KR101903173B1 - Austenitic stainless steel having excellent hot workability and corrosion resistance and method of manufacturing the same - Google Patents

Abstract

Austenitic stainless steels excellent in hot workability and corrosion resistance and a method for producing the same are disclosed. The austenitic stainless steel according to one embodiment of the present invention may contain not more than 0.05% C, 1.0 to 2.0% Si, not more than 1.0% Mn (excluding 0), 5.0 to 9.0% Ni, 5.0 to 9.0% Cr And the balance Fe and other unavoidable impurities, wherein the total amount of Fe and other inevitable impurities is 15.0 to 18.0%, N is 0.05% or less, Cu is 2.0 to 5.0%, Mo is 0.5 to 2.0%, Sn is 0.05 to 0.5% The SCI (sulfuric acid corrosion resistance index) defined by the following formula (1) is 50 or more.
SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1)

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to austenitic stainless steel having excellent hot workability and corrosion resistance, and a method for manufacturing the same. 2. Description of the Related Art Austenitic stainless steels,

The present invention relates to austenitic stainless steel and a method of manufacturing the same, and more particularly, to austenitic stainless steel excellent in hot workability and corrosion resistance and a method of manufacturing the same.

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.

In the case of a member such as a heat exchanger containing sulfur oxides due to problems such as sulfuric acid dew point corrosion or the like, the temperature of the exhaust gas is maintained at a high temperature of 150 ° C or more so that sulfuric acid does not condense on the surface of the member. However, in order to recover heat energy effectively from the point of view of increasing energy demand and energy efficiency in recent years, there is a tendency that the temperature of the exhaust gas containing sulfur oxides is kept at a temperature below the sulfuric acid dew point, The material tends to be kept at a temperature below the dew point, and a material excellent in sulfuric acid dew point corrosion property is required.

In the temperature range of the sulfuric acid exhaust gas temperature of about 140 ° C, high concentration sulfuric acid of about 80% causes condensation on the surface. Conventionally, a steel material exhibiting corrosion resistance in a sulfuric acid environment has been developed for such sulfuric acid dew point corrosion problem.

For example, Sb or Sn, which is effective for sulfuric acid corrosion, is added to a low alloy steel containing Sb and Cu, and copper which contains Sb or Sn to improve sulfuric acid resistance and hydrochloric acid resistance while improving sulfuric acid resistance and hydrochloric acid resistance. At the same time, development has been proceeding with steel containing Mo or B to improve hot workability.

Specifically, Patent Document 1 discloses a steel containing Sb or Sn in a copper-containing steel to improve the hydrochloric acid resistance while maintaining sulfuric acid resistance. Patent Document 2 discloses a steel having improved sulfuric acid resistance and hydrochloric acid resistance with the same steel component as that of Patent Document 1, and containing Mo or B to improve hot workability.

However, in the case of conventional carbon steel, sufficient corrosion resistance can not be exhibited in a facility using a high S-containing fuel having an S content exceeding 2%. When the S content is increased, the sulfuric acid concentration in the exhaust gas is increased, 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.

Japanese Laid-Open Patent Publication No. 1997-025536 (published Jan. 28, 1997) Japanese Laid-Open Patent Publication No. 1998-110237 (published on April 28, 1998)

The embodiments of the present invention can control the content of Sn and W of austenitic stainless steel to provide excellent corrosion resistance and resistance to sulfuric acid steel and STS 316L steel, which are mainly used in existing heat exchangers, Austenitic stainless steel capable of securing hot workability and a method for producing the same.

The austenitic stainless steel excellent in hot workability and corrosion resistance according to one embodiment of the present invention is austenitic stainless steel containing C: 0.05% or less, Si: 1.0-2.0%, Mn: 1.0% or less (excluding 0), Ni: 5.0 to 9.0% of Cr, 15.0 to 18.0% of Cr, 0.05 to 5.0% of N, 2.0 to 5.0% of Cu, 0.5 to 2.0% of Mo, 0.05 to 0.5% of Sn, 0.5 to 1.0% of W, The SCI (sulfuric acid corrosion resistance index) defined by the following formula (1) is in excess of 50, including unavoidable impurities.

SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1)

According to an embodiment of the present invention, the austenitic stainless steel may include 7.0 to 9.0% of Ni, 2.0 to 4.5% of Cu, 0.5 to 1.5% of Mo, and 0.05 to 0.1% of Sn .

Also, according to an embodiment of the present invention, the austenitic stainless steel may have a deactivation time of 10 hours or more in a 50% H ₂ SO ₄ solution.

According to an embodiment of the present invention, the austenitic stainless steel may have a corrosion loss of 20 mg / m ² hr or less in a mixed solution of 50% H ₂ SO ₄ and 0.2% HCl.

