KR101735003B1 - Lean duplex stainless steel with improved corrosion resistance and method of manufacturing the same - Google Patents

Abstract

A linseed duplex stainless steel having improved corrosion resistance and a method for manufacturing the same are disclosed. A method of manufacturing a lean duplex stainless steel according to an embodiment of the present invention includes the steps of cold rolling a lean duplex stainless steel and cooling the cold rolled steel sheet in a hydrogen atmosphere at a dew point of -70 to -40 占 폚, And controlling the temperature to 1,200 캜 and maintaining the temperature for 10 seconds or longer. Therefore, by omitting the cold rolling annealing and pickling step and annealing the hydrogen atmosphere, the Fe and Cr oxides in the surface oxide film of the stainless steel are sufficiently reduced, and Si is oxidized to form a rich Si oxide, whereby the corrosion resistance of the lean duplex stainless steel can be improved.

Description

TECHNICAL FIELD [0001] The present invention relates to a lean duplex stainless steel having improved corrosion resistance and a method of manufacturing the same. BACKGROUND ART [0002]

The present invention relates to a linseed duplex stainless steel having improved corrosion resistance and a method of manufacturing the same, and more particularly, to a method of manufacturing a linseed duplex stainless steel by strip casting in which a cold rolling annealing and pickling process is omitted, To a lean duplex stainless steel capable of securing an improved corrosion resistance and a method of manufacturing the same.

Austenitic stainless steels generally known to have good processability and corrosion resistance include iron (Fe) as a base metal and chromium (Cr) and nickel (Ni) as main raw materials, and molybdenum (Mo) and copper ) Are added to various kinds of steel to meet various applications.

STS 304 steel, which is advantageous in terms of surface quality and corrosion resistance, has been commonly used for interior and exterior materials of existing buildings. These austenitic stainless steels are excellent in corrosion resistance and pitting resistance, and contain low-carbon and more than 8 wt% of Ni components. As a result, the austenitic stainless steels are unstable in cost because of high fluctuation of cost due to rise in Ni price, resulting in low competitiveness. Therefore, in order to overcome this problem, various studies are being conducted to develop a new steel type capable of securing corrosion resistance equal to or higher than that of austenitic stainless steel while lowering the Ni content.

Accordingly, the use of ferritic grades that are close to the characteristics of the above austenitic stainless steels and that have good price competitiveness is increasing. However, ferritic stainless steels have lower surface hardness and lower corrosion resistance than austenitic stainless steels.

Duplex stainless steel is a stainless steel having an austenite phase and a ferrite phase with a volume fraction of about 35 to 65%, respectively. The duplex stainless steel is economical because it has a low Ni content while ensuring corrosion resistance equivalent to that of a conventional austenitic stainless steel, It is attracting attention as a steel material for industrial facilities such as desalination facilities, pulp, paper, and chemical equipment requiring corrosion resistance. In recent years, a duplex stainless steel has been used in which a high-cost alloy such as Ni and Mo is excluded and a low-cost alloy element is added in place of the element to increase the advantages of a low alloy cost, such as lean duplex stainless steel ) Is growing in interest.

Generally, the cold-rolled steel sheet of lean-duplex stainless steel has been noted for improving the formability of the lean-duplex stainless steel by cold-rolling annealing. When annealing is performed at a high temperature in cold rolling annealing, the surface of the lean duplex stainless steel has an irregular surface shape, and the surface gloss is poor, and the corrosion resistance is lowered.

Korean Patent Publication No. 10-2014-0082346

Embodiments of the present invention include the step of oxidizing Si while forming a rich Si oxide while sufficiently reducing the Fe and Cr oxides in the surface oxidation film of stainless steel through the heat treatment of the hydrogen atmosphere by omitting the cold rolling annealing and pickling process after the cold rolling of the lean duplex stainless steel To provide a method of manufacturing a lean duplex stainless steel capable of improving corrosion resistance and a lean duplex stainless steel produced thereby.

