KR20150053625A - Method for annealing-pickling ferritic stainless steel having high silicon content - Google Patents

Abstract

The present invention relates to a method for annealing and pickling a ferrite-based cold-rolled stainless steel sheet with a high silicon content comprising: an annealing step of thermally treating a ferrite-based cold-rolled stainless steel sheet composed of 13-15 wt% of Cr, 0.8-1.0 wt% of Si, 0.1-0.5 wt% of Nb, 0.1-0.3 wt% of Ti, 0.3-1.0 wt% of Mn, and the rest of Fe and other impurities at 950°C or higher but less than 1030°C of a target annealing temperature for 0-120 seconds of crack retention time; an electrolytic acid pickling step of electrolytic acid pickling the annealed ferrite-based cold-rolled stainless steel sheet with neutral salt and sulfuric acid; and a mixed-pickling step of dipping the electrolytic acid pickled ferrite-based cold-rolled stainless steel sheet in a mixed acid solution. The present invention can manufacture the ferrite-based cold-rolled stainless steel sheet with high quality by controlling the thickness of an Si oxide layer in accordance to the annealing conditions, and easily pickling the ferrite-based cold rolled stainless steel sheet even though the content of Si is high.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a ferritic stainless steel having high Si content,

The present invention relates to a method of annealing and pickling a hard Si ferrite stainless steel having a high Si content.

Generally, stainless steel cold-rolled steel sheet is subjected to heat treatment at 800 to 1150 ° C to obtain a predetermined mechanical property after cold rolling. In this heat treatment process, oxidized scale (SiO ₂ , (Cr, Fe) ₂ O ₃ ) is generated on the surface of the steel sheet. The oxide scale on the surface of the steel sheet not only deteriorates the appearance of the product but also causes corrosion of the steel sheet, Thereby deteriorating the quality of the steel sheet. In order to solve such problems, various methods such as physical descaling by brushes, shot ball blasting and the like, electrolytic descaling using sodium sulfate, sulfuric acid, nitric acid electrolyte, chemical descaling using a salt bath, Is removed.

In the case of neutral salt electrolytic pickling using sodium sulfate as an electrolyte in the stainless steel pickling process, the poorly soluble Cr oxide is oxidized to water ^- soluble CrO ₄ ^{2 -} by electrochemical reaction to dissolve Friendly high-speed pickling process. However, in the case of a stainless steel cold-rolled steel sheet used for an automobile exhaust system, the amount of silicon oxide in the oxide scale is increased to increase the Si content in order to improve the heat resistance and the oxidation resistance characteristic, so that the electrochemical removal is impossible.

As a result, in the case of a high-silicon-content ferritic stainless steel cold-rolled steel sheet, the scale is removed by immersing it in a mixed acidic solution consisting of nitric acid and hydrofluoric acid after passing through a neutral salt electrolytic pickling process. In this case, the acidity is enhanced by increasing the application current, concentration and temperature of the neutral salt electrolytic acid pickling process. When the acidic acidity is strengthened, the dissolution rate of the base material is increased and the base material is damaged. Also, in the mixed acid deposition process, since the concentration and temperature of hydrofluoric acid are increased to remove the silicon oxide, the surface can be seriously overcharged and the amount of NO _x gas, which is a harmful environmental substance, is increased.

Prior art related to a method of pickling a silicon-containing steel sheet is International Patent Application No. PCT / JP2010 / 059167 'METHOD FOR PICKLING STEEL PLATES AND PICKLING DEVICE'. The above patent provides a method of acid pickling of a steel sheet which can effectively remove Si annealing scale of a steel sheet by applying ultrasonic waves of two or more frequencies in a frequency range of 28.0 kHz to less than 1.0 MHz.

