JP6031606B2 - High speed pickling process for producing austenitic stainless cold rolled steel sheet - Google Patents

Description

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

The austenitic stainless steel cold-rolled steel sheet undergoes a heat treatment process at 1000 to 1150 ° C. in order to obtain predetermined mechanical properties after cold rolling. During this heat treatment process, the surface of the steel sheet reacts with high-temperature oxygen. Thus, oxide scale (SiO ₂ , (Cr, Mn) ₃ O ₄ ) is generated on the surface of the steel plate. The oxide scale generated on the surface of the steel sheet deteriorates the appearance of the product, deteriorates the quality of the steel sheet, and causes corrosion of the steel sheet, thereby reducing 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, 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 sheets by combining various methods such as chemical descaling using.

  The process of removing the scale on the surface of such a steel sheet is called a pickling process. In the above-mentioned stainless steel pickling process, in order to obtain a beautiful surface quality and to form a passive film uniformly and ensure corrosion resistance, a nitric acid electrolysis method in which a current is applied to the nitric acid solution while passing through the steel plate and descaling is performed. Pickling is performed by a chemical descaling method using a mixed acid of nitric acid (80 to 180 g / l) and hydrofluoric acid (2 to 40 g / l). At this time, the nitric acid in the electrolytic bath lowers the pH in the pickling tank to increase the activity of hydrofluoric acid, and oxidizes divalent iron ions dissolved on the surface of the steel plate to trivalent to pickle the bath. Maintain an appropriate redox potential.

However, when 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. Therefore, it is necessary to further install environmental pollution prevention equipment in the pickling process in order to satisfy the environmental regulation conditions such as total nitrogen restriction of discharged effluent due to strengthening environmental regulations in Japan and overseas, NOx concentration restriction of air discharge facility, etc. The problem of a significant increase in the unit production cost arises from the operational costs.

  In order to solve these problems, a pickling method that does not use nitric acid has been developed. As a technique, nitric acid is replaced with hydrochloric acid or sulfuric acid in the pickling process, and insufficient oxidizing power is reduced to hydrogen peroxide, excess acid. There is a pickling method that does not use nitric acid, supplemented by potassium manganate, trivalent iron ions and air injection.

  Specifically, German Patent No. 3937438 discloses a technique in which sulfuric acid, hydrofluoric acid, and iron sulfate are used as the pickling solution, and hydrogen peroxide is added to maintain the oxidation-reduction potential of the pickling solution at 300 mV or higher. Since the 1990s, including the above-mentioned technology, as disclosed in US Pat. No. 5,154,774 and European Patent 236354, mainly the oxidation-reduction potential (Oxidation) of hydrofluoric acid and iron ions, air, hydrogen peroxide, or solution. -Technology has been developed to identify the appropriate range of Reduction Potential (ORP). However, most of these methods have a limit that they are limitedly applied to products such as wire rods, steel bars, and thick plates whose product quality requirements are not strict.

  On the other hand, in US Pat. No. 5,908,511, sulfuric acid, hydrofluoric acid and iron salt are added to the pickling solution, hydrogen peroxide is periodically added, and the composition of the wetting agent, brightening agent, corrosion inhibitor and the like is adjusted. A technique of automatically controlling Fe (III) and the accompanying oxidation-reduction potential (ORP) in the management of pickling and pickling solutions is disclosed. The CLEANOX 352 product, which is a pickling solution, has been commercialized through the above technique and is most widely used around the world. However, although this method has been put to practical use for wire rods and hot rolled products, the product unit price of the product is 20% or more higher than conventional products, and there is a problem that a complicated solution composition and management method are used. .

  Further, European Patent Publication No. 1040211 and United States Patent Publication No. 2000-560982 improved from US Pat. No. 5,908,511 disclose a method of increasing the pickling rate by adding copper and chlorine ions to the pickling composition. When the surface potential (Open Circuit Potential, OCP) formed on the surface of the ferritic stainless steel sheet is lower than the redox potential of copper ions of 0.1 V, copper particles are deposited on the surface of the steel sheet during the pickling process. There is a risk of discoloring the steel sheet. In addition, when the pickling solution contains chlorine ions at a certain concentration or more, there is a risk of pitting corrosion.

  When pickling at a specific temperature or higher when using hydrogen peroxide, the amount of residual hydrogen peroxide is drastically reduced due to the self-decomposition of hydrogen peroxide. Since the amount of hydrogen increases remarkably, the economic loss due to the use of an excessive amount of hydrogen peroxide is large.

  As described above, various techniques relating to the pickling composition are known, but no pickling technique suitable for producing a high chromium austenitic cold-rolled steel sheet at a high speed is known.

  An object of the present invention is to provide an electrolytic pickling method suitable for high-speed production of austenitic stainless cold-rolled steel sheets while ensuring pickling performance without using nitric acid, and mixed acid acid suitable for removing silicon oxide at high speed It is to provide a washing method.

  Another object of the present invention is to provide a mixed acid solution suitable for high-speed production of austenitic stainless cold-rolled steel sheets that can ensure excellent pickling properties at low temperatures.

  Still another object of the present invention is to provide a mixed acid solution which does not contain nitric acid and is suitable for the above pickling method.

  Still another object of the present invention is to provide an austenitic stainless cold-rolled steel sheet obtained by the pickling method as described above and having a glossiness of 150 or more measured at a reflection angle of 60 ° with respect to the rolling direction. Is to provide.

  According to the present invention, there is provided a pickling method for high chromium austenitic stainless cold-rolled steel sheet which removes silicon oxide from a high chromium austenitic stainless cold-rolled steel sheet containing 16% by weight or more of chromium, which does not contain nitric acid. The cold rolled steel sheet is immersed in a mixed acid solution having an initial composition of 110 to 150 g / l of sulfuric acid, 15 to 30 g / l of free hydrofluoric acid and a hydrogen peroxide concentration of 4.5 g / l or more, and substantially free of iron ions. By doing this, the pickling method of the high chromium austenitic stainless steel cold-rolled steel plate which removes a silicon oxide from the said cold-rolled steel plate is provided.

  The hydrogen peroxide concentration of the mixed acid solution satisfies the following formula (1) in relation to iron ions in the mixed acid solution, and the oxidation-reduction potential (ORP) is expressed by the following formula (2) in relation to the iron ion concentration in the mixed acid solution. Or it is preferable to satisfy (3).

Hydrogen peroxide concentration ≧ 0.00736 + 10 × e ^{− [metal] /13.2} (1)

ORP ≧ 600 mV, however, iron ion concentration ≦ 10 g / l (2)

ORP ≧ 310 + 431 × e ^{− [metal] /25.24} provided that iron ion concentration> 10 g / l (3)
(In the above formulas (1) and (3), [metal] represents the iron ion concentration in the mixed acid solution.)

  Moreover, it is preferable to immerse so that the width (W) of the grain boundary after pickling may be formed to 1 μm or more. At this time, the immersion is preferably controlled so that the temperature (T), hydrofluoric acid concentration (C), and treatment time (t) of the mixed acid solution satisfy the following formula (4).

