JP5768141B2 - Eco-friendly high-speed pickling process for producing low chromium ferritic stainless steel cold rolled steel sheets with excellent surface quality - Google Patents

Description

  The present invention relates to a method of pickling a steel plate surface at a high speed when producing a ferritic stainless steel cold-rolled steel sheet that requires high surface quality, and specifically to a pickling method that does not use nitric acid during the pickling process. .

A stainless cold-rolled steel sheet undergoes a heat treatment process at 800 to 1150 ° C. in order to obtain predetermined mechanical characteristics after cold rolling. In such a heat treatment process, the surface of the steel sheet reacts with high-temperature oxygen inside the furnace. An oxide scale (SiO ₂ , (Cr, Fe) ₃ O ₄ ) is generated on the surface. Such an oxide scale on the surface of the steel sheet deteriorates the appearance of the product, deteriorates the quality of the steel sheet, and also serves as a starting point for causing corrosion of the steel sheet, thereby reducing the corrosion resistance of the steel sheet.

  Therefore, usually, to obtain beautiful surface quality and improve corrosion resistance, physical descaling with brush, shot ball blasting, electrolytic descaling using sodium sulfate, sulfuric acid, nitric acid electrolyte, salt bath, mixed acid, etc. Stainless steel cold-rolled steel sheets are manufactured by removing the oxide scale on the surface of the steel sheet by combining various methods such as chemical descaling using the steel, and the process of removing such scaling is called a pickling process. In such a stainless steel pickling process, in order to obtain a beautiful surface quality and to form a passive film uniformly and ensure corrosion resistance, it is descaled by applying current while passing the steel plate through the nitric acid solution. The pickling process has been performed by a nitric acid electrolysis method and a chemical descaling method using a mixed acid of nitric acid (80 to 180 g / L) and hydrofluoric acid (2 to 40 g / L). Nitric acid lowers the pH in the pickling tank to increase the activity of hydrofluoric acid, oxidizes divalent iron ions dissolved on the surface of the steel sheet to trivalent, and maintains an appropriate redox potential for pickling.

However, since nitric acid is used as the pickling solution, NOx, which is an atmospheric emission control substance, is generated, and nitrate nitrogen (NO ₃ -N) is contained in the waste acid and the washing water. The unit cost of production is significantly higher due to the additional installation and operation costs of environmental pollution prevention equipment in the pickling process to meet the environmental regulatory requirements such as total nitrogen limitation of discharged effluent due to stricter environmental regulations and NOx concentration limitation of air discharge facilities The problem of an increase occurs. Moreover, when producing low-chromium ferritic cold-rolled steel sheets containing 14 wt% or less chromium and having a corrosion resistance weaker than that of other stainless steels, the amount of dissolution in an acid solution increases due to the weak corrosion resistance, and NOx and nitric acid The problem arises that the generation of reactive nitrogen increases rapidly.

  In order to solve such problems, pickling methods that do not use nitric acid have been developed. In such a technique, in the pickling process, nitric acid is replaced with hydrochloric acid or sulfuric acid, etc., and the deficiency in oxidizing power is hydrogen peroxide, potassium permanganate, trivalent iron ions, and pickling without using nitric acid supplemented by air injection. A method was developed.

  Specifically, Patent Document 1 discloses a technology that uses sulfuric acid, hydrofluoric acid, and iron sulfate as the pickling solution, and adds hydrogen peroxide to maintain the oxidation-reduction potential of the pickling solution at 300 mV or higher. As disclosed in Patent Document 2 and Patent Document 3 since the 1990s, including the above-mentioned technology, mainly the oxidation-reduction potential (Oxidation-Reduction Potential) of hydrofluoric acid and iron ions, air, hydrogen peroxide, or a solution. , Techniques for identifying the appropriate range of ORP) continued to emerge. However, most of these methods are limited in that they can be limitedly applied to products such as wire rods, steel bars, and thick plates, which are not strict in product quality.

On the other hand, Patent Document 4 contains sulfuric acid, hydrofluoric acid, and an iron salt, and hydrogen peroxide is periodically added to adjust the composition of a wetting agent, a brightening agent, a corrosion inhibitor, and the like. As for the management of the solution, there is disclosed a technique that adopts an automatic control system based on Fe (III) and the ORP based thereon. As a result, the CLEANOX 352 product, which is a pickling solution, has been commercialized and is most widely used worldwide. This method has been put to practical use in the case of wire rods and hot-rolled products, but the product unit price of the product is 20% or more higher than that of existing products, and a complicated solution composition and management method is adopted. In addition, the pickling reduction rate has a relatively slow pickling rate of about 1.5 to 3 g / m ² · min, and the mixed pickling must be completed within 10 to 100 seconds. Not suitable for line.

  Further, in Patent Document 5 and Patent Document 6, which are improved patents of the above patent, a method of increasing the pickling speed by adding copper and chlorine ions to the pickling composition has been proposed. When the surface potential (Open circuit potential, OCP) formed is lower than 0.1 V, which is the oxidation-reduction potential of copper ions, copper particles may be deposited on the surface of the steel sheet during the pickling process, and the steel sheet may be discolored. In addition, when the pickling solution contains chlorine ions at a certain concentration or more, pitting corrosion may occur.

  As described above, various techniques are known for the non-nitric acid pickling composition, but no pickling technique suitable for high-speed production of ferritic cold-rolled steel sheets is known.