On the other hand, Korean Patent No. 1239473 discloses a method of pickling an acidic solution and a stainless steel in which a ferric stainless steel is immersed in an acidic solution to which hydrogen peroxide is added to a mixed acid solution of nitric acid and hydrofluoric acid. The surface potential of the ferritic stainless steel -0.2 to -0.3 V. The present invention also provides a pickling method comprising the steps of: Until recently, however, there has been no known technology for improving the pickling efficiency by controlling the generation of silicon oxide in the annealing process of a hard stainless steel ferritic stainless steel containing 0.5% or more of Si.

In the case of a ferritic stainless steel cold-rolled steel sheet, it is very important to remove the oxide scale, which is inevitably generated in annealing. This is because the properties such as gloss of the surface and corrosion resistance of the steel sheet can be changed by scaling. However, it is very difficult to effectively remove the oxide scale while minimizing the influence on the base metal. In particular, the technology for easily removing silicon oxide is currently applied to ferrite steels having a low Si content.

Accordingly, the present invention relates to a hot-rolled stainless steel cold-rolled steel sheet having a Si content of 0.8 to 1.0%, wherein the surface grain size of the steel sheet during annealing is controlled so as to satisfy the material of the steel sheet, Layer is prevented from being formed to a thickness exceeding a certain thickness, thereby providing a method of annealing and pickling which can more effectively remove Si oxide.

The inventors of the present invention have recognized that ferrite stainless steel cold-rolled steel sheets containing Si in a high content have a high Si oxide scale in the annealing process, and thus have a high product failure rate and a high acidity. . As a result, it was confirmed that the pickling was easy because the thickness of the Si oxide layer can be limited to 80 nm or less while maintaining the surface grain size of the steel sheet at 8 or less, and controlling the target temperature and time of the heat treatment for crack- And completed the present invention.

An embodiment of the present invention is a steel sheet comprising 13-15% by weight of Cr, 0.8-1.0% by weight of Si, 0.1-0.5% by weight of Nb, 0.1-0.3% by weight of Ti, 0.3-1.0% Annealing the ferritic stainless steel cold-rolled steel sheet containing other impurities by heat treatment at a temperature of 950 DEG C or higher and an annealing target temperature of lower than 1030 DEG C for a crack holding time of 0-120 seconds; An electrolytic pickling step of subjecting the annealed heat-treated ferritic stainless steel cold-rolled steel sheet to neutral salt electrolytic pickling and sulfuric acid electrolytic pickling; And a step of immersing the electrolytically pickled ferritic stainless steel cold-rolled steel sheet in a mixed acid solution, wherein the annealing target temperature and the crack holding time in the annealing heat treatment step are controlled so as to satisfy the following formulas 1 and 2: Containing ferritic stainless steel cold-rolled steel sheet.

Y? 0.011667X ² - 24.3167X + 12668 (Equation 1)

Y? -2.15X + 2220.42 (Equation 2)

In the above formulas 1 and 2, X = annealing target temperature (占 폚) and Y = crack holding time (second).

At this time, it is preferable that the surface grain size of the ferritic stainless steel cold rolled steel sheet subjected to the annealing treatment is 8 or less (excluding 0) and the thickness of the Si oxide layer as the annealing scale is 80 nm or less (excluding 0).

On the other hand, the neutral salt electrolytic acid pickling is performed by applying a current of 9-15 A / dm ² , and the sulfuric acid electrolytic pickling is preferably performed by applying a current of 9-20 A / dm ² .

The mixed acid solution contains 70-100 g / L of sulfuric acid, 5-10 g / L of hydrofluoric acid and 6-10 g / L of hydrogen peroxide, and the hydrolysis acid quantity can be performed by maintaining the strip surface potential at -0.1 to 0.05 V have.

It is possible to provide an annealing and pickling method capable of effectively removing Si oxide by controlling the thickness of the Si oxide layer of a hard-sintered ferritic stainless steel cold-rolled steel sheet having an Si content of 0.8-1.0% after controlling it in the annealing step. In particular, since the annealing and pickling method of the present invention minimizes damage to the base material and can easily remove the Si oxide layer while satisfying the surface grain size condition, the ferrite stainless steel cold rolled steel sheet for automobile exhaust system of high quality, Can be applied to manufacture.

