KR20180070136A - Method for acid pickling lean duplex stainless steel - Google Patents

Abstract

The present invention provides a pickling method capable of easily removing an oxide of manganese or silicon. The pickling method anneals lean duplex steel comprising 0.02-0.06 wt% of C, 0.55-0.8 wt% of Si, 2.8-3.2 wt% of Mn, 0.2-0.28 wt% of N, 19-21 wt% of Cr, 0.5-1.5 wt% of Ni, 0.3-1.2 wt% of Cu, 0.035 wt% or lower (excluding 0) of P, 0.003 wt% or lower (excluding 0) of S, and the remainder consisting of Fe and inevitable impurities, and then removes annealing scale of the steel. The pickling method comprises: a neutral salt electrolytic pickling step of removing chrome-rich scale on a surface by electrolyte solution using sodium sulfate as an electrolyte; a nitric acid electrolytic pickling step of removing an Mn oxide by the electrolyte solution using nitrogen and nitric acid as the electrolyte; and a mixed acid immersion step of immersing in a mixed acid solution including nitric acid and hydrofluoric acid. The nitric acid electrolytic pickling step is performed during a period of time which is at least 1.3 times and at most 2 times longer than a period of time during which the neutral salt electrolytic pickling step is performed.

Description

{METHOD FOR ACID PICKLING LEAN DUPLEX STAINLESS STEEL}

The steel according to the present invention is characterized in that the steel contains 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, In a pickling process of a linseed duplex stainless steel cold-rolled steel sheet containing 0.3 to 1.2% of Cu, 0.035% or less of P (exclusive of 0), S of 0.003% or less of S (excluding 0), and the balance of Fe and other unavoidable impurities, Mn The present invention relates to a pickling method for efficiently removing oxides.

Cold-rolled stainless steel will go through a heat treatment of 800 ~ 1150 ℃ to obtain the desired mechanical properties of after cold rolling, to react with the hot oxygen within the to the surface of the steel sheet in this thermal treatment the oxide scale on the surface (SiO _2, (Cr, Fe) ₂ O ₃ ) is produced. The oxidized scale on the surface of the steel sheet deteriorates the appearance of the steel sheet and deteriorates the quality of the steel sheet and also causes corrosion of the steel sheet as a starting point which causes corrosion of the steel sheet.

Therefore, in order to obtain a generally good surface quality and to improve corrosion resistance, physical descaling by brushes, shot ball blasting, etc., electrolytic descaling using sodium sulfate, sulfuric acid, nitric acid electrolytes, chemical descaling using a salt bath, Methods are combined to remove the oxide scale on the surface of the steel sheet to produce a stainless steel cold-rolled steel sheet. The process of removing such scaling is called a pickling process.

The pickling process including the neutral salt electrolytic pickling process using sodium sulfate in such a stainless steel pickling process is a process capable of making the surface beautiful by a high-speed environmentally friendly pickling process. In the neutral salt electrolytic pickling process, there is a scale removal mechanism for oxidizing and dissolving the poorly soluble Cr oxide through water-soluble CrO ₄ ^2- through an electrochemical reaction.

On the other hand, stainless steel can be classified into a ferritic stainless steel having a ferrite phase and austenitic stainless steel having an austenite phase depending on the microstructure constituting the steel, and a stainless steel having both of these phases. Among these, the above-mentioned abnormal stainless steels have many merits of ferritic stainless steel and austenitic stainless steel, and are widely used as structural materials requiring strength and corrosion resistance.

In order to stabilize both the austenite phase and the ferrite phase, the abnormal stainless steel contains a large amount of nickel, which is an austenite stabilizing element. In order to stabilize the ferrite phase, alloying elements such as chromium and molybdenum are contained as stabilizing elements have. Due to the large amount of these stabilizing elements, abnormal stainless steels are very expensive. Recently, lean duplex steels have been developed in which these alloying elements are replaced with other elements.

The lean duplex steel is produced by increasing the content of manganese instead of expensive nickel, which is an austenite stabilizing element, and by increasing the content of silicon instead of chromium or molybdenum, which is a ferrite stabilizing element.