  Width of crystal grain boundary after pickling (W) = − 0.184 + 0.0131 · t + 0.016 · C + 0.01 · T (4)

  Moreover, according to another embodiment of the present invention, the pickling method for high chromium austenitic stainless cold-rolled steel sheet which removes silicon oxide from high chromium austenitic stainless cold-rolled steel sheet containing 16% by weight or more of chromium. Being maintained at a temperature of 20 to 32 ° C., containing no nitric acid, and having an initial composition of 110 to 150 g / l of sulfuric acid, 25 to 40 g / l of free hydrofluoric acid, and a hydrogen peroxide concentration of 5.5 g / l or more, There is provided a pickling method for a high chromium austenitic stainless cold-rolled steel sheet, in which silicon oxide is removed from the cold-rolled steel sheet by immersing the cold-rolled steel sheet in a mixed acid solution substantially free of iron ions.

  At this time, the hydrogen peroxide preferably satisfies the following formula (5) in relation to the iron ion concentration.

Hydrogen peroxide concentration ≧ 0.805 + 9.2 × e ^{− [metal / 15.56]} (5)
(In the above formula (5), [metal] represents the iron ion concentration in the mixed acid solution.)

  In 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 0.1V.

  Moreover, it is preferable to immerse the cold-rolled steel sheet in the mixed acid solution for 10 to 100 seconds.

  The removal of silicon oxide from the cold-rolled steel sheet is performed after the neutral salt electrolysis treatment step and the sulfuric acid electrolysis treatment step, or the sulfuric acid electrolysis treatment step, and the neutral salt electrolysis treatment step is an electrolytic solution containing a sodium sulfate electrolyte. Cr-rich scale is electrolytically removed from the surface of the steel plate using the sulfuric acid, and the sulfuric acid electrolysis treatment step can electrolytically remove the residual scale of Cr and Fe using an electrolytic solution containing sulfuric acid as an electrolyte.

In the neutral salt electrolysis step, the austenitic stainless steel plate is immersed in a neutral salt electrolytic solution at a temperature of 50 to 90 ° C., and the surface potential of the steel plate is formed in the order of +, −, + to 10-30 A. / Dm ² can be applied by applying a current density of 30 seconds to 120 seconds. Further, the neutral salt electrolyte solution may contain 100 to 250 g / l of sodium sulfate electrolyte.

The sulfuric acid electrolysis step is performed so that the austenitic stainless steel plate that has undergone the neutral salt electrolysis step is immersed in a sulfuric acid electrolytic solution at a temperature of 30 to 60 ° C., and the surface potential of the steel plate is formed in the order of +, −, +. It can be performed by applying a current density of ˜30 A / dm ² for 5 to 50 seconds. The sulfuric acid electrolytic solution may contain 50 to 150 g / l of sulfuric acid.

  Further, according to the present invention, it is a high chromium austenitic stainless cold-rolled steel sheet containing 16% by weight or more of chromium obtained by the above method, which is measured at an angle of 60 ° with respect to the rolling direction of the steel sheet. A high chromium austenitic stainless cold-rolled steel sheet having a glossiness of 150 or more is provided.

  The high chromium austenitic stainless cold-rolled steel sheet preferably has a surface grain boundary width of 1 μm or more.

  Furthermore, according to the present invention, there is provided a mixed acid solution for removing silicon oxide from a high chromium austenitic stainless cold-rolled steel sheet containing 16% by weight or more of chromium, which does not contain nitric acid and has 110 to 150 g / sulfuric acid. 1 and free hydrofluoric acid 15 to 25 g / l and hydrogen peroxide, the hydrogen peroxide concentration satisfies the following formula (1) in relation to the iron ions in the mixed acid solution, and the oxidation-reduction potential (ORP) is A mixed acid solution that satisfies the following formula (2) or (3) in relation to the iron ion concentration in the solution is provided.

Hydrogen peroxide concentration ≧ 0.00736 + 10 × e ^{− [metal] /13.2} (1)

ORP ≧ 600 mV, however, iron ion concentration ≦ 10 g / l (2)

ORP ≧ 310 + 431 × e ^{− [metal] /25.24} provided that iron ion concentration> 10 g / l (3)
(In the above formulas (1) and (3), [metal] represents the iron ion concentration in the mixed acid solution.)

  Furthermore, according to another embodiment of the present invention, the composition contains no nitric acid, 110-150 g / l sulfuric acid, 25-40 g / l free hydrofluoric acid, and hydrogen peroxide, the hydrogen peroxide being iron in a mixed acid solution. A mixed acid solution that satisfies the following formula (5) in relation to the ion concentration is provided.

Hydrogen peroxide concentration ≧ 0.805 + 9.2 × e ^{− [metal / 15.56]} (5)
(In the above formula (5), [metal] represents the iron ion concentration in the mixed acid solution.)

  At this time, the mixed acid solution preferably has a temperature of 20 to 32 ° C.

  According to the present invention, when pickling an austenitic stainless cold-rolled steel sheet, the mixed acid solution in the pickling solution does not contain nitric acid, so NOx and nitrate nitrogen are not discharged. Therefore, it is possible to reduce the burden of installing the NOx removal facility and the denitrification facility.

  Moreover, since pickling can be adjusted by the hydrogen peroxide concentration, the oxidation-reduction potential (ORP) of the solution, and the hydrofluoric acid concentration, it is easy to control and suitable for high-speed production.

  Furthermore, since the quality after pickling is improved as compared with the existing pickling method, it is possible to produce an austenitic stainless cold-rolled steel sheet having excellent quality.

  In addition, by removing Fe and Cr oxides other than silicon oxide completely during the electrolytic pickling process, the silicon oxide can be easily removed by the mixed acid solution, and the silicon oxide can be removed and planarized by the mixed acid solution. It can be performed in a short time of 10 to 100 seconds.

  In addition, the solution composition and the management method are simple, the surface quality of the cold-rolled steel sheet can be ensured by preventing other reactions other than the pickling reaction with the surface of the steel sheet, and productivity by high-speed production. Can be improved.

  In addition, according to one embodiment of the present invention, since the hydrogen peroxide self-decomposition can be minimized by performing the pickling process at a low temperature, it is possible to improve productivity and economy by high-speed production. it can.

It is the electron micrograph which image | photographed the surface of the heat-treated high chromium austenitic stainless steel cold-rolled steel sheet, A) is the electron micrograph which image | photographed the cross section of the steel plate after performing an electrolytic treatment, B) is an electrolytic treatment. It is the electron micrograph which image | photographed the cross section of the steel plate which was not performed. It is the photograph which image | photographed the surface of the high chromium austenitic stainless cold-rolled steel plate pickled by Example 2 with the electron microscope, A) is the surface of the steel plate obtained by invention example 4, B) is a comparative example. 4 is the surface of the steel sheet obtained by 4. In order to maintain a surface potential of -0.2 to 0.1 V when a high chromium austenitic stainless steel cold-rolled steel sheet is immersed in a mixed acid, the minimum hydrogen peroxide concentration relative to the metal content and the redox potential of the solution at this time It is a graph which shows correlation. It is for corrosion resistance evaluation by the salt spray experiment after pickling in Example 8, and A) is a photograph after the corrosion resistance evaluation of the steel sheet obtained by Invention Example 1 having a grain boundary width of 1 micron or more. , B) is a surface photograph after the corrosion resistance evaluation of the steel sheet pickled by the pickling method according to Comparative Example 1. In Example 9, it is the graph which showed the content of the residual hydrogen peroxide by the time of the self-decomposition of hydrogen peroxide by the temperature of the mixed acid solution with time. The graph which shows the correlation of the minimum hydrogen peroxide concentration with respect to a metal content in order to maintain surface potential -0.2-0.1V when a high chromium austenitic stainless steel cold-rolled steel plate is immersed in mixed acid by Example 10. It is.

