KR20130014909A - Pickling agent and pickling process of stainless steel - Google Patents

Abstract

PURPOSE: A pickling solution and pickling method of stainless steel is provided to easily manufacturing ferritic stainless steel including high content Si and Cr and to have excellent surface roughness and glossiness. CONSTITUTION: A pickling method of ferritic stainless steel consists of a preprocess stage of ferritic stainless steel(S1), a pickling solution preparation stage preparing a pickling solution by adding hydrogen peroxide into a mixed acid solution including nitric acid and hydrofluoric acid(S2) and a pickling condition maintenance stage of maintaining a surface potential of ferritic stainless steel to be -0.2 to -0.3 V compared to a calomel electrode and temperature of the pickling solution to be 30-50°C(S3). The weight of the ferritic stainless steel includes Si greater than 0.4% and Cr greater than 16% in a weight% unit. The preprocess features electro-descaling or salt bath of ferritic stainless steel. [Reference numerals] (AA) Ferritic stainless steel; (S1) Preprocess stage of ferritic stainless steel; (S2) Pickling solution preparation stage; (S3) Pickling condition maintenance stage

Description

Pickling method and pickling process of stainless steel {pickling agent and pickling process of stainless steel}

The present invention relates to a pickling method and a pickling liquid of a ferritic stainless steel, and more particularly, a pickling method that can easily produce a ferritic stainless steel containing a high content of Si and Cr, and excellent surface roughness and gloss It relates to the amount of pickling that can be pickled to have.

Ferritic stainless steel is classified into low chromium ferritic stainless steel and high chromium ferritic stainless steel according to the chromium content. The chromium content of 11-14% is called low chromium ferritic stainless steel, and the chromium content of 16-14% is called high chromium ferritic stainless steel. The ferritic stainless steel is subjected to annealing heat treatment for the purpose of softening the material after cold rolling, in the process, an oxide scale is formed on the surface of the ferritic stainless steel, the scale deteriorates the appearance quality of the product, Corrosion starts at the oxidation scale, which reduces the corrosion resistance. Therefore, the ferritic stainless steel in which the oxide scale is formed is subjected to a step of removing the oxide scale formed on the surface through a pickling process.

In general, physical descaling, such as brushing or shot blasting, electrolytic descaling using sodium sulfate, sulfuric acid, or nitric acid electrolytes, and salt baths or mixed acids, in order to remove the scale of oxide to obtain a beautiful surface quality and to improve corrosion resistance. Chemical descaling, etc. may be used. A combination of these methods is used to remove the scale of oxidation of ferritic stainless steel, which is called a pickling process. On the other hand, with a silicon content of 0.4% or more, in the case of steel grades containing high content of Si, the content of silicon oxide is increased in the oxide scale generated in the annealing process. Such silicon oxide may not be removed electrochemically using an electrolytic pickling process, and may be removed through a chemical pickling process by immersion in nitric acid and hydrofluoric acid.

The pickling process is divided into one step pretreatment pickling to remove the external oxidation scale such as electrolytic descaling or salt bath, and two step mixing immersion step to remove silicon oxide by a mixture of nitric acid and hydrofluoric acid. In the pickling process of the ferritic stainless steel, the pickling process has been performed by using a mixed acid immersion process including nitric acid and hydrofluoric acid in order to secure corrosion resistance by forming a beautiful surface quality and a passivation film evenly.

On the other hand, for steel grades containing high content of Si and Cr, high concentration of hydrofluoric acid should be used to remove silicon oxide because of high corrosion resistance of the steel, and in this case, the surface may be unevenly eroded and thus glossiness after pickling process Surface quality, such as this falls.

An object of the present invention devised to solve the above problems is to provide a pickling method capable of manufacturing a ferritic stainless steel with improved surface quality.

In addition, another object of the present invention is to provide a pickling method capable of easily manufacturing a ferritic stainless steel containing a high content of Si and Cr.

Further, another object of the present invention is to provide a pickling liquid for pickling ferritic stainless steel to have excellent surface roughness and glossiness.

