KR20120075345A - High speed pickling method for surface improvement of high chromium ferritic stainless cold steel strip - Google Patents

Abstract

PURPOSE: A method for cleansing high chrome ferrite group stainless steel is provided to quantitively confirm an amount of oxidizer hydrogen peroxide being added, to have no residue of the oxidation scale of the stainless steel plate, and to enhance surface gloss. CONSTITUTION: A method for cleansing high chrome ferrite group stainless steel comprises next steps. A mixed acid pickling agent comprises acid 80-150g/l, free hydrofluoric acid 8-30g/l, and hydrogen peroxide 3-20g/l, however, does not comprises silver Fe+3 ion. A step for dipping into the acid pickling agent comprises next processes. Firstly, high chrome ferrite group stainless is dipped into the mixed acid pickling agent of 30 to 40°C. Secondly, the high chrome ferrite group stainless is dipped into the mixed acid pickling agent of 35 to 45°C. Whole time for the first and second dipping processes is 25 to 72 seconds.

Description

High Speed Pickling Method for Surface Improvement of High Chromium Ferritic Stainless Cold Steel Strip}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for pickling high chromium ferritic stainless steel cold rolled steel sheets requiring high surface quality at high speed, and provides a method for pickling using an acid solution containing sulfuric acid, hydrofluoric acid, and hydrogen peroxide.

In general, ferritic stainless steel is classified into low chromium ferritic stainless steel and high chromium ferritic stainless steel according to the chromium content. In general, a chromium content of 11-14% is called low chromium ferritic stainless steel, and a chromium content of 16-26% is referred to as high chromium ferritic stainless steel.

Since the characteristics of the scale formed during the annealing heat treatment vary according to the chromium content, the pickling method according to the chromium content should be performed differently. In general, in the case of low chromium ferrite steel, the scale is thickly formed during annealing heat treatment, and in the case of high chromium ferrite steel, the scale thickness is reduced during annealing heat treatment.

This scale deteriorates the appearance quality of the product, and furthermore, since corrosion starts at the oxidative scale and causes corrosion resistance, it is necessary to remove the scale formed on the surface through the pickling process.

In general, physical descaling, such as brushing or shot blasting, electrolytic descaling using sodium sulfate, sulfuric acid, or nitric acid electrolytes, salt baths or mixed acids, etc., in order to remove the oxidized scale to obtain beautiful surface quality and to improve corrosion resistance. By using a variety of methods such as chemical descaling or a combination thereof to produce a stainless steel cold rolled steel sheet, this process is called a pickling process.

This pickling process is divided into one-step electrolytic pickling such as electrolytic descaling and two-step mixed acid immersion such as chemical descaling.

In the pickling process of the stainless steel cold rolled steel sheet, in order to secure corrosion resistance by uniformly forming a beautiful surface quality and a passivation film, a pickling process is usually performed using a mixed acid containing 80 to 180 g / L nitric acid and 2 to 40 g / L hydrofluoric acid. Has been performed. Among these, nitric acid lowers the pH in the pickling bath to increase the activity of hydrofluoric acid, and serves to maintain a redox potential suitable for pickling by oxidizing divalent iron ions dissolved at the surface of the steel sheet to trivalent.

However, the nitric acid used in the mixed acid solution generates NOx, an air emission control substance, and contains nitrate nitrogen (NO ₃ -N) from waste acid and washing water. Due to the limitation of NOx concentration, there is a problem that the production cost increases considerably due to the cost of additional installation and operation of environmental pollution prevention facilities in the pickling process.

In order to solve this problem, conventional nitric acid pickling method has been developed in which the nitric acid is replaced with hydrochloric acid or sulfuric acid in the pickling process, and the insufficient oxidizing power is replaced by hydrogen peroxide, potassium permanganate, trivalent iron ion and air injection. come.