German Patent No. 3937438 US Pat. No. 5,154,774 European Patent No. 236354 US Pat. No. 5,908,511 European Patent Application No. 1040211 US Patent Application Publication No. 2000-560982

  The present invention provides an electrolytic pickling method suitable for high-speed production of low-chromium ferritic stainless cold-rolled steel sheets and a mixed pickling method suitable for high-speed removal of silicon oxide while ensuring pickling performance without using nitric acid. .

  Moreover, this invention provides the mixed acid solution which does not contain nitric acid suitable for the said pickling method.

  Furthermore, the present invention provides a low chromium ferrite stainless cold-rolled steel sheet obtained by the above pickling method.

In one embodiment of the present invention, according to one embodiment, silicon oxide is removed at high speed from the surface of a low chromium ferritic stainless steel cold rolled steel sheet containing 14 wt% or less of chromium using a mixed acid solution not containing nitric acid. A silicon oxide pickling method, wherein the cold-rolled steel sheet is immersed in a mixed acid solution containing hydrogen peroxide to perform a pickling step. The mixed acid solution contains 70 to 200 g / L sulfuric acid and 1 to 1 free hydrofluoric acid. Low chromium ferritic stainless steel containing 10 g / L, the first mixed acid solution containing hydrogen peroxide at a concentration of 7 g / L or more but not containing iron ions and having a pickling rate of 3 to 15 g / m ² · min A silicon oxide pickling method for cold-rolled steel sheets is provided.

を満たすことが好ましい。 According to another embodiment of the present invention, the relationship between the hydrogen peroxide concentration and the iron ion concentration during the pickling step is

It is preferable to satisfy.

  According to another embodiment of the present invention, the surface potential of the cold rolled steel sheet immersed in the mixed acid solution is preferably maintained at -0.2 to 0.2 V, and the cold rolled steel sheet is maintained for 10 to 100 seconds. Pickling can be performed by dipping in a mixed acid solution.

In another aspect of the present invention, there is provided a pickling method for a low chromium ferritic stainless steel cold-rolled steel sheet, wherein the ferritic stainless steel cold-rolled steel sheet containing 14 wt% or less of chromium that has undergone a degreasing and annealing process is pickled. As one embodiment, a sulfuric acid electrolysis step of electrolytically removing (Fe, Cr) scale from a low-chromium ferritic stainless steel cold-rolled steel sheet using an electrolytic solution using sulfuric acid as an electrolyte, sulfuric acid, hydrofluoric acid, and peroxidation There is provided a pickling method for a low-chromium ferritic stainless steel cold-rolled steel sheet having a pickling speed of 3 to 15 g / m ² · min, including a mixed acid dipping step of dipping in a mixed acid solution containing hydrogen.

  According to another embodiment of the present invention, the method may further include a neutral salt electrolysis step of electrolytically removing the chromium rich scale from the steel sheet surface using an electrolytic solution including a sodium sulfate electrolyte.

According to still another embodiment of the present invention, the neutral salt electrolysis step includes immersing the ferritic stainless steel plate in a neutral salt electrolytic solution at a temperature of 50 to 90 ° C. so that the surface potential of the steel plate is +, −, + May be applied by applying a current density of 10 to 30 A / dm ² for more than 0 seconds and not more than 90 seconds so that the sodium sulfate electrolyte is added to the neutral salt electrolyte solution in an amount of 100 to It may contain 250 g / L.

According to another embodiment of the present invention, the sulfuric acid electrolysis step includes immersing a ferritic stainless steel plate that has undergone an annealing or neutral salt electrolysis step in a sulfuric acid electrolytic solution having a temperature of 30 to 60 ° C. so that the surface potential of the steel plate is + May be performed by applying a current density of 10 to 30 A / dm ² for 5 to 50 seconds so as to be formed in the order of −, −, and the sulfuric acid electrolytic solution may contain 50 to 150 g / L of sulfuric acid. Good.

  Furthermore, according to an embodiment of the present invention, the mixed acid immersion step performs a pickling process by immersing the cold-rolled steel sheet in a mixed acid solution containing hydrogen peroxide for 10 to 100 seconds. ˜200 g / L and free hydrofluoric acid 1˜10 g / L, the first mixed acid solution contains hydrogen peroxide at a concentration of 7 g / L or more, but does not substantially contain iron ions, and in the mixed acid immersion step The surface potential of the steel sheet can be kept in the range of -0.2 to 0.2V.

を満たすことができる。 According to another embodiment of the present invention, the hydrogen peroxide in the pickling step is expressed by the following formula in relation to the iron ion concentration in the mixed acid solution.

Can be met.

  In still another aspect of the present invention, there is provided a low-chromium ferritic stainless steel cold-rolled steel sheet obtained by performing the pickling method, wherein the steel sheet has a glossiness of 130 or more.

を満たす、シリコン酸化物除去のための硝酸を含まない混酸溶液を提供する。 In still another embodiment of the present invention, a mixed acid solution for removing silicon oxide from a ferritic stainless steel cold-rolled steel sheet containing 14% by weight or less of chromium that has been subjected to a degreasing and annealing step, wherein sulfuric acid is 70 to 200 g / L. , 1 to 10 g / L of free hydrofluoric acid, and hydrogen peroxide. The hydrogen peroxide has the following formula in relation to the iron ion concentration in the mixed acid solution:

A nitric acid-free mixed acid solution for removing silicon oxide is provided.

  According to the present invention, when pickling ferritic stainless cold-rolled steel sheets, NOx and nitrate nitrogen are not discharged because the mixed acid solution in the pickling solution does not contain nitric acid. Therefore, the burden on the installation of the NOx removal facility and the denitrification facility can be reduced.