According to a feature of the present invention for achieving the object as described above, the present invention is a pickling of ferritic stainless steel containing a high content of Si and Cr containing at least 0.4% by weight Si, 16% Cr: In the process, pre-treatment of ferritic stainless steel; Pickling solution preparation step of preparing a pickling solution by adding hydrogen peroxide to the mixed acid solution consisting of nitric acid and hydrofluoric acid; And immersing the ferritic stainless steel in the pickling solution, wherein the surface potential of the ferritic stainless steel is -0.2 to -0.3V relative to the deep red electrode, and the pickling condition is maintained at 30-50 ° C; It includes a method of pickling ferritic stainless steel containing a high content of Si and Cr containing.

The pretreatment step may include electrolytic descaling or salt bath of the ferritic stainless steel.

In the pickling liquid preparation step, the pickling liquid may include 50-100 g / l nitric acid, 10-30 g / l hydrofluoric acid, and 4-7 g / l hydrogen peroxide.

In addition, according to another feature of the present invention, a pickling liquid for pickling ferritic stainless steel containing a high content of Si and Cr, including Si: 0.4% or more and Cr: 16% or more, by weight%, Pickling liquid for ferritic stainless steel containing a high content of Si and Cr, characterized in that it comprises 100g / l, 10-30g / l hydrofluoric acid and 4-7g / l hydrogen peroxide.

The pickling solution may be maintained at a temperature of 30-50 ℃ while pickling ferritic stainless steel.

According to the present invention as described above, it is possible to provide a pickling method for producing a ferritic stainless steel with improved surface quality.

In addition, according to the present invention, it is possible to provide a pickling method which can easily manufacture a ferritic stainless steel containing a high content of Si and Cr.

According to the present invention, it is possible to provide a pickling liquid for pickling ferritic stainless steel so as to have excellent surface roughness and glossiness.

1 is a flow chart of a method for pickling ferritic stainless steel containing high contents of Si and Cr according to a preferred embodiment of the present invention.
2a to 2c are SEM images of the surface of the ferritic stainless steel pickled according to Table 1;
Figure 3 is a graph showing the gloss of the roughness difference according to the grain crystal orientation of the comparative example and Example according to Table 1.

Specific details of other embodiments are included in the detailed description and the drawings.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments described below, but may be embodied in various forms. In the following description, it is assumed that a part is connected to another part, But also includes a case in which other elements are electrically connected to each other in the middle thereof. In the drawings, parts not relating to the present invention are omitted for clarity of description, and like parts are denoted by the same reference numerals throughout the specification.

Hereinafter, the present invention will be described with reference to the accompanying drawings.

1 is a flow chart of a method for pickling ferritic stainless steel containing high contents of Si and Cr according to a preferred embodiment of the present invention.

According to a preferred embodiment of the present invention, a method for pickling ferritic stainless steels containing high content of Si and Cr contains a high content of Si and Cr including at least 0.4% of Si and at least 16% of Cr. In the pickling process of the ferritic stainless steel, the pre-treatment step (S1) of the ferritic stainless steel; Pickling solution preparation step of preparing a pickling solution by adding hydrogen peroxide to the mixed acid solution consisting of nitric acid and hydrofluoric acid (S2); And immersing ferritic stainless steel in the pickling solution, wherein the surface potential of the ferritic stainless steel is -0.2 to -0.3V relative to the magenta electrode, and the pickling condition is maintained at 30-50 ° C. S3); In addition, the pickling liquid for ferritic stainless steel containing a high content of Si and Cr is a ferritic stainless steel containing a high content of Si and Cr containing at least 0.4% by weight Si and 16% Cr. The pickling liquid to be pickled may include 50-100 g / l nitric acid, 10-30 g / l hydrofluoric acid and 4-7 g / l hydrogen peroxide.

The pretreatment step S1 may electrolytic descale or salt bath ferritic stainless steel. In the pretreatment step (S1), electrolytic descaling was performed by immersing ferritic stainless steel in a 50-90 ° C. neutral salt electrolyte containing 100-250 g / L sodium sulfate, and then setting the current density to 100-200A / dm ² . By holding for -120 seconds. In addition, the salt bath may be performed by immersing ferritic stainless steel for 1-10 seconds in a molten salt such as 450-500 ℃ NaNO ₃ , NaOH.

