WO2018117488A1 - Austenitic stainless steel with excellent sulfuric acid corrosion resistance - Google Patents

Abstract

The present invention relates to austenitic stainless steel comprising, in percentage by weight: 0.05% or less (excluding 0%) of carbon (C); 0.05% or less (excluding 0%) of nitrogen (N); 1.0-2.0% of Si; 1.0% or less (excluding 0%) of manganese (Mn); 15-18% of chromium (Cr); 6.5-9.0% of nickel (Ni); 2.0-5.0% of copper (Cu); 0.5-2.0% of molybdenum (Mo); and the balance being Fe and inevitable impurities, wherein the austenitic stainless steel has a sulfuric acid corrosion index (SCI) of 40 or more that is represented by the formula: SCI = - [Cr] + 4[Ni] + 5[Mo] + 12 [Cu] (where [Cr], [Ni], [Mo], and [Cu] represent a weight percentage of Cr, Ni, Mo, and Cu, respectively). The present invention can provide austenitic stainless steel having excellent corrosion resistance to sulfuric acid dew point corrosion that causes various problems in a heat exchanger, stack, and chimney used for thermal power generation or industrial boilers, members for flue gas desulfurization facilities used in various industrial fields, or various members for equipment used in sulfuric acid environments.

Description

Austenitic stainless steel with excellent sulfuric acid corrosion resistance

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

So-called fossil fuels such as petroleum and coal, which are used as fuels for thermal power generation or industrial boilers, contain a large amount of sulfur (S). For this reason, when the fossil fuel is burned, 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 the flue gas desulfurization apparatus used in various industries, when the gas containing sulfur oxides (SO _X ) flows, sulfuric acid dew point corrosion occurs when the temperature decreases.

Due to the problem of dew point corrosion of sulfuric acid, in a member such as a heat exchanger containing sulfur oxide, the exhaust gas temperature is maintained at a high temperature of 150 ° C. or higher so that sulfuric acid does not condense on the member surface. However, in view of the recent increase in energy demand and increase in energy efficiency, there is a tendency to keep the temperature of the exhaust gas containing sulfur oxides lower than the dew point of sulfuric acid in order to effectively recover the thermal energy, and also to maintain the power generation equipment. There is a tendency to keep the temperature of the exhaust gas containing sulfur oxides at a temperature below the dew point of sulfuric acid in such an environment. Accordingly, development of steel materials capable of solving the problem of dew point corrosion of sulfuric acid, that is, exhibiting corrosion resistance in sulfuric acid environment has been in progress.

For example, Patent Document 1 (Unexamined-Japanese-Patent No. 1963-14585) discloses a low alloy steel in which Sb and Cu are compositely added to sulfuric acid corrosion resistance. Patent Literature 2 (JP-A-1997-25536) discloses a steel containing Sb or Sn in a copper-containing steel and improving hydrochloric acid resistance while maintaining sulfuric acid resistance. In addition, Patent Document 3 (Japanese Laid-Open Patent Publication No. 1998-110237) discloses steels having the same steel components as those described in Patent Document 2, improving sulfuric acid resistance and hydrochloric acid resistance, and containing Mo or B to improve hot workability. It is.

However, the low alloy steel disclosed in the above patent documents cannot exhibit sufficient corrosion resistance for a facility using a high S containing fuel having an S content of more than 2%. This is because the higher the S content, the higher the concentration of sulfuric acid in the exhaust gas, so that the amount of sulfuric acid condensation due to temperature decreases, so that the corrosive environment is severe. Due to these problems, the development of new steel grades that can secure stable corrosion resistance is required.

Accordingly, as a method for improving the corrosion resistance in an environment where sulfuric acid is concentrated (sulfuric acid dew point environment), Patent Document 4 (Korean Patent Laid-Open Publication No. 2000-0068736) has a Ni content of 12 to 27% and a Mo content of 2 to 5 Although high alloy austenitic stainless steel, which is%, has been disclosed, there are limitations in manufacturing and processing, such as poor workability and cracking during hot rolling.

(Patent Document 0001) JP 1963014585 A

(Patent Document 0002) JP 1997025536 A

(Patent Document 0003) JP 1998110237 A

(Patent Document 0004) KR 1020000068736 A

In order to solve such a conventional problem, the present invention, by adjusting the content of the component elements affecting the sulfuric acid corrosion characteristics appropriately, by controlling the sulfuric acid corrosion index (SCI) derived from the content of these elements, It is an object to provide austenitic stainless steel having excellent hot workability.