However, the components such as manganese and silicon which are added to the lean duplex steel to a large extent are difficult to remove by the pickling process including the neutral salt electrolytic pickling process, and these oxides are not picked up easily.

In addition, the ordinary abnormal stainless steel manufacturing process includes the steps of producing a cast steel through steelmaking and continuous casting, and subjecting the cast steel to hot rolling and hot rolling, cold rolling, cold rolling and annealing. In this process, the austenite phase and the ferrite phase are elongated at an intersection.

Such a structure appears in the form of stripes after the pickling due to the difference in dissolution characteristics between the austenite and ferrite steel in the pickling process, and there is a restriction on the use of such an outer material in which the surface quality is important because of such stripes.

An object of the present invention is to provide a pickling method of lean duplex steel containing high manganese which can easily remove oxides of manganese or silicon and inhibit the formation of stripe on the surface.

More specifically, the present invention relates to a steel comprising: a steel containing 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, : Pickling process of linseed duplex stainless steel cold rolled steel sheet containing 0.5 to 1.5% of Cu, 0.3 to 1.2% of Cu, 0.035% or less of P (excluding 0), S of 0.003% or less (excluding 0), the balance Fe and other unavoidable impurities , It is desired to effectively remove Mn oxide and to provide lean duplex stainless steels having uniform surface quality without stripes on the surface.

The steel according to the present invention is characterized in that the steel contains 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, Annealing scale of lean duplex steel containing 0.3% to 1.2% of Cu, 0.035% or less of P (excluding 0), S of 0.003% or less (excluding 0), the balance of Fe and other unavoidable impurities A neutral salt electrolytic acidic step of electrolyzing a chrome-rich scale on the surface with an electrolytic solution using sodium sulfate as an electrolyte, an electrolytic solution using nitric acid and metal nitrate as an electrolyte, And a mixed acid soaking step of immersing in a mixed acid solution containing nitric acid and hydrofluoric acid. It is preferable that the nitric acid electrolysis pickling step is performed for 1.3 times or more and 2.0 times or less as long as the time for performing the neutral salt electrolysis pickling step.

The electrolytic pickling, including the neutral salt electrolytic pickling step and the nitric acid electrolytic pickling step, is preferably carried out for a time ranging from 70 to 90% of the annealing heat treatment run time.

It is also preferable that the mixed acid soaking step is performed for a time ranging from 35 to 65% of the electrolytic acid pickling time including the neutral salt electrolysis pickling step and the nitric acid electrolysis pickling step.

The neutral salt electrolytic acid may be carried out in a neutral salt electrolyte containing 180 to 250 g / l of a sodium sulfate electrolyte.

The neutral salt electrolyte may have a temperature of 60 to 90 占 폚.

The pH of the neutral salt electrolyte is preferably in the range of 3.5 to 5.5.

The neutral salt electrolytic pickling can be performed by applying a current density in the range of 8 to 10 A / dm 2 to the neutral salt electrolytic solution so that the surface potential of the steel sheet is charged at least once in order of +, -, + .

The nitric acid electrolytic acid may be carried out in a nitric acid electrolytic solution containing 70 to 120 g / l of a nitric acid, a metal nitrate or a nitric acid electrolyte of nitric acid and metal nitrate.

The nitric acid electrolytic solution is preferably maintained at a temperature in the range of 45 to 60 ° C.

The nitric acid electrolytic acid treatment can be performed by applying a current density of 4 to 8 A / dm 2 to the nitric acid electrolytic solution so that the surface potential of the steel sheet is charged at least once in order of +, -, +.

The mixed acid solution may contain 130 to 180 g / l of nitric acid and 25 to 35 g / l of hydrofluoric acid.

The mixed acid solution preferably has a temperature of 45 to 55 캜.

According to one embodiment of the present invention, in the pickling of the high-Mn-containing lean duplex stainless steel sheet, the Mn oxide can be easily removed in the electrolytic pickling area.

It also provides a high-speed annealing pickling process capable of suppressing the generation of surface streaks while completely removing the remaining silicon oxide in the mixed acid immersion.