  Furthermore, since pickling can be controlled by the hydrogen peroxide concentration and the hydrofluoric acid concentration, it is easy to control and suitable for high-speed production. Moreover, the quality after pickling is improved as compared with existing pickling methods, and it is possible to produce ferritic stainless steel cold-rolled steel sheets of excellent quality.

Further, by completely removing Fe and Cr oxides excluding silicon oxide in the electrolytic pickling process, it is possible to easily remove silicon oxide with a mixed acid solution. Since it can be performed at a high speed of / m ² · min, removal and planarization of silicon oxide with a mixed acid solution can be performed within a short time of 10 to 100 seconds.

  And since the solution composition is simple, the management method is simple, the surface quality of the cold-rolled steel sheet can be ensured by preventing reaction other than the pickling reaction with the steel sheet surface, and production by high-speed production It is possible to improve the performance.

FIG. 1 is a photograph of the surface of a steel sheet obtained by heat-treating the low-chromium ferritic stainless steel cold-rolled steel sheet heat-treated according to Example 1, taken with an electron microscope. (A) is the surface of the steel sheet obtained according to Invention Example 2. (B) is the steel plate surface by the comparative example 4. FIG. FIG. 2 shows, in (a), an electron micrograph of a cross section of a steel plate after electrolytic treatment in a heat-treated low-chromium ferritic stainless steel cold-rolled steel plate, and (b) shows the electrolytic treatment being performed. The electron micrograph which photographed the cross section of the steel plate which has not been shown. FIG. 3 is a graph showing the correlation between the oxidation-reduction potential and the hydrogen peroxide concentration with respect to the surface potential of the steel sheet when the low-chromium ferritic stainless steel plate is immersed in a mixed acid. (B) shows the relationship between the surface potential and the hydrogen peroxide concentration. FIG. 4 is a graph showing the minimum hydrogen peroxide concentration obtained from the iron ion concentration in the mixed acid solution. FIG. 5 is a graph comparing the glossiness of ferritic stainless steel cold-rolled steel sheets that have been pickled according to Example 8.

  The present invention allows a low-chromium ferritic stainless steel cold-rolled steel sheet to pass through a neutral salt electrolytic cell using sodium sulfate as an electrolyte, a sulfuric acid electrolytic cell using sulfuric acid as an electrolyte, and a mixed acid bath of a non-nitric acid pickling composition. A method for removing oxide scale on a steel sheet surface is provided.

  The present invention will be described in detail below.

  In general, ferritic stainless steel cold-rolled steel sheet has an oxide scale with a thickness of usually 100 to 300 nm after heat treatment, and this oxide scale has a relatively high Cr oxide scale content than Fe oxide. It has a multilayer structure of a rich scale layer, a Fe rich scale layer having a relatively higher Fe oxide scale content than Cr oxide, and a Si oxide layer present at the interface between the oxide scale layer and the base material.

  Among the scales, the Cr rich scale layer is removed by a neutral salt electrolytic cell. The said neutral salt electrolysis tank contains the neutral salt electrolyte solution containing a sodium sulfate electrolyte, and contains the electrode for supplying an electric current to the steel plate surface. At this time, the electrode tank is configured such that the electric potential on the surface of the steel sheet is charged in the order of +, −, +.

When a current is applied to the neutral salt electrolyte in the pH range of 3 to ⁶ , Cr of the Cr rich scale is preferentially dissolved in the Cr ⁶⁺ state, and the Cr rich scale on the steel sheet surface can be removed. At this time, it is preferable to use sodium sulfate as the electrolyte. The sodium sulfate electrolyte causes dissolution of Cr-rich scale by increasing the electrical conductivity in the electrolytic solution and increasing the electrical conductivity to the steel sheet surface.

  At this time, it is preferable that the electrolyte solution contains 100 to 250 g / L of sodium sulfate electrolyte. When the sodium sulfate electrolyte is 100 g / L or more, appropriate conductivity for dissolution of chromium can be obtained. However, if it exceeds 250 g / L, sodium sulfate may be precipitated in the electrolyte, and the piping may be blocked to deteriorate the operation. Therefore, the sodium sulfate electrolyte is preferably contained at 250 g / L or less.

  The electrical conductivity of the electrolyte in the neutral salt electrolytic cell is closely related to the temperature of the electrolyte. Above 50 ° C., proper conductivity for dissolution of chromium can be obtained, and the higher the temperature, the higher the conductivity. However, if the temperature of the electrolytic solution exceeds 90 ° C., it is difficult to manage the temperature for operation. Therefore, the temperature of the electrolytic solution in the neutral salt electrolyzer is preferably in the range of 50 to 90 ° C.

On the other hand, the current applied through the electrode is preferably 10 A / dm ² or more, and Cr-rich scale Cr can be sufficiently eluted. However, if it exceeds 30 A / dm ² , the rectifier equipment for generating the current becomes large, and the initial equipment cost increases. Therefore, the current is preferably in the range of 10-30 A / dm ^2. .

  The neutral salt electrolysis time is 90 seconds or less. If necessary, the iron and chromium oxide scales can be removed only by sulfuric acid electrolysis without performing such neutral salt electrolysis. However, in this case, a burden may be imposed on the sulfuric acid electrolytic treatment and the mixed acid. Therefore, the pickling treatment time can be further shortened by performing the neutral salt treatment, which is more preferable. If the neutral salt electrolysis time exceeds 90 seconds, there is a possibility that the base material is partially eroded to cause peracid washing.