The method for producing an austenitic stainless steel excellent in hot workability and corrosion resistance according to one embodiment of the present invention is a method for producing an austenitic stainless steel having C: 0.05% or less, Si: 1.0-2.0%, Mn: 1.0% , Ni: 5.0 to 9.0%, Cr: 15.0 to 18.0%, N: 0.05% or less, Cu: 2.0 to 5.0%, Mo: 0.5 to 2.0%, Sn: 0.05 to 0.5% Hot-rolling an austenitic stainless steel slab having an SCR (sulfuric acid corrosion resistance index) of 50 or more, which is defined by the following formula (1), and cold rolling and cold-rolling annealing.

SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1)

Also, according to an embodiment of the present invention, the slab may be hot-rolled at a temperature of 1,100 to 1,200 ° C and cold-annealed at a temperature of 1,050 to 1,150 ° C.

According to an embodiment of the present invention, the cold-rolled and annealed steel sheet may further be subjected to cold salt pickling through neutral salt electrolysis and sulfuric acid electrolysis.

According to the embodiment of the present invention, the content of Sn and W of the austenitic stainless steel can be controlled to ensure excellent sulfuric acid corrosion resistance and good hot workability of the austenitic stainless steel in an environment of high concentration of sulfuric acid.

In addition, the austenitic stainless steel can be used as a heat exchanger, a chimney, a chimney, and the like applied to facilities for condensation and condensation of exhaust gas in boilers for thermal power generation and industrial boilers, The use of GGH (gas-gas-heater) ensures stable corrosion resistance when used.

1 is a graph illustrating anodic polarization characteristics in a 50% H ₂ SO ₄ solution of austenitic stainless steel according to an embodiment of the present invention.
FIG. 2 is a graph for explaining the change of the natural potential according to the immersion time in 50% H ₂ SO ₄ solution according to the W content of the austenitic stainless steel according to an embodiment of the present invention.
FIG. 3 is a graph for explaining the correlation between the W content of the austenitic stainless steel and the devolatilization time according to an embodiment of the present invention.
FIG. 4 and FIG. 5 are graphs for explaining the correlation between corrosion resistance and the sulfuric acid corrosion index of the austenitic stainless steel according to an embodiment of the present invention and the 50% H ₂ SO ₄ and 0.2% HCl mixed solution.
6 is a view showing a scale of a cold-rolled and annealed steel sheet of an austenitic stainless steel according to an embodiment.
7 is a photograph of the surface state of the austenitic stainless steel of FIG. 6 after evaluation of corrosion resistance.
8 is a graph showing the scale of the cold-rolled and annealed steel sheet of the austenitic stainless steel according to the comparative example.
9 is a photograph of the surface state of the austenitic stainless steel of FIG. 8 after evaluation of corrosion resistance.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

The austenitic stainless steel excellent in hot workability and corrosion resistance according to one embodiment of the present invention is austenitic stainless steel containing C: 0.05% or less, Si: 1.0-2.0%, Mn: 1.0% or less (excluding 0), Ni: 5.0 to 9.0% of Cr, 15.0 to 18.0% of Cr, 0.05 to 5.0% of N, 2.0 to 5.0% of Cu, 0.5 to 2.0% of Mo, 0.05 to 0.5% of Sn, 0.5 to 1.0% of W, Includes unavoidable impurities.

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.

C: not more than 0.05%, N: not more than 0.05%

C and N are present as intrinsic elements of Ti (C, N) carbonitride forming element. When C and N contents are increased, the contents of solid C and N which are not formed by carbonitride such as Ti (C, In addition, when used for a long time at 600 ° C or less after welding, Cr ₂₃ C ₆ carbide is generated and grain boundary corrosion occurs. Therefore, the content of C is not more than 0.05% for C, N Is limited to 0.05% or less.

Also, when the content of C + N is high, when the content of Ti is increased, the number of surface inclusions such as scabs is increased due to the increase of rigid inclusions, , The elongation and impact characteristics due to the increase in N decrease, so the C + N content is limited to 0.06% or less.

Si : 1.0 to 2.0% or less

Si is an element to be added as a deoxidizing element, and as the content of the ferrite phase-forming element increases, the stability of the ferrite phase increases.

The increase of Si content leads to the improvement of the formal potential and the oxidation resistance. Therefore, in the case of the present invention, it is preferable to add the Si content to at least 1.0% or more. When the content of Si is increased to 2.0% or more, problems such as increase of Si inclusions and surface defects occur. It is limited.