FIG. 1 is a Fow chart illustrating the steady state of equilibrium according to the potential (E) and acidity (pH) of a steel material component, in which (a) shows an E-pH chart for chromium, (b) E-pH diagram.
2 is a graph showing the results of the GDS analysis of the component in the depth direction of the annealing scale for the annealed, annealed, lean-duplex steel sheet.
3 is a photograph of a TEM section of an annealing scale for a lean-duplex steel sheet annealed in Example 1. Fig.
Fig. 4 is an optical photograph of a surface on which stripes are generated by pickling in Comparative Example 3. Fig.

The steel according to the present invention is characterized in that the steel contains 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, In a pickling process of a linseed duplex stainless steel cold-rolled steel sheet containing 0.3 to 1.2% of Cu, 0.035% or less of P (exclusive of 0), S of 0.003% or less of S (excluding 0), and the balance of Fe and other unavoidable impurities, Mn Thereby providing a means for efficiently removing the oxide. In addition, it provides a surface free from streaking on the surface after pickling.

The pickling process of the present invention includes a neutral salt electrolytic pickling process for electrolyzing a chrome-rich scale on the surface of a steel sheet using an electrolytic solution using sodium sulfate as an electrolyte, and an electrolytic solution using nitric acid and metal nitrate as an electrolyte, And a nitric acid electrolysis step of removing nitric acid and nitric acid, and immersing in a mixed acid solution containing nitric acid and hydrofluoric acid.

In the neutral salt electrolytic pickling step, the annealed lean duplex stainless steel sheet is immersed in a neutral salt electrolyte containing the sodium sulfate as an electrolyte and a current is applied to remove the chrome-rich scale on the surface of the steel sheet.

The sodium sulfate electrolyte is preferably contained in an amount of 180 to 250 g / l. When the content of the sodium sulfate electrolyte is less than 180 g / ℓ, there is a problem that the electric conductivity is lowered and the charging efficiency is lowered. When the content is more than 250 g / ℓ, there is a problem that the precipitation of sodium sulphate is costly. .

It is preferable that the neutral salt electrolytic acid pickling process is performed by maintaining the temperature of the neutral salt electrolytic solution within the range of 60 to 90 ° C. When the temperature of the neutral salt electrolytic solution is less than 60 ° C, there is a problem that the electric conductivity is lowered and the charging efficiency is lowered. When the temperature exceeds 90 ° C, the heat exchanger for heating the solution must be increased in size, It is preferable to carry out the neutral salt electrolytic pickling process at the above-mentioned range.

In the neutral salt electrolytic pickling process, the pH of the neutral salt electrolytic solution is preferably in the range of 3.5 to 5.5. If the pH of the neutral salt electrolyte is less than 3.5, the pH may drop sharply and electrolytic unevenness may occur. Also, since the solubility of the metal ion is increased, the metal may be electrodeposited on the surface during current application and yellowing may occur. And a large amount of sludge, which makes it difficult to manage the solution.

A lean duplex steel sheet is immersed in a neutral salt electrolyte solution under the above conditions and a chromium-rich scale on the surface of the steel sheet is removed by applying a predetermined current density. At this time, the current density applied to the electrolyte is preferably such that the potential of the surface of the steel sheet is charged at least once in the order of +, -, +, and a current density in the range of 8 to 10 A / dm 2 is preferably applied. If the current density applied to the neutral salt electrolyte is less than 8 A / dm 2, there is a problem that scale removal is difficult. When the current density exceeds 10 A / dm 2, there is a problem that the surface due to dissolution of the base material is damaged.

And a nitric acid electrolysis process for removing Mn oxide with an electrolytic solution using the nitric acid or metal nitrate as an electrolyte. In the nitric acid electrolysis process, the steel sheet on which the chromium rich scale is removed by the neutral salt electrolytic pickling process is immersed in the nitric acid electrolytic solution containing the nitric acid and the metal nitrate as electrolytes and the manganese oxide on the surface of the steel sheet is removed by applying an electric current .

