WO2012091412A2 - Eco-friendly high-speed pickling process for producing a low-chrome ferrite-based cold-rolled stainless steel sheet having superior surface quality - Google Patents

Abstract

The present invention relates to a method for pickling the surface of a steel sheet at a high speed during the manufacture of a ferrite-based cold-rolled stainless steel sheet that requires high surface quality. The method for pickling a low-chrome ferrite-based cold-rolled stainless steel sheet at a high speed involves pickling the ferrite-based cold-rolled stainless steel sheet which contains 14% or less of chrome and which has undergone a degreasing and annealing process, wherein the method comprises a neutral-salt electrolyzing step consisting of the electrolytic removal of chrome-rich scale from the surface of the steel sheet using an electrolytic solution in which sodium sulfate is used as an electrolyte; a sulfuric-acid electrolyzing step consisting of the electrolytic removal of iron-rich scale using an electrolytic solution in which sulfuric acid is used as an electrolyte; and a mixed-acid immersion step consisting of immersing the steel sheet in a mixed acid solution containing sulfuric acid, hydrofluoric acid, and hydrogen peroxide, wherein the whole pickling process is completed within 15 to 240 seconds. According to the present invention, the use of nitric acid in pickling a ferrite-based cold-rolled stainless steel sheet can be avoided, thus reducing the burden of installing an NOx removal facility and a denitrification facility, and further, pickling may be adjusted in terms of the hydrogen peroxide concentration and hydrofluoric acid concentration, thus making the process easy to control and suitable for high-speed production. The quality of the steel sheet having undergone the pickling process of the present invention is improved as compared to that of the steel sheet of existing pickling process, the result being that a high-quality ferrite-based cold-rolled stainless steel sheet may be produced.

Description

 【Specification】

 [Name of invention]

 Eco-friendly high-speed pickling process for manufacturing low-quality ferritic stainless steel leaded steel sheet with excellent surface quality

 Technical Field

 The present invention relates to a method for pickling a surface of a steel sheet at high speed in manufacturing a ferritic stainless steel cold rolled steel sheet requiring high surface quality, and more particularly, to a pickling method that does not use nitric acid during a pickling process.

 Background Art

 Stainless steel cold rolled steel sheet is used to obtain certain mechanical properties after rolling.

The heat treatment is performed ^at 800 ~ 1150 ^° C. In this heat treatment, the surface of the steel sheet reacts with high temperature oxygen in the furnace to generate an oxide scale (Si0 ₂ , (Cr, Fe) ₃ 0 ₄ ) on the surface. This oxidation scale on the surface of the steel sheet deteriorates the appearance of the product, deteriorates the quality of the steel sheet, and also becomes a starting point that causes corrosion of the steel sheet, and lowers the corrosion resistance of the steel sheet. Therefore, in order to obtain a beautiful surface quality and improve corrosion resistance, physical descaling by brush, shot ball blasting, etc., electrolytic descaling using sodium sulfate, sulfuric acid, nitrate electrolyte, chemical descaling using salt bath, common acid, etc. Stainless steel cold rolled steel sheet is manufactured by removing oxide scale on the surface of steel sheet by combining various methods, and the process of removing such scaling is called a pickling process. In this stainless steel pickling process, beautiful surface quality In order to ensure the corrosion resistance by uniformly forming the passivation film, the nitric acid electrolytic method and the nitric acid (80 to 180 g / «and the hydrofluoric acid (2 to 40 g / £)) are descaled by applying a current through a steel sheet to the nitric acid solution. The pickling process has been carried out by chemical descaling using common acid, nitric acid lowers the pH in the pickling bath to increase the activity of blenic acid, and oxidizes divalent iron or silver dissolved on the surface of steel sheet to trivalent to maintain the proper redox potential for pickling. However, as nitric acid is used as the pickling solution, NOx, an air emission control substance, is generated, and nitrate nitrogen (N0 ₃ -N) is contained in waste acid and washing water. Installation of additional environmental pollution prevention equipment in the pickling process to satisfy the environmental regulatory conditions such as total nitrogen restriction of discharged water and NOx concentration restriction of air discharge facility There is a problem that the production cost increases considerably according to the operating cost, and also in the production of low chromium ferritic mild steel sheet containing less than 14% by weight of chromium, which is more vulnerable to corrosion resistance than other stainless steels. In order to solve this problem, a pickling method that does not use nitric acid has been developed, and this technique has been developed using hydrochloric acid or sulfuric acid. In addition, a pickling method has been developed that does not use nitric acid, which is supplemented by hydrogen peroxide, potassium permanganate, trivalent iron ions, and air injection. Specifically, German Patent Publication No. 3937438 discloses a technique for maintaining a redox potential of a pickling solution of 300 mV or more by using sulfuric acid, fluoric acid, and iron sulfate as a pickling solution, and adding hydrogen peroxide. Since the 90's, as disclosed in U.S. Patent No. 5154774 and European Patent Publication No. 236354, the proper range of oxidation-reduction potential (ORP) of mainly hydrofluoric acid and iron ions, air, hydrogen peroxide or solution is specified. The technology to continue to emerge. However, most of these methods have a limitation in that they can be applied to products such as wire rods, steel bars and thick plates, which are not demanding in quality. On the other hand, U.S. Patent No. 5908511 contains sulfuric acid, hydrofluoric acid, iron salts and regularly injects hydrogen peroxide, pickling by adjusting the composition of the wetting agent polish, corrosion inhibitor, etc., the management of pickling solution is Fe (III) and accordingly ORP As a technique, an automatic control scheme is disclosed. Through this, CLEANOX352, a pickling solution, has been commercialized and is the most widely used worldwide. This method is used for wire rods and hot rolled products, but the production cost is 20% higher than the existing one, and it adopts complex solution composition and management method. In addition, it has a relatively slow pickling rate of 1.5-3 g / n-min, which is decisive, so it is not suitable for the high-speed pickling line where the common pickling should be completed within 10 100 seconds. In addition, European Patent Publication No. 1040211 and US Patent Publication No. 2000-560982, which are improved patents of the above patents, propose a method of increasing pickling rate by adding copper and chlorine ions to a pickling composition. If the open circuit potential (OCP) formed on the surface of the steel sheet is lower than 0.1 V, which is the redox potential of copper ions, copper particles may precipitate on the surface of the steel sheet and cause discoloration of the steel sheet. There is also a risk of pitting corrosion if the pickling solution contains more than a certain concentration of chlorine ions. As described above, various techniques are known for the nitric acid-free pickling composition, but there is no known pickling technique suitable for producing ferritic metal sheet at high speed.

