KR20220077472A - Stainless steel electrolytic pickling method using highly conductive sulfuric acid solution - Google Patents

Abstract

Disclosed is an electrolytic pickling method for stainless steel using a highly conductive sulfuric acid solution.
A method for electrolytic pickling of stainless steel using a high-conductivity sulfuric acid solution according to the present invention comprises the steps of: hot rolling stainless steel; and immersing the stainless steel in a sulfuric acid solution electrolyzer and applying an electric current to electrolytic pickling.
Equation (1) Conductivity (mS/m) = -177.192+5.164*C+4.092*T-0.0092*C ² -0.013*T ²
(Here, C is the concentration (g/L) of the sulfuric acid solution, and T is the temperature (°C) of the sulfuric acid solution when current is applied.)

Description

Stainless steel electrolytic pickling method using highly conductive sulfuric acid solution

The present invention relates to a stainless steel electrolytic pickling method using a sulfuric acid solution, and more particularly, in the pickling process of a hot-rolled steel sheet after hot rolling of stainless steel, high pickling efficiency can be secured and electricity cost can be reduced. It relates to a stainless steel electrolytic pickling method using a conductive sulfuric acid solution.

In general, stainless steel is subjected to a heat treatment process to soften the strengthened structure after hot rolling. In the case of austenitic stainless steel, it is carried out at a temperature of about 1,100° C. for 1 to 2 minutes, and in the case of a ferritic stainless steel, it is carried out at a temperature of 800 to 1,050° C. for 1-2 minutes.

However, the heat treatment process removes the stress of stainless steel and improves the mechanical properties due to re-dissolution of carbides, etc., but since the heat treatment process is performed in an oxidizing atmosphere, Cr-rich between the surface scale layer generated after hot rolling and the interface of the base material A Cr-rich scale is formed. Such a Cr-rich scale is formed depending largely on the content of Cr. When the Cr content is high, a scale layer containing about 60% Cr is formed, and even when the Cr content is low, a scale layer having a Cr concentration of about 40-50% is formed. And a Cr-depleted layer with a low Cr concentration is formed just below this Cr-rich scale layer, and a poorly soluble Si oxide layer is formed at the interface with the base material.

The scale of this surface layer is first mechanically removed through methods such as Scale Breaker and Shot Ball projection. However, 10-20 μm thick scale is almost removed through a mechanical descaling process, but 0.5 μm thick scale remains without being mechanically removed.

The remaining Cr oxide layer and Si oxide layer are poorly soluble, and a sophisticated pickling method is required to remove such a scale layer. The poorly soluble oxide layer is removed step by step through a chemical method. First, the scale weakly bound to the surface layer through the sulfuric acid immersion process is removed by mechanical exfoliation by hydrogen generation. The amount of scale dissolved in austenitic stainless steel in sulfuric acid solution is about 1 to 2 g/m ² , and the amount of scale dissolved in ferritic stainless steel is about 15 to 20 g/m ² . The difference in dissolution amount between austenitic stainless steel and ferritic stainless steel is due to the difference in corrosion resistance to sulfuric acid solution. That is, in the case of a steel with high corrosion resistance to sulfuric acid solution, since the dissolution rate of metal ions is slow, it takes time to react with hydrogen ions, so the amount of dissolution is small. On the other hand, in the case of steel with low corrosion resistance to sulfuric acid solution, metal dissolution occurs easily, and the generated electrons are consumed by hydrogen ions to generate a large amount of hydrogen bubbles, thereby increasing the amount of dissolution.

After the sulfuric acid immersion process, Cr oxide and Si oxide that are not completely removed are dissolved in a mixed acid tank made of hydrofluoric acid and nitric acid. In order to secure sufficient pickling properties in the mixed acid tank, it is necessary to sufficiently remove the scale of the surface layer from the sulfuric acid immersion tank. This is because, when the scale is transferred to the mixed acid tank in a state in which there is an excessive amount, the dissolution rate is significantly lowered. Therefore, in order to improve the pickling performance in the mixed acid tank, the Cr oxide layer and the Si oxide layer should be completely removed from the sulfuric acid tank.

As a pickling method for removing the Cr oxide layer and the Si oxide layer in the sulfuric acid bath, there are a direct current electrolytic pickling method and an immersion pickling method. The direct current electrolytic pickling method is widely used because it has superior workability compared to the conventional sulfuric acid immersion pickling method and the surface quality of the final steel product that has undergone the electrolytic pickling process is more beautiful than that of the immersion and pickling steel product. However, since the electrolytic pickling process uses electricity, there is a problem that the cost is excessive than that of the immersion pickling process, so a sulfuric acid electrolytic pickling method that can secure pickling properties while maximally increasing pickling efficiency is required.