  According to one embodiment of the present invention, an austenitic stainless cold-rolled steel sheet is prepared by using a neutral salt electrolytic cell using sodium sulfate as an electrolyte, a sulfuric acid electrolytic cell using sulfuric acid and metal sulfate as an electrolyte, and sulfuric acid, hydrogen peroxide and There is provided a method for removing oxide scale on the surface of a steel sheet by passing it through a mixed acid bath of a pickling composition containing hydrofluoric acid. According to another embodiment of the present invention, there is provided a high chromium austenitic stainless cold rolled steel sheet obtained by the method of the present invention. Furthermore, according to still another embodiment of the present invention, there is provided a mixed acid solution used for removing the oxide scale.

  Hereinafter, the present invention will be described in detail.

  In general, an austenitic stainless cold-rolled steel sheet is formed with an oxide scale having a thickness of 200 to 500 nm after heat treatment. The oxide scale includes a Cr-rich scale layer having a relatively higher Cr oxide content than Fe, an oxide scale, It has a multilayer structure of Si-oxide layers present at the interface of the base material.

  Among the scales, the Cr-rich scale layer can be removed in 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 surface of a steel plate. In this case, the electrode is configured such that the surface potential of the steel plate is charged at least once in the order of +, −, +.

The neutral salt electrolyte is preferably in the range of pH 3 to ⁶ , and when a current is applied to the neutral salt electrolyte having the above pH range, the Cr-rich scale layer Cr is preferentially in a state of Cr ^6+. By melting, Cr-rich scale Cr on the surface of the steel sheet can be removed. At this time, sodium sulfate is preferably used as the electrolyte in the neutral salt electrolyte. The sodium sulfate electrolyte dissolves Cr-rich scale Cr by increasing the electrical conductivity in the electrolytic solution and increasing the electrical conductivity to the surface of the steel sheet.

  At this time, the neutral salt electrolyte preferably includes a sodium sulfate electrolyte in a content of 100 to 250 g / l. When the sodium sulfate electrolyte is 100 g / l or more, proper conductivity for dissolution of chromium can be obtained. On the other hand, when it exceeds 250 g / l, sodium sulfate may be deposited in the electrolyte solution, block the equipment piping, and deteriorate the operation. Accordingly, 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 electrolytic solution. When the temperature of the electrolytic solution is 50 ° C. or higher, appropriate conductivity for dissolution of chromium in the Cr-rich scale layer is obtained, and the higher the temperature of the electrolytic solution, the higher the conductivity. However, when the temperature of the electrolytic solution exceeds 90 ° C, temperature management in operation is difficult. Therefore, it is preferable that the temperature of the electrolytic solution in the neutral salt electrolytic cell is 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. When the current applied through the electrode is 10 A / dm ² or more, Cr-rich scale Cr can be sufficiently eluted. However, if it exceeds 30 A / dm ² , the rectifier equipment for generating current becomes too large, and the initial equipment cost becomes large. Therefore, it is preferable that the current is in the range of 10~30A / dm ^2.

  The neutral salt electrolysis treatment time is preferably 30 seconds or longer and 120 seconds or shorter. By performing the treatment in a neutral salt electrolytic cell during the time in the above range, the effect of removing the chromium scale can be sufficiently obtained. However, when the neutral salt electrolysis time exceeds 120 seconds, there is no further scale removal effect, and metal ions eluted during the electrolytic pickling may be electrodeposited again, which is not preferable.

  On the other hand, residual scales such as Cr and Fe that have not been completely removed in the neutral salt electrolytic cell are removed by the sulfuric acid electrolytic cell. The sulfuric acid electrolytic cell comprises a sulfuric acid electrolyte composed of a metal sulfate containing at least one of sulfuric acid and iron, chromium, nickel, copper, manganese, and titanium, and an electrode for energizing the surface of the steel sheet with current. The electrode is configured such that the surface potential of the steel sheet is charged at least once in the order of +, −, +.

The sulfuric acid electrolyte is preferably in the range of pH 0 to 1. When a current is applied to the sulfuric acid electrolyte, Fe is dissolved in an ionic state of Fe ²⁺ . At this time, sulfuric acid H ⁺ and SO ₄ ²⁻ increase the electrical conductivity in the solution to increase the electrical conductivity from the electrode to the surface of the steel sheet, and chemically dissolve the residual scale iron by the lower pH. Let

  At this time, the sulfuric acid electrolyte preferably contains sulfuric acid in the range of 50 to 150 g / l. When sulfuric acid is 50 g / l or more, it can be maintained at an appropriate conductivity or higher, so that the current ratio to the surface of the steel sheet can be maintained. However, when sulfuric acid exceeds 150 g / l, chemical dissolution is greatly generated, so that the surface of the stainless cold-rolled steel sheet becomes rough.

  The sulfuric acid electrolyte may include a metal sulfate containing at least one of iron, chromium, nickel, copper, manganese, and titanium. When the metal concentration is low due to the equilibrium reaction and the electric conductivity is low, the metal sulfate is eluted from the sulfate to increase the electric conductivity. When the metal concentration is high and the electric conductivity is high, the sulfuric acid is again added. It is deposited as a salt to lower the electrical conductivity so that the electrical conductivity of the sulfuric acid electrolyte can be maintained constant.

  Like the neutral salt electrolytic cell, the sulfuric acid electrolyte preferably has a solution temperature of 30 ° C. or higher in order to obtain a minimum conductivity. However, when the solution temperature of the sulfuric acid electrolyte exceeds 60 ° C., the chemical dissolution is excessively performed, the surface of the stainless cold-rolled steel sheet becomes rough, and the black smut phenomenon in which the surface of the steel sheet turns black occurs. May occur. 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 that the current applied to the sulfuric acid electrolytic bath in the range of 10~30A / dm ^2. When the current is applied at 10 A / dm ² or more, the amount of scale elution due to the electrochemical reaction becomes sufficient, so that a sufficient electrolytic pickling effect can be obtained. However, when a current exceeding 30 A / dm ² is applied, the rectifier equipment for generating the current becomes large, and the initial equipment cost increases. Therefore, it is preferable that the current is in the range of 10~30A / dm ^2.

  The sulfuric acid electrolysis treatment is preferably performed for 5 seconds to 50 seconds. The scale can be sufficiently removed when the sulfuric acid electrolytic treatment is performed for 5 seconds or more. However, if the sulfuric acid electrolytic treatment time exceeds 50 seconds, it causes a problem of peracid washing. Therefore, it is preferable to perform sulfuric acid electrolysis treatment within the above range.

  As described above, the Si oxide layer and the chromium-depleted layer in the base metal remain on the surface of the steel plate that has undergone neutral salt electrolysis and sulfuric acid electrolysis, and the Si oxide layer and the chromium-depleted layer are formed by a mixed acid solution containing no nitrogen. Can be removed. The mixed acid solution contains sulfuric acid, free hydrofluoric acid, and hydrogen peroxide, and a steel sheet that has undergone neutral salt electrolysis-sulfuric acid electrolysis by immersing a steel sheet that includes a Si oxide layer and a chromium depleted layer in a mixed acid bath containing the mixed acid solution. From this, the Si oxide layer and the chromium depleted layer can be removed.

  Free 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).

