KR20180074408A - Austenitic stainless steel having excellent corrosion resistance to sulfuric acid - Google Patents

Abstract

The present invention relates to an austenitic stainless steel. The austenitic stainless steel may contain 0.05 wt% or less (excluding 0 wt%) of carbon (C), 0.05 wt% or less (excluding 0 wt%) of nitrogen (N), 1.0 to 2.0 wt% of Si, 1.0 wt% or less (excluding 0 wt%) of manganese (Mn), 15 to 18 wt% of chromium (Cr), 6.5 to 9.0 wt% of nickel (Ni), 2.0 to 5.0 wt% of copper (Cu), 0.5 to 2.0 wt% of molybdenum (Mo), and the balance iron (Fe) and unavoidable impurities, and its sulfuric acid corrosion index (SCI) is at least 40 that is defined by the calculation formula of ″SCI = -[Cr] + 4[Ni] + 5[Mo] + 12[Cu]″ (in the formula, [Cr], [Ni], [Mo], and [Cu] are wt% of Cr, Ni, Mo, and Cu, respectively). According to the present invention, it is possible to provide austenitic stainless steel having excellent corrosion resistance against sulfuric acid dew point corrosion, which is problematic in various members used in a heat exchanger, a stack, and a chimney for thermal power generation and industrial boilers and soot removal apparatus members for various industries, and facilities in sulfuric acid environments.

Description

TECHNICAL FIELD [0001] The present invention relates to austenitic stainless steels having excellent sulfuric acid corrosion resistance,

The present invention relates to an austenitic stainless steel excellent in sulfuric acid corrosion resistance.

So-called fossil fuels such as petroleum and coal used as fuels for thermal power generation and industrial boilers contain a large amount of sulfur (S). Therefore, when the fossil fuel is burnt, sulfur oxides (SO _x ) are generated in the exhaust gas. When the temperature of the exhaust gas decreases, and this reacts with moisture in the gas it is sulfur oxide (SO _X) sulfuric acid, at low temperatures below the dew point of the dew condensation on the surface of various members, and the sulfuric acid dew point corrosion develops. Similarly, in a flue gas desulfurization apparatus used in various industries, when a gas containing sulfur oxides (SO _x ) flows, when the temperature is lowered, sulfuric acid dew point corrosion occurs.

Due to the sulfuric acid dew point corrosion problem, members such as sulfuric acid-containing heat exchangers maintain the exhaust gas temperature at a high temperature of 150 ° C or more so as to prevent condensation of sulfuric acid on the surface of the member. However, in view of an increase in energy demand and an increase in energy efficiency in recent years, there is a tendency to keep the temperature of the exhaust gas containing sulfur oxides at a temperature below the sulfuric acid dew point in order to recover heat energy effectively, The temperature of the exhaust gas containing sulfur oxides tends to be kept at a temperature below the sulfuric acid dew point. Accordingly, the development of a steel material capable of solving the problem of sulfuric acid dew point corrosion, that is, exhibiting corrosion resistance in a sulfuric acid environment, has been developed.

 For example, Patent Document 1 (Japanese Unexamined Patent Publication (Kokai) No. 1963-14585) discloses a low alloy steel obtained by adding Sb and Cu, which are effective for sulfuric acid corrosion resistance. In addition, Patent Document 2 (Japanese Patent Application Laid-Open No. 1997-25536) discloses a steel containing Sb or Sn in a copper-containing steel to improve the hydrochloric acid resistance while maintaining sulfuric acid resistance. In addition, Patent Document 3 (Japanese Unexamined Patent Application Publication No. 1998-110237) discloses a steel having improved sulfuric acid resistance and hydrochloric acid resistance with the same steel component as that of Patent Document 2, and having improved hot workability by containing Mo or B, .

However, the low alloy steel disclosed in the above patent documents can not exhibit sufficient corrosion resistance for a facility using a high S-containing fuel having an S content exceeding 2%. This is because if the S content is high, the sulfuric acid concentration in the exhaust gas becomes high, and the amount of sulfuric acid condensation due to the temperature decrease is increased, so that the corrosive environment becomes severe. Due to these problems, it is required to develop a new steel type capable of securing stable corrosion resistance.

Patent Document 4 (Korean Patent Laid-Open Publication No. 2000-0068736) discloses a method for improving the corrosion resistance in an environment where sulfuric acid densifies in high concentration (sulfuric acid dew point environment). The Ni content is 12 to 27%, the Mo content is 2 to 5 % Of high-alloy austenitic stainless steels. However, there is a limit in manufacturing and processing such as cracking in the hot rolling process due to low hot workability.

JP 1963014585 A JP 1997025536 A JP 1998110237 A KR 1020000068736 A

DISCLOSURE OF THE INVENTION In order to solve such conventional problems, it is an object of the present invention to provide a sulfuric acid corrosion resistant material which can appropriately control the content of elemental elements affecting sulfuric acid corrosion characteristics and control the sulfuric acid corrosion index (SCI) And an object of the present invention is to provide an austenitic stainless steel excellent in hot workability.

In order to solve the above-mentioned problems, the austenitic stainless steel according to the present invention contains 0.05% or less (excluding 0%) and 0.05% or less (0% or less) of carbon (C) 1.0 to 2.0% of Si, 1.0% or less (excluding 0%) of Cr, 15 to 18% of Cr, 6.5 to 9.0% of Ni, (SCI), which is defined by the following formula (1), of not less than 40: 2.0 to 5.0%, molybdenum (Mo): 0.5 to 2.0% .

SCI = - [Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu]

In the above formula (1), [Cr], [Ni], [Mo] and [Cu] mean the weight percentages of Cr, Ni, Mo and Cu, respectively.