1 is a component analysis graph of an annealing scale measured after a high Si-content ferritic stainless steel cold-rolled steel sheet was subjected to crack holding treatment at 1010 캜 for 20 seconds.
2 is a graph showing an optimum target annealing temperature and a crack holding time region in annealing.
3 is a photograph showing the cross-sectional phenomenon of the base material according to electrolytic pickling conditions.
4 is a graph showing chromaticity and surface potential change according to hydrogen peroxide content.

According to an embodiment of the present invention, there is provided a method of manufacturing a steel sheet, comprising: 13-15 wt% of Cr; 0.8-1.0 wt% of Si; 0.1-0.5 wt% of Nb; 0.1-0.3 wt% And other impurities at a target annealing temperature of 950 DEG C or more and less than 1030 DEG C for a holding time of 0 to 120 seconds, and a step of annealing the annealed ferritic stainless steel cold rolled steel sheet for neutral salt electrolysis There is provided an annealing and pickling method of a high Si-content ferritic stainless steel cold rolled steel sheet comprising an electrolytic pickling step of pickling and sulphate electrolytic pickling, and a step of hydrolyzing the electrolytically pickled ferritic stainless steel cold rolled steel sheet in a mixed acid solution .

According to a preferred embodiment of the present invention, the ferritic stainless steel cold-rolled steel sheet of the present invention is preferably used for an automobile exhaust system. In the case of an automobile exhaust system, it is required to increase the Si content because heat resistance and oxidation resistance characteristics should be excellent. However, when the Si content is usually 0.5% by weight or more, a layer of Si oxide which is unavoidably generated in the annealing step is formed to be very thick, which is electrochemically stable and is very difficult to pick up. In particular, the thicker the Si oxide layer, the greater the pickling time and the amount of hydrofluoric acid used. Nevertheless, the present invention is meaningful in that it provides a method of easily annealing and pickling a high Si-content ferritic stainless steel cold-rolled steel sheet containing 0.8-1.0 wt% Si.

In this specification, the term 'annealing and pickling' refers to a continuous annealing process in which annealing for natural cooling after annealing and pickling for removing an oxide scale generated by annealing are continuously performed to improve the mechanical properties of cold-rolled stainless steel cold- A series of steel plate processing methods can be referred to collectively. In particular, during the annealing heat treatment, the temperature is gradually raised from room temperature to reach a certain temperature, and then the temperature is maintained for a predetermined time. The predetermined temperature reached at this time is referred to as an 'annealing target temperature' Maintenance time '.

According to a preferred embodiment of the present invention, the annealing target temperature is 950 ° C or more and less than 1030 ° C, and the crack holding time may be 0-120 seconds. Generally, the annealing target temperature may be different depending on the type of steel. In the case of the high Si-content ferritic cold-rolled steel sheet of the present invention, the temperature at which the material can be secured within the process time ranges from 950 ° C to 1030 ° C or 1030 ° C. It is also desirable to limit the crack holding time to 120 seconds in consideration of the minimum speed at which the strip passes in the annealing process. If the crack holding time exceeds 120 seconds, the passing speed is low and the control can not be easily carried out, so it may not be possible to put the plate. Particularly, since the annealing process is continuous with the pickling process, it is important to allocate the time considering the whole process. At this time, if cooling is performed as soon as the annealing target temperature is reached, it is preferable to regard the crack holding time as zero.

Generally, when the surface grain size of the steel sheet exceeds 8, the elongation of the steel sheet becomes poor and the workability is degraded. Further, when the thickness of the Si oxide layer generated through the annealing heat treatment exceeds 80 nm, the thickness of the oxide layer becomes too thick, which is not easy to pick up. However, as the temperature or time increases within the annealing target temperature and the crack holding time range, the possibility that the surface grain size is 8 or less is increased, but the possibility that the thickness of the Si oxide layer is 80 nm or less is low. Therefore, it is important to control appropriately within the temperature and time range.