U.S. Patent No. 5908511 contains sulfuric acid, hydrofluoric acid, iron salts and hydrogen peroxide regularly, and pickling by adjusting the composition of the wetting agent, the brightening agent, the corrosion inhibitor, etc., and the management of the pickling solution is based on Fe (III) and accordingly ORP ( A technique that takes an automatic control as oxidation-reduction potential is disclosed. Through this, CLEANOX352, a pickling solution, has been commercialized and is the most widely used worldwide. This method is used for wire rods and hot rolled products, but the production cost is over 20% higher than the existing one, and it adopts complex solution composition and management method. In addition, since the pickling reduction rate is decisively relatively low at about 1.5 to 3 g / m 2 · min, it is not suitable for a high speed pickling line in which mixed acid pickling should be completed within 10 to 100 seconds.

In addition, European Patent Publication No. 1040211 and US Patent Publication No. 2000-560982, which are improved patents of the US patent, proposed a method of increasing the pickling rate by adding copper and chlorine ions to the pickling composition, but a ferritic stainless steel sheet If the open circuit potential (OCP) formed on the surface is lower than 0.1 V, which is the redox potential of copper ions, copper particles may be deposited on the surface of the steel sheet during pickling to discolor the steel sheet. There is also a risk of pitting corrosion if the pickling solution contains more than a certain concentration of chlorine ions.

However, when a mixed solution of sulfuric acid and hydrofluoric acid is used, since the oxidizing property is weak, the dissolution rate on the scale is low and blackening may occur on the metal surface during the dissolution process. Therefore, an oxidizing agent is added to reach the strip surface potential having the best pickling property. At this time, the added oxidizing agent generally uses hydrogen peroxide that does not decompose and burden the environment.

However, it is not easy to find an appropriate amount of oxidant to be added, and in some cases, it is not possible to effectively and quantitatively cope with problems such as a result of unpickling or overpickling or blackening of the steel sheet surface. .

The present invention provides a steel sheet pickling method that exhibits excellent steel sheet surface properties by quantifying the surface potential in using a pickling liquid containing no nitric acid in the pickling process.

Furthermore, it is intended to provide a basis for quantitatively confirming the amount of hydrogen peroxide added as an oxidizing agent to be added.

The present invention is a pickling method for removing an oxidized scale by immersing a heat-treated ferritic cold-rolled steel sheet in a neutral salt electrolytic step, sulfuric acid electrolytic step, and then immersed in a mixed acid pickling solution containing sulfuric acid, free hydrofluoric acid, and hydrogen peroxide. , The mixed acid pickling liquid contains 80-150 g / l sulfuric acid, 8-30 g / l free hydrofluoric acid and 3-20 g / l hydrogen peroxide, the initial mixed acid pickling liquid does not contain Fe ^{3 +} ions, The dipping step includes a first dipping step of dipping in a mixed acid pickling solution at 30-40 ° C. and a second dipping step of dipping in a mixed acid pickling solution at 35-45 ° C., and the whole dipping step of the first and second dipping steps. It provides a pickling method of high chromium ferritic stainless steel sheet with a time of 25-72 seconds.

In another embodiment of the present invention, the hydrogen peroxide is represented by the following equation in relation to the iron ion concentration

It provides a pickling method of high chromium ferritic stainless steel cold rolled steel sheet characterized by satisfying the.

In still another embodiment of the present invention, there is provided a method for pickling a high chromium ferritic stainless steel cold rolled steel sheet, wherein the concentration of iron ions in the mixed acid pickling solution is maintained at 40 g / l or less.

Furthermore, as another embodiment of the present invention, there is provided a method for pickling a high chromium ferritic stainless steel cold rolled steel sheet, characterized in that to maintain the surface potential of the steel sheet in the range of -0.2 to 0V in the mixed acid immersion step.

According to one embodiment of the present invention, it is possible to quantitatively determine the pickling capacity according to the surface potential and to easily find the surface potential having excellent pickling ability in the mixed acid pickling liquid having a predetermined composition, thereby efficiently oxidizing the stainless steel sheet. The solution composition can be effectively controlled to obtain a stainless steel sheet having excellent surface characteristics without residue of scale, excellent surface glossiness, and no blacking phenomenon.

Furthermore, according to one embodiment of the present invention, when pickling a ferritic stainless steel cold rolled steel sheet, there is no burden of treatment for waste water treatment and exhaust gas since no nitric acid is used.

FIG. 1 is a graph showing the relationship between hydrogen peroxide concentration and iron ion concentration for obtaining a surface potential of -0.2 V in mixed acid pickling of a ferritic cold rolled steel sheet from the results of Example 2. FIG.