The sulfuric acid corrosion index (SCI) may preferably be 42 to 70. [

The above stainless steel contains not more than 0.05% phosphorus (excluding 0%), sulfur (S): not more than 0.03% (excluding 0%), niobium (Nb): not more than 1.0% (Ti): not more than 0.5% (excluding 0%), and tungsten (W): not more than 1.0% (excluding 0%).

In the boiling solution to which a ferric chloride (FeCl ₃ ) solution with a concentration of 0.2% is added to a sulfuric acid solution having a concentration of 50%, the corrosion amount of the above-mentioned austenitic stainless steel can be less than 20 mg / m ² hr. In the present specification, the term " gaseous corrosion reduction "means the amount of corrosion of the boiling solution in a condensed state on the surface of austenitic stainless steel after evaporation, and" dipping corrosion reduction "means austenitic stainless steel Means the amount of corrosion by immersion in a boiling solution.

In the boiling solution to which a 0.2% hydrochloric acid (HCl) solution is added to a sulfuric acid solution having a concentration of 50%, the weight loss of the gaseous phase portion and the dipping portion of the austenitic stainless steel may be less than 20 mg / m ² hr.

When the stainless steel is immersed in a sulfuric acid solution having a concentration of 50%, a continuous Cu-enriched layer may be formed on the surface of the stainless steel.

The Cu content ratio in the Cu concentrated layer region may be 3 to 10 times the Cu content ratio in the non-condensed layer region, and the content ratio of Cu is defined by the following formula 2.

Cu content ratio = [Cu] / ([Cr] + [Fe] + [Ni] + [Cu]

In the formula (2), [Cr], [Fe], [Ni] and [Cu] mean the weight percentages of Cr, Fe, Ni and Cu, respectively.

According to the present invention, it is possible to provide an austenitic stainless steel having excellent sulfuric acid corrosion resistance and good hot workability in an environment where a high concentration of sulfuric acid coagulates by appropriately controlling the content of each component element that affects sulfuric acid corrosion resistance .

Further, according to the present invention, by controlling the sulfuric acid corrosion index (SCI) derived from the content of Cr, Ni, Mo, and Cu to the corrosion resistance of the sulfuric acid atmosphere to 40 or more, Based stainless steel can be provided.

As a result, the austenitic stainless steel according to the present invention can be used as a member of a heat exchanger, a stack, and a chimney, which are facilities for condensation condensation of exhaust gas such as thermal power generation boilers and industrial boilers, Gas-gas-heater (GGH) of the soot removal apparatus of the present invention.

1 is a graph showing anodic polarization characteristics in a 50% aqueous H ₂ SO ₄ solution of a steel specimen having compositions of Example 6, Comparative Example 2, and Comparative Example 3;
Fig. 2 shows photographs of the steel specimens produced according to Example 1 and Comparative Example 7 and evaluation standards for hot workability.
FIG. 3 is a graph showing the weight loss of the gas phase corrosion and the weight loss of the dipping portion according to the SCI value in the corrosion solution (1) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution).
4 is a graph showing the amount of corrosion of the dipped part according to the SCI value in the corrosion solution (1) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution).
5 is a graph showing the amount of corrosion of the gas phase corrosion caused by the SCI value in the corrosion solution (2) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution).
6 is a graph showing the amount of corrosion of the dipped part according to the SCI value in the corrosion solution (2) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution).
7 is a graph showing the results of a comparison between Example 1, Example 5, Comparative Example 2, Comparative Example 5, Comparative Example 6, Comparative Example 9 and Comparative Example 10 in a Greendess solution (17% H ₂ SO ₄ + 0.35% HCl solution) Fig. 3 is a graph showing corrosion loss of the vapor phase portion and the dipping portion of the manufactured steel specimen. Fig.
8 is a graph showing the results of analysis of the surface coating composition according to the depth of the steel specimen after immersing the steel specimen of Example 3 (Cu content: 4.1%) in 50% H ₂ SO ₄ solution.
9 is a schematic view showing the surface film forming mechanism of FIG.
10 is a graph showing the results of analysis of the surface coating composition according to the depth of the steel specimen after immersing the steel specimen of Comparative Example 8 (Cu content: 1.6%) in 50% H ₂ SO ₄ solution.
11 is a schematic view showing the surface film formation mechanism of FIG.

Hereinafter, an austenitic stainless steel having excellent sulfuric acid corrosion resistance characteristics according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

The austenitic stainless steel having excellent sulfuric acid corrosion resistance characteristics of the present invention contains 0.05% or less (excluding 0%) of carbon (C), 0.05% or less (excluding 0%) of nitrogen (N) 1.0 to 2.0% of Si, 1.0% or less of Mn (excluding 0%), 15 to 18% of Cr, 6.5 to 9.0% of Ni, 2.0 to 5.0% of Cu, (SCI) as defined by the following formula (1) is 40 or more, which contains iron (Fe), molybdenum (Mo): 0.5 to 2.0%, iron (Fe) and unavoidable impurities.

SCI = - [Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu]

In the above formula (1), [Cr], [Ni], [Mo] and [Cu] mean the weight percentages of Cr, Ni, Mo and Cu, respectively.

Hereinafter, the component system of the austenitic stainless steel according to one embodiment of the present invention will be described in more detail. Unless otherwise stated, the content of each component means weight percent.

Carbon (C): 0.05% or less (excluding 0%)

Carbon (C) is an austenite-forming element and is an element effective for increasing the strength of a material by solid solution strengthening. However, when the content of carbon (C) is excessively high, it easily bonds with chromium (Cr) effective for improving corrosion resistance to form Cr carbide, thereby lowering the chromium (Cr) content around the grain boundary, . Therefore, in the present invention, the content of carbon (C) is limited to 0.05% or less in order to maximize the corrosion resistance.