The external oxidation scale of the ferritic stainless steel may be removed by the pretreatment step (S1). Such a pretreatment step (S1) is a step of roughly removing the external oxidation scale of the oxide scale formed on the surface of the ferritic stainless steel, which has undergone an annealing process, and can process a large amount in a short time. The pretreatment step (S1) may be preceded in order to efficiently carry out pickling of the ferritic stainless steel in which the thickness of the oxide scale is formed to be thick and the surface quality needs to be finely controlled. Pickling of the ferritic stainless steel can be used to remove the scale of oxidation as well as to control the surface quality of the ferritic stainless steel. Therefore, in order to have an excellent surface quality of ferritic stainless steel, the fermentation stainless steel must be finely pickled. When the oxide scale is removed in this manner, the pickling time is increased and the production cost is increased. . Therefore, the external oxidation scale in the oxidation scale of the ferritic stainless steel is roughly removed through the pretreatment step (S1), thereby finely removing the internal oxidation scale in the subsequent pickling process to produce a ferritic stainless steel having excellent surface quality. Can shorten the pickling time and improve the production efficiency.

In the pickling solution preparation step (S2), the pickling solution may be prepared by adding hydrogen peroxide to a mixed acid solution consisting of nitric acid and hydrofluoric acid, wherein the concentrations of nitric acid, hydrofluoric acid and hydrogen peroxide are 50-100 g / L nitric acid and 10-30 g hydrofluoric acid. / l and 4-7 g / l hydrogen peroxide.

The hydrofluoric acid may be included in a concentration of 10-30g / L, for example, the hydrofluoric acid is preferably in a free hydrofluoric state. The hydrofluoric acid has a pickling force in the undissociated free hydrofluoric acid (Free HF) state, and thus dissolves the silicon oxide, and further, penetrates the interface between the silicon oxide and the ferritic stainless steel to dissolve iron. The resulting iron and silicon ions may be removed from the surface of the ferritic stainless steel in the form of FeF _{x (3-x)} , H ₂ SiF ₆ and the like. Therefore, it is preferable that the said hydrofluoric acid is free hydrofluoric acid.

When the concentration of hydrofluoric acid is less than 10 g / L, the concentration of free hydrofluoric acid is low, so that the melting ability of the ferritic stainless steel containing high contents Cr and Si having high corrosion resistance is insufficient, and thus, the surface of the ferritic stainless steel Mistake problems can occur. In addition, when the concentration of the hydrofluoric acid exceeds 30g / l, the erosion rate of the ferritic stainless steel is increased, it is difficult to finely control the pickling degree of the oxide scale, the surface of the ferritic stainless steel may be rough.

The concentration of nitric acid is preferably 50-100g / l. As described above, the hydrofluoric acid provides a pickling force for removing silicon oxide on the surface of the waste-light stainless steel, so that the effective free hydrofluoric acid concentration is maintained in the pickling liquid above a certain acidity. Therefore, the pickling solution requires nitric acid more than a certain concentration in order not to dissociate the hydrofluoric acid, the concentration of nitric acid may be 50-100g / ℓ. If the concentration of nitric acid is less than 50g / ℓ because the effective free hydrofluoric acid concentration is not maintained, the dissociation of the hydrofluoric acid occurs and the pickling force is weakened, there may be a problem of the acid pickling, if the concentration exceeds 100g / ℓ caused, for example by an ion and hydrogen peroxide, such as HNO ₃ ⁺ reaction because the consumption of hydrogen peroxide by severe, the concentration of nitric acid is preferably maintained in 50-100g / ℓ.