 [Detailed Description of the Invention]

 [Technical problem]

 The present invention provides an electrolytic pickling method suitable for high-speed production of low chromium ferritic stainless steel mild steel sheet and a common pickling method for removing silicon oxide at high speed without securing nitric acid.

 In addition, the present invention is to provide a common acid solution containing no nitric acid suitable for the pickling method.

 Furthermore, the present invention provides a low cream ferritic stainless steel annealed steel sheet obtained through the pickling method as described above.

 Technical Solution

In one aspect of the invention, according to one embodiment, silicon oxide for rapidly removing silicon oxide from the surface of a low chromium ferritic stainless steel cold-rolled steel sheet containing less than 14% chromium using a common acid solution containing no nitric acid As a pickling method, the above-mentioned in a common acid solution containing hydrogen peroxide A pickling process is performed by dipping a cold rolled steel sheet, wherein the common acid solution is sulfuric acid

Low pickling rate of 3-15g / m ² Tiiin, containing 70-200g / i and free hydrofluoric acid l-10g /, the first common acid solution containing more than hydrogen peroxide concentration 이상 φ, but no iron ions Provided is a silicon oxide pickling method for a chromium ferritic stainless steel cold rolled steel sheet.

 According to another embodiment of the present invention, the relationship between hydrogen peroxide and iron ion concentration in the pickling process is

It is preferable to satisfy [H ₂ O ₂ ] ≧ 0.74 + 3.1e- ^{[Fe] / 11} + 3.2e- ^{[Fe] / 15} .

According to another embodiment of the present invention, the surface potential of the flexible steel sheet immersed in the common acid solution is preferably maintained at -0.2 to 0.2 V, and pickled by immersion in the common acid solution for 10 to 100 seconds in the common acid solution. Can be. In another aspect of the present invention, there is provided a method for pickling a low chromium ferritic stainless steel sintered steel sheet, which pickles a ferritic stainless steel cold rolled steel sheet containing less than 14% of the cracks subjected to degreasing and annealing. As a sulfuric acid electrolysis step of electrolytic removal of the (Fe, Cr) scale by using an electrolytic solution using sulfuric acid as an electrolyte from a low cream ferritic stainless steel stainless steel sheet; And a common acid immersion step of immersing in a common nitric acid-free acid solution including sulfuric acid, hydrofluoric acid, and hydrogen peroxide, and having a pickling rate of 3-15 g / m ² 'min. . According to another embodiment of the present invention, a neutral salt electrolysis step of electrolytically removing the crack-rich scale from the surface of the steel sheet using an electrolyte solution containing sodium sulfate electrolyte may be further included.

According to another embodiment of the present invention, the neutral salt electrolysis step is to immerse the ferritic stainless steel sheet in a neutral salt electrolyte solution of 50 ~ 9CTC temperature, the potential of the surface of the steel sheet is formed in the order of +,-, + 10 It can be carried out by applying a current density of ˜30 A / dm ² for more than 0 seconds and 90 seconds or less, and may further include 100-250 g / £ of sodium sulfate electrolyte in the neutral salt electrolyte solution.

According to another embodiment of the present invention, the sulfuric acid electrolysis step is immersed in a sulfuric acid electrolytic solution of the annealed or neutral salt electrolytic sulfuric acid solution of 30 ~ 60 ^° C temperature, the potential of the steel plate surface +, It can be carried out by applying a current density of 10 ~ 30A / dm ² for 5 ~ 50 seconds to be formed in the order of-, and the sulfuric acid electrolytic solution may contain 50-150g / sulfuric acid.

 Furthermore, according to one embodiment of the present invention, the common acid immersion step, the common acid immersion step is carried out a pickling process by immersing the cold rolled steel sheet in a common acid solution containing hydrogen peroxide for 10 to 100 seconds, the volatile acid solution is sulfuric acid 70-200 g and 1-10 g of free hydrofluoric acid; the first common acid solution contains a hydrogen peroxide concentration of 7 g / £ or more, substantially free of iron ions,

 In addition, the surface potential of the steel sheet in the common immersion step can be maintained in the range of -0.2 to 0.2V.

According to another embodiment of the present invention, the hydrogen peroxide in the pickling process in the relationship with the iron ion concentration in the common acid solution [H ₂ O ₂ ]> 0.74 + 3.1e- ^{[Fe] 1-1} + 3.2e- ^{[Fe] 15} can be satisfied. In still another aspect, the present invention relates to a steel sheet obtained by performing the pickling method, and provides a low chromium ferritic stainless steel cold rolled steel sheet having a glossiness of 130 or more. In still another aspect, the present invention relates to a common acid solution for removing silicon oxide from a ferritic stainless steel cold rolled steel sheet containing 14% or less of chromium, which has been subjected to a degreasing and annealing process. ˜10 g; And hydrogen peroxide, wherein the hydrogen peroxide is represented by the following equation in relation to the iron ion concentration in the common acid solution.