Mn: 1.0% or less (excluding 0)

Mn forms an austenite-phase stabilizing element or a precipitate such as MnS when contained in a large amount to lower the pitting resistance, so that its content is limited to 1.0% or less. However, excessive reduction of Mn causes an increase in refining cost and the like, and therefore it is more preferably limited to 0.6% or less.

Ni : 5.0 to 9.0%

Ni is an element for stabilizing the austenite phase and is an essential element for ensuring corrosion resistance in an environment where a high concentration of sulfuric acid is condensed. In order to sufficiently secure such an effect, at least 5.0% Ni content is required. As the Ni content increases, the corrosion resistance increases. When the content is increased by 9.0% or more, the effect is saturated, and Ni is an expensive element. Therefore, the content of Ni is limited to 5.0 to 9.0%. More preferably, the content of Ni can be limited to 7.0 to 9.0% in order to secure sufficient corrosion resistance.

Cr : 15.0 to 18.0%

Cr is an essential element for securing the corrosion resistance of the austenitic stainless steel. However, as in the present invention, an increase in the Cr content in an environment where a high concentration of sulfuric acid is condensed causes a problem of reducing the corrosion resistance of the sulfuric acid atmosphere. Therefore, in the present invention, the Cr content is preferably at least 15.0% or more, and when Cu and Mo are added at the same time, good corrosion resistance can be ensured in the sulfuric acid atmosphere in which the high concentration of sulfuric acid is condensed as described above. However, if the Cr content is more than 18.0%, the corrosion resistance decreases and the cost increases in a high sulfuric acid atmosphere.

Cu: 2.0 to 5.0%

Cu is an essential element for ensuring corrosion resistance in an environment where a high concentration of sulfuric acid is condensed as in the present invention. If the content of Cu is lower than 2.0%, it is impossible to secure corrosion resistance, so that at least 2.0% should be added. However, if the content of Cu is increased to 5.0% or more, corrosion resistance in a high concentration of sulfuric acid is increased, but problems such as rapid reduction in hot workability occur. More preferably, the content of Cu can be limited to 2.0 to 4.5% in order to ensure sufficient corrosion resistance and hot workability.

Mo : 0.5 to 2.0%

Mo is an effective element for securing the corrosion resistance of the austenitic stainless steel. Particularly, in the environment where high concentration of sulfuric acid is condensed, when 0.5% or more of Mo addition and Cu are simultaneously added, the corrosion resistance can be maximized. However, if the Mo content increases, the effect of increasing the corrosion resistance increases, but it is preferable not to add the Mo content exceeding 2.0% in view of the deterioration of the hot workability and the cost as an expensive element. More preferably, the Mo content can be limited to 0.5 to 1.5% in view of sufficient hot workability and cost reduction.

Sn : 0.05 to 0.5%

Sn is an indispensable element for securing the corrosion resistance in the sulfuric acid atmosphere and the composition and formation change of the target cold rolling annealing scale in the present invention. It is necessary to add at least 0.05% or more in order to suppress formation of Si oxide and ensure corrosion resistance after cold rolling annealing as in the present invention. However, if it is added excessively, the upper limit is limited to 0.5% due to deterioration of hot workability and reduction of efficiency of the manufacturing process. More preferably, the content of Sn can be limited to 0.05 to 0.1% in order to secure sufficient hot workability.

W: 0.5 to 1.0%

W is an effective element for securing the corrosion resistance of the sulfuric acid atmosphere. Particularly, in the environment where high concentration of sulfuric acid is condensed, addition of 0.5% or more of W and Cu simultaneously can maximize the corrosion resistance. However, when the W content is increased, the effect of increasing the corrosion resistance is increased, but it is preferable not to add more than 1.0% in view of the deterioration of hot workability and the cost element as an expensive element in the case of W.

The austenitic stainless steel excellent in hot workability and corrosion resistance according to one embodiment of the present invention has an SCI (sulfuric acid corrosion resistance index) of 50 or more, which is defined by the following formula (1).

SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1)

When the SCI is less than 50, corrosion resistance in a sulfuric acid atmosphere is reduced.

For example, the SCI is preferably 50 to 80. When the SCI is more than 80, the content of Mo, W, and Cu, which are elements for securing corrosion resistance, is increased to increase the corrosion resistance. However, there is a problem that the hot workability decreases as the number of these elements increases.

For example, the austenitic stainless steel may have austenitic single-phase structure when the structure is observed.

For example, the austenitic stainless steels may have a devolatilization time of at least 10 hours in a 50% H ₂ SO ₄ solution.

The deactivation deactivation time refers to the time after the passive film is formed on the cold-rolled annealed steel sheet and then the natural potential falls to -500 mV or less after immersing in a 50% H ₂ SO ₄ solution. When the time at which the natural potential falls to -500 mV or less is less than 10 hours, it is judged that the film characteristics and corrosion resistance are favorable. That is, when the deactivation time was 10 hours or longer, the passive film characteristics were not destroyed in the 50% H ₂ SO ₄ solution and the corrosion resistance was excellent.