HF⇔H ⁺ + F ⁻ (1)

H ₂ SO ₄ ⇔HSO ₄ ⁻ + H ⁺ ⇔SO ₄ ^{2 −} + 2H ⁺ (2)

That is, in the mixed acid solution, the free hydrofluoric acid is dissociated while being dissolved as in the formula (1), and the sulfuric acid is dissociated as in the formula (2). At this time, the equilibrium state of the mixed acid solution varies depending on the H ⁺ concentration provided by the dissociation of the free hydrofluoric acid and sulfuric acid, that is, the acidity.

In the case of free hydrofluoric acid, it has a pickling power in the state of free hydrofluoric acid (Free HF), dissolves Si oxide on the surface of the steel sheet, and permeates the interface between the Si oxide layer and the base material. Then, Fe is dissolved. The dissolved Fe 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 free hydrofluoric acid is preferably present in the mixed acid solution at a concentration in the range of 10 to 30 g / l. When the free hydrofluoric acid concentration is less than 10 g / l, the concentration present as free hydrofluoric acid is small, and the dissolving power for the Si and chromium depleted layers is insufficient. If it exceeds 1, the erosion rate of the base material is increased, so that the surface of the steel sheet after the mixed pickling process may be roughened. More preferably, it contains free hydrofluoric acid at a concentration of 15 to 30 g / l.

  In performing the said mixed acid process, although the solution temperature of a mixed acid tank can be set appropriately, it is not specifically limited. For example, the mixed pickling process can be performed by setting the temperature within a range of 20 to 95 ° C., 20 to 80 ° C., 20 to 65 ° C., 32 to 80 ° C.

  As described above, since free hydrofluoric acid provides pickling power 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 at a certain acidity or higher in the mixed acid solution. . Therefore, in the mixed acid solution, in order to prevent the free hydrofluoric acid from being dissociated, and to dissolve the chromium-depleted layer of the base material and remove the Si oxide generated at the grain boundary, a certain concentration or more is required. Sulfuric acid is required. Suitable sulfuric acid concentrations for this range from 110 to 150 g / l. When the sulfuric acid concentration is less than 110 g / l, it is difficult to sufficiently dissolve the chromium-depleted layer and the grain boundary of the austenitic steel base material, which may cause a problem of deterioration in corrosion resistance after pickling. When it exceeds 150 g / l, there is a problem that heat is generated during the sulfuric acid dilution operation and the operation becomes difficult. Therefore, it is preferable to contain sulfuric acid at a concentration in the above range.

  Of the oxide scale of austenitic stainless steel, Si oxides are all present on the surface and grain boundaries of austenitic crystals, and the Si oxides at the grain boundaries exist deep inside the base material. In the case of ferritic stainless steel, the corrosion resistance of the crystal is low, and there is no difference in the erosion rate between the inside of the crystal and the crystal grain, so the surface of the grain and the grain boundary are completely dissolved without selectivity, In the case of austenitic stainless steel, the corrosion resistance of the crystals is high, so that they are preferentially eroded from the grain boundaries. Therefore, in order to remove all the Si oxide, a considerable amount of crystal grain boundaries need to 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 combined with HF to form a complex compound in FeF _x ^(3-x). Is removed from the surface of the steel sheet. Such a reaction is represented by the following reaction formulas (3) to (6). The pickling rate can be increased when the above process proceeds smoothly.

Fe ⁰ ⇔Fe ²⁺ + 2e ⁻ (3)

Fe ²⁺ + H ₂ O ₂ → Fe ³⁺ + · H + OH ⁻ (4)

Fe ³⁺ + 3HF⇔FeF ₃ + 3H ⁺ (5)

Cu ²⁺ + 2e ⁻ → Cu ⁰ (6)

  Stainless steel has a correlation between the potential and current of a specific electrokinetic curve for each steel type, and the amount of current generated at this time can be expressed by the pickling rate. Therefore, the maximum pickling speed can be realized by controlling the surface potential. For this reason, it is preferable to maintain the surface potential of the cold-rolled steel sheet in the mixed acid tank in the range of -0.2 to 0.1V. If the surface potential of the cold-rolled steel sheet is out of the above range, it may not be pickled or may have poor pickling that has a partially unpickled surface. And good surface quality cannot be obtained.

On the other hand, pickling is conventionally 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, the present inventors have not been able to confirm the relationship between the redox potential (ORP) of the solution and the surface potential before the metal concentration reaches an appropriate level, and the mixed pickling with only the redox potential (ORP). I found that I can not adjust.

When the residual hydrogen peroxide concentration is insufficient before the metal concentration in the mixed acid solution reaches an appropriate level, the surface potential of the cold-rolled steel sheet in the mixed acid tank is not maintained at -2.0 V or more, and the reaction Since the reaction of the formula (4) is not performed, the Fe ²⁺ concentration on the surface of the steel sheet is locally increased, and the reaction in the left direction of the reaction formula (3) is increased. In this case, Cu or the like present as an additive or impurity in Fe and stainless steel is re-precipitated on the surface of the steel plate as in the reaction formula (6), and a blackening phenomenon occurs in which the surface of the steel plate turns black. Therefore, it is necessary that the residual hydrogen peroxide concentration always exists above a certain concentration.

However, after the metal concentration in the mixed acid solution reaches an appropriate level, the residual hydrogen peroxide concentration for maintaining the surface potential of −0.2 V or more has a correlation with the metal concentration in the solution. That is, the metal concentration in the solution increases as the pickling progresses, and Fe ²⁺ ions eluted by hydrogen peroxide are oxidized to Fe ³⁺ ions. This increase in Fe ³⁺ ion content induces an increase in surface potential at the same hydrogen peroxide concentration because Fe ³⁺ ions act as oxidizing agents. As the Fe ³⁺ ion increases, the hydrogen peroxide concentration for maintaining the surface potential of the steel sheet tends to decrease.

Accordingly, when the metal concentration is sufficient, a surface potential of −0.2 V or more can be sufficiently secured by Fe ³⁺ / Fe ²⁺ and the oxidation-reduction potential, and from this time, the surface potential of the steel sheet is determined from the oxidation-reduction potential of the solution. Can be controlled.

Such a relationship was obtained by the inventors' constant experiment and can be confirmed from the following examples. Based on the above experiments, the present inventors have found that the minimum hydrogen peroxide concentration and the oxidation-reduction potential of the solution due to the iron ion concentration in the solution for maintaining the surface potential of the steel sheet at −0.2 V or more are as follows. It was confirmed that there was such a relationship, and this was shown graphically in FIG.
0 g / L ≦ [metal] <10 g / L, [H ₂ O ₂ ] _min = 4.5 g / L, ORP 600 mV
10 g / L ≦ [metal] <20 g / L, [H ₂ O ₂ ] _min = 2.5 g / L, ORP 500 mV
20 g / L ≦ [metal] <40 g / L, [H ₂ O ₂ ] _min = 0.5 g / L, ORP 400 mV
40 g / L ≦ [metal] <60 g / L, [H ₂ O ₂ ] _min = 0 to 0.2 g / L, ORP 350 mV

  As can be seen from FIG. 3, when the iron ion concentration is 0, it must contain an effective hydrogen peroxide concentration of at least 4.5 g / l or more, but when the iron ion concentration is 40 g / l or more, it is peroxidized. Addition of hydrogen peroxide in order to maintain the oxidation-reduction potential (ORP) of the solution at a hydrogen potential of 0 to 0.2 g / l or more, which is a surface potential that can be pickled by the iron ions in the solution. Can be minimized. In other words, as the metal content increases, the surface potential of the steel sheet depends on the oxidation-reduction potential of the solution rather than the content of hydrogen peroxide, so that operation is possible at low cost even if the metal content in the solution is relatively high. Become. Therefore, in this region, only the minimum hydrogen peroxide for maintaining the redox potential (ORP) of the solution at 350 mV or higher is required.