A common acid solution containing no nitric acid for removal of silicon oxide that satisfies [H ₂ O ₂ ]> 0.74 + 3.1e- ^{[Fe] 1 -1} + 3.2e- ^{tFe] 15} is provided.

 Advantageous Effects

According to the present invention, in pickling a ferritic stainless steel mild steel sheet, since the common acid solution in the pickling solution does not contain nitric acid, ΝΟχ and nitrate nitrogen are not discharged. Therefore, the burden on the installation of a ΝΟχ removal apparatus and a denitrification apparatus can be reduced. Furthermore, pickling can be adjusted by the concentration of hydrogen peroxide and the concentration of hydrofluoric acid, so it is easy to control and is suitable for high speed production. In addition, the quality after pickling is also improved compared to the existing pickling method, it is possible to produce a high quality ferritic stainless steel sheet. In addition, by completely removing Fe and Cr oxides except silicon oxide in the electrolytic pickling process, it is possible to easily remove the silicon oxide by the common acid solution, and the pickling of the common acid tank at a high speed of 3 to 15 g / iif'min. It can be carried out within a short time of 10 100 seconds to remove and planarize the silicon oxide by the common acid solution. In addition, the solution composition is simple, so the management method is simple, and it is possible to secure the surface quality of the steel sheet by preventing any reaction other than pickling reaction with the surface of the steel sheet, and improve productivity by high-speed production. Can be.

 [Brief Description of Drawings]

1 is a photograph taken with an electron microscope of the surface of a steel sheet heat-treated with a low-cream ferritic stainless steel cold-rolled steel sheet heat treated according to Example 1, A) is the surface of the steel sheet obtained by the invention example 2, B) is a comparison Steel plate surface according to Example 4. Figure 2 shows an electron microscope photograph of the cross section of the steel plate after the electrolytic treatment in the heat-treated low chromium ferritic stainless steel thin steel sheet in A), and in B) the cross section of the steel sheet without electrolytic treatment is taken. An electron micrograph. 3 is a graph showing the correlation between the redox potential and hydrogen peroxide concentration with respect to the surface potential of the steel sheet when the low chromium ferritic stainless steel sheet is immersed in common acid, A) shows the relationship between the surface potential and the oxidation oxidation potential , B) shows the relationship between the surface potential and the hydrogen peroxide concentration. 4 is a graph showing the minimum hydrogen peroxide concentration required according to the iron ion concentration in the common acid solution. 5 is a graph comparing glossiness of the ferritic stainless steel mild steel sheet subjected to pickling according to Example 8. FIG.

 [Best form for implementation of the invention]

The present invention provides a method for removing an oxidized scale on the surface of a steel sheet by passing a low chromium ferritic stainless steel sheet 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 common acid bath of a nitric acid-free pickling composition. to provide. Hereinafter, the present invention will be described in detail. In general, ferritic stainless steel thin steel sheet has an oxidized scale having a thickness of 100 to 300 nm after heat treatment, and the oxidized scale has a Cr—rich scale layer having a relatively higher Cr oxide scale content than Fe and a Fe oxide than Cr. It has a multilayer structure of a Fe-rich scale layer having a relatively high scale content, and an Si-oxide layer present at the interface between the oxide scale and the base material. Of these scales, the Cr-rich scale layer is removed in a neutral salt electrolyzer. The neutral salt electrolyzer comprises a neutral salt electrolyte containing a sodium sulfate electrolyte, and includes an electrode for conducting a current to the surface of the steel sheet. At this time, the electrode assembly is configured such that the potential of the steel plate surface is charged in the order of +,-, +. When a current is applied to the neutral salt electrolyte in the pH 3-6 range, Cr of the Cr-rich scale preferentially dissolves in the state of Cr ⁶⁺ , thereby removing the Cr-rich scale on the surface of the steel sheet. At this time, it is preferable to use sodium sulfate as the electrolyte. The sodium sulfate electrolyte causes the Cr-rich scale to dissolve by increasing the electrical conductivity in the electrolyte and increasing the current carrying rate to the steel sheet surface. In this case, it is preferable that the electrolyte contains 100 to 250 g / £ sodium sulfate electrolyte. When the sodium sulfate electrolyte is 100 g / or more, an appropriate conductivity for dissolving chromium may be obtained. However, if it exceeds 250 _§ / £, sodium sulfate precipitates in the electrolyte solution, which may block the facility piping and worsen the operation. Therefore, the sodium sulfate electrolyte is preferably included at 250 g / or less. The electrical conductivity of the electrolyte in the neutral salt electrolyzer is closely related to the temperature of the electrolyte. Above 50 ^° C, proper conductivity for chromium dissolution can be obtained. The higher the temperature, the higher the conductivity. However, if the temperature of the electrolyte exceeds 90 ^° C, it is difficult to manage the silver in operation. Therefore, the temperature of the electrolyte in the neutral salt electrolyzer is preferably in the range of 50 to 9 CTC. On the other hand, the current applied through the electrode is preferably lOA / dm ² or more. Cr on the Cr-rich scale can be sufficiently eluted. However, if it exceeds 30A / dm ² there is a disadvantage that the rectifier (rectifier) equipment for generating a current is large, the initial equipment cost is large, the current is preferably in the range of 10 ~ 30A / diif. Neutral salt electrolytic treatment time is carried out to 90 seconds or less. Such neutral salt electrolytic treatment is not carried out as necessary, and the oxidation scale of iron and chromium can be removed only by the sulfuric acid electrolytic treatment. In this case, however, the sulfuric acid electrolytic treatment and the common acid may be burdened, and the neutral salt treatment may further shorten the pickling treatment time. If the neutral salt electrolytic treatment time is more than 90 seconds, there is a concern that the base metal may be partially eroded, resulting in peracid tax. 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 conducting a current to the surface of the steel sheet. At this time, the electrode assembly is configured such that the potential of the steel plate surface is charged in the order of +,-, +. When current is applied to the sulfuric acid electrolyte in the range of pH 0-1, Fe is in the state of Fe ²⁺ Dissolves. At this time, H ⁺ and SO ₄ ² — of sulfuric acid increase the electrical conductivity in the solution, thereby increasing the electrical conductivity from the electrode to the surface of the steel sheet, and chemically dissolving the iron in the Fe-rich scale layer by the lowered pH. At this time, the sulfuric acid is preferably 50 to 150g /. Sulfuric acid should be more than 50g / maintains the proper conductivity or higher, and thus it is possible to maintain the electrical conductivity to the surface of the steel sheet. However, when sulfuric acid exceeds 150g /, chemical dissolution is predominantly generated, which may cause a problem that the surface of the stainless steel annealed steel sheet is rough. Like the electrolyte solution of the neutral salt electrolyzer, the sulfuric acid electrolyte solution must have a solution temperature of 3CTC or more to obtain a minimum conductivity. However, in the case of exceeding 6CTC, the chemical dissolution may proceed excessively, resulting in a black smut phenomenon in which the surface of the stainless steel annealed steel sheet becomes rough, and the surface of the steel sheet turns black. Therefore, it is preferable that the electrolyte solution of the sulfuric acid electrolyzer has a temperature in the range of 30 to 6 CTC. On the other hand, it is preferable that the current applied to the sulfuric acid electrolyzer is in the range of 10 to 30 A / diif. In the case where the current is applied to 10 A / diif or less, active dissolution of the ferritic stainless steel sheet proceeds and the surface becomes rough. However, in the case of exceeding 30A / dm, the rectifier (rectifier) equipment for generating a current has a disadvantage that the initial equipment cost is large, the current is in the range of 10 ~ 30A / dm ² desirable. Such sulfuric acid electrolysis is preferably performed for 5 to 50 seconds. If the sulfuric acid electrolytic treatment for less than 5 seconds can not obtain a sufficient pickling effect, if it exceeds 50 seconds, causing the problem of peroxidation, it is preferable to carry out in the above range. As described above, only the Si oxide layer remains on the surface of the steel sheet subjected to the neutral salt electrolysis-sulfuric acid electrolysis, and this Si oxide layer can be removed by a common nitric acid free acid solution. The common acid solution includes sulfuric acid, free hydrofluoric acid, and hydrogen peroxide, and the Si oxide layer may be removed from the steel plate subjected to neutral salt electrolysis-sulfuric acid electrolysis by immersion in a common acid bath including the common acid solution. The hydrofluoric acid and sulfuric acid in the common acid solution are dissociated in the common acid solution as in the following reactions (1) and (2). That is, the hydrofluoric acid dissociates while dissolving in a common acid solution, and the equilibrium state is changed by H + concentration, that is, acidity, which dissociates sulfuric acid.