The concentration of hydrogen peroxide is preferably 4-7 g / l. In the mixed acid solution consisting of nitric acid and hydrofluoric acid, the surface potential of the immersed ferritic stainless steel is preferably maintained in a predetermined range, but in order to maintain this, an oxidizing agent is required. As the oxidizing agent, hydrogen peroxide, potassium permanganate, or the like can be generally used. However, in the case of potassium permanganate, hydrogen peroxide is suitable because it generates surface coloring of ferritic stainless steel or by-products of water and oxygen. Hydrogen peroxide increases the surface potential of the ferritic stainless steel to reduce the pickling rate difference for each crystal orientation. When the concentration of hydrogen peroxide is less than 4 g / l, the surface potential is not sufficiently increased, and therefore, the pickling rate difference for each crystal orientation of the surface of the ferritic stainless steel still exists so that the roughness difference cannot be reduced. In addition, when the concentration of the hydrogen peroxide is greater than 7g / ℓ, by excessively increasing the surface potential of the ferritic stainless steel, pickling speed for each crystal orientation is reversed, resulting in a roughness difference, thereby reducing the surface quality.

In the case of ferritic stainless steel containing high Cr and Si, ferritic stainless steel which has undergone an annealing process may contain a large amount of chromium oxide or silicon oxide in an oxide scale, which is difficult to remove with a conventional pickling solution and is also removed. If possible, the surface quality of stainless steel can be made poor. Particularly, in the case of silicon oxide, a high concentration of hydrofluoric acid is used. When the concentration of hydrofluoric acid in the pickling solution is high, the pickling rate is different depending on the grain orientation of the ferritic stainless steel. As such, the difference in pickling speed according to the grain crystal orientation may cause the roughness of the surface, which causes surface irregularity of the ferritic stainless steel after pickling. For example, when pickling ferritic stainless steel with a pickling solution composed of nitric acid / fluoric acid, the {1,0,0} plane of the ferritic stainless steel is first eroded, and {1,1,1}, {1, It can be pickled about 0.5-1.5μm more than the plane, causing surface roughness differences.

The pickling solution prepared in the pickling solution preparation step (S2) according to the present invention may be pickled so that the ferritic stainless steel containing a high content of Cr and Si has a uniform surface. The pickling solution has a constant pickling speed according to the grain crystal orientation of the ferritic stainless steel, so that the surface roughness does not occur. In addition, since the degree of removal of the oxidized scale can be finely controlled, the surface quality of the ferritic stainless steel can be improved.

Pickling condition maintaining step (S3) is immersed ferritic stainless steel in the pickling solution, the surface potential of the ferritic stainless steel is -0.2 to -0.3V compared to the red electrode, the temperature of the pickling solution is 30-50 ℃ And maintaining.

On the surface of the ferritic stainless steel, which has undergone the pretreatment step (S1), chromium oxide and iron oxide, which occupy most of the oxide scale, are removed, but silicon oxide remains. At this time, the silicon oxide can be removed by going through the pickling condition maintenance step (S3) of immersing the ferritic stainless steel in the pickling solution prepared in the pickling solution preparation step (S2). At this time, the temperature of the pickling solution is maintained at 30-50 ℃, the surface potential of the ferritic stainless steel may be -0.2 to -0.3V compared to the deep red electrode.

When the temperature of the pickling liquid is less than 30 ° C., the movement of ions in the pickling liquid is not active, and the pickling rate is lowered, thereby reducing the pickling efficiency. You will not be able to remove the scale. In addition, when the temperature of the pickling liquid is more than 50 ° C., the acid constituting the pickling liquid is a strong acid, which may cause safety problems. Therefore, it may be desirable to maintain the temperature of the pickling solution at 30-50 ° C.

In addition, the surface potential of the ferritic stainless steel immersed in the pickling solution may be -0.2 to -0.3V compared to the deep red electrode. The concentration of metal ions in the pickling solution is preferably maintained at 0-50 g / l.

Oxidation scale pickling rate in the ferritic stainless steel is different depending on the crystal orientation, wherein the difference in the pickling rate has been proved through experiments that can be affected by the surface potential. As described above, the rate difference pickled by crystal orientation by hydrofluoric acid used to remove silicon oxide can reduce the rate difference by increasing the surface potential, for example, by increasing the surface potential by adding hydrogen peroxide. .