Nitrogen (N): 0.05% or less (excluding 0%)

Nitrogen (N) is an element stabilizing the austenite phase and is an element that simultaneously improves high temperature strength and pitting resistance. However, when nitrogen (N) is excessively added, blowholes, pinholes, and the like are generated during casting and the isothermal workability causing surface defects is deteriorated. Therefore, the content of nitrogen (N) 0.05%.

silicon( Si ): 1.0 to 2.0%

Silicon (Si) is an element to be added as a deoxidizing element, and as the content of the ferrite phase-forming element increases, the ferrite phase stability increases. The increase of the content of silicon (Si) increases the formal potential and the oxidation resistance. Therefore, in the case of the present invention, it is preferable to add a silicon (Si) content of at least 1.0% or more, and when the content of silicon (Si) increases to 2.0% or more, problems such as increase in Si inclusions and surface defects Shall not be more than 2.0% maximum.

Manganese (Mn): 1.0% or less (excluding 0%)

Manganese (Mn) is an element effective for stabilizing the austenite phase like nitrogen (N). However, when the content of manganese (Mn) is increased, precipitates such as manganese sulfide (MnS) are formed and the pitting resistance is lowered, so that the content of manganese (Mn) is limited to 1.0% or less. On the other hand, when the content of manganese (Mn) is excessively reduced, problems such as an increase in purification cost may occur. Therefore, the content of manganese may be limited to 0.6% or more. Therefore, the preferable content range of the manganese in the present invention may be 0.6 to 1.0%.

chrome( Cr ): 15 to 18%

Chromium (Cr) is an essential element for securing the corrosion resistance of austenitic stainless steel. In order to improve the corrosion resistance, the higher the chromium (Cr) content is, the more effective. However, in the environment where the high concentration of sulfuric acid is condensed as in the present invention, the increase of the chromium (Cr) content may cause the decrease in the corrosion resistance in the sulfuric acid atmosphere.

Incidentally, when chromium (Cr) of at least 15% is contained as a constituent of the austenitic stainless steel according to the present invention at the same time with Cu and Mo in amounts described later, good corrosion resistance can be ensured in the sulfuric acid atmosphere in which the high concentration of sulfuric acid coagulates . However, if the content of chromium (Cr) exceeds 18%, the corrosion resistance decreases and the cost increases in a high concentration sulfuric acid atmosphere even if Cu and Mo are added. Therefore, in the present invention, the chromium (Cr) content is limited to 15 to 18%.

nickel( Ni ): 6.5 to 9.0%

Nickel (Ni) is an element for stabilizing the austenite phase and is an indispensable element for ensuring corrosion resistance in an environment where high concentration of sulfuric acid is condensed. When the content of nickel (Ni) is less than 6.5%, it is impossible to secure corrosion resistance in an environment of high concentration of sulfuric acid. On the other hand, when the content of nickel (Ni) exceeds 9% It does not appear and the economical efficiency is lowered due to the high price of nickel. Therefore, in the present invention, the content of nickel (Ni) is limited to 6.5 to 9.0%.

Copper (Cu): 2.0 to 5.0%

Copper (Cu) is an essential element for ensuring corrosion resistance in an environment where high concentration of sulfuric acid is condensed. When the content of copper (Cu) is less than 2.0%, it is impossible to secure corrosion resistance in an environment in which concentrated sulfuric acid is condensed. On the other hand, when the content of copper (Cu) exceeds 5.0% The corrosion resistance is increased but the problems such as the rapid deterioration of the hot workability occur. Therefore, in the present invention, the content of copper (Cu) is limited to 2.0 to 5.0%.

molybdenum( Mo ): 0.5 to 2.0%

Molybdenum (Mo) is an effective element for securing the corrosion resistance of austenitic stainless steel. If the content of molybdenum (Mo) is low, it is difficult to secure corrosion resistance. However, when molybdenum (Mo) of 0.5% or more is contained together with copper (Cu) in the above amount, corrosion resistance can be maximized in a high concentration of sulfuric acid . On the other hand, when the content of molybdenum (Mo) exceeds 2.0%, the corrosion resistance is increased but the hot workability is deteriorated and the manufacturing difficulty occurs, and the economical efficiency is lowered due to the high price of molybdenum (Mo). Therefore, in the present invention, the content of molybdenum (Mo) is limited to 0.5 to 2.0%.

In addition, the austenitic stainless steel according to the present invention may contain not more than 0.05% phosphorus (not 0%), not more than 0.03% sulfur (excluding 0%), 1.0% niobium (Nb) Or more of titanium (Ti): not more than 0.5% (excluding 0%), and tungsten (W): not more than 1.0% (excluding 0%). Hereinafter, the reasons for limiting the content of these components will be described.

Phosphorus (P): 0.05% or less (excluding 0%)

Phosphorus (P) is an element that can be incorporated as an impurity in the production of steel, and may cause deterioration of hot workability and corrosion resistance. Therefore, the content of phosphorus (P) is preferably as small as possible. Particularly, when the content of phosphorus (P) exceeds 0.05%, the corrosion resistance deteriorates remarkably in an environment of high concentration of sulfuric acid.

Sulfur (S): 0.03% or less (excluding 0%)

Sulfur (S) is an impurity inevitably contained in the steel, and is an element which deteriorates hot workability and corrosion resistance. Therefore, the content of sulfur is preferably as small as possible. Particularly, when the content of sulfur (S) exceeds 0.03%, corrosion resistance deteriorates remarkably in an environment where sulfuric acid at a high concentration is condensed.