Austenitic stainless steel according to an embodiment of the present invention, in weight percent, carbon (C): 0.05% or less (excluding 0%), nitrogen (N): 0.05% or less (excluding 0%), silicon (Si): 1.0 to 2.0%, manganese (Mn): 1.0% or less (excluding 0%), chromium (Cr): 15 to 18%, nickel (Ni): 6.5 to 9.0%, copper (Cu): 2.0 ~ 5.0%, molybdenum (Mo): 0.5 to 2.0%, residual iron (Fe) and inevitable impurities, characterized in that the sulfuric acid corrosion index (SCI) defined by the following formula (1) is 40 or more.

SCI =-[Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu] ------ Equation (1)

However, [Cr], [Ni], [Mo], and [Cu] in Formula 1 refer to the weight% of Cr, Ni, Mo, and Cu, respectively.

In addition, according to an embodiment of the present invention, the sulfuric acid corrosion index (SCI) may be preferably 42 to 70.

In addition, according to an embodiment of the present invention, the stainless steel, phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), niobium (Nb) : 1.0% or less (excluding 0%), titanium (Ti): 0.5% or less (excluding 0%) and tungsten (W): 1.0% or less (excluding 0%) Can be.

In addition, according to an embodiment of the present invention, the austenitic stainless steel is austenite-based in a boiling solution in which a 0.2% concentration of ferric chloride (FeCl ₃ ) solution is added to a 50% sulfuric acid solution The corrosion loss of the gas phase and immersion parts of the stainless steel may be 20 mg / m ² hr or less. As used herein, the term "corrosion loss" means an amount of evaporated boiling solution condensed on the surface of an austenitic stainless steel, and the term "immersion corrosion loss" refers to an austenitic stainless steel. It means the amount of corrosion by dipping in boiling solution.

In addition, according to an embodiment of the present invention, the austenitic stainless steel is a gas phase part of the austenitic stainless steel in a boiling solution in which 0.2% hydrochloric acid (HCl) solution is added to a 50% sulfuric acid solution And immersion corrosion loss may be 20 mg / m ² hr or less.

In addition, according to an embodiment of the present invention, the austenitic stainless steel, when the immersion in sulfuric acid solution of 50% concentration may be formed a continuous Cu concentration layer on the surface of the stainless steel.

In addition, according to an embodiment of the present invention, the austenitic stainless steel, the Cu content ratio in the Cu enriched layer region may be 3 to 10 times the Cu content ratio in the non-concentrated layer region, The content ratio is defined by Equation 2 below.

Cu content ratio (%) = [Cu] / ([Cr] + [Fe] + [Ni] + [Cu]) * 100 ------ Equation (2)

However, in the formula (2), [Cr], [Fe], [Ni], and [Cu] mean the weight% of Cr, Fe, Ni, and Cu, respectively.

According to the present invention, an austenitic stainless steel having excellent sulfuric acid corrosion resistance and good hot workability in an environment where sulfuric acid of high concentration condenses can be provided by appropriately adjusting the content of each component element affecting sulfuric acid corrosion resistance. .

In addition, according to the present invention, in particular, by controlling the sulfuric acid corrosion index (SCI) derived by using the content of Cr, Ni, Mo and Cu to the corrosion resistance of sulfuric acid atmosphere to 40 or more auster having a low corrosion loss value in sulfuric acid atmosphere Knight-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, a chimney, which is a facility in which exhaust gases are concentrated and condensed, such as a thermal power boiler and an industrial boiler. It is also suitable for use as a member of GGH (gas-gas-heater) of soot desulfurization equipment.

1 is a graph showing the polarization polarization characteristics of 50% H ₂ SO ₄ aqueous solution of steel specimens having the composition of Example 6, Comparative Example 2, and Comparative Example 3.

FIG. 2 shows photographs and hot work evaluation criteria for steel specimens prepared according to Example 1 and Comparative Example 7. FIG.

3 is a graph showing the corrosion loss of the gas phase and the immersion corrosion according to the SCI value in the corrosion solution (1) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution).

Figure 4 is a graph showing the corrosion loss of the immersion portion 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 corrosion loss of the gas phase according to the SCI value in the corrosion solution (2) (50% H ₂ SO ₄ + 0.2% HCl solution).

6 is a graph showing the corrosion loss of the immersion portion according to the SCI value in the corrosion solution (2) (50% H ₂ SO ₄ + 0.2% HCl solution).

FIG. 7 is according to Example 1, Example 5, Comparative Example 2, Comparative Example 5, Comparative Example 6, Comparative Example 9 and Comparative Example 10 in the Greendes solution (17% H ₂ SO ₄ + 0.35% HCl solution) It is a graph showing the corrosion loss of the gas phase portion and immersion portion of the manufactured steel specimens.

8 is a graph showing surface coating composition analysis results 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.

It is a schematic diagram which shows the surface film formation mechanism of said FIG.

10 is a graph showing the results of surface coating composition analysis 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.

It is a schematic diagram which shows the surface film formation mechanism of said FIG.