When the W content is 0.5% or more, the devolatilization time is more than 10 hours, and a 50% H ₂ SO ₄ aqueous solution is added to the austenitic stainless steels , The passive film is not destroyed and sufficient sulfuric acid corrosion resistance can be secured.

For example, the austenitic stainless steel may have a corrosion loss of 20 mg / m ² hr or less in a mixed solution of 50% H ₂ SO ₄ and 0.2% HCl.

The austenitic stainless steel may have a corrosion loss of 20 mg / m ² hr or less in the dipping portion and the gaseous phase in a mixed solution of 50% H ₂ SO ₄ and 0.2% HCl.

The correlation between the corrosion loss of the gaseous phase and the dipping part and the sulfuric acid atmosphere corrosion resistance index (SCI) in 50% H ₂ SO ₄ and 0.2% HCl mixed solution was observed. The mixed solution is a solution simulating an environment in which a very small amount of chloride is added to an environment in which a high concentration of sulfuric acid is condensed. The GGH environment solution of the desulfurization equipment is analyzed. As a result, .

The corrosion loss in the dipping portion was tested by placing the specimen at the bottom of the vessel and immersing it in the solution in order to evaluate the corrosion characteristics of the austenitic stainless steel specimen in the corrosive solution, Corrosion properties of dipped parts ". In order to evaluate the corrosion characteristics of the corrosion solution in the condensed state on the surface of the specimen, the austenitic stainless steel specimen was not immersed in the mixed solution but the specimen was placed at the top of the container, The test specimens were condensed and tested for corrosion, which was defined as "Corrosive properties of the vapor phase".

The method for producing an austenitic stainless steel excellent in hot workability and corrosion resistance according to one embodiment of the present invention is a method for producing an austenitic stainless steel having C: 0.05% or less, Si: 1.0-2.0%, Mn: 1.0% , Ni: 5.0 to 9.0%, Cr: 15.0 to 18.0%, N: 0.05% or less, Cu: 2.0 to 5.0%, Mo: 0.5 to 2.0%, Sn: 0.05 to 0.5% Hot-rolling an austenitic stainless steel slab having an SCR (sulfuric acid corrosion resistance index) of 50 or more, which is defined by the following formula (1), and cold rolling and cold-rolling annealing.

SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1)

For example, the slab can be hot-rolled at a temperature of 1,100 to 1,200 占 폚 and cold-annealed at a temperature of 1,050 to 1,150 占 폚.

Thereafter, the cold-rolled and annealed steel sheet can be subjected to cold salt electrolysis through neutral salt electrolysis and sulfuric acid electrolysis.

The austenitic stainless steel of the present invention contains 0.05% or more of Sn and is uniformly formed as a thin layer without forming a scale on the surface of the cold-rolled and annealed steel sheet.

That is, the formation of the SiO ₂ scale layer can be suppressed after the cold rolling annealing by containing a certain amount of Sn. Therefore, in the past, hydrofluoric acid has been essentially added in the cold-rolling pickling process in order to remove such scales as the SiO ₂ scale layer is formed in an annular shape. However, even if the hydrofluoric acid is not added, sufficient cold- The effect of pickling can be obtained, and the process cost can be reduced.

Hereinafter, the present invention will be described in more detail with reference to examples.

Example  And Comparative Example

Cast ingots of 120 mm in thickness containing the respective composition components according to Examples 1 to 9 and Comparative Examples 1 to 11 in Table 1 were 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.

Specifically, the comparative steel 1 is sulfuric acid steel used as a gas-gas-heater (GGH) material in the deactivation ratio of soot, and the comparative steel 6 is a high alloy steel containing 6% to be.