  On the other hand, the Fe rich scale layer is removed by a sulfuric acid electrolytic cell. The sulfuric acid electrolytic cell includes a sulfuric acid electrolytic solution containing sulfuric acid, and includes an electrode for passing a current to the steel sheet surface. At this time, the electrode tank is configured such that the electric potential on the surface of the steel sheet is charged in the order of +, −, +.

When a current is applied to a sulfuric acid electrolyte having a pH in the range of 0 to 1, Fe is dissolved in a Fe ²⁺ state. At this time, H ⁺ and SO ₄ ²⁻ of sulfuric acid increase the electrical conductivity in the solution and increase the electrical conductivity from the electrode to the steel sheet surface. Furthermore, the iron in the Fe rich scale layer is chemically dissolved by the lowered pH.

  At this time, it is preferable that the said sulfuric acid is 50-150 g / L. If sulfuric acid does not become 50 g / L or more, since the appropriate conductivity or more cannot be maintained, the current-carrying rate to the steel sheet surface cannot be maintained. However, when the sulfuric acid exceeds 150 g / L, there may be a problem that chemical dissolution occurs predominantly and the surface of the stainless cold-rolled steel sheet becomes rough.

  Like the neutral salt electrolytic cell, the sulfuric acid electrolyte must have a solution temperature of 30 ° C. or higher in order to obtain the minimum conductivity. However, when the temperature exceeds 60 ° C., chemical dissolution proceeds excessively, the surface of the stainless cold-rolled steel sheet becomes rough, and furthermore, a blackening phenomenon may occur in which the steel sheet surface changes to black. Therefore, it is preferable that the electrolytic solution of the sulfuric acid electrolytic cell has a temperature in the range of 30 to 60 ° C.

On the other hand, it is preferable current applied to the sulfuric acid electrolytic bath is in the range of 10~30A / dm ^2. When the current is applied less than 10 A / dm ^2, there is a problem that the active dissolution of the ferritic stainless steel plate proceeds and the surface becomes rough. However, if it exceeds 30 A / dm ² , the rectifier equipment for generating the current becomes large, and the initial equipment cost increases. Therefore, the current is preferably in the range of 10-30 A / dm ^2. .

  Such sulfuric acid electrolysis treatment is preferably performed for 5 to 50 seconds. In sulfuric acid electrolysis treatment for less than 5 seconds, a sufficient pickling effect cannot be obtained, and when it exceeds 50 seconds, a problem of peracid washing is caused.

  As described above, only the Si oxide layer remains on the surface of the steel sheet subjected to neutral salt electrolysis-sulfuric acid electrolysis, and this Si oxide layer should be removed with a non-nitric acid-free mixed acid solution that does not contain nitrogen. Can do. The mixed acid solution contains sulfuric acid, free hydrofluoric acid, and hydrogen peroxide. By immersing the mixed acid solution in a mixed acid tank containing such a mixed acid solution, the Si oxide layer is formed from the steel sheet subjected to neutral salt electrolysis-sulfuric acid electrolysis. Can be removed.

The hydrofluoric acid and sulfuric acid in the mixed acid solution are dissociated in the mixed acid solution as shown in the following reaction formulas (1) and (2). That is, in a mixed acid solution, hydrofluoric acid is dissociated while being dissolved, and the equilibrium state changes depending on the H ⁺ concentration provided by dissociating sulfuric acid, that is, the acidity.

HF → H ⁺ + F ⁻ (1)
H ₂ SO ₄ → HSO ₄ ⁻ + H ⁺ → SO ₄ ²⁻ + 2H ⁺ (2)

In the case of hydrofluoric acid, it has a pickling power in the state of free hydrofluoric acid that has not been dissociated and dissolves Si oxide, and further penetrates into the interface between the Si oxide layer and the base material to dissolve Fe. . The dissolved Fe ions and Si ions are removed from the surface of the steel sheet in the form of FeF _x ^(3-x) , H ₂ SiF ₆ or the like. The hydrofluoric acid is preferably present in the mixed acid solution at a concentration in the range of 1 to 10 g / L, more preferably 1 to 5 g / L. If it is less than 1 g / L, the concentration present in the free hydrofluoric acid is so small that the dissolving power for Si is insufficient, thereby causing a problem of unpickling the steel sheet surface. Moreover, when it exceeds 10 g / L, the erosion rate of a base material will become quick and the steel plate surface after a pickling process will become rough.

  As described above, in order that hydrofluoric acid provides pickling power for removing the Si oxide layer on the steel sheet surface, it is preferable that an effective free hydrofluoric acid concentration is maintained at a certain acidity or higher in the mixed acid solution. . Therefore, the mixed acid solution requires sulfuric acid having a certain concentration or more so that hydrofluoric acid does not dissociate. Suitable sulfuric acid concentrations are in the range of 50 to 150 g / L. If the sulfuric acid concentration is less than 50 g / L, an effective free hydrofluoric acid concentration is not maintained, and dissociation of hydrofluoric acid occurs, so that the pickling power becomes weak. Further, if it exceeds 150 g / L, there is a problem that, during the sulfuric acid dilution operation, heat is generated and the operation becomes difficult. Therefore, it is preferable that sulfuric acid is contained at a concentration in the above range.

  Among oxide scales of ferritic stainless steel, Si oxide exists both on the grain surface of ferrite crystals and on the grain system between grains, and the grain-based Si oxide is deeper inside the base material. Exists up to the position. In the case of austenitic stainless steel, the corrosion resistance of the crystal is high and it preferentially erodes from the crystal grain system, but since ferritic stainless steel has low crystal corrosion resistance, the erosion rate between the crystal interior and the crystal grain There is no difference and the grain surface and the grain system are completely dissolved without selectivity. Accordingly, in order to remove all the Si oxide, a considerable part of the base material must be dissolved.