It is preferable that the nitric acid electrolyte is contained in the nitric acid electrolyte in an amount of 70 to 120 g / l. When the content of the nitric acid electrolyte is less than 70 g / l, the electric conductivity is lowered and the charging efficiency is lowered. The higher the nitric acid concentration, the higher the descaling performance. However, when the nitric acid concentration exceeds 120 g / The surface of the stainless steel may be damaged, and it is preferable that the above range is satisfied.

The nitric acid electrolysis process is preferably performed by maintaining the temperature of the electrolytic solution in the range of 45 to 60 ° C. When the temperature of the nitric acid electrolytic solution is less than 45 ° C, there is a problem that the dissolution rate of the scale is lowered. When the temperature exceeds 60 ° C, the generation of NOx gas is increased. . ≪ / RTI >

The lean duplex steel sheet is immersed in the nitric acid electrolyte under the above conditions and a predetermined current density is applied to remove the chrome-rich scale on the surface of the steel sheet. At this time, the current density applied to the electrolyte is preferably such that the potential of the surface of the steel sheet is charged at least once in order of +, -, +, and a current density in the range of 4 to 8 A / dm 2 is preferably applied. If the current density applied to the nitric acid electrolytic solution is less than 4 A / dm 2, manganese scale removal is difficult. If the current density exceeds 8 A / dm 2, the surface dissolution amount becomes large and the surface may be damaged.

As shown in Fig. 1 (a), the Cr oxide is normally oxidized and dissolved in CrO ₄ ^{2 -} ions at a pH of 4 to 6 at a neutral salt of 0.6 V or higher relative to a reference electrode of SHE (standard hydrogen electrode). However, for the Mn oxide, the above-mentioned pH region is a stable region, and for dissolving the Mn oxide, 1.3 V or more is required. Therefore, a relatively large amount of energy is consumed to dissolve the Mn oxide compared with the dissolution of the Cr oxide.

Particularly, Cr ₂ O ₃ existing in the annealing scale is directly dissolved in the ions as shown in the following formulas (1) and (2), while in the case of Mn ₂ O ₃ , It is also disadvantageous in terms of speed since it is dissolved in MnO ₄ ^{2 -} after being oxidized with MnO ₂ .

Cr ₂ O ₃ + 5H ₂ O → 2CrO ₄ ^{2 -} + 8H ⁺ + 6e ^- (1)

Cr ₂ O ₃ + 4H ₂ O? Cr ₂ O ₇ ^{2 -} + 8H ⁺ + 6e ^- (2)

Mn ₂ O ₃ + H ₂ O → 2MnO ₂ + 2H ⁺ + 2e ^- (3)

MnO ₂ + 2H ₂ O ^- > MnO ₄ ^{2 -} + 4H ⁺ + 2e ^- (4)

In particular, in the case of lean duplex steel containing a high content of manganese, Mn oxide is present at the outermost angle in the annealing scale depth direction during annealing, so that it is difficult to remove Si oxide at the lower interface unless the Mn oxide is effectively removed.

Therefore, as shown in Fig. 1, in the case of the high-Mn-containing lean duplex steel, rather than dissolving in a neutral salt electrolytic pickling bath solution capable of easily dissolving Cr oxide, it dissolves in an acid solution capable of easily dissolving Mn oxide It is necessary to carry out the electrolytic pickling process as much as possible.

In the conventional neutral salt electrolytic pickling process, the immersion time in the neutral salt electrolytic pickling process is longer than the immersion time in the nitric acid electrolytic pickling process, and it is difficult to obtain a good pickling effect of the high-manganese duplex steel. Therefore, in order to facilitate the pickling process of the high manganese lean duplex steel, it is preferable that the nitrate electrolytic immersion time is longer than the immersion time of neutral salt electrolytic pickling.

More specifically, it is preferable to carry out the electrolytic pickling process for 1.3 times or more by immersing the electrolytic pickling bath in the nitric acid electrolytic pickling bath in comparison with the time of immersing the electrolytic pickling bath in the neutral salt electrolytic pickling bath. The manganese oxide can be sufficiently removed and the Si oxide can be effectively removed by the subsequent mixed acid immersion.

More preferably, the nitric acid electrolytic pickling process is performed at 1.3 times or more and 2.0 times or less as compared with the neutral salt electrolytic pickling process. The reason for setting the upper limit is that the longer the nitric acid electrolytic pickling time is, the more electrolytic polishing effect is exhibited on the base material, and the surface of the base material may be damaged.