  That is, the hydrogen peroxide concentration satisfies the following formula (1) in relation to iron ions in the mixed acid solution, and the oxidation-reduction potential (ORP) has the following formula (2) in relation to the iron ion concentration in the mixed acid solution. Or it is preferable to satisfy (3).

Hydrogen peroxide concentration ≧ 0.00736 + 10 × e ^{− [metal] /13.2} (1)

ORP ≧ 600 mV, however, iron ion concentration ≦ 10 g / l (2)

ORP ≧ 310 + 431 × e ^{− [metal] /25.24} provided that iron ion concentration> 10 g / l (3)
(In the above formulas (1) and (3), [metal] represents the iron ion concentration in the mixed acid solution.)

  In the mixed pickling process, if the hydrogen peroxide concentration of the mixed acid solution in the mixed acid tank and the oxidation-reduction potential satisfy the above relationship in relation to the iron ion concentration in the solution, an excellent pickling effect can be obtained. .

  From the above, in the mixed pickling process in the mixed acid tank, it is confirmed that the sulfuric acid and hydrofluoric acid concentrations in the pickling solution and the residual hydrogen peroxide concentration are the most important factors for the pickling effect increase and high-speed pickling. it can. Therefore, it is necessary to control the concentration of these components. In the concentration control of each component, the concentration of sulfuric acid and hydrofluoric acid can be adjusted using a commonly used acid analyzer, and the near infrared analysis method can be used. Alternatively, the residual hydrogen peroxide concentration can be analyzed and adjusted by automatic titration.

  On the other hand, it is preferable that the width of the grain boundary is 1 μm or more in the steel sheet that has been subjected to the pickling treatment according to the present invention. When the width of the crystal grain boundary on the surface of the steel sheet after the mixed pickling treatment is 1 μm or more, the Cr depleted layer is completely removed from the crystal grain boundary, and a steel sheet having excellent corrosion resistance is obtained. That is, when a Cr-depleted layer is present at the grain boundary, rusting is induced on the surface of the steel sheet by the Cr-depleted layer, whereas when there is no Cr-depleted layer, excellent corrosion resistance tends to be exhibited. The narrower the grain boundary after pickling, the smaller the Cr depletion layer at the grain boundary and the less corrosion resistance, whereas the wider the grain boundary, the fewer the Cr depletion layer at the grain boundary. Exists. Therefore, the presence of a Cr-depleted layer at the grain boundary can be confirmed by the degree of the grain boundary width, and thereby the degree of corrosion resistance of the steel sheet can be predicted.

  When the width of the grain boundary of the steel sheet is 1 μm or more, the Cr-depleted layer is completely removed by the pickling process to obtain a steel sheet having excellent corrosion resistance. However, when the grain boundary width is less than 1 μm, a Cr-depleted layer is present at the grain boundary and acts as a rusting nucleus, so that the corrosion resistance of the steel sheet tends to deteriorate. Preferably, the width of the crystal grain boundary is 1.5 μm or less. Although the corrosion resistance of the cold-rolled steel sheet obtained by pickling can be ensured even if it is outside the above range, the surface glossiness of the steel sheet may decrease, so the width of the grain boundary is 1.5 μm or less. Preferably there is. In addition, it is more preferable to obtain a high quality steel sheet having a grain boundary width that makes the glossiness measured at an angle of 60 ° with respect to the rolling direction 150 or more in the obtained cold rolled steel sheet.

  The width of the grain boundary of such a steel sheet is related to the process conditions of the mixed pickling process. In other words, the width of the crystal grain boundary after the pickling with the mixed acid has a relationship represented by the following formula (4) from the relationship with the temperature of the mixed acid solution, the concentration of free hydrofluoric acid and the pickling time when the mixed acid solution of the present invention is used. The grain boundary width (W) after pickling preferably satisfies 1 μm or more.

W = −0.184 + 0.0131 × t + 0.016 × C + 0.01 × T ≧ 1 (4)
(In the above formula (4), t represents the pickling time (seconds), C represents the HF concentration (g / l), and T represents the pickling temperature (° C.).)

  By using the above relational expression, the width of the grain boundary after pickling can be predicted from the process conditions without observing the surface state of the steel sheet, and the process conditions for obtaining a steel sheet having excellent corrosion resistance can be controlled. Can do.

  From the above-mentioned contents, it can be confirmed that the sulfuric acid and hydrofluoric acid concentrations 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. In the concentration control of each component, the concentration of sulfuric acid and hydrofluoric acid can be adjusted using a commonly used acid analyzer, and the near infrared analysis method can be used. Alternatively, the residual hydrogen peroxide concentration can be analyzed and adjusted by automatic titration.

  In the mixed pickling process, an excellent pickling effect can be obtained when the hydrogen peroxide concentration of the mixed acid solution in the mixed acid tank and the oxidation-reduction potential satisfy the above relationship in relation to the iron ion concentration in the solution. Is as described above. However, when the pickling process is performed with a relatively low-temperature mixed acid solution, it is more preferable to change the concentration of free hydrofluoric acid required.

  The temperature of the mixed acid solution of the present invention has a direct correlation with the amount of hydrogen peroxide input to maintain the minimum effective hydrogen peroxide content and needs to be particularly limited. That is, due to the characteristics of hydrogen peroxide that is self-decomposed by temperature, even if a stabilizer is added, the degree of decomposition of hydrogen peroxide due to an increase in temperature increases markedly. For example, as shown in FIG. 5, when the temperature of the mixed acid solution is increased by 20 ° C. from 30 ° C. to 50 ° C., the half-life of residual hydrogen peroxide is reduced by 6 times or more. Therefore, in order to suppress decomposition of residual hydrogen peroxide in the mixed acid solution, it is necessary to set the temperature of the mixed acid solution low, preferably 32 ° C. or less, more preferably 20 ° C. to 32 ° C. There is.

  When the mixed pickling process is performed at a low temperature as described above, the free hydrofluoric acid is preferably maintained at a concentration in the range of 25 to 40 g / l in the mixed acid solution. If it is less than 25 g / l, the pickling ability at low temperature is reduced and the dissolving power for the Si and chromium depleted layers is insufficient, so the problem of unpickling on the surface of the steel sheet occurs, and if it exceeds 40 g / l Since the erosion speed of the base material is increased, the surface of the steel sheet after the pickling process may be roughened. Preferably, it is good to maintain at 30-35 g / l.

As can be seen from FIG. 6, the minimum hydrogen peroxide concentration by the iron ion concentration in the low-temperature mixed acid solution for maintaining the surface potential of the steel sheet at −0.2 V or higher is as follows.
0 g / L ≦ [metal] <10 g / L, [H ₂ O ₂ ] _min = 10 to 5.5 g / L
10 g / L ≦ [metal] <20 g / L, [H ₂ O ₂ ] _min = 3.5 g / L
20 g / L ≦ [metal] <40 g / L, [H ₂ O ₂ ] _min = 1.5 g / L
40 g / L ≦ [metal] <60 g / L, [H ₂ O ₂ ] _min = 1 g / L

  Specifically, when the iron ion concentration is 0 g / l, the effective hydrogen peroxide concentration must be at least 5.5 g / l or more, but when the iron ion concentration is 40 g / l or more, 1 g / l. Even with an effective hydrogen peroxide concentration of 1 only, the addition of hydrogen peroxide can be minimized because the oxidation-reduction potential (ORP) of the solution maintains the surface potential at -0.2 V or more by metal ions. That is, as the metal content increases, the surface potential of the strip depends on the oxidation-reduction potential of the solution rather than the hydrogen peroxide content, so that operation is possible at low cost even if the metal content in the solution is relatively high. .