At this time, Fe ²⁺ is eluted from the base material, and the eluted Fe ²⁺ reacts with hydrogen peroxide to be oxidized to Fe ³⁺ , and then a complex compound that combines with HF and becomes FeF _x ^(3-x). It is produced and removed from the steel sheet surface. The above reaction can be expressed as the following reaction formulas (3) to (6). If such a process proceeds smoothly, the pickling rate can be increased.

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)

According to the experiments by the present inventors, in order to obtain a beautiful ferritic stainless steel cold-rolled steel sheet, a scale and base material of about 3 to 5 g / m ² must be removed in a mixed acid tank. Furthermore, in order to remove Si oxide by immersing in a mixed acid bath for 10 to 100 seconds, more preferably 20 to 60 seconds, and to produce a cold-rolled steel sheet at a high speed, it is about ^{3 to} 15 g / m ² · min. Pickling speed must be ensured.

Stainless steel has a dynamic potential curve having a specific correlation between potential and current for each steel type, and the amount of current generated at this time can be expressed as a pickling rate. That is, the maximum pickling speed can be realized by adjusting the surface potential. In order to obtain a high-speed pickling speed of ³ to 15 g / m ² · min required for producing a cold-rolled steel sheet at a high speed, the surface potential of the cold-rolled steel sheet in the mixed acid tank is set to −0.2 to 0.2 V. It is preferable to keep in this range. If the surface potential of the cold-rolled steel sheet is out of the above range, pickling may not occur or a pickling failure having a partially unpickled surface may be caused. Moreover, even if it pickles, the favorable surface quality of a cold-rolled steel plate cannot be obtained.

Conventionally, pickling was performed by adjusting the ratio of Fe ²⁺ / Fe ^{3+ in} the pickling solution to control the oxidation-reduction potential (ORP) of the pickling solution. However, as shown in FIG. 3 (a), since no relationship between the surface potential and the oxidation-reduction potential can be confirmed, in the acid pickling, the oxidation-reduction potential has any correlation with the surface potential. It turns out that it is a factor which does not have sex. Unlike this, the surface potential of ferritic stainless steel with low chromium content in the mixed acid is the concentration of metal ions generated in the pickling process in the mixed acid tank, that is, the concentration of the main component iron ions and the mixed acid solution. It can be seen that there is a correlation with the concentration of residual hydrogen peroxide, particularly the residual hydrogen peroxide concentration.

If the residual hydrogen peroxide concentration in the mixed acid solution is insufficient, the reaction of the above reaction formula (4) is not performed, so the Fe ²⁺ concentration on the steel sheet surface locally increases, The reaction becomes dominant. In this case, Cu or the like present as an additive or impurity in Fe and stainless steel is reprecipitated on the surface of the steel sheet as in reaction formula (6), and a blackening phenomenon occurs in which the steel sheet surface changes to black. Therefore, the residual hydrogen peroxide concentration must always exist above a certain concentration.

  The concentration of such residual hydrogen peroxide is correlated with the iron ion concentration present in the mixed acid solution. 3A is a graph showing the relationship between the surface potential and the oxidation-reduction potential, and FIG. 3B is a graph showing the relationship between the surface potential and the hydrogen peroxide concentration.

It can be seen that at the same hydrogen peroxide concentration, the surface potential of the steel sheet gradually increases as the iron ion concentration increases. This is because Fe ³⁺ ions act as an oxidizing agent, and as the Fe ³⁺ ions increase, the hydrogen peroxide concentration for maintaining the surface potential of the steel sheet tends to decrease. However, if hydrogen peroxide is insufficient even if Fe ³⁺ ions are present at a certain concentration or more, the surface potential of the steel sheet decreases to −0.2 V or less, resulting in a problem that the surface quality is deteriorated. From such a relationship, the minimum hydrogen peroxide concentration with respect to the iron ion concentration for maintaining the surface potential of the steel sheet at −0.2 V or higher is expressed by the following equation.

  Specifically, when the iron ion concentration is 0, an effective hydrogen peroxide concentration of at least 7 g / L must be included, and when the iron ion concentration is 40 or more, 1.0 g / L or more exists. There must be. On the other hand, the higher the concentration of hydrogen peroxide, the more time for adding additional hydrogen peroxide can be avoided, which helps simplify the process. However, since such hydrogen peroxide is expensive, the use of a high concentration of hydrogen peroxide increases the cost, and does not increase the pickling effect in proportion to the concentration of use, so that it exceeds 30 g / L. More preferably not.

  Further, the temperature of the mixed acid solution of the present invention is not particularly limited, and can be appropriately set by those skilled in the art. For example, the temperature can be set within a range of 20 to 95 ° C., 25 to 80 ° C. as another example, and 25 to 65 ° C. as another example.

  From the above-described contents, it is understood that the sulfuric acid concentration and hydrofluoric acid concentration and the residual hydrogen peroxide concentration in the pickling solution are the most important factors for increasing the pickling effect and high-speed pickling in the mixed acid bath process. Therefore, it is necessary to control the concentration of these components, and the concentration control of each component can be adjusted with sulfuric acid concentration and hydrofluoric acid concentration using a commonly used acid analyzer. The concentration can be analyzed and adjusted by a near infrared analysis method or an automated method.

  According to the present invention, the pickling process can be performed at high speed, the total pickling time is about 15 to 240 seconds, the time required for pickling can be greatly shortened, and the low chromium having excellent quality A ferritic stainless steel sheet can be obtained.