From this relationship, it can be understood that it is desirable to design the nitric acid electrolytic acid pickling process so that the time of the nitric acid electrolytic pickling process is longer than that of the neutral salt electrolytic pickling process in constructing the electrolytic pickling apparatus in which the electrolytic pickling process is performed. Therefore, for this purpose, for example, it is preferable to design the length of the nitric acid electrolysis pickling bath longer than the length of the neutral salt electrolysis pickling bath.

It is preferable that the length of the nitric acid electrolytic acid pickling bath is designed to be more than 1.3 times longer than the time during which the steel sheet is immersed in the neutral salt electrolytic pickling bath, and the specific length is passed through an electrolytic pickling process including the nitric acid electrolytic pickling and neutral salt electrolytic pickling And is not particularly limited as long as it depends on the moving speed of the steel sheet.

Furthermore, when the neutral salt electrolytic acid and the nitric acid electrolytic acid are combined to form an electrolytic pickling process, the electrolytic pickling process is preferably performed for a time ranging from 70 to 90% of the annealing heat treatment time. If the electrolytic pickling time is less than 70% of the total heat treatment time, the annealing scale can not be sufficiently removed. If the electrolytic pickling time exceeds 90%, surface damage due to pickling can be caused.

Therefore, the electrolytic pickling time is set within the above range based on the annealing heat treatment time, and then the total electrolytic pickling time is designed so that the nitric acid electrolytic pickling time is in the range of 1.3 to 2 times the neutral pickling time It is possible to efficiently remove the manganese oxide produced by the annealing heat treatment existing on the surface of the linseed duplex cold rolled steel sheet.

With respect to the steel sheet subjected to the pickling process by electrolysis of neutral salts and electrolysis of nitric acid A step of immersing in a mixed acid solution containing nitric acid and hydrofluoric acid to remove the oxide remaining after the last step like Si oxide.

On the other hand, even if the outer layer scale is fully dissolved in the electrolytic pickling process, the Si oxide is not easily removed at the interface, and is an insoluble oxide. Therefore, a mixed acid soaking process is performed to remove such Si oxide. The mixed impregnation process may be carried out in a mixed acid solution containing nitric acid and hydrofluoric acid.

Especially, in the case of lean duplex steel in which the austenite phase and the ferrite phase are distinguished, the Si oxide is mainly formed in the austenite phase along the grain boundary, and the ferrite phase is formed in the surface of the grain. Therefore, in the pickling step, the austenite phase is dissolved in the grain boundary phase and the ferrite phase is dissolved in the grain phase. Particularly, since the structure is elongated while hot rolling and cold rolling the slab made by the continuous casting process, streaks may occur due to the dissolution shape difference between the austenite phase and the ferrite phase.

Therefore, in order to minimize the difference in the amount of phase-to-phase solubility and to suppress the occurrence of streaks, dissolution of the base material should be minimized in mixed acid immersion. In order to achieve this, it is necessary to increase the nitric acid content of the mixed acid solution to 130 g / L or more in order to increase the oxidizing power, since the oxidizing power of the solution is large and the scale is removed and the passive film is well formed on the surface. However, if the amount of nitric acid is too large, surface nitrification may occur, so that the content of nitric acid is preferably limited to 180 g / l.

Further, it is preferable that the mixed acid solution has a sufficiently high free hydrofluoric acid concentration to remove Si oxide. Therefore, the free hydrofluoric acid concentration in the mixed acid solution is preferably 25 g / L or more. However, if the free hydrofluoric acid concentration is too high, the surface dissolution becomes severe and streaks may occur. Therefore, it is preferable that the free hydrofluoric acid concentration does not exceed 35 g / l.

It is preferable that the mixed acid-immersion is performed under the condition that the temperature of the mixed acid solution is 45 to 55 ° C. If the temperature is low, miscarriage may occur. If the temperature is too high, the amount of surface dissolution may become large and streaks may occur.