Austenitic stainless steel according to an embodiment of the present invention, in weight percent, carbon (C): 0.05% or less (excluding 0%), nitrogen (N): 0.05% or less (excluding 0%), Si : 1.0 to 2.0%, manganese (Mn): 1.0% or less (excluding 0%), chromium (Cr): 15 to 18%, nickel (Ni): 6.5 to 9.0%, copper (Cu): 2.0 to 5.0% , Molybdenum (Mo): 0.5 to 2.0%, residual iron (Fe) and inevitable impurities, characterized in that the sulfuric acid corrosion index (SCI) defined by the following formula (1) is 40 or more.

SCI =-[Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu] ------ Equation (1)

However, [Cr], [Ni], [Mo], and [Cu] in Formula 1 refer to the weight% of Cr, Ni, Mo, and Cu, respectively.

Hereinafter, an austenitic stainless steel having excellent sulfuric acid corrosion resistance 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 of the present invention is, in weight%, carbon (C): 0.05% or less (excluding 0%), nitrogen (N): 0.05% or less (excluding 0%), Si: 1.0 to 2.0%, manganese (Mn): 1.0% or less (excluding 0%), chromium (Cr): 15 to 18%, nickel (Ni): 6.5 to 9.0%, copper (Cu): 2.0 to 5.0 %, Molybdenum (Mo): 0.5 to 2.0%, residual iron (Fe) and inevitable impurities, characterized in that the sulfuric acid corrosion index (SCI) defined by the following formula (1) is 40 or more.

SCI =-[Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu] ------ Equation (1)

However, [Cr], [Ni], [Mo], and [Cu] in Formula 1 refer to the weight% of Cr, Ni, Mo, and Cu, respectively.

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

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

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

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

Nitrogen (N) is an austenite stabilizing element and is an element that simultaneously improves high temperature strength and pitting resistance. However, when the nitrogen (N) is excessively added, the blown (pin hole), pin holes (pin hole), etc. during the casting to cause surface defects, such as hot workability is poor, so the content of nitrogen (N) Limit to 0.05%.

Silicon (Si): 1.0 to 2.0%

Silicon (Si) is an element added as a deoxidation element, and the ferrite phase stability increases when the content is increased as a ferrite phase forming element. Increasing the content of silicon (Si) will increase the formula potential and oxidation resistance. Therefore, in the case of the present invention, it is preferable to add at least 1.0% of silicon (Si) content, and when the content of silicon (Si) is increased to 2.0% or more, problems such as an increase in steelmaking Si inclusions and surface defects occur. It is limited not to exceed 2.0% or more at maximum.

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

Manganese (Mn), like nitrogen (N), is an element effective for stabilizing austenite phase. However, when the content of the manganese (Mn) is increased to form a precipitate, such as manganese sulfide (MnS) to reduce the pitting resistance, so the content of the manganese (Mn) is limited to 1.0% or less. On the other hand, if the content of the manganese (Mn) is excessively reduced since problems such as an increase in the purification cost may occur, preferably the content of the manganese may be limited to 0.6% or more. Therefore, the preferred content range of the manganese in the present invention may be 0.6 ~ 1.0%.

Chromium (Cr): 15-18%

Chromium (Cr) is an essential element for securing corrosion resistance of austenitic stainless steel. The higher the chromium (Cr) content is effective to improve the corrosion resistance, but as in the present invention, an increase in the chromium (Cr) content in a high concentration of sulfuric acid condensation may cause a decrease in corrosion resistance in the sulfuric acid atmosphere.

However, when constituents of the austenitic stainless steel according to the present invention contain at least 15% of chromium (Cr) together with Cu and Mo in amounts described below, it is possible to ensure good corrosion resistance in a sulfuric acid atmosphere in which the high concentration of sulfuric acid condenses. Can be. However, when the content of chromium (Cr) is more than 18%, even if Cu and Mo are added in combination, the corrosion resistance decreases and the cost increases in a high sulfuric acid atmosphere. Therefore, in the present invention, the chromium (Cr) content is limited to 15 to 18%.

Nickel (Ni): 6.5-9.0%

Nickel (Ni) is an austenite stabilizing element, and at the same time, it is an essential element for securing 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 where high concentration of sulfuric acid is condensed. On the other hand, when the content of nickel (Ni) exceeds 9%, the effect of increasing the corrosion resistance compared to the input amount is It does not appear and the high price of nickel makes the economy less expensive. Therefore, in this invention, content of nickel (Ni) is restrict | limited to 6.5 to 9.0%.