  Hereinafter, the present invention will be described in detail through examples.

Example 1
In order to confirm the pickling property of the oxide scale of the ferritic cold rolled steel sheet depending on whether or not neutral salt electrolysis and sulfuric acid electrolysis are performed, from the ferritic stainless steel sheet having a chromium composition of 14% by weight or less, neutral salt electrolysis treatment and The Cr rich scale layer and the Fe rich scale layer were removed by sulfuric acid electrolytic treatment, respectively. At this time, the used neutral salt electrolytic solution contained 150 g / L of sodium sulfate electrolyte as an electrolyte, the temperature of the solution was 60 ° C., and a current of 150 A / dm ² was applied for 40 seconds. Further, the sulfuric acid electrolytic solution is a pH 1 solution containing 85 g / L of sulfuric acid, the solution temperature is 50 ° C., and a current of 20 A / dm ² is applied to the steel sheet surface in order of +, −, +. Applied for 2 seconds.

  The surface of the steel sheet subjected to neutral salt electrolysis and sulfuric acid electrolysis and the cross section of the untreated steel sheet were photographed with a scanning electron microscope (SEM) and shown in FIG. For comparison, a cross section of a steel plate subjected to electrolytic treatment is shown in (a), and a cross section of a steel plate not subjected to electrolytic treatment is shown in (b).

FIG. 1 shows that (Cr, Fe) ₃ O ₄ and silicon oxide remain simultaneously on the cross section of the steel sheet not subjected to neutral salt electrolysis and sulfuric acid electrolysis. It can be seen that only silicon oxide remains in the cross section of the steel plate.

Example 2
In order to observe the surface state of the cold-rolled steel sheet due to the surface potential in the mixed acid tank, the ferrite system having a chromium content of 14 wt% or less from which the Fe-rich scale layer and Cr-rich scale layer were removed in Example 1 above. A stainless steel plate was heat treated at 900 ° C. and then used as an experimental specimen.

  After immersing the above specimen in a mixed acid solution at 45 ° C. having a composition of 150 g / L sulfuric acid and 5 g / L free hydrofluoric acid, the potential is in the range of −0.6 to 0.6 V as described in Table 1. The mixed acid process was carried out by applying a current for 150 seconds while changing the pressure.

  After observing the surface state of the cold-rolled steel sheet after pickling using a scanning electron microscope (SEM), the presence or absence of pickling and the degree of surface roughness were evaluated. When the scale remains on the surface of the steel sheet, it is evaluated as unpickling, and the degree of surface roughness is judged to be not good when the surface roughness is 3 μm or more and judged to be good when the surface roughness is 3 μm or less. And displayed as ○. On the other hand, in the case of non-pickling, the degree of surface roughness was not evaluated.

  The evaluation results are shown in Table 1 below. Furthermore, the surface states of the steel plates of Invention Example 2 and Comparative Example 4 to which potentials of 0.1 V and −0.3 V were applied were photographed with an electron microscope (SEM) and shown in FIG.

  As can be seen from Table 1 above, when a surface potential of -0.3 to 0.2 V was applied, pickling with a mixed acid solution was possible (Invention Examples 1 to 3 and Comparative Example 4). However, in the case of Comparative Example 4, the degree of surface roughness was poor. That is, as can be seen from FIG. 2, in the case of (a) showing the steel sheet surface obtained by Invention Example 2, the steel sheet surface was uniformly dissolved along the crystal grains, but the steel sheet obtained by Comparative Example 4 was used. In the case of (b) indicating the surface, a good surface quality was not obtained, and a phenomenon that the crystal dropped off along the crystal plane of the crystal grain occurred.

  On the other hand, in Comparative Examples 1, 2, 3, 5 and 6, it was not pickled.

  From these results, it can be seen that application of the surface potential in the mixed acid bath in the range of −0.2 to 0.2 V is suitable for dissolution of the Si oxide layer.

Example 3
This example is for confirming the relationship between the hydrogen peroxide concentration and the iron ion concentration for obtaining a surface potential of -0.2 V in mixed pickling of ferritic cold rolled steel sheets.

  The ferritic stainless steel sheet having a chromium composition of 14 wt% or less from which the Fe rich scale layer and Cr rich scale layer were removed in Example 1 was heat-treated at 900 ° C. and used as an experimental specimen.

After the specimen was immersed in a mixed acid solution at 45 ° C. having a composition of 150 g / L sulfuric acid and 5 g / L free hydrofluoric acid, a steel plate was added while adding metal ions (Fe ³⁺ ) and hydrogen peroxide to the mixed acid solution. The surface potential of was measured.

  Measure the surface potential of the steel sheet due to the change in iron ion concentration, measure the minimum hydrogen peroxide concentration to keep the surface potential of the steel sheet above -0.2 V according to the change in iron ion concentration, and show the result It was shown in 3.

  FIG. 4 is a graph schematically showing the change in the minimum hydrogen peroxide concentration with respect to the change in the iron ion concentration in order to keep the surface potential of the steel sheet at −0.2 V or more. FIG. 4 shows that the minimum hydrogen peroxide concentration for maintaining the surface potential of the steel sheet gradually decreases as the iron ions increase. However, it can be seen that even if the iron ion concentration continues to increase to 40 g / L or more, the minimum hydrogen peroxide concentration does not decrease to 1 g / L or less. From these results, the relationship between the minimum hydrogen peroxide concentration and the iron ion concentration for maintaining the steel sheet surface potential at −0.2 V or higher is expressed as follows.

