KR20200055366A - High-speed pickling method of stainless steel - Google Patents

Abstract

The present specification discloses a high-speed pickling method of a stainless steel to remove a chromium-depleted layer formed during heat treatment of the stainless steel within a short time. According to one embodiment of the disclosed pickling method, the method includes a step of rolling and annealing the stainless steel, a step of immersing the annealed stainless steel in a pickling bath, and a step of applying a current density (I) satisfying equation (1), 40 A/dm^2 <= I <= 50 A/dm^2, to the stainless steel immersed in the pickling bath, wherein potential (V) created on the surface of the stainless steel satisfies equation (2), -2.6 V <= V <= 2.6 V.

Description

High-speed pickling method of stainless steel

The present invention relates to a method of pickling stainless steel.

Stainless steel is subjected to annealing heat treatment at a temperature of approximately 950 to 1100 ° C for stress relief and carbide solidification after hot rolling or cold rolling. During annealing heat treatment, iron oxide is formed when the temperature is low, and when the oxygen concentration is lowered below iron oxide, oxidation of concentrated chromium and manganese occurs, and chromium and manganese form a MnCr ₂ O ₄ spinel structure. Manganese is then depleted, and a chromium oxide layer is formed. As the chromium oxide layer diffuses to the outside during the heat treatment process, a chromium depletion layer having a lower chromium content is formed directly under the chromium oxide layer.

Since the chromium-depleted layer degrades the appearance quality of the product and further decreases corrosion resistance as a starting point for corrosion, a pickling method for removing the chromium-depleted layer is essential for the annealed stainless steel.

In addition, in the pickling process, the pickling rate is advantageous as it is a factor that significantly affects economic aspects and surface quality of steel. Therefore, a high-speed pickling method for removing the chromium-depleted layer is required.

Korean Patent Publication No. 10-2015-0053625 (released on May 18, 2015)

In order to solve the above-mentioned problems, the present invention is to provide a pickling method for removing the chromium-depleted layer formed in the heat treatment of the scaleless steel within a short time.

The method of pickling stainless steel according to an embodiment of the present invention includes rolling, annealing heat treatment of stainless steel, immersing the annealed stainless steel in a pickling tank, and formula (1) below for the stainless steel immersed in the pickling tank. And applying a satisfactory current density (I), and the potential (V) formed on the surface of the stainless steel satisfies Expression (2) below.

(1) 40A / dm ² ≤ I ≤ 50A / dm ²

(2) -2.6V ≤ V ≤ 2.6V

In addition, the average value (Vrms) of the potential may satisfy the following equation (3).

(3) -1.84V ≤ Vrms ≤ 1.84V

In addition, the pickling tank contains nitric acid and hydrofluoric acid, the concentration of the nitric acid is 80 ~ 200g / L, the concentration of the hydrofluoric acid may be 0.5 ~ 50g / L.

In addition, the pickling tank contains sulfuric acid and hydrofluoric acid, the concentration of the sulfuric acid is 80 ~ 200g / L, the concentration of the hydrofluoric acid may be 0.5 ~ 50g / L.

In addition, the temperature of the pickling tank may be 25 ~ 70 ℃.

In addition, at least one treatment step of brush treatment, shot blasting treatment, and scale breaker treatment may be further included.

According to the present invention, it is possible to promote the dissolution rate of the chromium depletion layer by applying a current density to a potential that can minimize the amount of oxygen generated by the oxidation reaction of water to the stainless steel immersed in the pickling tank.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the technical idea of the present invention is not limited to the embodiments described below. In addition, embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

Terms used in the present application are only used to describe specific examples. Thus, for example, a singular expression includes a plural expression unless the context clearly indicates that it should be singular. In addition, terms such as “comprise” or “include” as used in the present application are used to clearly indicate the existence of features, steps, functions, elements, or combinations thereof described in the specification, and other features. It should be noted that it is not used to preliminarily exclude the presence of a field or step, function, component, or combination thereof.

On the other hand, unless defined otherwise, all terms used in this specification should be regarded as having the same meaning as generally understood by a person having ordinary skill in the art to which the present invention pertains. Accordingly, unless specifically defined herein, certain terms should not be construed in excessively ideal or formal sense. For example, in this specification, a singular expression includes a plural expression unless the context clearly has an exception.