Copper (Cu): 2.0 to 5.0%

Copper (Cu) is an essential element for securing 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 where high concentration of sulfuric acid is condensed, whereas when the content of copper (Cu) exceeds 5.0%, Corrosion resistance increases, but problems such as a sudden drop in 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. When the content of molybdenum (Mo) is lowered, it is difficult to secure corrosion resistance, but when including more than 0.5% molybdenum (Mo) with the above-described copper (Cu) it can maximize the corrosion resistance in the environment of high concentration of sulfuric acid condensed. . On the other hand, when the content of molybdenum (Mo) exceeds 2.0%, the corrosion resistance is increased, but the hot workability is poor, manufacturing difficulties occur, and the economical price is low due to the high price of the 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 is phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), niobium (Nb): 1.0% Or less (excluding 0%), titanium (Ti): 0.5% or less (excluding 0%), and tungsten (W): 1.0% or less (excluding 0%) may further include one or more kinds. Hereinafter, the reason for numerical limitation regarding the content of these components is demonstrated.

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

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

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

Sulfur (S) is an impurity that is inevitably contained in steel and is an element that degrades hot workability and corrosion resistance, so the content thereof is preferably as small as possible. In particular, when the content of sulfur (S) exceeds 0.03%, corrosion resistance decreases significantly in an environment where sulfuric acid of high concentration is condensed.

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

When niobium (Nb) is included in the components of the austenitic stainless steel, carbon (C) may be fixed to act to increase corrosion resistance, intergranular corrosion resistance. However, when the content of niobium (Nb) is greater than 1.0%, the above-mentioned nitrogen (N) and nitride may be formed, thereby reducing corrosion resistance and hot workability.

Titanium (Ti): 0.5% or less (except 0%)

When titanium (Ti) is included in the components of the austenitic stainless steel, carbon (C) may be fixed as in niobium (Nb) to increase corrosion resistance, intergranular corrosion resistance. However, when the content of titanium (Ti) is greater than 1.0%, the above-mentioned nitrogen (N) and nitride may be formed, thereby reducing corrosion resistance and hot workability.

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

When tungsten (W) is included in the components of the austenitic stainless steel, it may act to increase corrosion resistance in an environment where high concentration of sulfuric acid is condensed. However, when the content of tungsten (W) exceeds 1.0%, the effect of increasing the corrosion resistance compared to the input amount may be insignificant and economic efficiency may be reduced.

On the other hand, 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 sulfuric acid atmosphere in the austenitic stainless steel element according to the present invention Is characterized in that more than 40.

SCI =-[Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu] ------ Equation (1)

However, [Cr], [Ni], [Mo] and [Cu] in the formula 1 refer to the weight percent of Cr, Ni, Mo and Cu, respectively.

When the sulfuric acid corrosion index (SCI) is less than 40, the corrosion loss value in an environment where high concentration of sulfuric acid is condensed may be higher than 20 mg / m ² hr. At this time, the corrosion reduction target value is determined by the value of corrosion loss in a high concentration of sulfuric acid boiling solution of a 6% Mo-containing austenitic stainless steel having excellent corrosion resistance among steels commonly used in the desulfurization equipment member.

Specifically, referring to FIGS. 3 to 6, when the sulfuric acid corrosion index (SCI) is 40 or more, the austenite in a boiling solution in which a 0.2% concentration of ferric chloride (FeCl ₃ ) solution is added to a 50% sulfuric acid solution The austenitic stainless steel in a boiling solution in which the gas phase and immersion corrosion loss of nitrous stainless steel is 20 mg / m ² hr or less, and 0.2% hydrochloric acid (HCl) solution is added to 50% sulfuric acid solution. It can be seen that the corrosion loss of the gas phase part and the immersion part is 20 mg / m ² hr or less.

In addition, when the sulfuric acid corrosion index (SCI) is 42 to 70, it is more preferable to provide an austenitic stainless steel having excellent corrosion resistance in an environment where high concentration of sulfuric acid is condensed, and at the same time, also excellent in hot workability. .

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

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

Cu content ratio (%) = [Cu] / ([Cr] + [Fe] + [Ni] + [Cu]) * 100 ------ Equation (2)

However, in the formula (2), [Cr], [Fe], [Ni], and [Cu] mean the weight% of Cr, Fe, Ni, and Cu, respectively.

The Cu concentrated layer is the film layer on the Cu of the austenitic stainless steel constituting elements in the H ₂ SO ₄ solution is preferentially dissolved by produced by forming a continuous Cu precipitates in the steel surface, the corrosion resistance in H ₂ SO ₄ solution It increases the role. Specifically, FIG. 9 schematically illustrates the above-described Cu thickening 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 Examples.

Example  1 to 6 and Comparative example  1 to 11

Ingots of 120 mm thickness containing each composition component according to Examples 1 to 6 and Comparative Examples 1 to 11 of Table 1 were then hot rolled at a temperature of 1,150 ° C. to 3.0 mm thick. A hot rolled steel sheet was produced. Thereafter, a cold rolled steel sheet having a thickness of 1.2 mm was manufactured through cold rolling, followed by cold rolling annealing and cold rolling at a temperature of 1,100 ° C. to prepare a final cold rolled pickling 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: An enamel coated specimen of the stainless steel specimen having the same chemical composition as that of Comparative Example 2.