Hereinafter, the present invention will be described in detail.

In general, electrolytic pickling processes, such as neutral salt electrolysis and sulfuric acid electrolysis, are used to remove the oxidized scale present on the surface of a ferritic stainless steel sheet.

In the present invention, the neutral salt electrolysis may be, for example, a solution containing 100-250 g / l sodium sulfate as an electrolyte. This neutral salt electrolysis step can be carried out by immersing the steel plate in a 50 ~ 90 ℃ neutral salt electrolyte solution, and then applying a current density of 10 ~ 30A / dm 2 for 15 to 120 seconds.

On the other hand, the sulfuric acid electrolysis process, for example, may be carried out using a sulfuric acid electrolytic solution containing 50 ~ 150g / l sulfuric acid as an electrolyte, this sulfuric acid electrolytic step is immersed steel plate in a sulfuric acid electrolyte solution of 30 ~ 60 ℃ After that, it can be carried out by adding a current density of 10 ~ 30A / dm 2 for 5 to 50 seconds.

The oxide layer of chromium and iron is removed on the surface of the ferritic stainless steel sheet which has undergone the neutral salt electrolysis and sulfuric acid electrolysis as described above, and only the Si oxide layer remains. The steel sheet is 80-150 g / L sulfuric acid and free hydrofluoric acid 8-. By immersing and passing through a mixed acid tank containing 30 g / L and 3-20 g / L hydrogen peroxide, all oxides except the base material can be removed.

At this time, the hydrofluoric acid is dissociated in the mixed acid solution as shown in Equation (1), and the equilibrium state is changed by H + concentration, that is, acidity, provided by dissociating sulfuric acid as in Equation (2).

^{HF ↔ H + + F - (} 1)

^{_{H 2 SO 4 ↔ HSO 4 -}} + H + ↔ SO 4 2 - + 2H + (2)

In the case of hydrofluoric acid, it has a pickling force in the free HF state, which is not dissociated, and the free hydrofluoric acid penetrates the interface between the Si oxide and the base material to dissolve the Si oxide and Fe. The free hydrofluoric acid dissolves the Si oxide and the base metal on the steel sheet, and combines the dissolved Fe and Si ions with FeF _x ^(3-x) , H ₂ SiF ₆ , and the like to remove them from the surface. For this purpose, the free hydrofluoric acid is preferably present at a concentration in the range of 8-30 g / l in the mixed acid pickling solution. If the concentration is less than 8 g / ℓ, the concentration of free hydrofluoric acid is low, so that the dissolving power for Si is insufficient, which may cause a problem of micro-acid washing on the surface of the steel sheet, when the erosion rate of the base material is higher than 30 g / ℓ It may accelerate and the surface of the steel sheet after the pickling process may become rough.

As mentioned above, the hydrofluoric acid provides a pickling force to remove the Si oxide layer on the surface of the steel sheet, but the effective free hydrofluoric acid concentration needs to be maintained above a certain acidity in the mixed acid solution. Therefore, in the mixed acid solution, sulfuric acid of a certain concentration or more is required to prevent hydrofluoric acid from dissociating. Suitable sulfuric acid concentrations range from 80 to 150 g / l. If the sulfuric acid concentration is less than 80g / ℓ, the effective free hydrofluoric acid concentration is not maintained, so that dissociation of hydrofluoric acid occurs and the pickling force is weakened. Since there is a problem that heat generation occurs and operation is difficult, it is preferable to include sulfuric acid in the concentration in the above range.

On the other hand, in the oxide scale of ferritic stainless steel, Si oxide is present at both the grain surface of the ferritic crystal and the grain boundary between the grain and the grain, and the Si oxide of the grain boundary is present at a deeper position inside the base material. In the case of austenitic stainless steels, the corrosion resistance of the crystal is high and therefore preferentially erodes from the grain boundary, whereas ferritic stainless steel has low corrosion resistance of the crystal, so there is no difference in the rate of erosion between the inside of the crystal and the grain, so that the grain surface and the grain boundary are selective. Completely dissolved without. Therefore, in order to remove all Si oxides, a substantial part of the base material needs to be dissolved.