Therefore, according to a more preferred embodiment of the present invention, the annealing target temperature and the crack holding time can be controlled so as to satisfy the following formulas 1 and 2 within the range of 950 캜 to less than 1030 캜 and 0 to 120 seconds, respectively.

Y? 0.011667X ² - 24.3167X + 12668 (Equation 1)

Y? -2.15X + 2220.42 (Equation 2)

In the above formulas 1 and 2, X = annealing target temperature (占 폚) and Y = crack holding time (second).

The surface grain size of the steel sheet and the thickness of the Si oxide layer are controlled to be 8 or less and 80 nm or less if the annealing target temperature and the crack holding time are controlled so as to satisfy the above formulas 1 and 2 within the range of 950 ° C to less than 1030 ° C and 0 to 120 seconds At the same time, it can be satisfied. In Equation (1), Equation (1) is a relational expression between the annealing target temperature and the crack holding time, in which the surface grain size of the annealed steel sheet is 8 or less, and Equation 2 shows that the annealing target temperature and crack And the holding time.

According to a preferred embodiment of the present invention, the electrolytic pickling agent includes a neutral salt electrolytic pickling agent and a sulfuric acid electrolytic pickling agent. In the case of electrolytic pickling, it is a pickling process for removing the oxide scale present on the surface of the high-Si-content ferritic stainless steel cold-rolled steel sheet. According to a preferred aspect of the invention, the neutral salt electrolytic pickling is performed by applying a current of 9-15A / dm ^2, and the sulfuric acid electrolytic pickling may be carried out by applying a current of 9-20A / dm ^2.

In the case of neutral salt electrolytic pickling, the steel sheet is preferably immersed in a neutral salt electrolytic solution at 50-90 DEG C, preferably at 50-90 DEG C, more preferably 200-250 g / L of sodium sulfate electrolyte, By applying a current density of 9-15 A / dm < ² > for 30-80 seconds so that the potential of the surface can be charged at least once in the order of +, -, +. The applied amount of current is 9 A / dm ² is less than when not formed, the surface potential as it can dissolve the scale, and the annealing is a problem in that the effect of the neutral salt electrolytic pickling incomplete, 15 A / dm ² There is a possibility that the base material in the lower part of the scale is damaged. Therefore, the amount of applied current is 9-15 A / dm ² .

On the other hand, the sulfuric acid electrolytic acid pickling treatment is preferably carried out using a sulfuric acid electrolytic solution at 40-55 ° C, more preferably 50-150 g / L sulfuric acid as an electrolyte and additionally containing at least one of iron, chromium, nickel, copper, manganese, It can be carried out using an electrolytic solution at 40-55 ° C. Most preferably, the sulfuric acid electrolysis step is carried out by immersing a steel sheet in an electrolytic solution, and then applying a current density of 9-20 A / dm ² so that the surface potential of the steel sheet can be charged at least once in the order of +, -, + It can be done by going for 15-40 seconds. When the current density is less than 9 A / dm ² , the surface forming potential is lowered during electrolytic pickling, which adversely affects the surface quality. When current exceeding 20 A / dm ² is applied, It may cause damage to the base material. Therefore, the applied current density of sulfuric acid electrolytic pickling is 9-20 A / dm ² .

The neutral salt electrolysis and the sulfuric acid electrolysis process are preferably carried out in a continuous process. Particularly, when the applied current density in the neutral salt electrolysis process is out of the above range, it may lead to damage of the ash or base material irrespective of the applied current density in the sulfuric acid electrolytic process, and even if the applied current density in the neutral salt electrolytic process is in the appropriate range, It is preferable that the neutral salt electrolysis applied current density and the sulfuric acid electrolysis applied current density do not deviate from the above range because the applied current density of the process does not exceed the above range.