  Such a relationship is expressed by the following formula (5).

Hydrogen peroxide concentration ≧ 0.805 + 9.2 × e ^{− [metal / 15.56]} (5)
(In the above formula (5), [metal] represents the iron ion concentration in the mixed acid solution.)

  From the above description, it can be confirmed that the sulfuric acid and hydrofluoric acid concentrations in the pickling solution and the residual hydrogen peroxide concentration are the most important factors for the pickling effect increase and the high-speed pickling in the mixed pickling step in the mixed acid bath. . Therefore, it is necessary to control the concentration of these components. In the concentration control of each component, the concentration of sulfuric acid and hydrofluoric acid can be adjusted using an acid analyzer that is normally used as described above. The residual hydrogen peroxide concentration can be analyzed and adjusted by a near infrared analysis method or an automatic titration method.

  According to the method of the present invention as described above, since the pickling process can be performed at a high speed, the total pickling time is about 15 to 240 seconds, and the time required for pickling can be greatly reduced. An austenitic stainless steel sheet having excellent quality is obtained.

  Hereinafter, an example is given and the present invention is explained in detail. However, the following examples are for explaining the present invention and are not intended to limit the present invention.

[Example 1]
In order to confirm the pickling property of the oxide scale of the austenitic cold rolled steel sheet with or without neutral salt electrolysis and sulfuric acid electrolysis, the austenitic stainless steel sheet having a chromium composition of 16% or more was subjected to neutral salt electrolysis treatment and sulfuric acid The scale layers made of Cr, Mn, and Fe were removed by electrolytic treatment.

At this time, the used neutral salt electrolytic solution contains 150 g / l of sodium sulfate electrolyte as an electrolyte, the temperature of the solution is 60 ° C., and the surface potential of the steel sheet is formed in the order of +, −, +. A current density of 20 A / dm ² was applied for 40 seconds.

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 so that the surface potential of the steel sheet is formed in the order of +, −, +. Was applied for 15 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 an electron microscope (SEM) and shown in FIG. For comparison, the cross section of the electrolytically treated steel sheet is shown in A), and the cross section of the non-electrolyzed steel sheet is shown in B).

As can be seen from FIG. 1, both (Cr, Mn) ₃ O ₄ and silicon oxide remain in the cross section of the steel sheet that was not subjected to neutral salt electrolysis and sulfuric acid electrolysis, whereas the electrolytic solution of the present invention. Only the silicon oxide remains on the cross section of the steel sheet subjected to neutral salt electrolysis and sulfuric acid electrolysis using the above.

[Example 2]
In order to observe the surface state of the cold-rolled steel sheet according to the surface potential in the mixed acid tank, the steel sheet from which the primary scale was removed by neutral salt electrolysis treatment and sulfuric acid electrolysis treatment obtained in Example 1 above was used. The presence or absence of pickling of the silicon oxide film due to the surface potential was evaluated.

  After the test piece was immersed in a mixed acid solution at 45 ° C. containing 120 g / l of sulfuric acid and 20 g / l of free hydrofluoric acid, the potential was changed in the range of −0.4 to 0.6 V as shown in Table 1. The mixed acid process was performed by applying for 30 seconds.

  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 dissolution of grain boundaries were evaluated.

  When the scale remains on the surface of the steel plate, it is evaluated as non-pickling. Judged to be defective and indicated by x. On the other hand, in the case of unpickling, the degree of dissolution of grain boundaries was not evaluated.

  The evaluation results are shown in Table 1 below. Moreover, the surface states of the steel plates of Invention Example 4 and Comparative Example 4 in which the applied potential was 0.0 V and −0.25 V were photographed with an electron microscope (SEM) and shown in FIG.

  As can be seen from Table 1 above, when a surface potential of −0.25 to 0.1 V was applied, pickling with a mixed acid solution was possible (Invention Examples 1 to 5 and Comparative Example 4). However, as can be seen from FIG. 2, in Comparative Example 4, the degree of pickling at the grain boundaries was poor. That is, in the case of A) showing the surface of the steel sheet obtained by Invention Example 4, the grain boundary on the surface of the steel sheet is normally dissolved and the grain boundary is clearly shown, whereas it is obtained by Comparative Example 4. In the case of B) indicating the surface of the steel plate, although there is no residual scale, it can be confirmed that the grain boundaries are discontinuously dissolved and not uniformly dissolved, and the width of the grain boundaries is small.

  On the other hand, Comparative Examples 1, 2, 3, 5 and 6 showed the results of unpickling.

  From these results, it can be seen that application of the surface potential in the mixed acid tank in the range of −0.2 to 0.1 V is more suitable for dissolution of the Si oxide layer and dissolution of the grain boundaries.

[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 an austenitic cold-rolled steel sheet. Using the steel sheet from which the primary scale was removed by neutral salt electrolysis treatment and sulfuric acid electrolysis treatment obtained by the above, the presence or absence of pickling of the silicon oxide film by the surface potential was evaluated.

The test piece was immersed in a mixed acid solution at 45 ° C. containing 120 g / l of sulfuric acid and 20 g / l of free hydrofluoric acid, and then the surface potential of the steel sheet was added while adding metal ions (Fe ³⁺ ) and hydrogen peroxide to the mixed acid solution. And the minimum hydrogen peroxide concentration and the oxidation-reduction potential (ORP) of the solution to maintain the surface potential of the steel sheet at −0.2 V or more are measured by changing the iron ion concentration. The results are shown in FIG. It was shown to.

  FIG. 3 is a graph schematically showing a change in the minimum hydrogen peroxide concentration due to a change in iron ion concentration and a redox potential (ORP) of the solution in order to maintain the surface potential of the steel sheet at −0.2 V or more. .

  As can be seen from FIG. 3, the minimum hydrogen peroxide concentration for maintaining the surface potential of the steel sheet gradually decreases as the iron ion concentration increases. As the metal concentration increases, the redox potential (ORP) of the solution decreases. When the metal concentration is 40 g / l or more, the surface potential of the strip can be increased even if the redox potential (ORP) of the solution is maintained at 400 mV. It can be seen that no additional hydrogen peroxide solution is required because it is formed at −0.2V.

  However, since the surface potential fluctuates in a region where the redox potential (ORP) of the solution is less than 400 mV, it is necessary to input a minimum amount of hydrogen peroxide for maintaining the redox potential (ORP) of the solution at 400 mV.

[Example 4]
This example is for confirming proper operation conditions when performing neutral salt electrolysis, and adjusting the solution temperature, applied current and sodium sulfate concentration of the neutral salt electrolysis tank as described in Table 2 below. The neutral salt electrolysis was performed in the same manner as in Example 1 except that the above was performed.

  The surface state of the steel sheet after the neutral salt electrolysis was observed, and the results are shown in Table 2 below.

  When the surface condition of the steel sheet after neutral salt electrolysis is manganese / chromium oxide scale, the case where the surface condition is poor is indicated by x, and the case where the surface condition of the steel sheet is good is indicated by ◯.