Niobium ( Nb ): 1.0% or less (excluding 0%)

When niobium (Nb) is included in the constituent components of the austenitic stainless steel, carbon (C) can be fixed to enhance the corrosion resistance, especially the intercalation corrosion resistance. However, when the content of niobium (Nb) exceeds 1.0%, the above-mentioned nitrogen (N) and nitride may be produced and corrosion resistance and hot workability may be deteriorated.

titanium( Ti ): 0.5% or less (excluding 0%)

When titanium (Ti) is included in the constituent components of the austenitic stainless steel, carbon (C) is fixed in the same manner as the niobium (Nb), thereby enhancing the corrosion resistance, especially the intercalation corrosion resistance. However, when the content of titanium (Ti) exceeds 1.0%, the above-mentioned nitrogen (N) and nitride may be produced and corrosion resistance and hot workability may be deteriorated.

Tungsten (W): 1.0% or less (excluding 0%)

When tungsten (W) is included in the constituents of the austenitic stainless steel, it acts to enhance the corrosion resistance in a high concentration sulfuric acid condensed environment. However, when the content of tungsten (W) exceeds 1.0%, the effect of increasing the corrosion resistance against the input amount is insignificant and the economical efficiency may be lowered.

Meanwhile, the sulfuric acid corrosion index (SCI) defined by the following formula (1) derived from the content of Cr, Ni, Mo and Cu on the corrosion resistance of the austenitic stainless steel among the elements constituting the austenitic stainless steel according to the present invention, Is 40 or more.

SCI = - [Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu]

In the above formula (1), [Cr], [Ni], [Mo] and [Cu] mean the weight percentages of Cr, Ni, Mo and Cu, respectively.

When the sulfuric acid corrosion index (SCI) is less than 40, it is not preferable since the corrosion weight loss value in the environment of high concentration of sulfuric acid may be higher than the target value of 20 mg / m ² hr which is the corrosion weight loss standard value. At this time, the target value of the corrosion weight loss criterion is determined as corrosion loss value in the high concentration sulfuric acid boiling solution of the 6% Mo-containing austenitic stainless steel having excellent corrosion resistance among the steels used in the desulfurization equipment member.

3 to 6, when the sulfuric acid corrosion index (SCI) is not less than 40, the ferric chloride solution (FeCl ₃ ) solution at a concentration of 0.2% is added to a sulfuric acid solution having a concentration of 50% Based stainless steel in a boiling solution containing 0.2% hydrochloric acid (HCl) solution in a sulfuric acid solution of 50% concentration and 20 mg / m ² hr or less in both the vapor phase portion and the dipping portion corrosion reduction of the nitrate- It can be seen that the amount of corrosion loss in the gaseous and dipped parts is less than 20 mg / m ² hr.

When the sulfuric acid corrosion index (SCI) is in the range of 42 to 70, an austenitic stainless steel excellent in corrosion resistance in an environment of high concentration of sulfuric acid and having excellent hot workability is more preferable .

When the austenitic stainless steel according to the present invention is immersed in a sulfuric acid solution having a concentration of 50%, a continuous Cu-enriched layer may be formed on the surface of the stainless steel.

The Cu content ratio in the Cu concentrated layer region may be 3 to 10 times the Cu content ratio in the non-condensed layer region, and the content ratio of Cu is defined by the following formula 2.

Cu content ratio = [Cu] / ([Cr] + [Fe] + [Ni] + [Cu]

In the formula (2), [Cr], [Fe], [Ni] and [Cu] mean the weight percentages of Cr, Fe, Ni and Cu, respectively.

The Cu-enriched layer is a coating layer formed by preferentially dissolving Cu among the constituent elements of the austenitic stainless steel in the H ₂ SO ₄ solution to form a continuous Cu precipitate on the surface of the steel. The corrosion resistance in the H ₂ SO ₄ solution . Specifically, FIG. 9 schematically shows the Cu enriched layer generating mechanism.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these embodiments.

Example  1 to 6 and Comparative Example  1 to 11

A ingot having a thickness of 120 mm including the composition components according to Examples 1 to 6 and Comparative Examples 1 to 11 shown in Table 1 below was cast and hot-rolled at a temperature of 1,150 ° C to obtain a 3.0 mm thick Hot-rolled steel sheets were produced. Thereafter, a cold-rolled steel sheet having a thickness of 1.2 mm was produced through cold-rolling, and then cold-rolled annealed and cold-rolled at a temperature of 1,100 ° C to produce a final cold rolled pickled steel sheet.

Si Mn Cr Ni Mo Cu N C Sb Example 1 1.1 0.6 15.1 7.8 1.1 3.1 0.03 0.03 - Example 2 1.1 0.6 16.1 7.1 1.1 3.1 0.03 0.03 - Example 3 1.1 0.6 16.1 7.1 1.1 4.1 0.03 0.03 - Example 4 1.1 0.6 16.1 8.1 1.5 3.1 0.03 0.03 - Example 5 1.5 0.9 17.5 8.5 0.8 2.0 0.03 0.03 - Example 6 1.5 0.8 17.8 7.6 0.6 3.0 0.03 0.03 - Comparative Example 1 ^* 0.03 0.03 0.03 0.03 0.03 0.5 0.03 0.03 0.1 Comparative Example 2 0.03 0.03 0.03 0.03 0.03 0.5 0.03 0.03 0.1 Comparative Example 3 0.6 One 18 8 0.2 0.1 0.03 0.04 - Comparative Example 4 0.6 One 18 10 2 0.2 0.03 0.03 - Comparative Example 5 17.5 0.3 17 0.1 One 0.2 0.01 0.01 - Comparative Example 6 0.6 0.3 17 0.1 0.1 0.1 0.01 0.01 - Comparative Example 7 0.6 One 23 22 6 0.1 0.03 0.03 - Comparative Example 8 1.1 0.6 20.1 15.1 3.1 1.6 0.16 0.03 - Comparative Example 9 1.1 0.6 15.1 6.1 1.1 2.1 0.03 0.03 - Comparative Example 10 1.1 0.6 15.1 6.1 1.1 3.1 0.03 0.03 - Comparative Example 11 1.1 0.6 15.1 6.1 1.1 5.1 0.03 0.03 -

* Comparative Example 1: Enamel-coated specimen of stainless steel specimen having the same chemical composition as Comparative Example 2.