Evaluation Example 1: Evaluation of Polarization Characteristics in Aqueous Sulfuric Acid Solution According to Cu Content

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

In this experiment, the 50% H ₂ SO ₄ aqueous solution environment simulates the section showing the maximum corrosion rate as a result of measuring the corrosion resistance according to the sulfuric acid concentration change. In addition, the steel having a composition of Comparative Example 2 is a carbon steel base sulfuric acid resistant steel (Cu content: 0.5%) mainly used in gas gas heater (GGH) parts of a conventional soot desulfurization facility, and a steel having a composition of Comparative Example 3 Is an STS 304 steel (Cu content: 0.1%), and the steel having the composition of Example 6 is an austenitic stainless steel in which the alloy component is configured so that the Cu content is 3.0%.

Details that can be confirmed from FIG. 1 are as follows.

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

Referring to FIG. 1, it can be seen that sulfuric acid resistant steel of Comparative Example 2 is -350 mV, STS 304 of Comparative Example 3 is -300 mV, and the steel manufactured according to Example 6 exhibits a natural potential value of -250 mV. From this, it can be seen that in the case of the steel having a composition of Example 6 having a Cu content of 3%, the natural potential is higher than that of the steel having the compositions of Comparative Examples 2 and 3.

(2) Corrosion current density at natural potential

Referring to Figure 1, the sulfuric acid resistant steel (Cu content: 0.5%) of Comparative Example 2 is 500㎂ level, STS 304 steel (Cu content: 0.1%) of Comparative Example 3 is 10mA level, the steel produced according to Example 6 (Cu content: 3.0%) can be seen that represents the corrosion current density in the state of natural potential of 50 mA level. That is, the STS 304 steel (Comparative Example 3) is about 10 times higher than the sulfuric acid resistant steel (Comparative Example 2), and the steel manufactured according to Example 6 is about 10 times higher than the sulfuric acid resistant steel (Comparative Example 2). Corrosion current density value in the natural potential state about twice as low, it can be seen that the corrosion current density in the state at the natural potential tends to increase with increasing Cu content.

In addition, referring to FIG. 1, it can be seen that in case of sulfuric acid resistant steel (Comparative Example 2), the current density tends to increase as the potential is gradually applied at the natural potential. However, in the steels of Comparative Examples 3 and 6, which are stainless steels, it can be seen that the current density rapidly decreases as a potential is gradually applied in a region of -200 mV or more, which results in the formation of a passive film on the surface of the stainless steel in the dislocation region. Because.

(3) Maximum current density due to potential application

1, the sulfuric acid resistant steel of Comparative Example 2 is 0.5A level, the STS 304 steel of Comparative Example 3 is 0.1A level, and the steel produced according to Example 6 has a maximum current density according to potential application of 0.02A level. It can confirm that it shows. From this, it can be seen that the steel having a composition of Example 6 having a Cu content of 3% has a maximum current density value several tens of times lower than that of steels having the compositions of Comparative Examples 2 and 3.

From the anode polarization measurement results of (1) to (3), the corrosion current density of the STS 304 steel (Comparative Example 3) is higher than that of sulfuric acid resistant steel (Comparative Example 2). Its corrosion resistance is inferior to that of sulfuric acid steel (Comparative Example 2), and its use is not appropriate. In addition, in the case of the sulfuric acid resistant steel (Comparative Example 2), the corrosion current density in the natural potential state has a low value, but the current density rapidly increases as the potential is applied. In the case of a soot desulfurization facility, the corrosion rate of sulfuric acid resistant steels increases rapidly because H ₂ SO ₄ does not exist alone, but metal ions, which are oxidants, coexist. On the other hand, in the case of the steel produced according to Example 6 (Cu content: 3%), since the corrosion current density and the maximum current density show lower values than the sulfuric acid resistant steel (Comparative Example 2), the corrosion resistance in sulfuric acid dew point atmosphere Not only is it excellent, it also has excellent corrosion resistance by creating a passivation film on the steel surface even in the presence of metal ions as oxidants.

Evaluation Example 2: Evaluation of SCI, Hot Workability and Structure

Sulfuric acid corrosion index (SCI), hot workability and structure were evaluated according to the following method, and the results are shown in Tables 2 and 3 below.

(1) Sulfuric acid corrosion index (SCI)

The sulfuric acid corrosion index (SCI) is to index the effect of the Cr, Ni, Mo and Cu content on the corrosion resistance of the steel specimen in the environment of high concentration sulfuric acid through various experiments, when the SCI is 40 or more in sulfuric acid atmosphere It was judged that the corrosion resistance was excellent. At this time, the sulfuric acid corrosion index (SCI) is a value calculated according to the following formula (1).