In addition, "about", "substantially", and the like in the present specification are used in the sense of or close to the value when manufacturing and substance tolerances unique to the stated meaning are presented, and are used to help understand the present invention. Or, an absolute value is used to prevent unscrupulous use of the disclosed content by unscrupulous intruders.

The chromium-depleted layer formed during the heat treatment of stainless steel degrades the appearance quality of the product and further decreases the corrosion resistance as a starting point for corrosion. Therefore, for the annealed stainless steel, a pickling process for removing the chromium-depleted layer is essential Is required.

Hereinafter, a method of pickling stainless steel according to the present invention will be described.

The pickling method according to an example of the present invention includes rolling, annealing heat treatment of stainless steel, immersing the annealed stainless steel in a pickling tank, and applying a current density to the stainless steel immersed in the pickling tank. . In addition, at least one treatment step of brush treatment, shot blasting treatment, and scale breaker treatment may be further included.

According to the present invention, the step of rolling and annealing stainless steel may be performed in various ways by appropriately using a method known to a person skilled in the art, and hereinafter, each step of the pickling method of the present invention except for this will be described.

Step of immersing annealing heat-treated stainless steel in pickling tank

In the present invention, the step of immersing the annealing heat-treated stainless steel in the pickling tank is a step of chemically descaling the stainless steel with a scale formed on the surface due to the annealing heat treatment. In this step, the Fe and Si oxide scales, including the Cr oxide scale, are removed by chemical descaling.

According to an example of the present invention, the pickling tank includes nitric acid, sulfuric acid, and hydrofluoric acid. Nitric acid and sulfuric acid lower the pH to increase the activity of hydrofluoric acid and oxidize divalent iron ions dissolved on the surface of the steel plate to trivalent to maintain the appropriate redox potential for pickling. Accordingly, the pickling solution of the pickling tank may be composed of nitric acid, hydrofluoric acid or a mixed solution of sulfuric acid and hydrofluoric acid.

However, when nitric acid is included in the mixed solution of the pickling tank, NO _{x, which} is an air emission regulating substance, may occur, so the pickling solution of the pickling tank may be a mixed solution containing sulfuric acid and hydrofluoric acid.

When a mixed acid solution of nitric acid or hydrofluoric acid is used as the pickling solution of the pickling tank, the nitric acid concentration may be 80 g / L to 200 g / L, and the hydrofluoric acid concentration may be 0.5 g / L to 50 g / L. When the concentration of nitric acid and the concentration of hydrofluoric acid is lower than this, the dissolution rate is lowered. When the concentration is higher than this, NOx is generated in the case of nitric acid, and it is expensive because it is an expensive compound.

When a mixed solution of sulfuric acid and hydrofluoric acid is used as the pickling solution of the pickling tank, the concentration of sulfuric acid is 80 g / l to 200 g / l, and the concentration of hydrofluoric acid is 0.5 g / l to 50 g / l. When the concentration of sulfuric acid and the concentration of hydrofluoric acid are low, the dissolution rate decreases, and when the concentration is high, the heat of reaction becomes very large, and the stability of the pickling tank is significantly lowered, and in the case of hydrofluoric acid, it is expensive because it is an expensive compound.

The temperature range of the pickling tank is limited to 25 ° C to 70 ° C. The pickling reaction is very slow at 25 ° C or lower, and occurs very rapidly at 70 ° C or higher to lower the stability of the pickling tank.

Applying current density to the stainless steel immersed in the pickling tank

The step of applying the current density to the stainless steel immersed in the pickling tank of the present invention is a step for easily removing the chromium depletion layer, which is difficult to oxidize due to its compact structure, and corresponds to a key step for improving the pickling rate of the present invention. do.

In the above step, the pH range of the pickling tank is preferably pH 3 to 6, and when the current density and potential are applied to the pickling solution having such a pH range, the chromium depletion layer Cr of the steel surface is preferentially in the state of Cr ⁶⁺ . It can be dissolved to remove the chromium-depleted layer.

However, in order to promote the chromium depletion layer, it is necessary to suppress the oxidation reaction of water represented by the following Reaction Formula 1. This is because oxygen bubbles formed by excessive oxidation of water surround the surface of the steel, so that the dissolution of the chromium depletion layer is suppressed. Therefore, it is necessary to control by applying a current density to a potential capable of promoting the dissolution rate of the chromium depletion layer while minimizing the amount of oxygen generated by the oxidation reaction of water.