HF → H ⁺ + F— (1)

H ₂ S0 ₄ → HS0 ₄ ^" + H ⁺ → S0 ₄ ² 2H ⁺ (2) In case of hydrofluoric acid, it has an acid force in the undissociated free hydrofluoric acid (Free HF) state to dissolve the Si oxide, and furthermore, Penetrates the interface between the layer and the base material Fe is dissolved, and the thus dissolved Fe and Si ions are removed from the steel sheet surface in the form of FeF _x ^{(3 x)} , H ₂ SiF _6, or the like. The hydrofluoric acid is preferably present in a concentration of 1 ~ 10g i, more preferably 1 ~ 5g / in the common acid solution. If the concentration is less than Ig / H, the concentration of free hydrofluoric acid is low, so that the dissolving power for Si is insufficient, and this causes a problem of micro pickling on the surface of the steel sheet. The steel plate surface after that becomes rough. As described above, since hydrofluoric acid provides a pickling force for removing the Si oxide layer on the surface of the steel sheet, it is preferable that the effective free hydrofluoric acid concentration is maintained in the common acid solution above a certain acidity. Therefore, the sulfuric acid solution requires a certain concentration or more of sulfuric acid so as not to dissociate the hydrofluoric acid. Suitable sulfuric acid concentrations range from 50 to 150 g /. If the sulfuric acid concentration is less than 50g, the effective free hydrofluoric acid concentration is not maintained, so that dissociation of hydrofluoric acid occurs and the pickling force is weakened. There is a problem such that operation is difficult, it is preferable to include sulfuric acid in the concentration of the above range. Among the oxide scales of ferritic stainless steel, Si oxide is present both at the grain surface of the ferritic crystal and at grain boundaries between grains and grains, and the Si oxide at the grain boundary is present at a deeper position inside the base material. In the case of austenitic stainless steel, the corrosion resistance of the crystal is high, so it is preferentially eroded from the grain boundary. However, ferritic stainless steel has low corrosion resistance. There is no difference in rate of erosion between the grains and the grains, so that the grain surface and grain boundaries are completely dissolved without selectivity. Therefore, in order to remove all the Si oxides, a substantial portion of the base metal must be dissolved. At this time, and the Fe ²⁺ eluted from the base material, the eluted Fe ²⁺ is produced a complex with FeF _x ^(3_x), in combination with HF after the reaction with the hydrogen peroxide oxidation to Fe ³⁺ is removed from the steel sheet surface. The reaction can be expressed as the following reaction formulas (3) to (6), it is possible to increase the pickling rate only when such a process proceeds smoothly.