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

[Example 5]
This example is for confirming proper operating conditions when performing sulfuric acid electrolysis, and was performed by adjusting the solution temperature, applied current, and sulfuric acid concentration of the sulfuric acid electrolysis tank as described in Table 3 below. The neutral salt electrolysis was performed in the same manner as in Example 1 except for the above.

  The surface state of the steel sheet after sulfuric acid electrolysis was observed, and the results are shown in Table 3 below.

  If the surface condition is poor, such as iron or manganese / chromium oxide scale remaining, it is indicated by x, and all the oxide scale of iron or manganese / chromium is removed and only the silicon oxide scale remains. Good cases are indicated by ○.

As can be seen from Table 3 above, the solution temperature in the sulfuric acid electrolytic tank 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. Excellent surface quality of steel sheet when sulfuric acid electrolysis is performed.

[Example 6]
This example is for confirming the proper processing time of neutral salt electrolysis and sulfuric acid electrolysis, and was carried out except that the neutral salt electrolysis time and sulfuric acid electrolysis time were adjusted as shown in Table 4. In the same manner as in Example 1, neutral salt electrolytic treatment and sulfuric acid electrolytic treatment were performed.

  The surface state of the steel sheet after carrying out neutral salt electrolysis and sulfuric acid electrolysis was observed, and the results are shown in Table 4 below.

  When the scale of manganese / chromium / iron other than silicon oxide does not remain is indicated by ○, and when the scale of manganese / chromium / iron other than silicon oxide remains, it is not pickled and the base material is eroded Was determined to be per-acid pickling, and the case where unpickling and per-pickling were indicated was indicated by x.

As can be seen from Table 4 above, when the neutral salt electrolysis is performed in the range of 30 to 120 seconds and the sulfuric acid electrolysis is performed in the range of 5 to 50 seconds, as shown in FIG. In the case of Comparative Examples 1 to 4 that are out of the above range, the surface state of FIG. 1 has a surface state as shown in FIG. 1B), so the scale of (Cr · Mn) ₃ O ₄ May be present or the surface may be damaged.

[Example 7]
This example is for confirming the proper treatment conditions in the mixed acid tank, and for the steel plate obtained by Example 1 above, from which the primary scale has been removed by the neutral salt electrolysis treatment and the sulfuric acid electrolysis treatment, Mixed acid pickling was performed with a mixed acid solution having the composition shown in Table 5 and processing conditions. At this time, the treatment temperature of the mixed acid solution was 40 to 45 ° C., and the hydrogen peroxide concentration was maintained at −0.1 V based on the iron concentration in the solution based on Example 3 and FIG. Hydrogen peroxide was adjusted at a concentration to achieve.

  The presence or absence of pickling of the silicon oxide film on the surface of the steel sheet obtained by the mixed acid treatment was observed, and the results are shown in Table 5 below.

  The case where silicon oxide does not remain is indicated by ○, the case where silicon oxide remains is indicated by × as non-pickling, and the gloss measured at a reflection angle of 60 ° with respect to the rolling direction even when pickled. When the degree was less than 150, it was judged as peracid washing and indicated by x.

  As can be seen from Table 5 above, in the pickling composition in which sulfuric acid is 110 to 150 g / l, free hydrofluoric acid is 15 to 30 g / l, and the minimum hydrogen peroxide based on iron ion concentration is as in Example 3. It is preferable to deposit for 10 to 100 seconds and to perform pickling with mixed acid.

[Example 8]
This example is for confirming proper processing conditions in a mixed acid tank for improving corrosion resistance. The primary scale obtained by Example 1 was removed by neutral salt electrolysis and sulfuric acid electrolysis. The steel sheet was treated with a mixed acid solution under the conditions shown in Table 5 below. At this time, as for the hydrogen peroxide concentration, the hydrogen peroxide was adjusted at a concentration for maintaining the surface potential of the steel sheet at -0.1 V based on the iron concentration in the solution, based on Example 3 and FIG. On the other hand, the sulfuric acid concentration was adjusted to 120 g / l.

  In the evaluation of the surface corrosion resistance, 5% salt water was repeatedly sprayed-wet-dried in a combined spraying experiment and observed for the presence or absence of rusting. The results are shown in Table 6.

  As a result of the evaluation of surface corrosion resistance, the case where there is no rust is indicated by ○, the case where rust is present is indicated by ×, and even if there is no rust, if the gloss is less than 150, it is determined that the gloss is poor. Displayed.

  The surface photograph of the steel plate obtained by Invention Example 1 and Comparative Example 1 as a result of the corrosion resistance evaluation by the salt spray experiment after pickling is shown in FIG. 4A is a photograph after the corrosion resistance evaluation of the steel sheet obtained by Invention Example 1 having a grain boundary width of 1 micron or more, and B) is the corrosion resistance of the steel sheet pickled by the pickling method according to Comparative Example 1. FIG. It is the surface photograph after evaluation.

  As can be seen from Table 6 above, the width (W) of the grain boundary after pickling for improving the corrosion resistance is expressed using the pickling time (t), the HF concentration (C), and the mixed acid solution temperature (T). Is represented by the following formula (4).

  W = −0.184 + 0.0131 × t + 0.016 × C + 0.01 × T (4)

  As a result of calculating the width of the grain boundary after pickling from the above formula (4), Invention Examples 1 to 4 having a value of 1 or more are excellent in corrosion resistance, but Comparative Examples 1 to 9 having a value of less than 1 are shown. Was inferior in corrosion resistance.

  From the above, it is preferable that the width of the grain boundary after pickling is 1 or more, and when the value calculated from the above relational expression is 1 or more, it can be confirmed that the corrosion resistance is excellent.

[Example 9]
In order to observe the degree of self-decomposition of hydrogen peroxide according to the temperature of the mixed acid solution, 35 g / l hydrofluoric acid, 120 g / l sulfuric acid and 10 g / l hydrogen peroxide were used, and p-toluene was used as a stabilizer for the hydrogen peroxide. In a mixed acid solution containing sulfonic acid (PTSA) at 1.5% by weight of the hydrogen peroxide content, the residual hydrogen peroxide content due to the temperature change of the solution was measured, and the results are shown in FIG.

  As can be seen from FIG. 5, the half-life of hydrogen peroxide is significantly reduced by the following formula.

t = 10.289 × 10 ³ · e −0.14 ^{· T}
(In the above formula, t represents the half-life (min) of the residual hydrogen peroxide concentration, and T represents the temperature (° C.).)

  When the temperature of the mixed acid solution rises from 32 ° C. to 50 ° C. as in this example, the half-life of hydrogen peroxide is reduced by 13 times. Therefore, the temperature of the mixed acid solution is preferably set to 32 ° C. or lower.

[Example 10]
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 low temperature mixed pickling of an austenitic cold rolled steel sheet. Using the steel sheet from which the primary scale was removed by neutral salt electrolysis treatment and sulfuric acid electrolysis treatment obtained by the above, the presence or absence of pickling of the silicon oxide film by the surface potential was evaluated.

The test piece is immersed in a mixed acid solution at 32 ° C. containing 120 g / l sulfuric acid and 35 g / l free hydrofluoric acid, and then the surface potential of the steel sheet is added while adding metal ions (Fe ³⁺ ) and hydrogen peroxide to the mixed acid solution. Was measured, and the minimum hydrogen peroxide concentration for maintaining the surface potential of the steel sheet at −0.2 V or higher was measured according to the change of the iron ion concentration. The result is shown in FIG.