Evaluation example  1: Cu content in aqueous sulfuric acid solution Polarization property  evaluation

In order to analyze the effect of Cu on the corrosion resistance in the sulfuric acid dew point corrosion environment containing sulfur oxides, anodic polarization experiments of the steel specimens prepared according to Example 6, Comparative Example 2 and Comparative Example 3 were carried out at 70 ° C and 50% H ₂ SO ₄ aqueous solution. FIG. 1 shows a polarization curve of the anode showing the above experimental results.

In this experiment, the environment of 50% H ₂ SO ₄ aqueous solution is the environment simulating the zone showing the maximum corrosion rate as a result of the corrosion resistance measurement according to the sulfuric acid concentration change. The steel having the composition of Comparative Example 2 is carbon steel-based sulfuric acid steel (Cu content: 0.5%) mainly used for GGH (Gas Gas Heater) parts of a conventional soot desulfurization equipment, Is steel of STS 304 (Cu content: 0.1%), and the steel having the composition of Example 6 is an austenitic stainless steel which constitutes an alloy component such that the Cu content is 3.0%.

The matters to be confirmed from FIG. 1 are as follows.

(1) Natural potential in 50% H ₂ SO ₄ solution

1, it can be seen that the sulfuric acid steel of Comparative Example 2 is -350 mV, the STS 304 of Comparative Example 3 is -300 mV, and the steel produced according to Example 6 has a natural potential value of -250 mV. From these results, it can be seen that the steel having the composition of Example 6 having a Cu content of 3% exhibited a higher natural potential than the steel having the compositions of Comparative Examples 2 and 3.

(2) Corrosion current density in the natural potential state

1, the sulfuric acid steel (Cu content: 0.5%) and the STS 304 steel (Cu content: 0.1%) of Comparative Example 3 were 10 mA respectively, (Cu content: 3.0%) shows a corrosion current density at a natural potential of 50 μA. That is, the STS 304 steel (Comparative Example 3) was about 10 times higher than the sulfuric acid resistant steel (Comparative Example 2), and the steel produced according to Example 6 was about 10 The corrosion current density value at the natural potential lower than that of the natural potential is shown. As the Cu content increases, the corrosion current density at the natural potential state tends to increase.

In addition, as shown in FIG. 1, in the case of sulfuric acid (Comparative Example 2), the current density tends to increase continuously as the potential is gradually applied at the natural potential. However, in the case of the steels of Comparative Example 3 and Example 6, which are stainless steel, the current density was abruptly decreased as the dislocation was gradually applied in the region of -200 mV or more. This shows that a passive film is formed on the surface of the stainless steel in the potential region Because.

(3) Maximum current density due to potential application

1, the sulfuric acid steel of Comparative Example 2 had a maximum current density of 0.5 A, the STS 304 steel of Comparative Example 3 had a maximum current density of 0.1 A, and the steel prepared according to Example 6 had a maximum current density of 0.02 A . It can be seen from this that the steel having the composition of Example 6 having a Cu content of 3% has a maximum current density value several tens of times lower than that of the steel having the compositions of Comparative Examples 2 and 3.

From the results of the anode polarization measurement of the above (1) to (3), the corrosion current density of the STS 304 steel (Comparative Example 3) is higher than that of the sulfuric acid refractory steel (Comparative Example 2) The corrosion resistance is not suitable to be used to heat the sulfuric acid steel (Comparative Example 2). Also, in the case of the sulfuric acid-containing sulfuric acid (Comparative Example 2), the corrosion current density in the natural potential state is low, but the current density rapidly increases with the potential application. In actual flue gas desulfurization facilities, the corrosion rate of sulfuric acid steel increases rapidly because H ₂ SO ₄ does not exist alone but metal ions as oxidants coexist. On the other hand, in the case of the steel (Cu content: 3%) produced according to Example 6, the corrosion current density and the maximum current density are lower than those of the sulfuric acid steel (Comparative Example 2) It is not only excellent but also has excellent corrosion resistance by generating a passive film on the surface of steel even in the presence of metal ions which are oxidants.

Evaluation example  2: SCI, Hot  Evaluation of processability and structure

Sulfuric Acid Corrosion Index (SCI), hot workability and tissue structure were evaluated according to the following methods, and the results are shown in Tables 2 and 3 below.

(1) Sulfuric Acid Corrosion Index (SCI)

The sulfuric acid corrosion index (SCI) is an index of the effect of Cr, Ni, Mo, and Cu contents on the corrosion resistance of steel specimens in a high concentration sulfuric acid condensation environment through various experiments. Corrosion resistance. Here, the sulfuric acid corrosion index (SCI) is a value calculated according to the following formula (1).

SCI = - [Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu]

In the formula 1, [Cr], [Ni], [Mo] and [Cu] refer to the weight percentages of Cr, Ni, Mo and Cu, respectively.

(2) Hot workability

The hot workability was evaluated by the width and depth of the cracks generated at the edges of the hot-rolled steel sheets subjected to the hot rolling process according to Examples 1 to 6 and Comparative Examples 1 to 11. At this time, in the case of the steel specimen having a width and a depth of 10 mm or less, it was judged that the hot workability was ensured and the steel specimen having a crack width and depth of 10 mm or more was judged not to secure the hot workability Quot; X ".