The time for carrying out the mixed impregnation process can be appropriately performed in consideration of the time of the electrolytic pickling process performed previously. Specifically, it is preferably carried out for a time ranging from 35 to 65% of the electrolytic acid pickling time including the neutral salt electrolysis pickling step and the nitric acid electrolysis pickling step.

In the case of carrying out in the above range, the Si oxide can be removed without any problem of scum and / or pickling, while suppressing occurrence of streaks after the pickling process, and thus a lean duplex cold rolled steel sheet having good pickling quality can be secured.

In the case of carrying out the mixed impregnation process under such conditions, it is possible to suppress the generation of streaks appearing after the pickling due to the difference in dissolution characteristics of the austenite and ferrite steels in the lean duplex stainless steel. Therefore, Can be used.

Example

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are illustrative of the present invention and are not intended to limit the present invention.

Example  One

0.3% of Si, 0.7% of Mn, 3.0% of Mn, 0.25% of N, 20% of Cr, 1.0% of Ni, 0.7% of Cu, 0.7% of P, 0.035% or less (excluding 0), S: 0.003% or less (excluding 0), the balance Fe and other unavoidable impurities were heat-treated at 1060 캜 for 80 seconds.

The component in the depth direction of the annealing scale of the lean duplex cold rolled steel sheet formed by the heat treatment was analyzed by GDS, and the TEM sectional shape was photographed.

GDS analysis results and TEM cross-sectional photographs are shown in Figs. 2 and 3, respectively.

2 and 3, the annealing scale had a thickness of 120 to 230 nm, and the annealing scale outer angle contained Mn in an amount of 60% by weight and Cr in an annealing scale interface side in an amount of 40%.

In order to remove the annealing scale on the surface of the lean duplex cold rolled steel sheet, a neutral salt electrolytic pickling process and a nitric acid electrolytic pickling process were carried out.

The neutral salt electrolytic acid was performed by applying 10 A / dm 2 to a 200 g / l sodium sulfate solution at pH 4 and 80 ° C. The nitric acid electrolysis was performed by applying a current of 6 A / dm ² to a 100 g / Respectively.

The total electrolytic time was fixed to 64 seconds, which is 80% of the annealing time, and the time of the neutral salt electrolytic pickling and the nitric acid electrolytic pickling were changed while changing the time of the neutral salt electrolytic pickling process and the nitric acid electrolytic pickling process, The ratio was changed.

The content of Mn components remaining on the surface of the steel sheet after the electrolytic pickling step was analyzed, and the results are also shown in Table 1.

division Electrolytic process time (sec) Electrolytic pickling process time ratio
(Nitric acid electrolysis / neutral salt electrolysis) Surface residual Mn (%) Neutral salt electrolysis Nitric acid electrolysis Comparative Example 1 64 0 0 20 Comparative Example 2 43 21 0.5 17 Comparative Example 3 38 26 0.7 15 Comparative Example 4 32 32 One 7.5 Comparative Example 5 29 35 1.2 5 Inventory 1 27 35 1.3 3.2 Inventory 2 27 37 1.4 3.2 Inventory 3 23 41 1.8 3.0 Honorable 4 21 43 2 3.0 Inventory 5 18 46 2.5 3.0

As can be seen from Table 1, when the nitric acid electrolytic acid pickling time was 1.2 or less as compared with the neutral salt electrolytic pickling time, the residual Mn content remained to be 5% or more in a large amount as in Comparative Examples 1 to 5.

However, as in Examples 1 to 5, when the time ratio of the electrolytic pickling process was 1.3 or more, it was 3.2% or less, which was lowered to the Mn content of the base metal.

Therefore, it is preferable that the nitric acid electrolytic pickling process is performed longer than the neutral salt electrolytic pickling process when the high Mn steel is pickled in the pickling process composed of the neutral salt-nitric acid electrolysis process.

Example  2

The electrolytically pickled linseed duplex cold-rolled steel sheet obtained in Inventive Example 2 of Example 1 was immersed in a mixed acid solution to carry out acidic pickling.

The mixed acid solution and the mixed acid immersion condition were performed under the conditions shown in Table 2 below, and the mixed acid immersion time was 32 seconds.