[Scheme 1]

H ₂ O (aq) → 4H ^{+ _{+ 4e - + O 2 (g}} ) ↑

Accordingly, the average (Root Mean Square) current density (I) applied to the stainless steel can preferably satisfy the following equation (1).

(1) 40A / dm ² ≤ I ≤ 50A / dm ²

If the current density is less than 40A / d㎡, the amount of chromium-depleted layer is very small, and the chromium-depleted layer cannot be sufficiently removed. When it exceeds 50A / d㎡, the chromium-depleted layer is formed by oxygen formed by excessive oxidation of water. The dissolution rate may be lowered.

Due to the current density applied, the potential formed on the surface of the stainless steel may preferably satisfy the following equation (2).

(2) -2.6V ≤ V ≤ 2.6V

When the potential exceeds ± 2.6 V, the dissolution rate of the chromium-depleted layer may be lowered by oxygen formed by excessive oxidation of water.

The potential may more preferably satisfy the following equation (3) on the basis of the average value (Root Mean Square).

(3) -1.84V ≤ Vrms ≤ 1.84V

In the above range, the pickling process can be performed more stably and quickly, and when the potential average value exceeds ± 1.84V, the dissolution rate of the chromium depletion layer may be lowered by oxygen formed by excessive oxidation of water.

Brush treatment, shot blasting treatment, scale breaker treatment

The brush treatment step is a treatment step of mechanically separating and removing the reactants formed after the scale and pickling treatment remaining on the surface of the steel.

The shot blasting treatment step is a treatment step for removing a scale by spraying a shot of an abrasive or the like into the steel at an air pressure or a centrifugal force of about 2000 rpm.

The scale breaker treatment step is a treatment step in which a scale breaker is generated on a scale surface by using a scale breaker, thereby making it easier to remove scale by a pickling process.

In the above, each processing step has been briefly described, but each processing step is not necessarily interpreted as being limited to the described content, and may be expanded and interpreted within a range understood by a person skilled in the art.

Hereinafter, the present invention will be described in more detail through examples. However, it is necessary to note that the following examples are only intended to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and reasonably inferred therefrom.

{Example}

In each of the following invention examples and comparative examples, the hot rolled 3 mm thick 304 stainless steel was annealed at 1100 ° C. The annealing heat treatment controlled the set temperature of the heat treatment furnace so that the actual temperature of the steel reached 1100 ° C in 2 minutes.

Thereafter, the 304 stainless steel was taken out of the heat treatment furnace and then cooled through air cooling. After heat treatment, the surface of the scale was partially removed using a shot ball.

Subsequently, some scale was removed by immersion in a sulfuric acid solution at 70 ° C. for 200 g / L for 30 seconds. Table 1 summarizes the results of removing the chromium-depleted layer through AC electrolytic pickling.

Nitric acid + folic acid Sulfuric acid + hydrofluoric acid Current density
(A / dm ² ,
medium) Authorized potential
(V,
Average value (rms)) chrome
Depletion
density
(%) nitric acid
density
(g / ℓ) Foshan
density
(g / ℓ) Sulfuric acid
density
(g / ℓ) Foshan
density
(g / ℓ) Comparative Example 1 80 30 - - 0 0 15 Comparative Example 2 80 30 - - 0 ± 0.8 15.5 Comparative Example 3 80 30 - - 15 ± 0.85 16 Comparative Example 4 - - 150 30 15 ± 0.85 16.5 Comparative Example 5 - - 150 30 60 ± 2.25 17 Inventive Example 1 80 30 - - 40 ± 1.63 17.5 Inventive Example 2 - - 150 30 50 ± 1.84 18 Inventive Example 3 - - 150 30 40 ± 1.63 17.5

Inventive Example 1 and Comparative Examples 1 to 3 were measured by varying the current density and potential applied from a mixed solution of nitric acid and hydrofluoric acid under the same conditions. Inventive Example 1 applied a current density of 40 A / d m 2, and the potential formed on the surface of the steel was ± 1.63 V. The chromium concentration of the measured chromium depletion layer is 17.5%, and considering that the chromium concentration of 304 steel is usually 18%, it can be seen that the chromium depletion layer is almost removed.