One aspect of the present invention is to provide a stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution capable of securing pickling properties while maximally increasing pickling efficiency in a hot-rolled steel sheet subjected to heat treatment (or heat treatment omitted) after hot rolling. .

A method for electrolytic pickling of stainless steel using a high-conductivity sulfuric acid solution according to an embodiment of the present invention includes the steps of: hot rolling stainless steel; and immersing the stainless steel in a sulfuric acid solution electrolyzer and applying an electric current to electrolytic pickling.

Equation (1) Conductivity (mS/m) = -177.192+5.164*C+4.092*T-0.0092*C ² -0.013*T ²

(Here, C is the concentration (g/L) of the sulfuric acid solution, and T is the temperature (°C) of the sulfuric acid solution when current is applied.)

In addition, according to an embodiment of the present invention, the amount of sludge is 10 g/L or less.

In addition, according to an embodiment of the present invention, the metal ion concentration may be 60 g / L or less.

In addition, according to an embodiment of the present invention, the temperature of the sulfuric acid solution may be 50 to 70 ℃.

In addition, according to an embodiment of the present invention, when a current is applied, the current density may be 20 to 55A/dm ² .

In addition, according to an embodiment of the present invention, the current application time may be within 5 to 50 seconds.

In addition, according to an embodiment of the present invention, the amount of scale dissolved after pickling may be 9g/m ² or more.

In addition, according to an embodiment of the present invention, after electrolytic pickling, immersing the electrolytically pickled stainless steel in a mixed acid tank containing nitric acid and hydrofluoric acid for pickling; may further include.

In addition, according to an embodiment of the present invention, the amount of scale dissolved by the mixed acid tank immersion pickling may be 23 g/m ² or more.

In addition, according to an embodiment of the present invention, the amount of scale dissolved by electrolytic pickling and mixed acid bath pickling may be more than 30 g/m ² .

In addition, according to an embodiment of the present invention, the mixed acid tank immersion may be performed for 40 seconds to 60 seconds.

In addition, according to an embodiment of the present invention, after hot rolling, a hot rolling annealing step; may further include.

In the stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution according to an embodiment of the present invention, by applying a high-conductivity sulfuric acid solution to the direct current electrolytic pickling method, as the resistance of the pickling solution is reduced, the electrolysis efficiency is improved and the electricity cost is reduced can do.

This specification does not describe all elements of the embodiments, and general content in the technical field to which the present invention pertains or content that overlaps among the embodiments is omitted.

Also, when a part "includes" a component, it means that other components may be further included, rather than excluding other components, unless otherwise stated.

The singular expression includes the plural expression unless the context clearly dictates otherwise.

Hereinafter, the present invention will be described in detail.

The following examples are presented to sufficiently convey the spirit of the present invention to those of ordinary skill in the art to which the present invention pertains. The present invention is not limited to the embodiments presented herein, and may be embodied in other forms.

The present inventors have found that when the conductivity of the sulfuric acid solution is increased in the sulfuric acid electrolyzer and the metal ion concentration and the sludge concentration of the sulfuric acid solution are limited, high pickling efficiency can be secured, and it is advantageous for improving the pickling performance in the mixed acid tank, which is a subsequent process. discovered and came to propose the present invention.

The present invention relates to a sulfuric acid electrolytic pickling method for removing a Cr oxide layer and a Si oxide layer produced by heat treatment after hot rolling, the method comprising the steps of: hot rolling stainless steel; and immersing the stainless steel in a sulfuric acid solution electrolyzer and electrolytic pickling by applying an electric current.

Hereinafter, a stainless steel electrolytic pickling method using a highly conductive sulfuric acid solution according to an embodiment of the present invention will be described in detail.

First, a stainless steel is prepared and hot-rolled, and the rolled stainless steel is heat-treated. In the present invention, the step of hot rolling and the step of heat treatment are not particularly limited. In particular, the step of heat-treating the rolled stainless steel may be omitted depending on the type of stainless steel or when a separate means for relieving residual stress is provided.

Then, the heat-treated stainless steel mechanically removes the Cr oxide layer and the Si oxide layer generated by hot rolling and heat treatment through a scale breaker, shot ball projection, or the like.

Thereafter, in order to remove the remaining poorly soluble Cr oxide layer and Si oxide layer, the stainless steel is immersed in a sulfuric acid solution electrolytic bath and electrolytic pickling is performed by applying an electric current.

In this case, the sulfuric acid solution may have a conductivity value of 680 or more defined by the following formula (1).