After the steel plate was immersed in the mixed acid solution, the surface of the steel plate was observed with a microscope, and the results of the pickling were evaluated as follows.

- If the Si oxide is identified:

- If there is no residual Si oxide but streaks occur: 'Stripe'

- no residual oxides and streaks: 'normal'

division Solution concentration (g / l) Mixed acid immersion temperature
(° C) Results of the pickling nitric acid Foshan Comparative Example 1 100 15 50 Misanthus tax Comparative Example 2 100 20 45 Misanthus tax Comparative Example 3 100 25 50 stripe Comparative Example 4 100 30 45 stripe Comparative Example 5 130 20 45 Misanthus tax Inventory 1 130 25 55 normal Comparative Example 6 150 15 50 Misanthus tax Inventory 2 150 25 50 normal Inventory 3 150 30 45 normal Comparative Example 7 180 20 45 Misanthus tax Honorable 4 180 25 45 normal Inventory 5 180 35 45 normal Comparative Example 8 200 25 60 stripe Comparative Example 9 200 40 50 stripe Comparative Example 10 200 40 45 stripe

As can be seen from the inventive Examples 1 to 5 of Table 2, the mixed acid immersion process was carried out in a mixed acid solution having a temperature range of 45 to 55 ° C and a concentration of 130 to 180 g / l of nitric acid and 25 to 35 g / , It was found that there was no fine lines and no streaks.

On the other hand, in the case of Comparative Examples 1 and 2, the concentrations of nitric acid and hydrofluoric acid were all low, and the results were inferior.

However, as in Comparative Examples 3 and 4, when the concentration of nitric acid was low but the concentration of hydrofluoric acid was sufficiently high, pickling was observed, but it was found that stripes were generated due to lack of oxidizing power. Fig. 4 shows the surface optical photograph after the pickling obtained in Comparative Example 3, and it can be seen from Fig. 4 that a stripe pattern appears.

On the other hand, in the case of Comparative Examples 8 to 10, although the oxidizing power was sufficient, it could be confirmed that the temperature of the mixed acid solution was too high or the stripes appear again when the hydrofluoric acid concentration was excessive.

From the above results, it is necessary to have a sufficient hydrofluoric acid concentration to dissolve Si oxide and to sufficiently oxidize the solution so that the austenite phase and the ferrite phase can be uniformly dissolved. To provide such oxidizing power, It should be contained at a concentration of 180 g / l.

Example  3

Annealing heat treatment was performed under the same temperature conditions as in Example 1 using the same lean duplex cold rolled steel sheet as in Example 1. [ At this time, annealing heat treatment time was changed as shown in Table 1 below.

Thereafter, an electrolytic pickling process of neutral salt electrolytic pickling and nitric acid electrolytic pickling was carried out.

The electrolytic pickling of the neutral salt electrolytic pickling and nitric acid electrolytic pickling was carried out under the same conditions as in Inventive Example 2 of Example 1. However, the process time of the electrolytic pickling was changed as shown in Table 3, and the execution time of the nitric acid electrolytic pickling process was set at 1.4 times the execution time of the neutral salt electrolytic pickling process. The ratio of the time of electrolytic pickling to the time of annealing in this case is shown in Table 1.

Then, mixed acid pickling by mixed acid immersion was performed. As the mixed acid solution of the mixed acid pickling, a solution having the same conditions as in Inventive Example 3 of Example 2 was used. At this time, the mixed acid soaking time was changed as shown in Table 1 below, and in this case, the ratio of mixed acid soak time to electrolytic time is shown in Table 3.

After the pickling process of the annealed steel sheet, the pickling property was evaluated by observing the surface of the steel sheet. The results are also shown in Table 3.