In this case, the Fe ^{2 +} is eluted from the base material, the eluted Fe ^{2 +} is generated by a complex FeF ^{_{x (3-x),}} in combination with HF after the reaction with the hydrogen peroxide oxidation to Fe ³⁺ is removed from the surface of the steel sheet . The reaction can be expressed as in the following reaction formulas (3) to (6), it is possible to increase the pickling rate only when such a process proceeds smoothly.

^{Fe 0 → Fe 2 + + 2e} - (3)

^{_{Fe 2 + + H 2 O 2}} → Fe 3 + + OH + OH -? (4)

Fe ^{3 +} + 3HF → FeF ₃ + 3H ⁺ (5)

^{Cu 2 + + 2e - → Cu} 0 (6)

At this time, the left direction reaction of the mixed acid acid when the hydrogen peroxide concentration is low in the amount of a tax, the formula (4) the reaction does not occur to the Fe ^{2 +} concentration in the surface of steel sheet increased locally formula (3) it becomes dominant. In this case, as shown in Equation (6), Cu, which is present as an additive or an impurity in Fe and stainless steel, is reprecipitated on the steel sheet surface, resulting in a blackening phenomenon in which the steel sheet surface turns black. Therefore, it is preferable that the residual hydrogen peroxide concentration always be present at a certain concentration or more, and the suitable hydrogen peroxide concentration in the pickling liquid is preferably present in the range of 3-20 g / l.

It was found experimentally that the hydrogen peroxide concentration correlated with the concentration of the iron solution present in the pickling solution, this correlation can be expressed as follows.

At this time, the concentration of the metal ion in the mixed acid pickling solution is preferably controlled to 40 g / l or less. When the concentration of metal ions such as iron ions exceeds 40 g / l, as can be seen from the above formula, the amount of hydrogen peroxide used increases rapidly, resulting in an increase in cost. desirable. For this purpose, for example, a means for circulating the mixed acid pickling solution through a suitable filtration device such as an ion exchange resin filter and then removing the metal ions into the mixed acid bath may be added.

Furthermore, it was found by the experiment of the present inventors to obtain a beautiful ferritic stainless steel cold rolled steel sheet, the scale and the base material of about 3 ~ 4g / ㎡ ~ min should be removed from the mixing tank, 8 ~ to produce a cold rolled steel sheet at high speed It was necessary to secure a pickling rate of about 20 g / m 2 min. Stainless steel has a unique potential-current correlation for each steel grade, and the current generated can be expressed by pickling rate. As a result, the maximum pickling rate can be achieved by adjusting the surface potential.

For ferritic stainless steels, the surface potential is not correlated with the redox potential (ORP) of the solution, but with the temperature of the solution, the concentration of iron ions generated during the pickling process, and most closely the concentration of residual hydrogen peroxide in the pickling solution. I found out to have a relationship.

That is, the hydrogen peroxide acts to raise the surface potential of the strip, and at this time, the dissolution rate of the scale and the base material changes according to the surface potential of the strip. In the case of the pickling solution composition as described above, the surface potential of the strip showing the highest dissolution rate was experimentally determined to be in the range of -0.2 to 0V relative to the red electrode. From such a result, the amount of hydrogen peroxide can be determined under the pickling solution composition as described above, and the temperature and strip potential of the mixed acid bath for preventing surface peroxidation can be determined.

The process of immersing in such mixed acid pickling solution may be performed in two steps. When the mixed acid immersion process is performed by a single process, when the steel sheet for pickling treatment is supplied to the mixed acid bath, a rapid temperature rise of the mixed acid solution due to the reaction heat occurs, thereby actively decomposing hydrogen peroxide in the mixed acid solution. Proceed. Therefore, there is a problem in that the consumption of hydrogen peroxide is increased to increase the required cost of the process. Therefore, it is preferable to carry out the process of immersion in the mixed acid solution in two steps.