Equation (1) Conductivity (mS/m) = -177.192+5.164*C+4.092*T-0.0092*C ² -0.013*T ²

(However, C is the concentration (g/L) of the sulfuric acid solution, and T is the temperature (°C) of the sulfuric acid solution when current is applied)

When the current is applied, the current density may be 20 to 55A/dm ² , and the current application time may be 5 to 50 seconds.

In addition, the amount of scale dissolved after electrolytic pickling may be 9 g/m ² or more.

The inventors of the present invention control the concentration and temperature of the sulfuric acid solution, and as the conductivity of the sulfuric acid solution is increased, the electrolysis efficiency is improved as the resistance of the solution is decreased, and when the conductivity value defined by Equation (1) is 680 or more, It was confirmed that the total dissolved amount by pickling can be secured to 30 g/L or more, and it is effective in removing the Cr-rich scale with high Cr concentration.

According to Equation (1), looking at the relationship between the sulfuric acid concentration and the conductivity, when the sulfuric acid concentration is 200 g/L at a solution temperature of 65 ° C, the conductivity of the solution is about 700 mS/m, and the sulfuric acid concentration is 100 g/L. When decreasing, the conductivity decreases to 458 mS/m. As the conductivity decreases from 700 mS/m to 458 mS/m, the voltage for electrolytic pickling increases by 153%. That is, power required to apply the same amount of current at P=I*V increases by 1.53 times. Therefore, in the present invention, the sulfuric acid concentration may be limited to 200 to 300 g/L in order to secure electrolysis efficiency.

In addition, looking at the relationship between the temperature and conductivity of the sulfuric acid solution, under the condition that the sulfuric acid concentration is 200 g/L, when the temperature of the solution is 70 ° C, the conductivity is 710 mS/m, and when the temperature of the solution decreases to 40 ° C, Conductivity decreases from 710 mS/m to 630 mS/m. That is, a voltage increase of 13% occurs. Accordingly, in the present invention, the temperature of the sulfuric acid solution may be limited to 50° C. or higher to ensure electrolysis efficiency, and preferably, the temperature of the solution may be limited to 50° C. to 70° C.

In addition, in the method for electrolytic pickling of stainless steel using a high-conductivity sulfuric acid solution according to an embodiment of the present invention, the amount of sludge in the solution may be 10 g/L or less. As the amount of sludge in the solution increases to 35 g/L, the conductivity decreases by 20 mS/m, and accordingly, the resistance of the solution may increase, and the sulfuric acid electrolytic pickling efficiency may decrease. Therefore, in the present invention, the upper limit of the amount of sludge in the solution is limited to 10 g/L.

In addition, in the stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution according to an embodiment of the present invention, the concentration of metal ions in the solution may be 60 g/L or less. The higher the concentration of metal ions in the solution, the lower the conductivity, so it is desirable to control it as low as possible. However, the conductivity hardly changes in the range of 0 to 35 g/L of metal ion concentration, but when the metal ion concentration reaches 35 g/L, it decreases by about 10 mS/m and does not decrease significantly up to 60 g/L. In addition, if the metal ion concentration is lowered to increase the conductivity, the solution must be continuously replaced during actual operation. Therefore, in the present invention, the upper limit of the concentration of metal ions in the solution is limited to 60 g/L.

In addition, the electrolytic pickling method for stainless steel using a high-conductivity sulfuric acid solution according to an embodiment of the present invention may further include a mixed acid tank immersion pickling process after the sulfuric acid electrolytic pickling process.

In the mixed acid tank immersion pickling process, electrolytic pickling stainless steel is immersed in a mixed acid tank containing nitric acid and hydrofluoric acid for pickling treatment.

At this time, the mixed acid tank immersion may be performed for 30 seconds to 120 seconds.

The stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution according to an embodiment of the present invention can secure high pickling properties not only in the sulfuric acid electrolytic pickling process but also in the mixed acid tank pickling process, which is a subsequent process, and is dissolved by the mixed acid tank immersion pickling The amount of scale used may be 23 g/m ² or more.

Finally, the total amount of scale dissolved by the electrolytic pickling and the mixed acid tank immersion pickling may be greater than 30 g/m ² .

Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is only for illustrating the practice of the present invention, and the present invention is not limited by the description of these examples. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

Example

Prepare a sulfuric acid solution electrolyzer to satisfy the sulfuric acid solution conditions in Table 1 below, and prepare a stainless stainless steel with a 3mm thick STS 304 hot-rolled steel sheet similar to that used in heat treatment and pickling processes, heat-treat at 1,100°C, and then shot ball After removing the scale on the surface by projecting the pickling Example and Comparative Example specimens were prepared.