SCI =-[Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu] ------ Equation (1)

In this case, [Cr], [Ni], [Mo] and [Cu] in Formula 1 refer to the weight% of Cr, Ni, Mo, and Cu, respectively.

(2) hot workability

According to Examples 1 to 6 and Comparative Examples 1 to 11, the hot workability was evaluated by the width and the depth of cracks generated at the edges of the hot rolled plates subjected to the hot rolling. In this case, it is determined that the hot workability is secured in the case of steel specimens having a width and depth of 10 mm or less, and is marked with "◎", and the hot workability is not determined in the case of steel specimens having a width and depth of cracks of 10 mm or more. As indicated by "X".

Specifically, FIG. 2 shows photographs and hot work evaluation criteria for steel specimens prepared according to Example 1 and Comparative Example 7.

(3) organization

According to Examples 1 to 6 and Comparative Examples 1 to 11, the structure of the steel specimens completed with pickling was observed using a ferrite scope (Fisher, Ferritescope MP30) and an optical microscope. At this time, the steel specimen having a ferrite single-phase structure "F", the steel specimen having an austenitic single-phase structure "A", the specimen having an austenitic + martensite structure is marked as "A + M".

Evaluation Example 3: Corrosion Loss Evaluation

The steel specimens prepared according to Examples and Comparative Examples were cut into a width of 30 mm × length of 30 mm, and the surface of the cut steel specimens were ground with # 120 abrasive paper, and then a mother liquor solution containing a high concentration of sulfuric acid (corrosive solution) ) Was tested for corrosion. At this time, the corrosion test was carried out in the ISO 28706 method.

First, a glass ball was placed on the bottom of the 3L beaker, and a condenser was installed on the top of the beaker to condense hot gas and water vapor. And, the corrosion solution was added to the extent that only a part of the glass ball in the beaker is submerged.

In this evaluation example, the corrosion loss of the immersion part and the gaseous part was measured. First, in order to evaluate the erosion loss of the immersion part, a steel specimen was placed at the bottom of the beaker (the lower glass ball), and the specimen was placed in a boiling corrosion solution. Immersion was allowed. In order to evaluate the loss of corrosion of the gas phase, a steel specimen is placed on the top of the beaker (upper glass ball), and hot gas and water vapor formed by evaporation of a boiling corrosion solution are condensed on the surface of the steel specimen. This was to occur. At this time, the corrosion test used only one steel grade in each beaker to minimize the mutual interference between the specimens.

In this evaluation example, the corrosion test was carried out using three kinds of corrosion solutions, each of the corrosion solution composition and the corrosion test conditions are as follows.

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

Corrosion solution (1) is a solution in which 0.2% FeCl ₃ solution is added to 50% sulfuric acid, which simulates an environment in which a small amount of oxidant is added to an environment where high concentration of sulfuric acid is condensed. The corrosion solution (1) is a solution that simulates the environment, found that the Fe ions generated from trace amounts of oxidant is contained as a result of analyzing the gas gas heater (GHG) environmental solution of the desulfurization facility.

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 unit area (m2), and the results are shown in Table 2 below.

-Corrosion solution: boiling

Corrosion time: 24 hours

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

(2) 50% H ₂ SO ₄ + 0.2% HCl solution

Corrosion solution (2) is a solution in which 0.2% HCl solution is added to 50% sulfuric acid, which simulates an environment in which a small amount of chloride is added to an environment where a high concentration of sulfuric acid is condensed. The corrosion solution (2) is a solution that simulates the environment found to contain Cl ions generated from a small amount of chloride as a result of analyzing the gas gas heater (GHG) environmental solution of the desulfurization facility.

In addition, the corrosion test conditions, corrosion loss measurement method and the measurement target are the same as in the corrosion solution (1), the corrosion loss side diameter results are shown in Table 2 below.

(3) 17% H ₂ SO ₄ + 0.35% HCl solution

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

The corrosion test conditions are as follows, and 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 3 below.

-Corrosion solution temperature: 80 ℃

Corrosion time: 6 hours

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

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

	SCI	Hot workability		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)
					Immersion	Meteorology	Immersion	Meteorology
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

Looking at Table 2, in the case of the steel specimens prepared according to Examples 1 to 6, it can be seen that the hot workability and sulfuric acid corrosion resistance is excellent. On the other hand, steel specimens prepared according to Comparative Examples 7, 8 and 11 are not suitable for use because they are inferior in hot workability and cause manufacturing problems. And, in the case of steel specimens prepared according to Comparative Examples 1 to 6, the corrosion loss of the gas phase in the corrosion solution (1) (50% H ₂ SO ₄ + 0.2% FeCl ₃ solution) and corrosion solution (2) (50% H ₂ SO ₄ + 0.2% HCl solution) in the immersion / gas phase corrosion loss can be seen that significantly higher than the above examples.