2 shows photographs of the steel specimens produced according to Example 1 and Comparative Example 7 and evaluation standards for hot workability.

(3) Organization

The structure of the steel specimens having undergone the pickling operation according to Examples 1 to 6 and Comparative Examples 1 to 11 was observed using a ferrite scope (Fisher, Ferritescope MP30) and an optical microscope. At this time, steel specimens with ferrite single phase structure are indicated as "F", steel specimens with austenite single phase structure as "A", and specimens with austenite + martensite structure as "A + M".

Evaluation example  3: Evaluation of corrosion loss

The steel specimens produced according to Examples and Comparative Examples were cut into a width of 30 mm and a length of 30 mm and the surfaces of the cut steel specimens were polished with # 120 abrasive paper. Thereafter, a simulated solution in which a high concentration of sulfuric acid was concentrated ) Was used for the corrosion test. At this time, the corrosion test was conducted according to the ISO 28706 method.

First, a glass ball was placed on the bottom of a 3L beaker, and a condenser was installed at the top of the beaker to condense hot gas and water vapor. Then, the corrosion solution was injected so that only a part of the glass ball in the beaker was locked.

In this evaluation example, the weight loss of the dipping portion and the gaseous phase portion was measured. First, in order to evaluate the corrosion loss of the dipping portion, the steel specimen was placed on the bottom (bottom of the glass ball) of the beaker, So as to be immersed. In order to evaluate the amount of corrosion of the vapor phase portion, the steel specimen is placed on the uppermost part of the beaker (the upper part of the glass ball), and the hot gas and water vapor formed by evaporating the corrosion solution in the boiling state are condensed on the surface of the steel specimen, . At this time, the corrosion test minimizes mutual interference between specimens by using only one kind of steel for each beaker.

In this evaluation example, three types of corrosion solutions were used for the corrosion test, and the compositions of the corrosion solutions and the corrosion test conditions were as follows.

(1) 50% H ₂ SO ₄ + 0.2% FeCl ₃ solution

The corrosion solution (1) is a solution in which 0.2% FeCl ₃ solution is added to 50% sulfuric acid. The solution is a solution simulating an environment in which a small amount of oxidizing agent is added to an environment where a high concentration of sulfuric acid is condensed. The corrosion solution (1) is a solution simulating the above environment, finding out that Fe ions generated from a small amount of oxidizing agent are contained as a result of analyzing a GGH (Gas Gas Heater) environment solution of a desulfurization facility.

The corrosion test conditions were as follows. The weight loss before and after the corrosion test was measured to calculate the corrosion loss per unit time (hr) and unit area (m2), and the results are shown in Table 2 below.

- Corrosive solution state: Boiling state

- Corrosion time: 24 hours

At this time, the corrosion test in the corrosion solution (1) was performed on a steel having austenite structure and a steel having a ferrite structure.

(2) 50% H ₂ SO ₄ + 0.2% HCl solution

The corrosion solution (2) is a solution in which 0.2% HCl solution is added to 50% sulfuric acid. The solution is a solution simulating an environment in which a trace amount of chloride is added to an environment where a high concentration of sulfuric acid is condensed. The corrosion solution (2) is a solution simulating the environment described above by finding that Cl ion generated from a trace amount of chloride is contained in the GGH (Gas Gas Heater) environmental solution of the desulfurization facility.

The corrosion test conditions, the method of measuring corrosion loss and the measurement object are the same as those of the corrosion solution (1), and the results of the corrosion loss sagging are shown in Table 2 below.

(3) 17% H ₂ SO ₄ + 0.35% HCl solution

Corrosion solution (3) is a solution of 0.35% hydrochloric acid solution in 17% sulfuric acid solution and simulates the actual corrosive environment of GGH (Gas Gas Heater) part of the soot desulfurization facility. It is also called Green death solution .

The corrosion test conditions are as follows. The weight difference before and after the corrosion test was measured to calculate the corrosion loss per unit time (hr) and the unit area (m2), and the results are shown in Table 3 below.

- Corrosive solution temperature: 80 ℃

- Corrosion time: 6 hours

At this time, the corrosion test in the corrosion solution (3) was carried out on the steel specimens of Examples 1, 5, 2, 5, 6, 9 and 10.

Table 2, which shows the results of the corrosion loss in the corrosion solutions (1) and (2), is shown below.

SCI Hot
Processability group 50% H ₂ SO ₄ + 0.2% FeCl ₃ solution Corrosion loss (mg / cm ² hr) 50% H ₂ SO ₄ + 0.2% HCl solution Corrosion loss (mg / cm ² hr) Dipping portion Mover Dipping portion Mover Example 1 58.8 ◎ A 0.66 1.52 5.65 1.65 Example 2 55 ◎ A 0.85 1.05 6.32 1.95 Example 3 67 ◎ A 0.71 2.09 5.88 2.38 Example 4 61 ◎ A 0.75 2.39 4.48 2.00 Example 5 44.5 ◎ A 0.28 4.94 15.23 7.87 Example 6 51.6 ◎ A 0.71 10.42 10.35 9.11 Comparative Example 1 6.24 ◎ F 137.16 30.02 139.10 65.00 Comparative Example 2 6.24 ◎ F 127.96 35.28 167.56 73.02 Comparative Example 3 16.2 ◎ A 0.52 61.51 95.20 63.32 Comparative Example 4 34.4 ◎ A 0.27 28.00 65.20 13.66 Comparative Example 5 -9.2 ◎ F 0.71 69.47 177.74 93.13 Comparative Example 6 -14.9 ◎ F 1.89 83.92 175.31 95.37 Comparative Example 7 96.2 × A 1.29 18.95 16.45 17.45 Comparative Example 8 75 × A 0.52 3.53 10.00 3.69 Comparative Example 11 76 × A 1.22 1.72 6.12 2.4

Table 2 shows that the steel specimens prepared according to Examples 1 to 6 have excellent hot workability and sulfuric acid corrosion resistance. On the other hand, the steel specimens produced according to Comparative Examples 7, 8 and 11 are not suitable for use because they cause manufacturing problems to open the hot workability. In the case of the steel specimens prepared according to Comparative Examples 1 to 6, the amount of gaseous corrosion loss in the corrosion solution 1 (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution) and the corrosion solution 2 (50% H ₂ SO ₄ + 0.2% HCl solution) was remarkably higher than that of the above examples.