Specimens for Examples and Comparative Examples for pickling were immersed in a sulfuric acid solution electrolyzer in which sulfuric acid concentration, solution temperature, sludge amount and metal ion concentration were changed according to the conditions in Table 1 below, and a current density of 50A/dm ² was applied for 12 seconds. Thus, the first pickling was performed. Thereafter, secondary pickling was performed by immersing the first pickled stainless steel in a mixed acid tank consisting of a mixed acid of 80 g/L or more of nitric acid and 20 g/L of hydrofluoric acid at a temperature of 50° C. for at least 40 seconds.

division Sulfuric Acid Concentration (g/L) temperature
(℃) Formula (1)
(mS/m) Sludge amount
(g/L) metal ion concentration
(g/L) solution conductivity
(mS/m) Example 1 200 60 686.3 10 60 686.3 Example 2 200 60 686.3 10 50 686.3 Example 3 220 70 736.3 10 50 736.3 Example 4 250 75 772.6 10 40 772.6 Example 5 300 70 766.7 10 40 766.7 Comparative Example 1 50 40 200.9 35 80 160.9 Comparative Example 2 100 40 390.1 35 70 360.1 Comparative Example 3 120 40 452.9 25 70 432.9 Comparative Example 4 150 40 533.3 25 80 503.3 Comparative Example 5 180 40 597.1 25 80 567.1 Comparative Example 6 180 40 597.1 25 70 577.1

Pickling properties were evaluated by measuring the weight loss of stainless steel after primary and secondary pickling, and the results are shown in Table 2 below. Weight loss corresponds to the amount of scale dissolved, and in the case of STS 304 steel, the amount of scale dissolved in the pickling process is generally estimated to be 28-30 g/m ² .

division 1st pickling secondary pickling total weight loss
(g/m ² ) Sulfuric acid electrolyzer weight loss
(g/m ² ) Honsanjo weight loss
(g/m ² ) Example 1 9.2 25 34.2 Example 2 9.1 24.8 33.9 Example 3 9.5 23.9 33.4 Example 4 9.5 24.2 33.7 Example 5 10.0 24.8 34.8 Comparative Example 1 4.2 19 23.2 Comparative Example 2 4.8 19 23.8 Comparative Example 3 4.9 18.4 23.3 Comparative Example 4 5.8 18.5 24.3 Comparative Example 5 6.2 19.5 25.7 Comparative Example 6 6.5 19.8 26.3

Examples 1 to Examples in which the conductivity value of Formula (1) of the present invention is 680 mS/m or more, the sulfuric acid solution temperature of 50° C. or more, the amount of sludge of 10 g/L or less, and the metal ion concentration of 60 g/L or less are satisfied 5 indicates that the weight loss after the first pickling is 9g/m ² or more, and the weight loss after the second pickling is 23g/m ² or more, and the total weight loss after pickling exceeds 30g/m ² It was confirmed that the pickling property was excellent. In the sulfuric acid electrolytic pickling according to the present invention, not only was it very effective to remove scale with high Cr concentration, but it was also possible to secure pickling properties in the subsequent mixed acid tank pickling process. On the other hand, the value of Equation (1) of the present invention In the case of Comparative Examples 1 to 6, which did not reach the lower limit of 680, the sludge amount and metal ion concentration were higher than those of the present invention, the conductivity was low, and the total weight loss after the second pickling was 23.2 to 26.3. was able to confirm This is considered to be because the sulfuric acid electrolytic pickling efficiency is lowered due to the high resistance of the solution because the conductivity of the sulfuric acid solution is low. In addition, the actual conductivity values of Comparative Examples 1 to 6 were derived lower than the value of Equation (1) according to the present invention, which is due to the high sludge amount and high metal ion concentration.

Specifically, Comparative Example 1 has a sulfuric acid concentration of 50 g/L, a sulfuric acid solution temperature as low as 40° C., a sludge amount of 35 g/L, which exceeds the upper limit of 10 g/L, and a metal ion concentration of 80 g/L as an upper limit. In the case of exceeding 60 g/L of phosphorus, the conductivity was very low as 160.9 mS/m, so the weight loss in the sulfuric acid electrolyzer was 4.2 g/L, and it was confirmed that the pickling performance was inferior.

In Comparative Example 2, the sulfuric acid concentration is 100 g/L, the sulfuric acid solution temperature is as low as 40 ° C, the amount of sludge is 35 g/L, which exceeds the upper limit of 10 g/L, and the metal ion concentration is also 70 g/L, which is the upper limit. In excess of 60 g/L, the conductivity was very low as 360.1 mS/m, and the weight loss in the sulfuric acid electrolyzer was 4.8 g/L, confirming that the pickling performance was inferior.