On the surface of the ferritic stainless steel sheet subjected to the neutral salt electrolysis and sulfuric acid electrolysis step, the chromium and iron oxide layers are removed and only the Si oxide layer is left. The hydrolysis acidity is to remove all the oxides except the base metal Is a pickling process. The mixed acid solution contains 70-100 g / L sulfuric acid, 5-10 g / L hydrofluoric acid, and 6-10 g / L residual hydrogen peroxide in the solution, and the temperature may be 33-40 ° C. According to a more preferred embodiment, the hydrolysis acid may be carried out while maintaining the surface potential of the strip at -0.1 to 0.05 V.

The strip surface potential can be pickled even in the range of -0.2 to 1.0 V. If the surface potential of the strip is less than -0.2 V, the dissolution rate may be low, resulting in poor acidity or surface irregularity, and if the strip potential is higher than 0.1 V, the dissolution rate may not be appropriate. However, in order to improve the surface quality, it is more preferable to maintain the surface potential at -0.1 to 0.05 V.

At this point it is important to include 6-10 g / L hydrogen peroxide in the mixed acid solution. As can be seen from FIG. 4, the preferable surface potential can be formed when the content of hydrogen peroxide is 6-10 g / L. In particular, when the number of hydrogen peroxide is 6 g / L or more, the surface color difference reaches 14 level. Acceptability can be confirmed by the surface color difference value. Generally, the color difference value is 14 or less. That is, it is possible that the content of hydrogen peroxide is 6 g / L or more in order to form a preferable range of pickling and surface potential, but it is preferable not to exceed 10 g / L because excessive residual hydrogen peroxide causes an increase in cost .

Example

Hereinafter, the present invention will be described in more detail with reference to Examples. These examples are for the purpose of illustrating the present invention more specifically, and the present invention is not limited thereto.

Experimental Example  One. Annealing  Particle size and Pickling castle  evaluation

13 of the same ferritic stainless steel cold-rolled steel sheet, the contents of which were 14 wt% of Cr, 0.9 wt% of Si, 0.15 wt% of Ti, 0.45 wt% of Nb and the balance of Fe and other impurities, 13, and then annealed. At this time, the annealing temperature and the crack holding time were changed as shown in Table 1, and the surface particle size of the steel sheet and the thickness of the Si oxide layer were measured by the annealing treatment. The results are shown in Table 1 below.

No. Annealing target temperature (캜) Crack holding time (sec) Granularity Si annealed oxide layer thickness (nm) Steel plate 1 950 100 6 59 Steel plate 2 950 50 9.3 51 Steel plate 3 970 60 7.8 63 Steel plate 4 970 30 8.5 56 Steel plate 5 990 70 6 78 Steel plate 6 990 50 6.5 73 Steel plate 7 990 20 7.6 65 Steel plate 8 1010 50 6.0 87 Steel plate 9 1010 20 7.0 80 Steel plate 10 1010 10 7.5 74 Steel plate 11 1030 30 5.5 96 Steel plate 12 1030 10 6.0 90 Steel plate 13 1030 0 6.2 86

As a result of the annealing, the surface grain size of the steel sheet tends to decrease as the annealing target temperature and the crack holding time increase. On the other hand, the Si oxide layer thickness tends to increase as the annealing target temperature and the crack holding time increase. At this time, in the case of steel sheets 2 and 4, the surface grain size of the steel sheet exceeded 8. Normally, when the surface grain size exceeded 8, the elongation was lowered, so that it was judged that the steel sheets 2 and 4 had poor quality.