On the other hand, in the case of the steel specimens produced according to Comparative Examples 3 to 6, the corrosion amount of the dipping portion in the corrosion solution (1) was equivalent to or lower than those of the Examples. In the actual GGH corrosion atmosphere of the desulfurization equipment, Since corrosion and gaseous corrosion occur repeatedly, even if the corrosion loss is low in the gaseous phase, the corrosion loss in the dipping area is large, so the actual service life is shortened.

In the case of the steel specimen produced according to Comparative Example 7, although the hot workability was poor, it was a steel material containing 6% Mo having the highest sulfuric acid corrosion resistance characteristic used in a member such as a desulfurization equipment. It was judged whether or not the sulfuric acid corrosion property was excellent based on the steel specimen of Comparative Example 7. [ Therefore, the target value of the corrosion weight loss standard was set at a level of 20 mg / m ² hr, and all of the steel specimens manufactured according to Examples 1 to 6 of the present invention exhibited a lower value than the target value of the corrosion weight loss, .

On the other hand, FIGS. 3 and 4 show graphs depicting the amount of gaseous corrosion reduction and the amount of dipping corrosion reduction according to the SCI value in the corrosion solution (1) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution). At this time, the graph was created using the data of Examples 1 to 6, Comparative Examples 1 to 8, and Comparative Example 11 described in Table 2 above.

3, which is a graph showing the graph of the amount of gaseous corrosion reduction according to the SCI value in the corrosion solution (1), the corrosion loss decreases gradually as the sulfuric acid corrosion index (SCI) increases and the sulfuric acid corrosion index (SCI) In case of 40 or more, it can be confirmed that the corrosion weight loss value is lower than the target value of 20 mg / m ² hr which is the corrosion weight loss standard. From the above results, the present inventors have found that the sulfuric acid corrosion index (SCI) value should be 40 or more in order to have excellent corrosion resistance in the gas phase portion of the corrosion solution (1).

4, which shows a graph of the corrosion amount of the dipping portion according to the SCI value in the corrosion solution (1), the corrosion loss was 20 mg / m ² hr or less in all the steel specimens except for the steel specimens of Comparative Examples 1 and 2 appear. As shown in Fig. 1, in the case of stainless steel (Examples 1 to 6, Comparative Examples 3 to 8, and Comparative Example 11), dislocations gradually increase with the addition of an oxidizing agent, . AgCl or higher results from the phenomenon that the corrosion rate decreases because the passive film is formed on the stainless steel surface and the corrosion rate is drastically reduced. On the other hand, in the case of the sulfuric acid steel of the carbon steel component (Comparative Examples 1 and 2), the corrosion rate is rapidly increased with the addition of the oxidizing agent, and the corrosion loss is increased.

5 and 6 show graphs depicting the amount of gaseous corrosion reduction and the amount of dipping corrosion reduction according to the SCI value in the corrosion solution 2 (50% H ₂ SO ₄ + 0.2% HCl solution), respectively. At this time, the graph was created using the data of Examples 1 to 6, Comparative Examples 1 to 8, and Comparative Example 11 described in Table 2 above.

5 showing graphs depicting the depression of gaseous corrosion according to the SCI value in the corrosion solution (2), the corrosion loss decreases gradually as the sulfuric acid corrosion index (SCI) increases, and the sulfuric acid corrosion index (SCI) In case of 40 or more, it can be confirmed that the corrosion weight loss value is lower than the target value of 20 mg / m ² hr which is the corrosion weight loss standard. From the above results, the present inventors have found that the sulfuric acid corrosion index (SCI) value should be 40 or more in order to have excellent corrosion resistance in the gas phase portion of the corrosion solution (2).

6, which shows a graph of the corrosion amount of the dipping part according to the SCI value in the corrosion solution (2), the corrosion loss is gradually decreased as the sulfuric acid corrosion index (SCI) is increased Able to know. When chloride is added to the high concentration of sulfuric acid concentrated environmental corrosion occurs, but promoted by chlorides, when the value of sulfuric acid corrosion index (SCI) of more than 40 in the solution, the corrosion weight loss reference target value of 20mg / m ² lower than hr As shown in Fig.

Meanwhile, Table 3, in which the corrosion test evaluation data in the corrosion solution (3) is described, is shown below.

SCI Hot workability group Green Death Solution Corrosion loss (g / cm ² hr) Dipping part ^* Weather Department ^** Example 1 58.8 ◎ A 1.162 0.517 Example 5 44.5 ◎ A 2.245 0.523 Comparative Example 2 6.24 ◎ F 89.685 8.116 Comparative Example 5 -9.2 ◎ F 72.56 12.56 Comparative Example 6 -14.9 ◎ F 76.45 15.65 Comparative Example 9 40 ◎ A + M 32.25 2.56 Comparative Example 10 52 ◎ A + M 35.45 4.23

As shown in Table 3, the steel specimens prepared according to Examples 1 and 5 had a smaller value of dipping and gaseous corrosion loss than the steel specimens prepared according to Comparative Examples 2, 5, 6, 9 and 10 .