Fe ° → Fe ²⁺ + 2e ^" (3)

Fe ²⁺ + H ₂ 0 ₂ → Fe ³⁺ + · ΟΗ + (4)

Fe ³⁺ + 3HF → FeF ₃ + 3H ⁺ (5)

Cu ²⁺ + 2e ^" → Cu ° (6) According to the experiments of the present inventors, in order to obtain a beautiful ferritic stainless steel cold rolled sheet, a scale and a base material of about 3 to 5 g / m ² should be removed from a common tank. -Pickling speed of 3 ~ 15g / iif'min should be ensured to remove Si oxide by immersing in a common tank for 100 seconds, more preferably 20-60 seconds to produce lead steel plate at high speed. Each steel grade has a unique coincidence potential-current and the amount of current generated can be expressed by pickling rate. As a result, the maximum pickling speed can be achieved by adjusting the surface potential. In order to obtain a high pickling speed of 3-15 g / m 'required to produce a thin steel sheet at a high speed, it is preferable to maintain the surface potential of the cold rolled steel sheet in a common tank in the range of -0.2 to 0.2V. If the surface potential of the mild steel sheet is out of the above range, pickling may not occur or a pickling failure may be caused in part with an unpickled surface. In addition, even if it is pickled, the annealed steel sheet cannot obtain good surface quality. Conventionally, pickling was performed by controlling the ratio of Fe ²⁺ / Fe ³⁺ in the pickling solution and controlling the oxidation-reduction potential ( ³ ) of the pickling solution. However, as shown in FIG. 3A, no correlation between the surface potential and the redox potential cannot be confirmed, and the redox potential is not an important factor having any correlation with pickling. It can be seen. In contrast, the surface potentials in common acids of ferritic stainless steels with low creep content are the concentrations of metal ions generated during the pickling process in the common bath, that is, the iron ions concentrations and the residual hydrogen peroxide concentrations in the common acid solution, especially the residual hydrogen peroxide concentration. It can be confirmed that it has a correlation with. When the residual hydrogen peroxide concentration in the common acid solution is insufficient, reaction of the reaction formula (4) is not performed so that the concentration of Fe ^{2+ on the} surface of the steel sheet increases locally, and the reaction in the left direction of the reaction formula (3) predominates. do. In this case, Cu, which is present as an additive or an impurity in Fe and stainless steel, is reprecipitated on the surface of the steel sheet as in Banung equation (6), thereby causing a blackening of the steel sheet surface. therefore Residual hydrogen peroxide concentration should always be above a certain concentration. This residual hydrogen peroxide concentration correlates with the iron ion concentration present in the common acid solution. 3 is a graph showing the minimum hydrogen peroxide concentration to maintain the surface potential of the strip according to the iron ion concentration in the pickling solution, the surface potential of the steel sheet in the common acid bath to obtain a high pickling rate, as described above. It can be seen that the surface potential of the steel sheet gradually increases as the iron ion concentration increases at the same hydrogen peroxide concentration. This is because Fe ³⁺ ion acts as an oxidizing agent, and as the Fe ³⁺ ion increases, the concentration of hydrogen peroxide to induce the steel plate surface potential tends to decrease. However, even when Fe ³⁺ ions are present at a certain concentration or more, when the hydrogen peroxide is deficient, the surface potential of the steel sheet is reduced to -0.2V or less, resulting in a problem of deterioration of surface quality. From this relationship, the minimum hydrogen peroxide concentration according to the iron ion concentration for maintaining the steel sheet surface potential of -0.2V or more is expressed as follows.

[H ₂ 0 ₂ ] min = 0.74 + 3.1 e- ^ ^{/ 1-1} + 3.2e— ^{[Fe] / 15}

Specifically, when the iron ion concentration is 0, an effective hydrogen peroxide concentration of at least 7 g / £ or more is to be included, and when the iron ion concentration is 40 or more, it must be present in an amount of l.Og / or more. On the other hand, the higher the concentration of hydrogen peroxide can avoid the effort of the addition of additional hydrogen peroxide helps to simplify the process. However, these Since hydrogen peroxide is expensive, the use of a large concentration of hydrogen peroxide leads to an increase in cost and does not lead to an increase in pickling effect proportional to the concentration used.

It is more preferable not to exceed 30 g /. Furthermore, since the temperature of the common acid solution of this invention does not need to specifically limit, it can be performed by the person skilled in the art as it sets suitably. For example, 20 to 95 ^° C., as another example, may be set to 25-80 ^° C, another example to 25 to 65 ^° C range. From the foregoing, it was confirmed that the concentration of sulfuric acid and hydrofluoric acid in the pickling solution and the concentration of residual hydrogen peroxide in the pickling tank process are the most important factors for increasing pickling effect and high speed pickling. Therefore, it is necessary to control the concentration of these components, the concentration control of each of these components, it is possible to adjust the sulfuric acid and sulfuric acid concentration using a conventionally used acid analyzer, the residual hydrogen peroxide concentration through the near infrared analysis method or automatic titration method Concentration can be analyzed and adjusted. According to the present invention, it is possible to perform the pickling process at a high speed, it is possible to significantly shorten the time required for pickling as a whole pickling time is 15 to 240 seconds, it is possible to obtain a low cream ferritic stainless steel sheet having excellent quality have. [Form for implementation of invention]

Hereinafter, the present invention will be described in detail with reference to Examples. Example 1

In order to confirm the pickling properties of the oxide scale of ferritic thin steel sheet with or without neutral salt electrolysis and sulfuric acid electrolysis, ferritic stainless steel sheets having a chromium composition of 14% or less were subjected to Cr salt electrolytic treatment and sulfuric acid electrolytic treatment, respectively. -Rich scale layer and Fe—rich scale layer were removed. In this case, the neutral salt electrolytic solution used includes 150 g of sodium sulfate electrolyte as an electrolyte, the temperature of the solution is 60 ^° C., and a current of 150 A / duf was applied for 40 seconds. Furthermore, the sulfuric acid electrolytic solution is a solution of pH 1 containing 85 g of sulfuric acid, the solution temperature is 50 ^° C, and a current of 20 A / dm ² is applied for 15 seconds so that the potential of the steel plate surface is formed in the order of +,-, +. It was.