Example 4
This example is for confirming operating conditions suitable for carrying out neutral salt electrolysis.

  Neutral salt electrolysis was performed in the same manner as in Example 1 above. However, the solution temperature, applied current, and sodium sulfate concentration in the electrolytic cell are as described in Table 2 below.

  The surface state of the steel plate subjected to neutral salt electrolysis was observed, and the results are shown in Table 2. A case where the surface state is good is indicated as ◯, and a case where the surface state is poor such as the presence of a chromium oxide scale is indicated as x.

As can be seen from the above results, the solution temperature in the electrolytic cell is 50 to 90 ° C., and the electrolytic solution contains sodium sulfate 100 to 250 g / L as an electrolyte and has a current density of 10 to 30 A / dm ^2. When neutral salt electrolysis was performed, the surface quality of the steel sheet was excellent.

Example 5
This example is for confirming operating conditions suitable for carrying out sulfuric acid electrolysis.

  Sulfuric acid electrolysis was performed in the same manner as in Example 1 above. However, the solution temperature, applied current, and sodium sulfate concentration in the electrolytic cell are as described in Table 3 below.

  The surface state of the steel plate subjected to sulfuric acid electrolysis was observed, and the results are shown in Table 3. When the Fe oxide scale and Cr oxide scale are all removed and only the Si oxide scale remains and the surface state is good, the surface state is poor, such as an iron or chromium oxide scale remaining. The case is displayed as x.

As can be seen from the above results, the solution temperature in the electrolytic cell is 30 to 60 ° C., the sulfuric acid concentration in the electrolytic solution is 50 to 150 g / L, and the current density is 10 to 30 A / dm ^2. When sulfuric acid electrolysis was performed, the surface quality of the steel sheet was excellent.

Example 6
This example is for confirming the processing time suitable for neutral salt electrolysis and sulfuric acid electrolysis.

  Neutral salt electrolysis and sulfuric acid electrolysis conditions were the same as in Example 1. However, the processing time was as shown in Table 4 below.

  The presence or absence of pickling by the treatment time of neutral salt electrolysis and sulfuric acid electrolysis was observed. When there is no chromium or iron oxide scale remaining in addition to silicon oxide, it is indicated as ◯, and when silicon oxide or chromium or iron oxide scale remains, it is not pickled and the base material is eroded. A case where it was determined that the pickling was peracid pickling and a case where unpickling and per pickling appeared was indicated as x, and the results are shown in Table 4.

From Table 4 above, when performing neutral salt electrolysis on the steel sheet surface in the range of 0 to 90 seconds and sulfuric acid electrolysis in the range of 5 to 50 seconds, the surface of the stainless cold-rolled steel sheet as shown in FIG. In the case of Comparative Examples 1 to 4 outside the above range, the surface state as shown in (b) of FIG. 2 is obtained, and the scale of (Cr, Fe) ₃ O ₄ is You can see that it exists. Thereby, the processing time in the mixed acid tank can be minimized.

Example 7
This example is for confirming suitable processing conditions in a mixed acid tank.

  As in Example 1, the steel sheet subjected to neutral salt electrolysis and sulfuric acid electrolysis was treated with a mixed acid solution under the conditions shown in Table 5 below. At this time, the treatment temperature of the mixed acid solution was normal temperature, and the hydrogen peroxide concentration was adjusted as in Example 3.

  The presence or absence of pickling by this was observed, and the result was shown by (circle) and x, and was shown in Table 5. The case where silicon oxide does not remain is indicated as ◯, the case where silicon oxide remains is determined as non-acid pickling, and the case where there is erosion of the base material is determined as per-acid picking, and non- pickling and per-acid picking appear The case was indicated as x.

  As can be seen from Table 5 above, the pickling composition comprising 70 to 200 g / L of sulfuric acid, 1 to 10 g / L of free hydrofluoric acid, and 1.0 g / L or more of the minimum hydrogen peroxide based on iron ion concentration is 10 to 100. It is preferable to pickle for 2 seconds.

Example 8
In this example, ferritic cold rolled steel sheets are pickled using a conventional nitric acid-hydrofluoric acid mixed acid solution and pickled using a sulfuric acid-hydrofluoric acid-hydrogen peroxide mixed acid solution according to the present invention. It is for comparing the pickling quality of the steel plate in the case.

  A ferritic stainless steel cold-rolled steel sheet having a chromium content of 14% by weight or less was pickled with a mixed acid solution, and the glossiness was measured (n = 15). The mixed acid conditions and the mixed acid solution are as follows.

  Comparative Example 1: A ferritic stainless steel cold-rolled steel sheet was subjected to neutral salt electrolysis and sulfuric acid electrolysis according to Example 1, and then immersed in a mixed acid solution containing 100 g / L nitric acid and 3 g / L hydrofluoric acid for 30 seconds for pickling. Thereafter, the glossiness was measured.

  Invention Example 1: The glossiness of the steel sheet obtained by Invention Example 4 of Example 7 was measured.

  Invention Example 2: The glossiness of the steel sheet obtained by Invention Example 2 of Example 7 was measured.

  The glossiness measured for each steel sheet is shown in FIG.

  From FIG. 5, when the steel plate of Comparative Example 1 by conventional nitric acid pickling is compared with the steel plate in which the mixed pickling method of the present invention is applied to a non-pickled steel plate, 130 or more steel plates are obtained by the present invention. It can be seen that the effect of increasing the glossiness of about 40 to 60 is obtained from the steel plate of Comparative Example 1.