(weight%) Si Mn Cr Ni Mo Cu N C Sb Sn W Example 1 1.1 0.6 15.1 7.8 1.1 3.1 0.03 0.03 - 0.06 0.5 Example 2 1.1 0.6 16.1 7.1 1.1 3.1 0.03 0.03 - 0.09 One Example 3 1.1 0.6 16.1 7.1 1.1 4.1 0.03 0.03 - 0.07 0.7 Example 4 1.1 0.6 16.1 8.1 1.1 3.1 0.03 0.03 - 0.08 0.6 Example 5 1.5 0.9 17.5 8.5 0.8 2 0.03 0.03 - 0.1 0.5 Example 6 1.5 0.8 17.8 7.6 0.6 3 0.03 0.03 - 0.15 0.5 Example 7 1.1 0.6 16.1 7.1 0.5 3.1 0.03 0.03 - 0.1 0.7 Example 8 1.1 0.6 16.1 8.1 0.5 3.1 0.03 0.03 - 0.05 0.6 Example 9 1.1 0.6 15.1 7.8 0.5 3.1 0.03 0.03 - 0.06 0.5 Comparative Example 1 0.03 0.03 0.03 0.03 0.03 0.5 0.03 0.03 0.1 - - Comparative Example 2 0.6 One 18 8 0.2 0.1 0.03 0.04 - - - Comparative Example 3 0.6 One 18 10 2 0.2 0.03 0.03 - - - Comparative Example 4 17.5 0.3 17 0.1 One 0.2 0.01 0.01 - - - Comparative Example 5 0.6 0.3 17 0.1 0.1 0.1 0.01 0.01 - - - Comparative Example 6 0.6 One 23 22 6 0.1 0.03 0.03 - 0.03 - Comparative Example 7 1.1 0.6 20.1 15.1 3.1 1.6 0.16 0.03 - 0.04 - Comparative Example 8 1.1 0.6 15.1 6.1 1.1 2.1 0.03 0.03 - 0.04 0.4 Comparative Example 9 1.1 0.6 15.1 6.1 1.1 3.1 0.03 0.03 - 0.04 0.3 Comparative Example 10 1.1 0.6 15.1 6.1 1.1 5.1 0.03 0.03 - 0.04 0.2 Comparative Example 11 1.1 0.6 15.1 4.6 1.1 4.6 0.03 0.03 - 0.03 0.2

1 is a graph illustrating anodic polarization characteristics in a 50% H ₂ SO ₄ solution of austenitic stainless steel according to an embodiment of the present invention.

FIG. 1 shows the anodic polarization curves of Example 7 containing Cu as an essential element of the present invention, Comparative Example 2 which is a conventional sulfuric acid steel, and Comparative Example 3 which is STS 304 hardness in an aqueous solution of 50% H ₂ SO ₄ at 70 ° C Respectively.

Comparative Example 2 is sulfuric acid steel of carbon steel component mainly used for GGH parts of a conventional soot desulfurization equipment, and detailed components are shown in Table 1 above. Comparative Example 3 is STS 304 steel, and Example 7 is a steel to which about 3% of Cu is added, and the detailed components are shown in Table 1 above.

% H ₂ SO ₄ aqueous solution environment 50 in this experiment is a simulation environment that the interval represents the corrosion measurement result up to the corrosion rate of the sulfuric acid concentration.

As a result of comparing the spontaneous potentials in 50% H ₂ SO ₄ aqueous solution, the sulfuric acid steel of Comparative Example 2 was about -350 mV, the STS 304 steel of Comparative Example 3 was about -300 mV, and the steel of Example 7 was about -250 mV It is found that the natural potential tends to increase with the addition of the element Cu and the corrosion potential increases and the corrosion resistance increases.

As a result of measuring the corrosion current density in the natural potential state, it was found that about 10 ^-5 A / cm ^{2 in} Comparative Example ² and about 10 ^-4 A / cm ^{2 in} Comparative Example 3, It is found that the current density is about 10 times higher than the current density. On the other hand, in the case of Example 7 containing about 3% of Cu, the current density is about 10 ^-6 A / cm ^2, which is about 10 times lower than that of sulfuric acid.

It can be seen that as the potential is gradually applied at the natural potential, current density tends to increase continuously in case of sulfuric acid steel. However, in the case of stainless steel, in the region of -200 mv or more, as the potential is gradually applied, the passive film is formed on the surface, and the current density is rapidly decreased.

As a result of measuring the maximum current density according to the application of the potential, the current density in Comparative Example 2 was about 0.5 A / cm ^2, and in Comparative Example 3 about 0.1 A / cm ² , the austenitic stainless steel of Comparative Example 3 had a sulfuric acid It can be seen that it has a somewhat lower value than steel. On the other hand, in the case of Example 7, the current density is 0.02 A / cm ^2, which is ten times lower than that of Comparative Example 2 and Comparative Example 3.

As a result of the measurement of the anode polarization characteristics described above, the corrosion current density of the ordinary STS 304 steel (Comparative Example 3) is higher than that of the sulfuric acid refractory steel (Comparative Example 2) It can be seen that the use is not suitable for heating.

In the case of normal sulfuric acid steel (Comparative Example 2), the corrosion current density has a low value, but the current density sharply increases according to the potential application. The actual desulfurization ratio does not exist solely by H ₂ SO ₄ but because the metal ions are present simultaneously as an oxidizing agent, the corrosion rate of the sulfuric acid steel rapidly increases.