A method of manufacturing a lean duplex stainless steel having improved corrosion resistance according to an embodiment of the present invention includes: (a) a step (b) in which carbon (C) is 0.08% or less (more than 0), silicon (Si) is 0.2 to 3.0% (Ni), 0.2 to 0.3% of nitrogen (N), 0.5 to 2.5% of copper (Cu) and 0.1 to 1.0% of tungsten (W) (Fe), and other unavoidable impurities, and a step of cold-rolling the cold-rolled steel sheet at a dew point of -70 to -40 占 폚 in a hydrogen atmosphere and a heat treatment temperature of 1,000 to 1,200 占 폚 And maintaining the atmosphere for at least 10 seconds to perform an atmosphere heat treatment.

According to an embodiment of the present invention, the method may further include a step of hot-annealing the lean-duplex stainless steel at a temperature of 1,000 to 1,250 ° C for 2 to 40 minutes before cold-rolling the lean-duplex stainless steel.

In one embodiment of the present invention, a linseed duplex stainless steel having improved corrosion resistance is manufactured according to the above manufacturing method.

According to an embodiment of the present invention, silicon (Si) may be contained in an amount of 10 wt% or more with respect to the total weight of oxides contained in a region within 0.02 mu m in depth direction from the surface of the lean duplex stainless steel.

Also, according to one embodiment of the present invention, the lean duplex stainless steel may contain 45 to 75% by volume of the austenite phase and the remainder of the ferrite phase.

According to an embodiment of the present invention, the lean area ratio may be less than 1% when the lean area ratio of the lean duplex stainless steel is ASTM D610.

Embodiments of the present invention can eliminate the cold annealing and pickling process in the production of lean duplex stainless steel and oxidize Si while sufficiently reducing Fe and Cr oxides in the surface oxidation film of the stainless steel through heat treatment in a hydrogen atmosphere to form a rich Si oxide, The corrosion resistance of the duplex stainless steel can be improved.

1 is a flowchart illustrating a method of manufacturing a lean duplex stainless steel according to an embodiment of the present invention.
FIG. 2 is a graph for explaining oxidation and reduction reaction curves of oxides according to a heat treatment temperature and a dew point during a heat treatment in a hydrogen atmosphere.
FIGS. 3 and 4 are photographs of a surface of a lean duplex stainless steel specimen according to an embodiment and a comparative example of the present invention after the evaluation of the surface area ratio according to ASTM D610. FIG.
5 to 8 are graphs showing the elemental distribution of oxides on the surface of a stainless steel specimen of a lean duplex stainless steel according to Examples and Comparative Examples of the present invention by GDS (Glow Discharge Spectrometer) analysis.

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.

1 is a flowchart illustrating a method of manufacturing a lean duplex stainless steel according to an embodiment of the present invention.

Referring to FIG. 1, a linseed duplex stainless steel having improved corrosion resistance according to an embodiment of the present invention is manufactured by cold-rolling a linseed duplex stainless steel, dew pointing the cold-rolled steel sheet in a hydrogen atmosphere at -70 to -40 ° C., and the heat treatment temperature is controlled to 1,000 to 1,200 ° C. and maintained for 10 seconds or longer.

The lean duplex stainless steel comprises 0.08% or less of carbon (C), 0.2-3.0% of silicon (Si), 2.0-3.0% of manganese (Mn), 19.0-23.0% of chromium (Cr) (Fe) and other unavoidable impurities, in an amount of 0.3 to 2.5%, 0.2 to 0.3% of nitrogen (N), 0.5 to 2.5% of copper (Cu) and 0.1 to 1.0% of tungsten (W)

The content of carbon (C) is 0.08% or less (more than 0).

C is an austenite-forming element, which is effective in increasing the strength of the material by solid solution strengthening. However, when the content is excessive, carbide formation such as Cr which is effective for corrosion resistance at the ferrite-austenite phase boundary is easily combined and the Cr content around the grain boundary is lowered It is preferable to limit the content of C to 0.08% or less in order to maximize the corrosion resistance.

The content of silicon (Si) is 0.2 to 3.0%.

Si is added for some deoxidizing effect, but it is also an element which is enriched in ferrite when annealing with a ferrite forming element. Therefore, it is preferable to add at least 0.2% in order to secure a proper ferrite phase fraction. However, when it is excessively added in an amount exceeding 3.0%, the hardness of the ferrite phase is rapidly increased, which affects the lowering of the elongation, making it difficult to secure the austenite phase for securing sufficient elongation. Also, when the content of Si is excessive, the slag fluidity is deteriorated at the time of steelmaking, and it forms an inclusive bond with oxygen to adversely affect the corrosion resistance. Therefore, the Si content is preferably limited to 0.2 to 3.0%.