  Therefore, when applying this invention, it turns out that the surface quality of the steel plate after pickling can be improved.

Claims (14)

A silicon oxide pickling method for removing silicon oxide at a high speed from the surface of a low-chromium ferritic stainless steel cold-rolled steel sheet containing 14 wt% or less of chromium using a mixed acid solution containing no nitric acid,
A pickling step is performed by immersing the cold-rolled steel sheet in a mixed acid solution containing hydrogen peroxide,
The mixed acid solution comprises sulfuric acid 70~200g / L and the free hydrofluoric acid 1 to 10 g / L, when the mixed acid solution does not contain iron ions, the mixed acid solution seen contains hydrogen peroxide in a concentration 7 g / L or more, A silicon oxide pickling method for low chromium ferritic stainless steel cold-rolled steel sheets having a pickling rate of 3 to 15 g / m ² · min. The relationship between the hydrogen peroxide concentration and the iron ion concentration during the pickling step is

The silicon oxide pickling method for a low chromium ferritic stainless steel cold-rolled steel sheet according to claim 1, wherein:   3. The silicon oxidation of a low chromium ferrite stainless cold-rolled steel sheet according to claim 1 or 2, wherein the surface potential of the cold-rolled steel sheet immersed in the mixed acid solution is maintained at -0.2 to 0.2V. Pickling method.   The method for pickling a low chromium ferritic stainless steel cold rolled steel sheet according to claim 1 or 2, wherein the cold rolled steel sheet is immersed in a mixed acid solution for 10 to 100 seconds. A pickling method for a low chromium ferritic stainless steel cold-rolled steel sheet, wherein the ferritic stainless steel cold-rolled steel sheet containing 14% by weight or less of chromium that has been subjected to a degreasing and annealing process,
A sulfuric acid electrolysis step of electrolytically removing (Fe, Cr) ₃ O ₄ scale from a low chromium ferrite stainless steel cold rolled steel sheet using an electrolytic solution using sulfuric acid as an electrolyte;
In the mixed acid immersion step of immersing in a mixed acid solution, the mixed acid solution has a concentration of 7 g of hydrogen peroxide when sulfuric acid is 70 to 200 g / L , free hydrofluoric acid is 1 to 10 g / L, and the mixed acid solution does not contain iron ions. / L or more in viewing contains, does not contain nitric acid, and a step <br/>, it has a pickling rate of 3~15g ^/ m 2 · min, pickling method of low chromium ferritic stainless cold-rolled steel sheet.   The method further includes a neutral salt electrolysis step of electrolytically removing chromium-rich scale having a relatively higher Cr oxide content than the Fe oxide content from the steel sheet surface using an electrolytic solution containing a sodium sulfate electrolyte. The pickling method of the low-chromium ferritic stainless steel cold-rolled steel sheet according to claim 5. In the neutral salt electrolysis step, the ferritic stainless steel plate is immersed in a neutral salt electrolytic solution at a temperature of 50 to 90 ° C., and the electric potential of the steel plate surface is formed in the order of +, −, and +10 to 30 A / The pickling method for a low-chromium ferritic stainless steel cold-rolled steel sheet according to claim 6, wherein the current density of dm ² is applied for more than 0 seconds and not more than 90 seconds.   The method for pickling a low chromium ferritic stainless steel cold-rolled steel sheet according to claim 6, wherein the neutral salt electrolytic solution contains a sodium sulfate electrolyte in an amount of 100 to 250 g / L. In the sulfuric acid electrolysis step, the ferritic stainless steel plate that has undergone the annealing or neutral salt electrolysis step is immersed in a sulfuric acid electrolytic solution having a temperature of 30 to 60 ° C., so that the surface potential of the steel plate is formed in the order of +, −, +. The pickling method for a low-chromium ferritic stainless steel cold-rolled steel sheet according to claim 5, wherein a current density of 10 to 30 A / dm ² is applied for 5 to 50 seconds.   The method for pickling a low chromium ferritic stainless steel cold-rolled steel sheet according to claim 9, wherein the sulfuric acid electrolytic solution contains 50 to 150 g / L of sulfuric acid. In the mixed acid immersion step, the cold-rolled steel sheet is immersed in a mixed acid solution containing hydrogen peroxide for 10 to 100 seconds to perform a pickling process. The mixed acid solution includes 70 to 200 g / L of sulfuric acid and 1 to 10 g of free hydrofluoric acid. / contains L, and when said mixed acid solution does not contain iron ions, the mixed acid solution is characterized by the early days containing hydrogen peroxide in a concentration 7 g / L or more, a low chromium ferritic stainless cold according to claim 5 Pickling method for rolled steel sheets.   The pickling method for a low-chromium ferritic stainless steel cold-rolled steel sheet according to claim 11, wherein the surface potential of the steel sheet is maintained in the range of -0.2 to 0.2V in the mixed acid immersion step. The hydrogen peroxide in the pickling step is expressed by the following formula in relation to the iron ion concentration in the mixed acid solution.

The pickling method for a low-chromium ferritic stainless steel cold-rolled steel sheet according to claim 10, wherein: A mixed acid solution for removing silicon oxide from a ferritic stainless cold-rolled steel sheet containing 14 wt% or less of chromium that has undergone a degreasing and annealing process,
Including sulfuric acid 70-200 g / L, free hydrofluoric acid 1-10 g / L, and hydrogen peroxide,
The hydrogen peroxide has the following formula in relation to the iron ion concentration in the mixed acid solution:

A mixed acid solution containing no nitric acid for removing silicon oxide.

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