In Comparative Example 3, the sulfuric acid concentration is 120 g/L, the sulfuric acid solution temperature is as low as 40 ° C, the amount of sludge is 25 g/L, which exceeds the upper limit of 10 g/L, and the metal ion concentration is also 70 g/L, which is the upper limit. In excess of 60 g/L, the conductivity was 432.9 mS/m, so the weight loss in the sulfuric acid electrolyzer was 4.9 g/L, and it was confirmed that the pickling performance was inferior.

In Comparative Example 4, the sulfuric acid concentration was 150 g/L, the sulfuric acid solution temperature was as low as 40 ° C, the amount of sludge was 25 g/L, which exceeded the upper limit of 10 g/L, and the metal ion concentration was also 80 g/L, which was the upper limit. In excess of 60 g/L, the conductivity was low as 503.3 mS/m, and the weight loss in the sulfuric acid electrolyzer was 5.8 g/L, confirming that the pickling property was inferior.

In Comparative Example 5, the sulfuric acid concentration was 180 g/L, the sulfuric acid solution temperature was as low as 40 ° C, the amount of sludge was 25 g/L, which exceeded the upper limit of 10 g/L, and the metal ion concentration was also 80 g/L, which was the upper limit. In excess of 60 g/L, the conductivity was low as 567.1 mS/m, and the weight loss in the sulfuric acid electrolyzer was 6.2/L, confirming that the pickling property was inferior.

In Comparative Example 6, the sulfuric acid concentration was 180 g/L, the sulfuric acid solution temperature was as low as 40 ° C, the amount of sludge was 25 g/L, which exceeded the upper limit of 10 g/L, and the metal ion concentration was also 70 g/L, which was the upper limit. In excess of 60 g/L, the conductivity was low as 577.1 mS/m, and the weight loss in the sulfuric acid electrolyzer was 6.5/L, confirming that the pickling performance was inferior.

In the foregoing, exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and those of ordinary skill in the art are within the scope not departing from the concept and scope of the claims described below. It will be understood that various changes and modifications are possible.

Claims (12)

hot rolling stainless steel; and
Including; immersing the stainless steel in a sulfuric acid solution electrolyzer and applying an electric current to electrolytic pickling;
The sulfuric acid solution has a conductivity value of 680 or more defined by the following formula (1), a stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution.
Equation (1) Conductivity (mS/m) = -177.192+5.164*C+4.092*T-0.0092*C ² -0.013*T ²
(Here, C is the concentration (g/L) of the sulfuric acid solution, and T is the temperature (°C) of the sulfuric acid solution when current is applied.) According to claim 1,
The amount of sludge is
An electrolytic pickling method for stainless steel using a high-conductivity sulfuric acid solution of 10 g/L or less. According to claim 1,
The metal ion concentration is
An electrolytic pickling method for stainless steel using a high-conductivity sulfuric acid solution of 60 g/L or less. According to claim 1,
The temperature of the sulfuric acid solution is,
A stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution at 50 to 70°C. According to claim 1,
A stainless steel electrolytic pickling method using a highly conductive sulfuric acid solution having a current density of 20 to 55A/dm ² when the current is applied. According to claim 1,
A stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution with a current application time of 5 to 50 seconds. According to claim 1,
The amount of scale dissolved after the electrolytic pickling is,
An electrolytic pickling method for stainless steel using a highly conductive sulfuric acid solution of 9 g/m ² or more. According to claim 1,
After the electrolytic pickling,
The electrolytic pickling method for stainless steel using a high-conductivity sulfuric acid solution further comprising; immersing the electrolytically pickled stainless steel in a mixed acid tank containing nitric acid and hydrofluoric acid for pickling. 9. The method of claim 8,
The amount of scale dissolved by the mixed acid tank immersion pickling is 23 g/m ² or more. A stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution. 9. The method of claim 8,
The amount of scale dissolved by the electrolytic pickling and the mixed acid tank immersion pickling is more than 30 g/m ² A stainless steel electrolytic pickling method using a high-conductivity sulfuric acid solution. 9. The method of claim 8,
The electrolytic pickling method for stainless steel using a high-conductivity sulfuric acid solution in which the mixed acid bath immersion is performed for 30 seconds to 120 seconds. According to claim 1,
After the hot rolling, a hot rolling annealing step; stainless steel electrolytic pickling method using a high conductivity sulfuric acid solution further comprising.