The content of manganese (Mn) is 2.0 to 3.0%.

Mn is an element that increases the nitrogen solubility and is an austenite forming element. When it is used as an expensive substitute for Ni, when it is added in an amount exceeding 3.0%, it has an effect on nitrogen solubility but forms MnS by binding with S It is difficult to secure corrosion resistance at STS 304 level. When the content of Mn is less than 2%, it is difficult to secure a proper austenite phase fraction even if Ni, Cu, N and the like as the austenite forming elements are controlled, and the solubility of nitrogen added is low so that sufficient employment of nitrogen is obtained at normal pressure I can not. Therefore, the content of Mn is preferably limited to 2.0 to 3.0%.

The content of chromium (Cr) is 19.0 to 23.0%.

Cr is a ferrite phase stabilizing element together with Si, and plays an important role in securing ferrite phase of two-phase stainless steel and is an essential element for securing corrosion resistance. However, in order to maintain the phase fraction of the two-phase stainless steel and ensure the corrosion resistance of STS 304 or more, the content of Cr Is preferably limited to 19.0 to 23.0%.

The content of nickel (Ni) is 0.3 to 2.5%.

Ni plays a major role in securing the austenite phase of the two-phase stainless steel as an austenite stabilizing element together with Mn, Cu, and N. [ In order to reduce the cost, instead of decreasing the Ni content which is high in price, the content of other austenite phase forming elements Mn and N can be increased sufficiently to maintain the phase fraction balance by reduction of Ni sufficiently. However, it is preferable to add at least 0.3% in order to secure sufficient austenite stability in order to suppress the formation of fired organic martensite which occurs during cold working. When a large amount of Ni is added, austenite phase fraction is increased and it is difficult to secure a proper austenite phase fraction. Especially, it is difficult to secure competitiveness compared to STS 304 steel due to an increase in manufacturing cost due to expensive Ni. Therefore, it is preferable to limit the content of Ni to 0.3 to 2.5%.

The content of nitrogen (N) is 0.2 to 0.3%.

N is an element contributing to the stabilization of the austenite phase together with Ni in the two-phase stainless steel, and is one of the elements in which austenite is concentrated in the annealing heat treatment, and the increase of the N content can increase the corrosion resistance and increase the strength have. However, the solubility of N varies depending on the content of Mn added. On the other hand, in order to improve the corrosion resistance, it is preferable to add 0.2% or more of nitrogen, and if the N content is too low, it becomes difficult to secure a proper phase fraction. Therefore, it is preferable that the N content is limited to 0.2 to 0.3%.

The content of copper (Cu) is 0.5 to 2.5%.

Cu is an element that stabilizes the austenite phase such as Ni, Mn and N, and increases the corrosion resistance of the stainless steel in a sulfuric acid atmosphere. If the content of Cu is less than 0.5%, corrosion resistance may be deteriorated and a problem may arise. On the other hand, if the content of Cu exceeds 2.5%, the hot workability of the two-phase stainless steel is lowered and cracks occur during hot working Thereby making it difficult to substantially operate the engine. Therefore, it is preferable to limit the Cu content to 0.5 to 2.5%.

The content of tungsten (W) is 0.1 to 1.0%.

W can be added as an austenite forming element to replace Mo as an element improving corrosion resistance. On the other hand, W may induce intermetallic compound formation at 700 to 1,000 at the time of heat treatment, resulting in deterioration of corrosion resistance and mechanical properties. When the content of W is more than 1%, there is a problem that the corrosion resistance and especially the elongation rate are drastically lowered due to the formation of an intermetallic compound. On the other hand, in order to secure the predetermined corrosion resistance of the two-phase stainless steel, Do. Therefore, it is preferable to limit the content of W to 0.1 to 1.0%.

A lean duplex stainless steel according to an embodiment of the present invention produces a lean duplex stainless steel slab containing the above composition through a steelmaking process (1) and a strip casting (2). Thereafter, the slab is subjected to hot rolling, hot rolling annealing (3), cold rolling (4), and then cold rolling the cold rolled steel sheet to atmosphere heat treatment (5).