That is, when the surface potential is less than -0.2V compared with the deep red electrode, the pickling speed difference by crystal orientation due to the hydrofluoric acid cannot be sufficiently compensated, and there is still a pickling speed difference by grain orientation, which causes a roughness difference by grain crystal orientation. Can be generated. In addition, when the surface potential is greater than -0.3 V compared to the deep red electrode, the amount of hydrogen peroxide is increased to relatively reduce the concentration of hydrofluoric acid, thereby lowering the removal ability of silicon oxide. That is, when the surface potential is -0.2 to -0.3V compared to the deep red electrode by adding hydrogen peroxide, the pickling rate difference for each crystal orientation can be minimized. Therefore, the roughness difference for grain crystal orientation is within 0.2μm, and has a uniform surface. Ferritic stainless steels containing contents Cr and Si can be obtained.

Hereinafter, examples and comparative examples of the present invention will be described. However, the following examples are only a preferred embodiment of the present invention, and the scope of the present invention is not limited by the following examples.

(Examples and Comparative Examples)

The following Examples and Comparative Examples are acidized as shown in Table 1 by using a ferritic stainless steel containing a high content of Si and Cr, characterized in that it comprises Si: 0.4% or more, Cr: 16% or more by weight% The surface roughness of pickling and ferritic stainless steels was checked by varying the washing solution, pickling time and surface potential.

 Prior to the experiment as shown in Table 1, the ferritic stainless steel containing Si: 0.4% or more, Cr: 16% or more by weight by cold rolling to make a cold-rolled steel sheet, the cold-rolled steel sheet was heat-treated at 1050 ℃. The heat-treated ferritic stainless steel was subjected to a pretreatment step, and the entire post-treatment step was performed. At this time, the pretreatment step was performed using electrolytic descaling.

As shown in Table 1, the pickling liquor is in the range of 40-100 g / l nitric acid, 5-30 g / l free hydrofluoric acid and 0-10 g of hydrogen peroxide, and hydrogen peroxide is added to the mixed acid solution consisting of nitric acid and hydrofluoric acid. The pickling solution was prepared. After immersing ferritic stainless steel in the pickling solution, the temperature of the pickling solution is immersed in the pickling solution at 20-45 ° C. for 30 seconds, and after confirming pickling and surface potential and surface roughness, the results are shown in Table 1 below. Indicated.

nitric acid
(g / ℓ) Foshan
(g / ℓ) Hydrogen peroxide
(g / ℓ) Temperature
(℃) Pickling time
(second) Surface potential
(V) Pickling
(O: pickling, X: not pickling) Surface roughness difference
(Of {100} and {111}
Height difference) (μm) Comparative Example 1 100 5 0 45 30 -0.28 X - Comparative Example 2 100 10 0 45 30 -0.30 O 1.1 Comparative Example 3 100 30 0 45 30 -0.33 O 1.5 Comparative Example 4 40 30 0 45 30 -0.38 X - Comparative Example 5 100 30 5 45 30 -0.19 X - Comparative Example 6 100 30 5 20 30 -0.40 X - Comparative Example 8 100 30 3 45 30 -0.32 O 1.2 Comparative Example 9 100 30 8 45 30 -0.16 O 0.35 Example 1 100 10 4 45 30 -0.21 O 0.15 Example 2 100 30 7 45 30 -0.22 O 0.2

Referring to Table 1, it was confirmed that the surface roughness difference of Examples 1 and 2, which are pickling solutions containing 50-100 g / l nitric acid, 10-30 g / l hydrofluoric acid, and 4-7 g / l hydrogen peroxide, was 0.2 μm or less. . In addition, when the surface potential of the ferritic stainless steel immersed in the pickling solution is -0.2 to -0.3V compared to the deep red electrode, the stripped oxide scale does not remain, and the surface roughness difference, which is the roughness difference for each grain crystal orientation, is 0.15 μm and 0.2 μm. It was possible to obtain a uniform surface quality.

Specifically, in Comparative Examples 1 to 4 using the pickling solution without hydrogen peroxide added, the surface roughness difference was 1.1 μm even if the pickling did not proceed (Comparative Example 1 and Comparative Example 4), or the pickling proceeded. 2) and 1.5μm (Comparative Example 3) was generated large, it was confirmed that the surface quality is poor. In addition, even when hydrogen peroxide was added, as in Comparative Example 8 and Comparative Example 9, when the concentrations of hydrogen peroxide were 3 g / L and 8 g / L, respectively, even when pickling proceeded, the surface roughness difference was 1.2 μm and 0.35 μm, respectively, exceeding 0.2 μm. It was confirmed that the surface quality is reduced.