FIG. 7 is a graph showing the weight loss of the gas phase corrosion and the weight loss of the dipping portion in the Greendess solution for the steel specimens of the Examples and Comparative Examples described in Table 3 above. Referring to FIG. 7, it can be seen that, in the case of the steel specimens of the comparative examples, the corrosion amount of the dipped part is larger than the value of the corrosion amount of the vapor part.

However, in the case of GGH corrosion atmosphere of actual desulfurization facilities, it is repeatedly occurred in the subterranean corrosion where the corrosion is occurred by the immersion in the high concentration sulfuric acid solution and the subterranean corrosion where the corrosion is caused by the exposure to the sulfuric acid fume. Therefore, even though the steel specimens prepared according to the above comparative examples have excellent properties with low corrosion loss in the gas phase, the corrosion resistance is reduced in the dipping portion immersed in a high concentration of sulfuric acid, so that the service life is shortened. On the other hand, the austenitic stainless steels having the compositions of Examples 1 and 5 have excellent corrosion loss characteristics in the dipping portion and the vapor phase mode environment, so they can be suitably used in the GGH corrosion atmosphere of the desulfurization facility.

Evaluation example  4: Evaluation of coating composition analysis

Figs. 8 and 10 show the results of analysis of the film composition according to the depth after immersion in the H ₂ SO ₄ solution of the steel specimen of Example 3 and the steel specimen of Comparative Example 8 where the Cu content is 1.6%, which has a Cu content of 4.1% have. Specifically, the coating composition was analyzed by XPS analysis after immersing each steel specimen in a 50% H ₂ SO ₄ solution at room temperature for about 5 minutes, followed by drying.

Referring to FIG. 8, in the case of the steel specimen having the composition of Example 3, it was found that Cu and Ni were concentrated to about 3 to 10 times as much as the base material within about 6 nm in the depth direction from the surface after dipping in the H ₂ SO ₄ solution. The present inventors observed the characteristics of the steel specimen film of Example 3 through TEM observation and found that Cu is continuously distributed on the surface of the steel specimen. 9 is a schematic diagram showing a continuous film forming mechanism.

Referring to FIG. 10, it can be seen that in the case of the steel specimen having the composition of Comparative Example 8, the Cu and Ni are somewhat thickened compared to the base material within about 2 nm from the surface in the depth direction after immersing in the H ₂ SO ₄ solution. As a result of observing the characteristics of the steel specimen film of Comparative Example 8 through TEM observations, the present inventors found that Cu, corrosion oxide, and base material are discontinuously distributed on the surface. Fig. 11 shows a schematic diagram showing a discontinuous infiltration forming mechanism.

If the above results seen in, the Cu content is more than 2% is to increase the corrosion resistance of the Cu in the preferentially dissolved, to form a continuous Cu precipitates on the surface of steel specimens H ₂ SO ₄ solution in H ₂ SO ₄ solution. On the other hand, if less than the Cu content is 2% Cu precipitates are deposited discretely on the surface of steel specimens, corrosion oxide was observed by a corrosion occurs where Cu does not precipitate and, as a result, the corrosion resistance in H ₂ SO ₄ solution .

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, Should be construed as being included in the scope of the present invention.

Claims (7)

(C): not more than 0.05% (excluding 0%), nitrogen (N): not more than 0.05% (excluding 0%), Si: 1.0 to 2.0%, manganese (Mn): not more than 1.0% (Except for 0%), 15 to 18% of Cr, 6.5 to 9.0% of Ni, 2.0 to 5.0% of Cu, 0.5 to 2.0% of molybdenum, ) And unavoidable impurities,
An austenitic stainless steel having a sulfuric acid corrosion index (SCI) of 40 or more as defined by the following formula (1):
SCI = - [Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu]
In the above formula (1), [Cr], [Ni], [Mo] and [Cu] mean the weight percentages of Cr, Ni, Mo and Cu, respectively. The method according to claim 1,
And austenitic stainless steel having a sulfuric acid corrosion index (SCI) of 42 to 70. The method according to claim 1,
The above stainless steel contains not more than 0.05% phosphorus (excluding 0%), sulfur (S): not more than 0.03% (excluding 0%), niobium (Nb): not more than 1.0% (Ti): not more than 0.5% (excluding 0%), and tungsten (W): not more than 1.0% (excluding 0%). The method according to claim 1,
The 0.2% concentration in the sulfuric acid solution of 50% concentration of chloride iron (FeCl ₃₎ a portion boiling solution of the austenitic stainless steel of the gas phase was added to the solution and a needle portion corrosion decrease the austenitic 20mg / m ² or less hr Stainless steel. The method according to claim 1,
An austenitic stainless steel having a corrosion loss of 20 mg / m ² hr or less in a boiling solution to which a 0.2% hydrochloric acid (HCl) solution is added to a 50% sulfuric acid solution. The method according to claim 1,
Austenitic stainless steel in which a continuous Cu thick layer is formed on the surface of the stainless steel when the stainless steel is dipped in a sulfuric acid solution having a concentration of 50%. The method according to claim 6,
Wherein the Cu content ratio in the Cu thickened layer region is 3 to 10 times the Cu content ratio in the non-thickened layer region, and the content ratio of Cu is an austenitic stainless steel defined by the following formula 2:
Cu content ratio = [Cu] / ([Cr] + [Fe] + [Ni] + [Cu]
In the formula (2), [Cr], [Fe], [Ni] and [Cu] mean the weight percentages of Cr, Fe, Ni and Cu, respectively.

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