Then, pickling was carried out on the remaining steel except for the steel plates 2 and 4 which showed poor grain quality. Acid pickling was treated with neutral salt electrolytic acid, sulfuric acid electrolytic acid, and mixed acid. At this time, the neutral salt electrolytic acid was carried out in a pickling solution at 80 ° C containing sodium sulfate 200 / L for 85 seconds under an applied current of 9 A / dm ² , and the sulfuric acid electrolytic pickling treatment was carried out in a pickling solution containing 45 g / Under an applied current of 15 A / dm < ² > for 35 seconds. The hydrolysis was carried out by immersing in a mixed acid solution at 40 DEG C containing 60 g / L of sulfuric acid, 10 g / L of hydrofluoric acid and 6 g / L of hydrogen peroxide for 60 seconds. The pickling results are shown in Table 2 below.

Annealing target temperature (캜) Crack holding time (sec) Si annealed oxide thickness (nm) Pickling or without ¹⁾ Steel plate 1 950 100 59 ○ Steel plate 3 970 60 63 ○ Steel plate 5 990 70 78 ○ Steel plate 6 990 50 73 ○ Steel plate 7 990 20 65 ○ Steel plate 8 1010 50 87 X Steel plate 9 1010 20 80 ○ Steel plate 10 1010 10 74 ○ Steel plate 11 1030 30 96 X Steel plate 12 1030 10 90 X Steel plate 13 1030 0.1 86 X

1) ○: Full pickles, X: Missan tax

As a result of the pickling, it was confirmed that the steel plates 8 and 11 to 13 exhibited a weak acidity. This was a common result when the Si oxide thickness exceeded 80 nm. That is, it is preferable to control the Si oxide layer thickness to 80 nm or less when the high-Si-content ferritic stainless steel cold rolled steel sheet is annealed and pickled.

As a result of the comprehensive analysis of the annealing and pickling results, it was found that complete pickling of the Si oxide was easy when the annealing target temperature and the crack holding time were constant. In order to grasp more clearly, a graph was drawn based on the above data. As a result, the graph shown in FIG. 2 was obtained. The graph of the annealing temperature and the cracking time Is the same as the graph area of Fig. 2 below. The following equations (1) and (3) can be derived through the region shown in the graph of FIG.

Y? 0.011667X ² - 24.3167X + 12668 ... Equation 1

Y? -2.15X + 2220.42      ... Equation 2

950? X <1030, 0? Y? 120      ... Equation 3

In the above formulas 1 to 3, X = annealing target temperature (占 폚) and Y = crack holding time (second).

Experimental Example  2. Electrolytic  Optimal test

In the steel sheet annealed in Experimental Example 1, a steel sheet 9 having a Si thickness of 80 nm was selected to analyze the surface annealing scale component. As a result, it was confirmed that oxides such as Cr, Mn and Si were formed on the surface of the steel sheet (see Fig. 1). Therefore, electrolytic pickling of Cr, Mn, and Fe scale before the oxidation of Si oxide was required.

In the electrolytic pickling, neutral hydrochloric acid and sulfuric acid pickling were continuously carried out. By changing the current condition, it was decided to understand the pickling conditions that minimize the whiteness and the damage of the parent material in the electrolytic pickling of the high Si containing ferritic stainless steel cold rolled steel sheet. Sodium sulfate 200g / L, pH 80 ° C, and current application time 85 seconds were set under the same conditions. Sulfuric acid was set at 80g / L, temperature 45 ^° C, and current application time 35 seconds. The whiteness of the surface of the sample subjected to electrolytic pickling by changing the electric current under the above conditions and whether or not the parent material was damaged are respectively observed, and the results are shown in Table 3.

division Neutral salt current (A / dm ² ) Sulfuric acid current (A / dm ² ) Whiteness Whether the base material is damaged ^{1 )} Current condition 1 21 15 65.93 ○ Current condition 2 21 10 63.77 ○ Current condition 3 20 30 57.66 ○ Current condition 4 20 12 68.04 ○ Current condition 5 20 20 66.01 ○ Current condition 6 15 25 59.64 ○ Current condition 7 15 5 70.71 △ Current condition 8 15 7 67.98 △ Current condition 9 15 9 68.45 × Current condition 10 15 20 69.89 × Current condition 11 10 25 59.68 ○ Current condition 12 9 15 73.01 × Current condition 13 8 15 56.01 ●