In the case of Comparative Example 5 and Comparative Example 6 in which the concentration of hydrogen peroxide was 5 g / L, it was confirmed that pickling did not proceed. This means that even when the pickling liquids are the same, the surface potentials of ferritic stainless steels are -0.19V and -0.4V, respectively, and when they fall outside the range of -0.2 to -0.3V according to the present invention, high content Cr and Si excellent in corrosion resistance are obtained. It was confirmed that the ferritic stainless steel was not suitable for pickling.

2A to 2C are SEM images of the surface of the ferritic stainless steel pickled according to Table 1;

2A is a surface of Example 2, and FIGS. 2B and 2C are surfaces of Comparative Example 3 and 9, respectively. Referring to FIG. 2, in the case of FIG. 2A, which is an embodiment having a low surface roughness difference, it may be confirmed that the surface roughness difference is smaller than that of FIGS. 2B and 2C which are comparative examples.

FIG. 3 is a graph showing glossiness for roughness differences of grain crystal directions in Comparative Examples and Examples according to Table 1. FIG.

Referring to FIG. 3, it can be seen that the surface glossiness of the ferritic stainless steel is increased as the roughness difference according to grain crystal orientation decreases. That is, the glossiness, which is one of the properties required for ferritic stainless steel, may be determined by the surface roughness difference, and it may be confirmed that excellent glossiness may be obtained as the surface roughness difference is smaller. That is, when the roughness difference of each grain orientation is 0.2μm or less, it was confirmed that the glossiness (based on GS60C) of ferritic stainless steel can be secured to 140 or more.

That is, the ferritic stainless steel containing Si: 0.4% or more and Cr: 16% or more by weight by the pickling solution and the pickling method according to the present invention has a roughness difference according to grain crystal orientation of 0.2 μm or less, and glossiness 140 In this case, the pickling solution may be 50-100 g / L nitric acid, 10-30 g / L hydrofluoric acid and 4-7 g / L hydrogen peroxide, the surface potential of the ferritic stainless steel in the pickling section compared to the deep red electrode It is preferable to keep the temperature of the pickling solution at -30 to -50 to -0.2 to -0.3V.

It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the foregoing detailed description, and all changes or modifications derived from the meaning and scope of the claims and the equivalents thereof are included in the scope of the present invention Should be interpreted.

Claims (5)

In the pickling process of ferritic stainless steel containing a high amount of Si and Cr containing at least 0.4% by weight of Si and 16% by weight of Cr,
Pretreatment of ferritic stainless steel;
Pickling solution preparation step of preparing a pickling solution by adding hydrogen peroxide to the mixed acid solution consisting of nitric acid and hydrofluoric acid; And
Immersing ferritic stainless steel in the pickling solution, the surface potential of the ferritic stainless steel is -0.2 to -0.3V relative to the deep red electrode, the pickling conditions maintaining step of maintaining the temperature of the pickling solution at 30-50 ℃; A method of pickling ferritic stainless steel containing a high content of Si and Cr. The method of claim 1,
The pretreatment step is a method for pickling ferritic stainless steel, characterized in that the electrolytic descaling or salt bath (salt bath). The method of claim 1,
The pickling solution in the pickling solution preparation step is a pickling method of ferritic stainless steel, characterized in that the nitric acid 50-100g / L, hydrofluoric acid 10-30g / L and hydrogen peroxide 4-7g / L. A pickling solution for pickling ferritic stainless steel containing a high amount of Si and Cr containing at least 0.4% by weight of Si and 16% by weight of Cr, with 50-100 g / l nitric acid and 10-30 g / l hydrofluoric acid. And 4-7 g / l of hydrogen peroxide, and the pickling solution for ferritic stainless steel containing high content of Si and Cr. 5. The method of claim 4,
The pickling liquid is a pickling liquid for ferritic stainless steel, characterized in that the temperature of the pickling liquid is maintained at 30-50 ℃ while pickling the ferritic stainless steel.

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