At this time, the solution temperature of the first pickling bath is preferably lower than the solution temperature of the second pickling bath. In the first pickling bath, since the oxide layer on the steel sheet is relatively large, the reaction between the hydrogen peroxide and the oxide layer becomes active. Therefore, even if the pickling reaction is performed at a relatively low temperature, a high level of pickling force can be secured. Furthermore, in the second pickling bath, since the oxide layer is relatively small, the pickling force may be weakened when pickling with the pickling solution having the same temperature. Therefore, in the second pickling bath, it is preferable to increase the pickling force by increasing the temperature of the solution with respect to the first pickling bath. As a result, an equal level of pickling force can be obtained in each pickling tank, thereby obtaining a steel sheet of excellent quality.

On the other hand, when the solution temperature of the first mixing tank is higher, the temperature rises due to a rapid reaction, leading to an increase in the consumption of hydrogen peroxide. On the other hand, in the second mixing tank, the reaction is relatively inactive, so that the surface of the steel sheet finally obtained after pickling Oxide may remain due to the acid pickling, and there is a problem of increasing the consumption amount of hydrogen peroxide in comparison with the quality of the steel sheet as a whole.

The pickling liquid temperature in each pickling tank is preferably set in the range of 30-40 ° C for the pickling liquid in the first pickling tank, and the pickling liquid in the second oxygen agent has a range of 35-45 ° C. When the temperature is higher than the above range, the hydrogen peroxide consumption may be increased, and when lower than the above range, sufficient pickling effect may not be obtained.

The immersion time in the pickling liquid may vary depending on the operating conditions, and is not particularly limited, but may be performed for 25 to 72 seconds as a whole, and the operating time in each pickling tank may be the same or different. It is preferable that the immersion time in the first pickling bath is shorter than the immersion time in the second pickling bath in consideration of the rapid reaction in the first pickling bath and the increase in the amount of hydrogen peroxide used.

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

Example

Example  One

In this embodiment, to confirm the effect of sulfuric acid concentration and free hydrofluoric acid on the pickling properties of cold rolled steel sheet, after removing the scale of the outer layer by electrolytic pickling, sulfuric acid and hydrofluoric acid 50 ~ 200g / L The free hydrofluoric acid was changed to 5-30 g / L. Hydrogen peroxide, on the other hand, contained 15 g / L. This pickling solution was maintained at 35 ℃, pickling was performed by immersing the steel plate in the pickling solution for 30 seconds.

The residual amount of scale was observed for the steel sheet obtained by the mixed acid, and the results are shown in Table 1. In Table 1, when the scale remains on the surface of the steel sheet, it is represented by fine pickling and when the surface is severely eroded by overpicking.

Sulfuric acid concentration (g / L) Hydrofluoric Acid Concentration (g / L) Pickling evaluation Comparative Example 1 50 5 Misan tax Comparative Example 2 60 15 Misan tax Comparative Example 3 70 10 Misan tax Comparative Example 4 80 25 Pickling Comparative Example 5 90 15 Pickling Inventory 1 100 10 Pickling Inventory 2 120 10 Pickling Inventory 3 150 15 Pickling Comparative Example 6 180 20 Taxation Comparative Example 7 200 30 Taxation

Example  2

This embodiment is to confirm the relationship between the hydrogen peroxide concentration and the iron ion concentration to obtain a surface potential of -0.2V in the mixed pickling of ferritic cold rolled steel sheet. In order to confirm the effect of the applied potential on the pickling properties of the cold rolled steel sheet, it was evaluated after removing the scale of the outer layer by electrolytic pickling.

The conditions of the mixed acid tank were observed in the scale of the steel sheet obtained after immersing in 100 g / L sulfuric acid, 10 g / L free hydrofluoric acid, 15 g / L hydrogen peroxide, and pickling solution at 35 ° C. for 30 seconds.

The results are shown in Table 2 and FIG. In Table 1, when the scale did not remain on the surface of the steel plate, it was indicated by ○, and when the scale remained, it was evaluated as unpickled and represented by x.

Applied potential (V) Pickling Comparative Example 1 0.6 × Comparative Example 2 0.4 × Comparative Example 3 0.3 × Comparative Example 4 0.2 × Comparative Example 5 0.1 × Inventory 1 0.0 ○ Inventory 2 -0.1 ○ Inventory 3 -0.2 ○ Comparative Example 6 -0.3 × Comparative Example 7 -0.4 ×

On the other hand, the relationship between the minimum hydrogen peroxide concentration according to the iron ion concentration for maintaining the steel surface surface potential from -0.2V or more from the result of Figure 1 can be expressed as follows.