 The surface of the steel plate subjected to the neutral salt electrolysis and the sulfuric acid electrolysis and the cross section of the untreated steel pipe were photographed with an electron microscope (SEM) and are shown in FIG. 1. For comparison, the cross section of the electrolytically treated steel sheet is shown in A), and the cross section of the non-electrolytically treated steel sheet is shown in B).

As can be seen from FIG. 1, it can be seen that (Cr, Fe) ₃ 0 ₄ and silicon oxide remain at the same time on the cross section of the steel sheet which is not subjected to neutral salt electrolysis and sulfuric acid electrolysis. Only silicon oxide remained in the cross section of the electroplated steel plate. Example 2

In order to observe the surface state of the cold rolled steel sheet according to the surface potential in the common tank, the ferritic stainless steel having a chromium content of 14% or less in which the Fe-rich scale layer and the Cr-rich scale layer were removed in Example 1 After heat-treating the steel sheet at 9CX C The test specimen was used.

The specimen was immersed in a 45 ^° C. common solution having a composition of 150 g sulfuric acid and 5 g / £ free hydrofluoric acid, and then subjected to a localization process by applying an electric potential for 150 seconds while changing a potential in the range of -a5 to 0.5 V as shown in Table 1. Was performed.

 After pickling, the surface state of the cold rolled steel sheet was observed using a scanning electron microscope (SEM), and then the presence of pickling and the degree of surface roughness were evaluated. When the scale remained on the surface of the steel sheet, it was evaluated by acid pickling, and the degree of surface roughness was judged to be X when it was judged to be not good when the surface roughness was not good, and indicated by O when it was judged to be good or less. On the other hand, in the case of non-acid 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 sheets of Inventive Example 2 and Comparative Example 4 to which potentials of 0.1 V and −0.3 V were applied are photographed by electron microscope (SEM), and are shown in FIG. 2.

Table 1

표 1로부터 알 수 있는 바와 같이， 표면전위가 -0.3~0.2V 인가된 경우에는 흔산 용액에서의 산세가 가능하였다 (발명예 1 내지 3 및 비교예 3). 그러나， 비교예 3의 경우에는 표면 거침 정도가 불량하였다. 즉, 도 2로부터， 발명예 2에 의해 얻어진 강판 표면을 나타내는 A)의 경우에는 강판 표면이 결정립에 따라 균일하게 용해된 반면， 비교예 3에 의해 얻어진 강판 표면을 나타내는 B)의 경우에는 양호한 표면품질이 얻어지지 않고, 결정립의 결정면을 따라 결정이 탈락하는 현상이 일어났음을 확인할 수 있다.

As can be seen from Table 1, the surface potential of -0.3 ~ 0.2V In this case, pickling in a common acid solution was possible (Invention Examples 1 to 3 and Comparative Example 3). However, in the case of Comparative Example 3, the surface roughness was poor. That is, in the case of A) which shows the steel plate surface obtained by Inventive Example 2 from FIG. 2, the steel plate surface was melt | dissolved uniformly according to the crystal grain, whereas in the case of B) which shows the steel plate surface obtained by the comparative example 3, a favorable surface is shown. It could be confirmed that the quality was not obtained and crystallization occurred along the crystal plane of the crystal grain.

 On the other hand, in the case of Comparative Examples 1, 2, 4 and 5, the results of the acid pickling.

 From this result, it can be seen that it is suitable for dissolution of the Si oxide layer that the surface potential in the common acid tank is applied in the range of -0.2 to a2V. Example 3

This embodiment is to confirm the relationship between the hydrogen peroxide concentration and the iron or silver concentration to obtain a surface potential of -0.2V in common pickling of ferritic thin steel sheet. In Example 1, a ferritic stainless steel plate having a chromium composition of 14 ^« 3/4 or less from which the Fe-rich scale layer and the Cr—rich scale layer were removed was heat-treated at 90 C and used as an experimental specimen.

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

The surface potential of the steel sheet was measured according to the iron ion concentration change, and the minimum hydrogen peroxide concentration was measured to maintain the surface potential of the steel sheet at -0.2V or more according to the iron ion concentration change. The results are shown in FIG. 3. 4 is a graph illustrating the change in the minimum hydrogen peroxide concentration according to the change in iron ion concentration in order to maintain the surface potential of the steel sheet -0.2V or more. As can be seen from Figure 4, as the iron ions increase, the minimum hydrogen peroxide concentration for maintaining the surface potential of the steel sheet gradually decreased. However, even if the iron ion concentration continues to increase above 40 g / £, it can be seen that the minimum hydrogen peroxide concentration does not decrease below. From these results, the relationship between the minimum hydrogen peroxide concentration according to the iron ion concentration for maintaining the steel plate surface potential above -0.2V can be expressed as follows.

[H ₂ 0 ₂ ] _min = 0.74 + 3.1 θ ^{"[Ρβί 1} · ¹ + 3.2e- ^{[Fe] / 15}

Example 4

 This example is intended to confirm suitable operating conditions when neutral salt electrolysis is performed.