The acidity was evaluated by observing the surface of the steel sheet after the pickling process with the naked eye and showing the case where there was no pickle and pickle. The results are shown in Table 3 together with the case of pickle or pickle.

division Process time (sec) Time ratio Time ratio Pickling castle Annealing Electrolytic sunshine Mixed impregnation Electrolytic pickling / annealing Mixed immersion / electrolytic pickling Inventory 1 60 48 28.8 0.80 0.60 ○ Comparative Example 1 60 48 80 0.80 1.67 × Comparative Example 2 60 56 42 0.93 0.75 × Comparative Example 3 60 60 33.6 1.00 0.56 × Inventory 2 80 60 39 0.75 0.65 ○ Comparative Example 4 80 56 45 0.70 0.80 × Comparative Example 5 80 72 22 0.90 0.31 × Inventory 3 100 95 42 0.90 0.44 ○ Honorable 4 100 75 45 0.75 0.60 ○ Comparative Example 6 100 60 25.6 0.60 0.43 × Inventory 5 140 98 34 0.70 0.35 ○ Comparative Example 7 140 126 40 0.90 0.32 × Comparative Example 8 140 64 25.2 0.46 0.39 ×

Claims (12)

The steel according to claim 1, wherein the steel contains 0.02 to 0.06% of C, 0.55 to 0.8% of Si, 2.8 to 3.2% of Mn, 0.2 to 0.28% of N, 19.0 to 21.0% of Cr, 0.5 to 1.5% of Ni, : A pickling method for removing an annealing scale of a steel after annealing a lean duplex steel containing 0.3 to 1.2%, P: not more than 0.035% (excluding 0), S: not more than 0.003% (excluding 0), the balance Fe and other unavoidable impurities As a result,
A neutral salt electrolytic acid pickling step for electrolytically removing chromium-rich scale on the surface with an electrolytic solution using sodium sulfate as an electrolyte;
A nitric acid electrolytic acid pickling step of removing Mn oxide with an electrolytic solution using nitric acid and metal nitrate as an electrolyte; And
Nitric acid, and hydrofluoric acid,
Wherein the nitric acid electrolysis pickling step is carried out for a time of 1.3 times or more and 2.0 times or less as compared with the time of performing the neutral salt electrolytic pickling step.
The pickling method of annealed annealed lean duplex steel according to claim 1, wherein the electrolytic pickling treatment including the neutral salt electrolytic pickling step and the nitric acid electrolytic pickling step is performed for a time ranging from 70 to 90% of the annealing heat treatment performing time.
The pickling method of the annealed annealed lean duplex steel according to claim 1, wherein the mixed acid-immersion step is performed for a time ranging from 35 to 65% of the electrolytic pickling performance time including the neutral salt electrolytic pickling step and the nitric acid electrolytic pickling step .
The pickling method according to claim 1, wherein said neutral salt electrolytic pickling is carried out in a neutral salt electrolytic solution containing 180 to 250 g / l of a sodium sulfate electrolyte.
5. The pickling method according to claim 4, wherein the neutral salt electrolyte has a temperature of 60 to 90 占 폚.
5. The pickling method of a lean duplex steel according to claim 4, wherein said neutral salt electrolyte has a pH in the range of 3.5 to 5.5.
The method of claim 4, wherein the neutral salt electrolytic pickling is performed by applying a current density in the range of 8 to 10 A / dm 2 to the neutral salt electrolyte so that the surface potential of the steel sheet is charged at least once in order of +, -, + In the annealed annealed lean duplex steel.
The pickling method according to claim 1, wherein the nitric acid electrolytic pickling is carried out in a nitric acid electrolytic solution containing nitric acid, metal nitrate, or a nitric acid electrolyte of nitric acid and metal nitrate in an amount of 70 to 120 g / l.
The pickling method according to claim 8, wherein the nitric acid electrolytic solution is carried out while maintaining the temperature of the electrolytic solution in the range of 45 to 60 ° C.
The method according to claim 8, wherein the nitric acid electrolytic acid treatment is carried out by applying a current density of 4 to 8 A / dm 2 to the nitric acid electrolytic solution so that the surface potential of the steel sheet is charged at least once in order of +, -, + Lt; RTI ID = 0.0 > annealed < / RTI > annealed duplex steel.
The pickling method of a lean duplex steel according to claim 1, wherein the mixed acid solution contains 130 to 180 g / l of nitric acid and 25 to 35 g / l of hydrofluoric acid.
12. The pickling method of a lean duplex steel according to claim 11, wherein the mixed acid solution has a temperature of 45 to 55 占 폚.

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