² or more, and the applied current amount of the sulfuric acid electrolytic pickling is 0 or 8 A / dm ² or more. Way.

The sulfuric acid solution of claim 1 or 2, wherein the mixed acid solution has a silicon content of 0.3 wt% or less, or a molybdenum content of 0.5 wt% or less based on the total weight of the cold rolled steel sheet. And 5-12 g / L free hydrofluoric acid, and a redox potential (ORP) of 550 mV or more, wherein the high chromium ferritic stainless steel cold rolled sheet is pickled.

The method of claim 4, wherein hydrogen peroxide is added to the mixed acid solution so that the redox potential of the mixed acid solution is maintained at 550 mV or more.

The mixed acid solution of claim 1 or 2, wherein the mixed acid immersion step comprises: when the silicon (Si) content exceeds 0.3% by weight or the molybdenum (Mo) content exceeds 0.5% by weight based on the total weight of the cold rolled steel sheet, A pickling method for a high chromium ferritic stainless steel cold rolled steel sheet having a redox potential (ORP) of ˜150 g / L sulfuric acid, 20-30 g / L free hydrofluoric acid, and a mixed acid solution of 320 mV or more.

The method for pickling a high chromium ferritic stainless steel cold rolled steel sheet according to claim 6, wherein hydrogen peroxide is added to maintain the redox potential of the mixed acid solution at 320 mV or more.

The method of claim 1, wherein the neutral salt electrolytic pickling is performed using an electrolyte solution containing sodium sulfate at a concentration of 100 to 250 g / L.

The method of claim 8, wherein the temperature of the electrolytic solution used in the neutral salt electrolytic pickling is 50 ~ 90 ℃, pickling method of high chrome ferritic stainless steel cold rolled steel sheet.

The method of claim 1, wherein the neutral salt electrolytic pickling is performed for 24 to 100 seconds.

According to claim 1 or 2, wherein the sulfuric acid electrolytic pickling is at least one metal selected from the group consisting of iron, chromium, nickel, copper, manganese and titanium in 50-150g / l sulfuric acid or 50-150g / l sulfuric acid A method of pickling a high chromium ferritic stainless steel cold rolled steel sheet, which is carried out using an electrolyte solution containing a metal sulfate formed by melting.

The method of claim 11, wherein the temperature of the electrolytic solution used during the electrolytic acid pickling of sulfuric acid is 30 ~ 60 ℃, pickling method of high chrome ferritic stainless steel cold rolled steel sheet.

The method of claim 1, wherein the sulfuric acid electrolytic pickling is performed for 10 to 50 seconds, the pickling method of high chromium ferritic stainless steel cold rolled steel sheet.

The method of claim 1, wherein the mixed acid immersion step is performed for 25 to 90 seconds, pickling method of high chrome ferritic stainless steel cold rolled steel sheet.

The method of claim 1, wherein the surface potential of the cold rolled steel sheet during the mixed acid immersion step is maintained in the range of -0.2 to 0V, pickling method of high chromium ferritic stainless steel cold rolled steel sheet.