1) ∘: occurrence of base material damage, △: surface roughness, x: good, ⊚:

(Current condition 2), a sample in which surface roughness was observed (current condition 7), and a sample picked up in good condition (current condition 12) in the electrolytic pickled sample according to Experimental Example 2 were selected Sectional images were taken. As a result, the photograph shown in FIG. 3 was obtained. FIG. 3 (a) shows a case where the base material is damaged, (b) shows surface roughness caused by damage to the base material and sulfuric acid electrolysis, .

As a result of the experiment, it was found that the damage of the base material through the current conditions 1 to 5 and 13 was greatly influenced by the amount of neutral salt current application than the amount of application of sulfuric acid. That is, if the current value of the neutral salt electrolytic cell exceeds 15 A / dm ² , the base material damage occurs (current condition 1 to 5) regardless of the current of the sulfuric acid electrolytic cell, and if the neutral salt electrolytic cell current value is less than 9 A / dm ^2, (Current condition 13).

On the other hand, if the current of the neutral salt electrolytic cell is 9 A / dm ² or more and 15 A / dm ² (Current conditions 6 to 12), the whiteness and the presence or absence of the substrate material were different depending on the applied current value of the sulfuric acid electrolytic cell. That is, the current of the neutral salt electrolytic bath is 9 to 15 A / dm ² If the sulfuric acid current in the range exceeds 20 A / dm ² , the substrate damage will occur (current conditions 6 and 11) and 9 A / dm ² , Surface roughness occurred (current conditions 7 and 8). In conclusion, in order to obtain a good pickling surface by electrolytic pickling, it is found that both the applied current condition of the neutral salt electrolytic pickling process and the applied current condition of the sulfuric acid electrolytic process must be satisfied at the same time.

Experimental Example  3. Monsoon  Optimal test

According to the current condition 12 of Experimental Example 2, optimum conditions of the blending and acidification according to the applied electric potential were obtained for all the taxed samples. The experiment was carried out by artificially applying electric potential for 60 seconds in a mixed acid solution of 40 g / L containing 80 g / L of sulfuric acid and 10 g / L of hydrofluoric acid to observe the surface morphology. The results according to the applied potential are shown in Table 4 below.

division The applied potential (V) Whether or not pickling Surface quality ^{1 )} Dislocation condition 1 -0.4 Misanthus tax × Dislocation Condition 2 -0.35 Misanthus tax × Dislocation Condition 3 -0.3 And pickles × Dislocation Condition 4 -0.25 And pickles × Dislocation Condition 5 -0.2 Pickle △ Dislocation Condition 6 -0.15 Pickle △ Dislocation Condition 7 -0.1 Pickle ○ Dislocation condition 8 -0.05 Pickle ○ Potential condition 9 0 Pickle ○ Potential condition 10 0.05 Pickle ○ Potential condition 11 0.1 Pickle △ Dislocation condition 12 0.15 Misanthus tax × Potential condition 13 0.2 Misanthus tax × Potential condition 14 0.3 Misanthus tax ×

1)?: Good surface quality,?: Non-uniform surface quality, x: Decreased surface quality

As can be seen from the above Table 4, when the surface formation potential is as low as -0.35 V or lower or as high as 0.15 V or higher (dislocation conditions 1, 2, 12, 13 and 14) On the other hand, when the surface potential was in the range of -0.3 to -0.25 V (potential conditions 3 and 4), the dissolution rate was too high to cause over-acidification.

However, when the surface potential was formed in the range of -0.2 to 0.1 V (dislocation conditions 5 to 11), it was found that the pickling was normally possible. However, in the case of dislocation conditions 5, 6, and 11, surface irregularity occurs. Therefore, it is preferable to control the surface formation potential in the range of -0.1 to 0.05 V in order to improve the surface quality 10).