On the other hand, Comparative Examples 1 to 3 was applied to the lower current density than 40A / dm ² as a current density of ^{0A / dm 2, 0A / dm} 2, 15A / dm 2 , respectively, the potential formed in the river surface is 0V, ± each 0.8V and ± 0.85V. The chromium depletion layer has a chromium concentration of 15%, 15.5%, and 16.5%, respectively, and considering that the chromium 304 chromium concentration is usually 18%, it can be seen that the chromium depletion layer was hardly removed.

Inventive Examples 2 and 3 and Comparative Examples 4 and 5 were measured by varying the current density and potential applied in a mixed solution of sulfuric acid and hydrofluoric acid under the same conditions. Inventive Examples ² and 3 applied current densities of 50 A / dm ² and 40 A / dm ² , respectively, and the potentials formed on the surface of the steels were ± 1.84 V and ± 1.63 V, respectively. The chromium concentration of the measured chromium depletion layer is 18 and 17.5%, respectively, and considering that the chromium concentration of 304 steel is 18%, the chromium depletion layer is almost removed.

On the other hand, Comparative Examples 4 and 5 each applied a current density greater than 50 A / dm ² as a current density of 15 A / dm ² or less than 40 A / dm ² or a current density of 60 A / dm ² and formed on the surface of the steel. The potentials were ± 0.85V and ± 2.25V, respectively. When the chromium concentration of the measured chromium depletion layer is 16.5% and 17%, respectively, considering that the chromium concentration of 304 steel is 18%, it can be seen that the chromium depletion layer was hardly removed.

Referring to Table 1, it can be seen that the larger the applied potential formed on the surface of the steel, the more the chromium depletion layer was removed. However, the current density applied to Comparative Example 5 was excessive as 60 A / dm ² , so that the potential formed on the surface of the steel was ± 2.25 V in excess of ± 1.84 V. In Comparative Example 5, the chromium depletion layer had a smaller chromium concentration than the inventive example, although the magnitude of the current density and dislocation was larger, and thus, Comparative Example 5 had a chromium depletion layer due to oxygen bubbles generated by the oxidation reaction of water. It can be seen that the dissolution rate was inhibited.

According to the present invention from the above results, when the equations (1) and (2) are satisfied, the chromium-depleted layer is dissolved by applying a current density to a stainless steel immersed in the pickling tank to a potential that can minimize the amount of oxygen generated by the oxidation reaction of water. It can be seen that it can speed up. In addition, it can be seen that a stainless steel without corrosion deterioration can be produced by removing the chromium depletion layer.

As described above, although exemplary embodiments of the present invention have been described, the present invention is not limited thereto, and a person having ordinary skill in the art does not depart from the concept and scope of the following claims. It will be understood that various modifications and variations are possible.

Claims (6)

Rolling stainless steel, annealing heat treatment;
Immersing the annealing heat-treated stainless steel in a pickling tank; And
Including the step of applying a current density (I) satisfying the following formula (1) to the stainless steel immersed in the pickling tank;
The potential V formed on the surface of the stainless steel is a pickling method of stainless steel that satisfies the following formula (2):
(1) 40A / dm ² ≤ I ≤ 50A / dm ²
(2) -2.6V ≤ V ≤ 2.6V. According to claim 1,
The average value of the potential (Vrms) is a pickling method of stainless steel that satisfies the following formula (3):
(3) -1.84V ≤ Vrms ≤ 1.84V. According to claim 1,
The pickling tank includes nitric acid and hydrofluoric acid,
The concentration of the nitric acid is 80 ~ 200g / L, the concentration of the hydrofluoric acid is 0.5 ~ 50g / L of the pickling method of stainless steel. According to claim 1,
The pickling tank contains sulfuric acid and hydrofluoric acid,
The concentration of the sulfuric acid is 80 ~ 200g / L, the concentration of the hydrofluoric acid is 0.5 ~ 50g / L of the pickling method of stainless steel. According to claim 1,
The pickling method of the stainless steel is 25 ~ 70 ℃ temperature of the pickling tank. According to claim 1,
A method for pickling stainless steel, further comprising at least one treatment step of brush treatment, shot blasting treatment, and scale breaker treatment.