On the other hand, in the case of Example 7, the corrosion current density and the maximum current density are lower than that of the sulfuric acid steel, and the passivation film is formed on the surface even in the presence of the metal ion as the oxidizing agent.

Accordingly, the properties of the cold-rolled steel sheets of the steels 1 to 9 and comparative steels 1 to 11 were measured and are shown in Table 2 below.

In order to confirm the hot workability of the steel sheet, an ingot manufactured to a thickness of 120 mm was hot-rolled at a temperature of 1,150 ° C. to produce an annual rolled steel sheet having a thickness of 3.0 mm. The criterion of hot workability was judged to be excellent in hot workability when the crack width and depth of the hot rolled plate were 10 mm or less and indicated as O. When the crack width and depth were more than 10 mm, Respectively.

Thereafter, the hot-rolled steel sheet having a thickness of 3.0 mm was cold-rolled and cold-annealed. The thickness of the cold rolled steel sheet was 1.2 mm, and cold annealing was performed for 1 minute at a temperature of 1,100 ° C. Thereafter, the cold-rolled and annealed steel sheet was immersed in a mixed acid solution of nitric acid and hydrofluoric acid to remove the cold-annealed scale to secure a cold-rolled and annealed steel sheet. The structure was observed through a ferrite scope and optical observation. At this time, specimen with ferrite single phase structure is indicated as F, specimen with austenite single phase structure is indicated as A, and specimen having austenite and martensite phase structure is indicated as A + M.

Sulfuric acid corrosion index of the formula (1) it was calculated using the de-passivating time is cold-rolled annealed steel sheets and then polished with # 600 1 il maintained after forming the passivation film 50% H ₂ SO ₄ solution for The corrosion loss was measured by measuring the amount of corrosion loss in the gaseous and dipped parts after immersing in a mixed solution of 50% H ₂ SO ₄ and 0.2% HCl for 24 hours.

Hot workability group Sulfuric Acid Corrosion Index (SCI) Deactivation time (hours) Corrosive weight loss in immersed parts (mg / m ² hr) Warming Corrosion Reduction (mg / m ² hr) Example 1 O A 67.35 over 10 5.65 1.65 Example 2 O A 66.5 over 10 6.32 1.95 Example 3 O A 77.75 over 10 5.88 2.38 Example 4 O A 68.5 over 10 4.48 2.00 Example 5 O A 54.75 over 10 15.23 7.87 Example 6 O A 63.05 over 10 10.35 9.11 Example 7 O A 62.75 over 10 5.88 2.38 Example 8 O A 65.5 over 10 4.48 2.00 Example 9 O A 64.35 over 10 5.65 1.65 Comparative Example 1 O F 6.24 0 167.56 73.20 Comparative Example 2 O A 26.2 One 95.20 63.32 Comparative Example 3 O A 42.4 1.5 65.20 13.66 Comparative Example 4 O F 7.7 0.3 177.74 93.13 Comparative Example 5 O F 2 0.2 175.31 95.37 Comparative Example 6 X A 97.2 2.1 16.45 17.45 Comparative Example 7 X A 80 1.32 10.00 3.69 Comparative Example 8 O A + M 50 7 - - Comparative Example 9 O A + M 61.75 4.3 - - Comparative Example 10 X A 85.5 4.1 6.12 2.34 Comparative Example 11 X A + M 75 1.4 - -

As a result of checking the hot workability of the steel sheet, it was found that the hot workability of Comparative Examples 6, 7, 10, and 11 was not ensured, As a result of observation of the structure of the cold-rolled and annealed steel sheet, it can be seen from Table 2 that all the embodiments of the present invention have austenitic single phase structure. In addition, the sulfuric acid corrosion index (SCI) calculated according to the steel component system was found to be 50 or more in all of the embodiments of the present invention.

FIG. 2 is a graph for explaining the change of the natural potential according to the immersion time in 50% H ₂ SO ₄ solution according to the W content of the austenitic stainless steel according to an embodiment of the present invention.

Referring to FIG. 2, the behavior of the natural potential change with the immersion time change in the 50% H ₂ SO ₄ solution according to the W content among the alloying elements of 15Cr-6Ni-3Cu steel is shown.

Since the stainless steel has a passive film, the natural potential at the initial stage of immersion is as high as about 400 mV, but the natural potential rapidly drops to -300 to -200 mV within about 10 minutes as time elapses.

When the content of W in the alloy elements is less than 0.5%, specifically, when 0.2% W and 0.4% W are contained, the natural potential drops sharply to about -500 mV during the immersion time within 3 hours.

However, when the content of W in the alloy elements is 0.5% or more, specifically 0.5% of W, 0.7% of W and 1.0% of W, the spontaneous potential is about -300 To -200 mV. As the content of the alloy element W increases, the natural potential has a higher value.