Neutral salt electrolysis was performed in the same manner as in Example 1. However, the solution temperature, applied current, and sodium sulfate concentration of the electrolyzer were performed as described in Table 2 below. The surface state of the steel plate subjected to the neutral salt electrolysis was observed, and the results are shown in Table 2. If the surface state is good, it is indicated by O. If the surface state is bad, such as the presence of a chromium oxide scale, it is indicated by X. Table 2

상기 결과로부터 알 수 있는 바와 같이, 전해조 내의 용액 온도가

As can be seen from the above results, the solution temperature in the electrolytic cell is

When the neutral salt electrolysis was performed under the conditions of 50 to 90 ^° C., containing 100 to 250 g / of sodium sulfate as an electrolyte in the electrolyte solution, and having a current density of 10 to 30 A / diif, the surface quality of the steel sheet was excellent. Example 5

 This embodiment intends to confirm suitable operating conditions when sulfuric acid electrolysis is performed.

 Sulfuric acid electrolysis was carried out in the same manner as in Example 1. However, the solution temperature, applied current and sulfuric acid concentration of the electrolyzer were performed 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. If both the Fe and Cr oxide scales are removed and only the Si oxide scale remains, and the surface state is good, it is indicated by O. If the surface state is poor, such as iron or crust oxide scale remains, it is indicated by X.

Table 31 Temperature rc) Current (A / dni ^! ) Sodium Sulfate Concentration (g / «) Surface Quality Comparative Example 1 30 5 50 X Comparative Example 2 70 5 150 X Comparative Example 3 70 30 150 X Comparative Example 4 30 10 30 X Invention Example 1 30 10 50 O Inventive Example 2 60 20 150 O As can be seen from the above results, the solution temperature in the electrolytic cell

When the sulfuric acid electrolysis was carried out under the conditions of 30 to 60 ^° C, sulfuric acid concentration in the electrolytic solution is 50 ~ 150g /, current density of 10 ~ 30A / dm ² , the surface quality of the steel sheet was excellent. Example 6

 This example is intended to identify suitable treatment times for neutral salt electrolysis and sulfuric acid electrolysis.

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

 Neutral salt electrolysis and sulfuric acid electrolysis were observed for pickling according to the treatment time. If the scale of chromium or iron other than silicon oxide does not remain, it is indicated by O. If the scale of chromium or iron other than silicon oxide remains, it is considered as acid pickling. And in the case of over-acid tax, X is indicated and the results are shown in Table 4.

비교예 1 0 5 X Table 4

Comparative Example 1 0 5 X

 Comparative Example 2 0 60 x (Acid Tax) Comparative Example 3 90 0 X

 Comparative Example 4 90 60 x (Acid Tax) Invention Example 1 0 15 O

 Inventive Example 2 0 50 O

 Inventive Example 3 30 5 O

Inventive Example 4 90 50 O As can be seen from Table 4 above, in the case where neutral salt electrolysis is performed in the range of 0 to 120 seconds, and sulfuric acid electrolysis is performed in the range of 5 to 50 seconds, it is added to the steel sheet surface and is shown in FIG. As shown in FIG. 2, only Si oxide was present on the surface of the stainless steel sintered steel sheet. However, in Comparative Examples 1 to 4 outside the range, the surface state of (Cr, Fe) ₃ 0 ₄ was reduced. It could be confirmed that it exists. Thereby, the processing time in a common tank can be minimized. Example 7

 This embodiment intends to confirm suitable processing conditions in a common tank. As in Example 1, the steel plate subjected to neutral salt electrolysis and sulfuric acid electrolysis was treated with a common acid solution under the conditions shown in Table 5 below. At this time, the treatment temperature of the common acid solution is room temperature, the hydrogen peroxide concentration was adjusted as in Example 3.

The pickling was observed accordingly, and the results are shown in Table 5 as indicated by O and X. The case where silicon oxide does not remain is represented by O, and the case where silicon oxide remains is considered to be an acid pickling, when there is erosion of a base material, it is considered as an over-acid tax, and when an acidic acid and an over-acid tax appear, it is represented by X. Table 5

상기 표 5로부터 알 수 있는 바와 같이， 황산 70~200g/i, 자유 불산 \~\0g/t 및 철 이온 농도에 따른 최소 과산화수소가 1.0 이상으로 이루어지는 산세 조성물에 10~100초 동안 침적하여 산세하는 것이 바람직함을 알 수 있다. 실시예 8

As can be seen from Table 5, sulfuric acid 70 ~ 200g / i, free hydrofluoric acid \ ~ \ 0g / t and pickling by pickling 10 minutes to 100 seconds to pickling composition consisting of a minimum hydrogen peroxide according to the iron ion concentration It can be seen that it is preferable. Example 8

 This embodiment is to compare the pickling quality of the steel sheet when pickling using a conventional nitric acid-fluoric acid common solution and a sulfuric acid-fluoric acid-hydrogen peroxide common solution according to the present invention for the ferritic cold-rolled steel sheet.

 The glossiness was measured after pickling the ferritic stainless steel annealed steel sheet having a crack content of 14% or less in a common acid solution (n = L5). In the case of the common acid conditions and the common acid solution, the following solutions were used.

 Comparative Example 1: After carrying out neutral salt electrolysis and sulfuric acid electrolysis according to Example 1, ferritic stainless steel cold rolled steel sheet was immersed in a common acid solution containing 100 g / £ nitric acid and 3 g / i hydrofluoric acid for 30 seconds, and then polished. Was measured.

Inventive Example 1: Glossiness of the steel sheet obtained by Inventive Example 4 of Example 7 Measured.

 Inventive Example 2: The glossiness of the steel sheet obtained by Inventive Example 2 of Example 7 was measured. The glossiness measured about each said steel plate is shown in FIG. As can be seen from FIG. 5, when the steel sheet of Comparative Example 1 according to the conventional nitrate pickling or the steel sheet to which the acid pickling method of the present invention is applied to the steel sheet in an unpickled state, the steel sheet obtained according to the present invention is As having a gloss of 130 or more, it can be confirmed that a glossiness raising effect of about 40 to 60 is obtained as compared to the steel sheet of Comparative Example 1.