On the other hand, in the case of steel specimens prepared according to Comparative Examples 3 to 6, although the corrosion loss of the immersion portion in the corrosion solution (1) showed the same or lower level than the examples, the immersion portion in the GGH corrosion atmosphere of the actual desulfurization equipment Corrosion and gaseous phase corrosion occurs repeatedly, so even if the loss of corrosion in the gas phase is reduced corrosion life in the immersion part, the actual service life is shortened.

In the case of the steel specimen prepared according to Comparative Example 7, the hot workability was inferior, but the steel specimen contained 6% Mo having the best sulfuric acid corrosion resistance property used in a member such as a desulfurization facility. Based on the steel specimens of Comparative Example 7, it was determined whether the sulfuric acid corrosion resistance is excellent. Accordingly, the corrosion reduction standard target value was set to 20 mg / m ² hr, and all steel specimens prepared according to Examples 1 to 6 of the present invention exhibited low values below the corrosion reduction target value, so that excellent sulfuric acid corrosion resistance was achieved. It can be said that.

Meanwhile, FIGS. 3 and 4 show graphs of gas phase corrosion loss and immersion corrosion loss depending on the SCI value in the corrosion solution 1 (50% H ₂ SO ₄ + 0.2% FeCl ₃ 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.

First, referring to FIG. 3, which shows a graph of corrosion loss of the gas phase according to the SCI value in the corrosion solution (1), as the sulfuric acid corrosion index (SCI) increases, the corrosion loss gradually decreases, and the sulfuric acid corrosion index (SCI) value increases. In the case of 40 or more, it can be seen that the corrosion loss value is lower than the 20 mg / m ² hr target value of corrosion reduction. From the above results, the present inventors 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 of the corrosion solution (1).

In addition, referring to FIG. 4 which shows a graph of corrosion loss of the immersion part according to the SCI value in the corrosion solution (1), the corrosion loss is less than the reference value of 20 mg / m ² hr in all steel specimens except 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), the potential was gradually increased with the addition of the oxidizing agent, and the potential was -0.2 V vs. . If AgCl or higher, passivation film is formed on the surface of stainless steel, and the corrosion rate is drastically reduced, resulting from the reduction of corrosion loss. On the other hand, in the case of sulfuric acid resistant steels (Comparative Examples 1 to 2) of the carbon steel component, the corrosion rate increases rapidly with the addition of the oxidizing agent, thereby increasing the corrosion loss.

5 and 6 show graphs of gas phase corrosion loss and immersion corrosion loss according to SCI values 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.

First, referring to FIG. 5 which shows a graph of corrosion loss of gas phase according to SCI value in corrosion solution (2), as the sulfuric acid corrosion index (SCI) increases, the corrosion loss gradually decreases, and the sulfuric acid corrosion index (SCI) value increases. In the case of 40 or more, it can be seen that the corrosion loss value is lower than the 20 mg / m ² hr target value of corrosion reduction. From the above results, the present inventors 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 of the corrosion solution (2).

In addition, referring to FIG. 6 which shows a graph of immersion corrosion loss according to the SCI value in the corrosion solution (2), the corrosion loss gradually decreases as the sulfuric acid corrosion index (SCI) increases, unlike the result of immersion in FIG. Able to know. If chloride is added to the environment where sulfuric acid is concentrated in high concentration, corrosion is promoted by chloride, but even if the sulfuric acid corrosion index (SCI) value is above 40, it is lower than 20mg / m ² hr. You can see that it has.

On the other hand, Table 3 showing the corrosion test evaluation data in the corrosion solution (3) is shown below.

	SCI	Hot workability	group	Green Death Solution Corrosion Loss (g / cm 2 hr)
				Immersion *	Meteorological 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

Referring to Table 3, in the case of the steel specimens prepared according to Examples 1 and 5, compared to the steel specimens prepared according to Comparative Examples 2, 5, 6, 9 and 10, the value of corrosion loss of the immersion part and the gas phase part is small. It can confirm that it shows.

In addition, FIG. 7 is a graph showing the corrosion loss value of the gas phase portion and the immersion portion corrosion value in the Greendes solution for the steel specimens of the Examples and Comparative Examples described in Table 3. Referring to FIG. 7, it can be seen that in the steel specimens of the comparative examples, the immersion corrosion loss value is larger than the gas phase corrosion loss value.