  FIG. 6 is a graph schematically showing the change in the minimum hydrogen peroxide concentration due to the change in the iron ion concentration in order to maintain the surface potential of the steel sheet at −0.2 V or more.

  As can be seen from FIG. 6, as the iron ion concentration increases, the minimum hydrogen peroxide concentration for maintaining the surface potential of the steel sheet gradually decreases. Therefore, when the metal concentration is 40 g / l or more, the surface potential of the steel sheet is reduced. Only 1 g / l of hydrogen peroxide is required to form -0.2V.

[Example 11]
This example is for confirming the proper treatment conditions in the mixed acid tank, and for the steel plate obtained by Example 1 above, from which the primary scale has been removed by the neutral salt electrolysis treatment and the sulfuric acid electrolysis treatment, Mixed acid pickling was performed with a mixed acid solution having the composition shown in Table 7 and processing conditions. At this time, the treatment temperature of the mixed acid solution was 15 to 32 ° C., and the hydrogen peroxide concentration was maintained at −0.1 V based on the iron concentration in the solution based on Example 10 and FIG. Hydrogen peroxide was adjusted at a concentration to achieve.

  The presence or absence of pickling of the silicon oxide film on the surface of the steel sheet obtained by the mixed acid treatment was observed, and the results are shown in Table 7 below.

  The case where silicon oxide does not remain is indicated by ○, the case where silicon oxide remains is indicated by × as non-pickling, and the gloss measured at a reflection angle of 60 ° with respect to the rolling direction even when pickled. When the degree was less than 150, it was judged as peracid washing and indicated by x.

As can be seen from Table 7 above, the temperature is 20 to 32 ° C., the sulfuric acid is 110 to 150 g / l, the free hydrofluoric acid is 25 to 40 g / l, and the minimum hydrogen peroxide according to the iron ion concentration is composed as in Example 10. The pickling composition is preferably deposited for 10 to 100 seconds for pickling.

Claims (11)

A method for pickling high chrome austenitic stainless cold-rolled steel sheet, which removes silicon oxide from a high chrome austenitic stainless cold-rolled steel sheet containing 16 wt% or more of chromium,
The pickling method includes a sulfuric acid electrolytic treatment step and a mixed acid solution immersion step,
In the sulfuric acid electrolysis process, residual scale of Cr and Fe is electrolytically removed using a sulfuric acid electrolytic solution containing sulfuric acid as an electrolyte,
The step of immersing the mixed acid solution does not include nitric acid, the initial composition includes 110 to 150 g / l sulfuric acid, 15 to 30 g / l free hydrofluoric acid, and a hydrogen peroxide concentration of 4.5 g / l or more, and substantially contains iron ions. The silicon oxide is removed from the cold-rolled steel sheet by immersing the cold-rolled steel sheet in no mixed acid solution,
The concentration of the hydrogen peroxide in the mixed acid solution satisfies the following formula (1) in relation to the iron ion concentration in the mixed acid solution, and the oxidation-reduction potential (ORP) of the mixed acid solution is the iron ion concentration in the mixed acid solution. The pickling method of the high chromium austenitic stainless steel cold-rolled steel sheet which satisfy | fills following formula (2) or (3) by the relationship.
Hydrogen peroxide concentration ≧ 0.00736 + 10 × e ^{− [metal] /13.2} (1)
ORP ≧ 600 mV, provided that iron ion concentration ≦ 10 g / l (2)
ORP ≧ 310 + 431 × e ^{− [metal] /25.24} , provided that iron ion concentration> 10 g / l (3)
(In the above formulas (1) to (3), [metal] represents the iron ion concentration in the mixed acid solution.)   The pickling method for a high chromium austenitic stainless cold-rolled steel sheet according to claim 1, wherein the width (W) of the grain boundary after pickling is formed to be 1 µm or more. The high-chromium austenitic stainless steel according to claim 2, wherein the immersion is controlled such that the temperature (T), hydrofluoric acid concentration (C), and treatment time (t) of the mixed acid solution satisfy the following formula (4). Pickling method for cold-rolled steel sheet.
Width of crystal grain boundary after pickling (W) = − 0.184 + 0.0131 · t + 0.016 · C + 0.01 · T (4) A method for pickling high chrome austenitic stainless cold-rolled steel sheet, which removes silicon oxide from a high chrome austenitic stainless cold-rolled steel sheet containing 16 wt% or more of chromium,
The pickling method includes a sulfuric acid electrolytic treatment step and a mixed acid solution immersion step,
In the sulfuric acid electrolysis process, residual scale of Cr and Fe is electrolytically removed using a sulfuric acid electrolytic solution containing sulfuric acid as an electrolyte,
The mixed acid solution immersion step is maintained at a temperature of 20 to 32 ° C., does not contain nitric acid, has an initial composition of 110 to 150 g / l of sulfuric acid, 25 to 40 g / l of free hydrofluoric acid, and a hydrogen peroxide concentration of 5.5 g / l or more. And immersing the cold rolled steel sheet in a mixed acid solution substantially free of iron ions to remove silicon oxide from the cold rolled steel sheet,
The hydrogen peroxide is a pickling method for a high chromium austenitic stainless cold-rolled steel sheet, which satisfies the following formula (5) in relation to the concentration of iron ions.
Hydrogen peroxide concentration ≧ 0.805 + 9.2 × e ^{− [metal / 15.56]} (5)
(In the above formula (5), [metal] represents the iron ion concentration in the mixed acid solution.)   The high-chromium austenitic stainless cold-rolled steel sheet according to any one of claims 1 to 4, wherein a surface potential of the cold-rolled steel sheet immersed in the mixed acid solution is maintained at -0.2 to 0.1V. Pickling method.   The pickling method of the high chromium austenitic stainless cold-rolled steel sheet according to any one of claims 1 to 5, wherein the cold-rolled steel sheet is immersed in the mixed acid solution for 10 to 100 seconds. The removal of silicon oxide from the cold-rolled steel sheet further includes a neutral salt electrolytic treatment step before the sulfuric acid electrolytic treatment step,
The high chromium austenite according to any one of claims 1 to 6, wherein the neutral salt electrolytic treatment step performs electrolytic removal of Cr-rich scale from the surface of the steel sheet using a neutral salt electrolytic solution containing a sodium sulfate electrolyte. Pickling method for stainless steel cold-rolled steel sheet. In the neutral salt electrolysis treatment step, the austenitic stainless steel plate is immersed in a neutral salt electrolytic solution at a temperature of 50 to 90 ° C., and the surface potential of the steel plate is formed in the order of +, −, +. The pickling method of the high chromium austenitic stainless cold-rolled steel sheet according to claim 7, which is performed by applying a current density of 30 A / dm ² for 30 seconds to 120 seconds.   The pickling method of the high chromium austenitic stainless cold-rolled steel sheet according to claim 7 or 8, wherein the neutral salt electrolytic solution contains a sodium sulfate electrolyte in an amount of 100 to 250 g / l. In the sulfuric acid electrolytic treatment step, the austenitic stainless steel plate that has been subjected to the neutral salt electrolytic treatment step is immersed in a sulfuric acid electrolytic solution at 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 high chromium austenitic stainless cold-rolled steel sheet according to any one of claims 7 to 9, which is carried out by applying a current density of 10 to 30 A / dm ² to 5 to 50 seconds.   The pickling method for a high chromium austenitic stainless cold-rolled steel sheet according to any one of claims 1 to 10, wherein the sulfuric acid electrolytic solution contains 50 to 150 g / l of sulfuric acid.

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