 Therefore, when applying the present invention it can be seen that the surface quality of the steel sheet after pickling can be improved.

Claims

[Range of request]  [Claim 1]  Silicon oxide pickling method for rapidly removing silicon oxide from the surface of low chromium ferritic stainless steel mild steel sheet containing 14% or less of chromium by using a common acid solution containing no nitric acid.  The pickling process is performed by immersing the quenched steel sheet in a common acid solution containing hydrogen peroxide, The common acid solution contains 70-200 g of sulfuric acid and l-10 g of free hydrofluoric acid, and the first common acid solution contains a hydrogen peroxide concentration of 7 g / or more, but does not contain iron ions, and pickling of 3-15 g / m ² 'min. Silicon oxide pickling method of a low chromium ferritic stainless steel annealed steel sheet having a speed. [Claim 2]  The relationship between the hydrogen peroxide and the iron ion concentration in the pickling process according to claim 1, wherein A method of pickling silicon oxide of a low chromium ferritic stainless steel mild steel sheet characterized by satisfying [H ₂ O ₂ ]> 0.74 + 3.1e- ^{[Fe] / 1 -1} + 3.2e- ^{[Fe] 15} . [Claim 3] The surface of the natural steel sheet immersed in a common acid solution according to claim 1 or 2 A method of pickling silicon oxide of a low chromium ferritic stainless steel leaded steel sheet, the potential of which is maintained at -0.2 to 0.2V. [Claim 4]  The method of claim 1, wherein the cold rolled steel sheet is immersed in a common acid solution for 10 to 100 seconds. [Claim 5]  In the method of pickling ferritic stainless steel cold rolled steel sheet containing less than 14% of the cracks which have undergone degreasing and annealing,  A sulfuric acid electrolysis step of electrolytically removing the (Fe, Cr) scale using an electrolytic solution using sulfuric acid as an electrolyte from a low chromium ferritic stainless steel stainless steel sheet; A pickling method of a low chromium ferritic stainless steel cold rolled steel sheet, comprising a common acid immersion step immersed in a common acid solution containing sulfuric acid, hydrofluoric acid, and hydrogen peroxide, and having a pickling rate of 3-15 g.min. [Claim 6] The neutral salt electrolysis step according to claim 5, further comprising electrolytic removal of chromium-rich scale from the surface of the steel sheet using an electrolytic solution containing sodium sulfate electrolyte. A pickling method of a low chromium ferritic stainless steel cold rolled steel sheet comprising a. [Claim 7] The method of claim 6, wherein the neutral salt electrolysis step is to immerse the ferritic stainless steel sheet in a neutral salt electrolytic solution at a temperature of 50 ~ 90 ^° C, 10 ~ 30A so that the potential of the surface of the steel sheet is formed in the order of +, —, + A pickling method for a low chromium ferritic stainless steel cold rolled steel sheet, characterized in that it is carried out by applying a current density of / dm ² for more than 0 seconds and not more than 90 seconds. [Claim 8]  The method of claim 6, wherein the sodium sulfate electrolyte in the neutral salt electrolyte solution A pickling method for a low chromium ferritic stainless steel cold rolled steel sheet comprising 100 ~ 250g / i. [Claim 9] The method of claim 5, wherein the sulfuric acid electrolysis step is immersed in a sulfuric acid electrolytic solution of 30 ~ 60 ^° C annealed ferritic stainless steel sheet subjected to an annealed or neutral salt electrolysis step, the potential of the surface of the steel sheet +,-, + A method of pickling low-ferrous ferritic stainless steel cold rolled steel sheet, characterized in that the step is applied by applying a current density of 10 to 30 A / dm ² for 5 to 50 seconds in order. [Claim 10]  10. The method of claim 9, wherein the sulfuric acid electrolytic solution contains 50-150 g / £ sulfuric acid. [Claim 11]  The method of claim 5, wherein the common acid immersion step is a pickling process by immersing the molten steel sheet in a common acid solution containing hydrogen peroxide for 10 100 seconds, the common acid solution is sulfuric acid 70 ~ 200 g / and free hydrofluoric acid l ~ 10 g / To include, the first common acid solution contains a hydrogen peroxide concentration of 7g / 이상 or more, and iron ions do not substantially contain the pickling method of a low-cream ferritic stainless steel cold-rolled steel sheet. [Claim 12]  12. The method of claim 11, wherein the surface potential of the steel sheet is maintained in the range of -0.2 to 0.2V in the common acid immersion step. [Claim 13]  The method according to claim 10, wherein the hydrogen peroxide in the pickling process is expressed by the following equation in relation to the iron ion concentration in the common acid solution. [H ₂ 0 ₂ ]> 0.74 + 3.1 e- ^{[Fe] l} - ¹ + 3.2e- ^{[Fe] / 15} Pickling method of low-crack ferritic stainless steel cold rolled steel sheet characterized in that it satisfies. [Claim 14]  A steel sheet obtained by carrying out the pickling method according to any one of claims 5 to 13, wherein the glossiness of the steel sheet is 130 or more. [Claim 15]  A common acid solution for removing silicon oxide from a ferritic stainless steel cold rolled steel sheet containing 14% or less of chromium after degreasing and annealing, including sulfuric acid 70-200 g, free hydrofluoric acid l-10 g / £ and hydrogen peroxide. Is the following equation in relation to the iron ion concentration in the common acid solution. A common acid solution containing no nitric acid for removing silicon oxide, satisfying [H ₂ O ₂ ] ≥0.74 + 3.1e— ^{[Fe] / 11} + 3.2e- ^{[Fe] / 15} .