This is because the passive film containing the Cr / Fe oxide formed on the normal stainless steel surface dissolves slowly in the sulfuric acid solution. If the passive film completely dissolves over time, the natural potential value of -500 mV, which is the potential value of the stainless steel base material, I have.

2 shows that when the content of the alloy element W is 0.5% or more, the natural potential does not drop to -500 mV even when immersed in a 50% H ₂ SO ₄ solution for a dipping time of 10 hours or more. That is, when the content of W as an alloying element in the stainless steel is 0.5% or more, it can be seen that the passive film containing the Cr / Fe oxide is retained without being dissolved.

FIG. 3 is a graph for explaining the correlation between the W content of the austenitic stainless steel and the deactivation time according to an embodiment of the present invention.

The passivation annealing time was obtained by polishing the cold-rolled annealed steel sheet prepared according to the above Examples and Comparative Examples at # 600 for 1 day to form a passive film, immersing it in 50% H ₂ SO ₄ solution, Was measured to fall below -500 mV.

The time required for the natural potential to drop below -400 to -500 mV with immersion time was defined as the passivation time and the passivation film characteristics and corrosion resistance were considered to be favorable when the passivation time was less than 10 hours. That is, when the devolatilization time was more than 10 hours, the passive film characteristics were not broken in the 50% H ₂ SO ₄ solution and the corrosion resistance was evaluated to be excellent.

Referring to Table 2 and FIG. 3, there is shown a correlation between the deactivation time and the deactivation time according to the content of the alloying element W in the examples and the comparative examples of the present invention. The deactivation time increases gradually with the increase of the content of the alloy element W in the steel. When the content of the alloy element W is 0.5% or more, the passivation film is destroyed in the aqueous solution of 50% H ₂ SO ₄ . As a result, it can be seen that the content of the alloy element W in the steel should be at least 0.5% or more so that it has sufficient corrosion resistance in an aqueous 50% H ₂ SO ₄ solution.

FIG. 4 and FIG. 5 are graphs for explaining the correlation between corrosion resistance and the sulfuric acid corrosion index of the austenitic stainless steel according to an embodiment of the present invention and the 50% H ₂ SO ₄ and 0.2% HCl mixed solution.

4 and 5 show the correlation between the corrosion loss amount of the gas phase portion and the dipping portion and the sulfuric acid corrosion resistance index (SCI) in a mixed solution containing 0.2% HCl in 50% H ₂ SO ₄ as a corrosion test solution.

The mixed 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. This is a solution simulating this environment based on the analysis of the GGH environment solution of the desulfurization facility and containing Cl ion as a trace amount of chloride.

Referring to FIGS. 4 and 2, it can be seen that as the sulfuric acid corrosion index (SCI) increases, the corrosion loss of the gaseous phase tends to decrease gradually as a result of the corrosion reduction of the gaseous phase. For example, when the sulfuric acid corrosion resistance index (SCI) is 50 or more, it can be understood that the corrosion loss amount in the gaseous phase has a value of 20 mg / m ² hr or less, specifically 10 mg / m ² hr or less.

Referring to FIGS. 5 and 2, it can be seen that the corrosion loss of the dipped portion tends to decrease as the sulfuric acid corrosion index (SCI) increases, similar to the corrosion loss result of the gas phase portion. For example, when the sulfuric acid corrosion index (SCI) is 50 or more, it can be seen that the corrosion loss of the dipped portion has a value of 20 mg / m ² hr or less.

As a result, it was found that the sulfuric acid corrosion index (SCI) value should be above 50 in order to have excellent corrosion resistance in the mixed solution of 50% H ₂ SO ₄ and 0.2% HCl.

6 is a view showing a scale of a cold-rolled and annealed steel sheet of an austenitic stainless steel according to an embodiment. 7 is a photograph of the surface state of the austenitic stainless steel of FIG. 6 after evaluation of corrosion resistance. 8 is a graph showing the scale of the cold-rolled and annealed steel sheet of the austenitic stainless steel according to the comparative example. 9 is a photograph of the surface state of the austenitic stainless steel of FIG. 8 after evaluation of corrosion resistance.

6 and 7 are views of an austenitic stainless steel cold-rolled annealed steel sheet according to an embodiment of the present invention. Specifically, the cold-rolled annealed steel sheet according to Example 1 containing Sn at 0.06% After annealing for 1 minute, annealing scale was observed through TEM observation.

8 and 9 show a comparative example of the present invention in which an austenitic stainless steel cold rolled annealed steel sheet was observed. Specifically, a cold rolled annealed steel sheet according to Comparative Example 9 containing 0.04% Sn was heated at 1,100 DEG C for 1 minute After the heat treatment, the annealing scale was observed through TEM observation.