Example  3

The present Example was evaluated using a test piece whose scale of the outer layer was removed by electrolytic pickling in order to observe the effect of metal ion concentration on the decomposition of the fruit water during pickling operation of cold rolled steel sheet.

The conditions of the mixed acid bath were immersed in a pickling solution at 35 ° C. for 30 seconds after varying the concentration of 100 g / L sulfuric acid, 10 g / L free hydrofluoric acid, 15 g / L hydrogen peroxide and metal ion concentration as shown in Table 1, and then hydrogen peroxide was decomposed to half of the initial concentration. The rate at which it is measured was measured.

The results in Table 1 show relative values of 1 as the time taken for the fruit tree to decompose in half when 10 g / L is present.

Metal ion concentration (g / L) Time to Decrease in Half Comparative Example 1 60 0.1 Comparative Example 2 50 0.3 Inventory 1 40 0.7 Inventory 2 30 0.8 Inventory 3 20 0.9 Honorable 4 10 One

As can be seen from Table 3, the hydrogen peroxide was slowly decomposed when the concentration of the metal ion was 40 g / l or less, but the decomposition rate of the fruit water rapidly increased when it exceeded 40 g / l.

Example  4

This Example is to observe the effect on the temperature increase in the mixed acid bath and the hydrogen peroxide consumption when the temperature of the mixed acid tanks 1 and 2 are adjusted during the pickling operation of cold rolled steel sheet. It evaluated using. The conditions of the mixed acid tank were immersed for 30 seconds in a solution adjusted to 100 g / L sulfuric acid, 10 g / L free hydrofluoric acid, 15 g / L hydrogen peroxide and 10 g / L metal ion concentration while changing the temperature of the mixed acid tanks 1 and 2, followed by mixing acid tank 1 The degree of temperature rise at and the total hydrogen peroxide consumption were evaluated.

Mixed acid tank 1 temperature
(℃) Mixed acid tank 2 temperature
(℃) Mixed acid tank 1 temperature rise
(℃) Hydrogen Peroxide Consumption Comparative Example 1 40 30 15 medium Comparative Example 2 40 35 15 medium Comparative Example 3 45 30 20 versus Inventory 1 30 35 3 small Inventory 2 30 40 3 small Inventory 3 35 45 5 small

As can be seen from Table 4, when the temperature of the mixed acid tank 1 is high, as in Comparative Examples 1 to 3, it shows a large temperature rise of the mixed acid tank, and therefore it can be seen that the consumption of hydrogen peroxide is large.

Claims (4)

As a pickling method in which a heat-treated ferritic cold rolled steel sheet is subjected to a neutral salt electrolysis process and a sulfuric acid electrolysis process, and then immersed in a mixed acid pickling solution containing sulfuric acid, free hydrofluoric acid, and hydrogen peroxide to remove an oxidized scale.
The mixed acid tax acid include, but sulfate 80-150g / l, the free hydrofluoric acid 8-30g / l and hydrogen peroxide 3-20g / l, the initial mixed acid acid tax does not contain Fe ^{3 +} ion,
The immersion in the mixed acid pickling solution comprises a first immersion step immersed in a mixed acid pickling liquid at 30-40 ° C. and a second immersion step immersed in a mixed acid pickling liquid at 35-45 ° C., wherein the first and second Pickling method of high chromium ferritic stainless steel cold rolled steel sheet with a total immersion time of 25-72 seconds.
The method of claim 1, wherein the hydrogen peroxide is represented by the following equation in relation to the iron ion concentration

Pickling method of high chromium ferritic stainless steel cold rolled steel sheet characterized by satisfying the.
The method for pickling high-chromium ferritic stainless steel cold rolled steel sheet according to claim 1 or 2, wherein the concentration of iron ions in the mixed acid pickling solution is maintained at 40 g / l or less.
The method of pickling high-chrome ferritic stainless steel cold rolled steel sheet according to claim 1 or 2, wherein the surface potential of the steel sheet is maintained in the range of -0.2 to 0 V in the mixed acid immersion step.

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