Experimental Example  4. Test for optimal content of hydrogen peroxide

According to the results of Experimental Example 3, the surface potential and the surface color difference value according to the residual hydrogen peroxide content were confirmed in order to confirm the amount of residual hydrogen peroxide suitable for forming the surface potential of 0.001-0.5 V. In the case of the color difference value, it is an index indicating pickling property. Generally, when the surface color difference value is 14 or less, it can be regarded as pickling. Results according to Experimental Example 4 were as shown in Table 5 and FIG.

H ₂ O ₂ (g / L) Color difference (dE) Surface potential (V) 0 32 -0.48 2 22.5 -0.46 4 17 -0.3 5 15 -0.14 6 14.5 -0.06 8 14.2 -0.01 10 14.3 -0.001 12 14 0

As shown in Table 5 and FIG. 4, the surface potential increases depending on the residual and the amount of water, and the dislocation does not change much even if the residual hydrogen peroxide content is increased when the constant potential is reached. Also, it can be seen that the surface chrominance value does not deviate significantly from the 14 level above the constant potential.

As a result of the above Experimental Examples 3 and 4, in order to improve the surface quality, the surface potential should be set to -0.1 V or higher, and at least 6 g / L of hydrogen peroxide is required. However, excessive residual hydrogen peroxide causes an increase in the cost of the pickling process, and therefore it has been confirmed that it is desirable not to exceed the residual water content of 10 g / L.

Claims (6)

A ferritic stainless steel containing 13-15 wt% of Cr, 0.8-1.0 wt% of Si, 0.1-0.5 wt% of Nb, 0.1-0.3 wt% of Ti, 0.3-1.0 wt% of Mn and the balance iron and other impurities Annealing the stainless steel cold-rolled steel sheet at a temperature of 950 ° C or more and less than 1030 ° C for a holding time of 0 to 120 seconds at an annealing target temperature;
An electrolytic pickling step of subjecting the annealed heat-treated ferritic stainless steel cold-rolled steel sheet to neutral salt electrolytic pickling and sulfuric acid electrolytic pickling; And
A step of immersing the electrolytically pickled ferritic stainless steel cold-rolled steel sheet in a mixed acid solution;
Wherein the annealing target temperature and the crack holding time of the annealing heat treatment step are controlled so as to satisfy the following formulas (1) and (2).
Y? 0.011667X ² - 24.3167X + 12668 (Equation 1)
Y? -2.15X + 2220.42 (Equation 2)
In the above equations (1) and (2), X = annealing target temperature (占 폚), Y =
The method according to claim 1,
Wherein the annealing heat-treated ferritic stainless steel cold-rolled steel sheet has a surface particle size of 8 or less (excluding 0).
The method according to claim 1,
Wherein the annealing heat-treated ferritic stainless steel cold-rolled steel sheet has a Si oxide layer thickness of 80 nm or less (excluding 0).
The method according to claim 1,
The neutral salt electrolytic acid pickling is performed by applying a current of 9-15 A / dm ² ,
Wherein the sulfuric acid electrolytic pickling is performed by applying a current of 9-20 A / dm &lt; ² &gt; to the annealing / pickling method of a high-Si-content ferritic stainless steel cold rolled steel sheet.
The method according to claim 1,
Wherein the mixed acid solution contains 70-100 g / L of sulfuric acid, 5-10 g / L of hydrofluoric acid and 6-10 g / L of hydrogen peroxide, and a method of annealing and pickling a ferritic stainless steel cold-rolled steel sheet containing high Si.
6. The method of claim 5,
Wherein the hydrolysis acid is carried out by maintaining the surface surface potential of the strip at -0.1 to 0.05 V. 2. A method for annealing and pickling a ferritic stainless steel cold-

Images