Referring to FIGS. 6 and 8, in the case of Comparative Example 9 in which the content of Sn is less than 0.05%, for example, 0.04%, the annealing scale of SiO ₂ after the cold rolling annealing is annularly formed on the surface as a whole. On the other hand, in the case of Example 1 in which the content of Sn is 0.05% or more, for example, 0.06%, the SiO ₂ annealing scale is not formed on the surface in an annular shape, Respectively.

Therefore, when the austenitic, including the content of Sn to 0.05% or more of stainless steel, if the bar that is formed of SiO ₂ annealed scale inhibition in the cold-rolled and annealed, or no addition of Sn comparison containing less than 0.05%, Steel, SiO ₂ The annealing scale is formed thick on the surface, and hydrofluoric acid must be added in the cold rolling to remove it. On the other hand, in the embodiments of the present invention, the SiO ₂ annealing scale is formed thin on the surface so that even when the hydrofluoric acid is not added during cold rolling annealing, sufficient cold rolling pickling effect can be obtained even through neutral salt electrolysis and sulfuric acid electrolysis, Can be saved.

7 and 9, FIG. 7 shows the cold-rolled annealed steel sheet of Example 1, FIG. 9 shows the cold-rolled annealed steel sheet of Comparative Example 9 after neutralizing salt electrolysis and cold- Respectively.

The corrosion resistance was evaluated by using a combined cycle corrosion tester. The combined cycle corrosion test conditions were as follows: salt spray (5% NaCl solution sprayed for 2 hours at 30 ° C), dry (4 hours drying at 25% temperature and 60 ° C), wet (relative humidity 90% ) Was repeated for 1 cycle. In this condition, the corrosion resistance was evaluated by observing the surface of the specimen after repeating 30 cycles.

9, in Comparative Example 9 in which the content of Sn was 0.04%, the scale of cold rolling annealing formed on the surface through neutral salt electrolysis and sulfuric acid electrolysis was not sufficiently removed, have. On the other hand, referring to FIG. 7, in the case of Example 1 in which the content of Sn is 0.06%, Cr / Fe oxide and Si oxide formed on the surface through neutral salt electrolysis and sulfuric acid electrolysis are sufficiently removed, Can be seen.

That is, the austenitic stainless steel cold-rolled annealed steel sheet according to the embodiment of the present invention can completely remove the cold rolling annealing scale through neutral salt electrolysis and sulfuric acid electrolysis, It is possible to obtain a sufficient cold rolling pickling effect even if the pickling process is not carried out at a later time.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will readily obviate modifications and variations within the spirit and scope of the appended claims. It will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims (7)

(Excluding 0), Ni: 5.0 to 9.0%, Cr: 15.0 to 18.0%, N: 0.05% or less, Cu: 2.0% or less (Sulfuric acid corrosion resistance index), defined by the following formula (1), containing 0.05 to 5.0%, Mo: 0.5 to 2.0%, Sn: 0.05 to 0.1%, W: 0.5 to 1.0%, balance Fe and other unavoidable impurities. Is 50 to 80 and is excellent in hot workability and corrosion resistance.
SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1) delete The method according to claim 1,
The austenitic stainless steel is excellent in hot workability and corrosion resistance with a deactivation time of 10 hours or more in a 50% H ₂ SO ₄ solution. The method according to claim 1,
The austenitic stainless steels are excellent in hot workability and corrosion resistance in a mixed solution of 50% H ₂ SO ₄ and 0.2% HCl with a corrosion loss of 20 mg / m ² hr or less. (Excluding 0), Ni: 5.0 to 9.0%, Cr: 15.0 to 18.0%, N: 0.05% or less, Cu: 2.0% or less (Sulfuric acid corrosion resistance index), defined by the following formula (1), containing 0.05 to 5.0%, Mo: 0.5 to 2.0%, Sn: 0.05 to 0.1%, W: 0.5 to 1.0%, balance Fe and other unavoidable impurities. Hot-rolling an austenitic stainless steel slab having a thickness of 50 to 80;
Cold rolling and cold-rolling annealing; And
A process for producing an austenitic stainless steel excellent in hot workability and corrosion resistance, which comprises cold rolling a cold-rolled and annealed steel sheet through neutral salt electrolysis and sulfuric acid electrolysis.
SCI = -Cr + 4Ni + 5 (Mo + 0.5W) + 12Cu - ????? (1) 6. The method of claim 5,
Wherein the slab is hot-rolled at a temperature of 1,100 to 1,200 占 폚 and cold-annealed at a temperature of 1,050 to 1,150 占 폚, thereby exhibiting excellent hot workability and corrosion resistance. delete

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