However, in the case of the GGH corrosion atmosphere of the actual desulfurization equipment, the corrosion of the gas phase part that is exposed to corrosion by immersion in high concentration sulfuric acid solution and exposure to sulfuric acid fume and the like occur repeatedly. Therefore, even in the case of steel specimens prepared according to the comparative examples, even though the corrosion loss in the gas phase portion has excellent characteristics of low corrosion loss in the immersion portion deposited in high concentration of sulfuric acid, the actual service life is shortened.

On the other hand, austenitic stainless steels having compositions of Examples 1 and 5 have excellent corrosion loss characteristics in immersion and gas phase mode environments, and thus can be suitably used in a GGH corrosion atmosphere of a desulfurization facility.

Evaluation example  4: film composition analysis evaluation

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

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 about 3 to 10 times compared to the base metal within about 6 nm in the depth direction from the surface after being immersed in H ₂ SO ₄ solution. The present inventors observed the steel specimen coating characteristics of Example 3 through TEM observation, and found that Cu was continuously distributed on the surface of the steel specimen. 9 is a schematic diagram showing a continuous film formation mechanism.

Referring to FIG. 10, in the case of the steel specimen having the composition of Comparative Example 8, after the immersion in the H ₂ SO ₄ solution, it can be seen that the concentration of Cu and Ni in about 2 nm in the depth direction from the surface appeared somewhat thicker than the base metal. As a result of observing the steel specimen coating property of Comparative Example 8 through TEM observations, the inventors found that Cu, corrosion oxide, and base metal were discontinuously distributed on the surface. 11 is a schematic diagram illustrating a discontinuous peeling 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, when the Cu content is 2% or less, the Cu precipitates are discontinuously deposited on the surface of the steel specimen, and corrosion occurs in the place where Cu is not deposited, resulting in corrosion oxides, and as a result, corrosion resistance in the H ₂ SO ₄ solution. Is lowered.

As described above, the embodiments disclosed in the present invention are not intended to limit the technical idea of the present invention but to explain the present invention, and the scope of the present invention should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto. Should be construed as being included in the scope of the present invention.

Austenitic stainless steel having excellent sulfuric acid corrosion resistance according to embodiments of the present invention is used in heat exchangers, stacks and chimneys, and soot degassing devices used in thermal power generation or industrial boilers, and in sulfuric acid environments. It is applicable to various equipment.

Claims (7)

1. By weight, carbon (C): 0.05% or less (except 0%), nitrogen (N): 0.05% or less (except 0%), silicon (Si): 1.0-2.0%, manganese (Mn): 1.0 % Or less (excluding 0%), chromium (Cr): 15 to 18%, nickel (Ni): 6.5 to 9.0%, copper (Cu): 2.0 to 5.0%, molybdenum (Mo): 0.5 to 2.0%, balance Contains iron (Fe) and unavoidable impurities,

   Austenitic stainless steel having a sulfuric acid corrosion index (SCI) of 40 or more, which is defined by the following formula (1):

   SCI =-[Cr] + 4 [Ni] + 5 [Mo] + 12 [Cu] ------ Equation (1)

   However, [Cr], [Ni], [Mo], and [Cu] in Formula 1 refer to the weight% of Cr, Ni, Mo, and Cu, respectively.
2. The method of claim 1,

   The sulfuric acid corrosion index (SCI) is 42 ~ 70 austenitic stainless steel.
3. The method of claim 1,

   The stainless steel is phosphorus (P): 0.05% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), niobium (Nb): 1.0% or less (excluding 0%), titanium (Ti): Austenitic stainless steel further comprising at least one kind from the group consisting of 0.5% or less (excluding 0%) and tungsten (W): 1.0% or less (excluding 0%).
4. The method of claim 1,

   Austenite-based austenite-based corrosion loss of the austenitic stainless steel in the boiling solution of 0.2% concentration of ferric chloride (FeCl ₃ ) solution in 50% sulfuric acid solution of 20 mg / m ² hr or less Stainless steel.
5. The method of claim 1,

   An austenitic stainless steel having a corrosion reduction of the austenitic stainless steel in a boiling solution in which a 0.2% concentration of hydrochloric acid (HCl) is added to a 50% sulfuric acid solution of 20 mg / m ² hr or less.
6. The method of claim 1,

   An austenitic stainless steel in which a continuous Cu thickening layer is formed on the surface of the stainless steel when the stainless steel is immersed in a 50% sulfuric acid solution.
7. The method of claim 6,

   The Cu content ratio in the Cu enriched layer region is 3 to 10 times the Cu content ratio in the non-concentrated layer region, and the content ratio of Cu is an austenitic stainless steel defined by Equation 2 below:

   Cu content ratio (%) = [Cu] / ([Cr] + [Fe] + [Ni] + [Cu]) * 100 ------ Equation (2)

   However, in the formula (2), [Cr], [Fe], [Ni], and [Cu] mean the weight% of Cr, Fe, Ni, and Cu, respectively.

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