Specifically, in the steelmaking step (1), the raw material is compounded and dissolved so that the lean duplex stainless steel satisfies the target composition. Thereafter, molten steel melted in the steelmaking step (1) is cast through the strip casting (2) to produce a slab having a thickness of 5 mm or less. The slab is hot-rolled (3) to produce a hot-rolled coil, and the hot-rolled coil is cold-rolled (4) to produce a cold-rolled steel sheet having a thickness of 2 mm or less. According to the manufacturing method of a lean duplex stainless steel according to an embodiment of the present invention, the cold-rolled steel sheet is subjected to an atmosphere heat treatment (5) To produce a lean duplex stainless steel.

The lean duplex stainless steel is subjected to a hot-rolled annealing (3) step of hot-annealing the lean-duplex stainless steel slab at a temperature of 1,000 to 1,250 ° C for 2 to 40 minutes before cold rolling (4).

During the hot annealing process, the temperature is maintained at 1,000 to 1,250 DEG C for 2 to 40 minutes.

If the heat treatment temperature for hot rolling annealing is less than 1,000 占 폚, defects are generated during the cold rolling of the lean duplex stainless steel, and there is a tendency that the heat treatment ferrite and the austenite are recovered and recrystallization is insufficient and the elongation rate is lowered. Further, when the heat treatment temperature for hot-rolling annealing exceeds 1,250 占 폚, there is a problem that it is difficult to obtain a tensile strength of 800 MPa or more with the crystal grains of the ferrite and the austenite being too coarse.

When the heat treatment time of the hot-rolled annealing is short, that is, less than 2 minutes, diffusion in which alloying elements are distributed during heat treatment becomes insufficient and consequently the alloying elements are not sufficiently concentrated in the austenite phase. There is a problem that it is difficult to control the formation rate of the transformed organic martensite. In addition, when the hot-annealing time is longer than 40 minutes, deflection of the hot-rolled steel sheet occurs, resulting in lowering of productivity and loss of heat source unit. Therefore, it is preferable that the optimum annealing heat treatment condition is maintained at 1,000 to 1,250 캜 for 2 to 40 minutes and then heat-treated.

Then, the linseed duplex stainless steel in which the hot-rolled annealing process is completed is cold-rolled at a total reduction ratio of 50% or more.

Thereafter, the cold-rolled steel sheet is subjected to a cold-rolling annealing process, and the cold-rolled steel sheet is maintained at a dew point of -70 to -40 ° C and a heat treatment temperature of 1,000 to 1,200 ° C in a hydrogen atmosphere for 10 seconds or longer Atmosphere heat treatment to produce the above-mentioned lean duplex stainless steel.

In the atmosphere heat treatment (5), heat treatment is continuously performed in a hydrogen atmosphere at -70 ° C to -40 ° C at a temperature of 1,000 to 1,200 ° C so that the holding time becomes 10 seconds or more.

For example, the atmosphere heat treatment (5) can be performed in a brass annealing furnace. The heat treatment in the hydrogen atmosphere reduces the oxides in the oxide film formed on the surface of the stainless steel duplex stainless steel and the Si and Cr components in the oxide film are enriched and the corrosion resistance is improved.

The corrosion resistance can be improved by constituting the main component of the oxide film formed on the surface of the lean duplex stainless steel with the oxides rich in Si and Cr.

Thus, in order to constitute the main component of the oxide film with oxides rich in Si and Cr, a hydrogen atmosphere having excellent reducing properties is required. Here, the hydrogen atmosphere is a condition in which an impurity gas which is unavoidably involved in the industrial production process and a heat treatment of the atmosphere allow the inclusion of a gas which is inevitably involved in the process of replacing the inside with hydrogen, It says.

FIG. 2 is a graph for explaining oxidation and reduction reaction curves of oxides according to a heat treatment temperature and a dew point during a heat treatment in a hydrogen atmosphere.

Referring to FIG. 2, FIG. 2 shows oxidation and reduction reaction curves of oxides according to the heat treatment temperature and dew point during the hydrogen atmosphere heat treatment. Fig. 2 shows the influence of the dew point and temperature of hydrogen gas on the oxidation-reduction of Fe, Cr and Si.

In order to form an oxide having excellent corrosion resistance on the surface of the stainless steel of the lean duplex stainless steel, the Fe and Cr oxides formed after the cold rolling should be reduced and the surface should be replaced with an oxide rich in Si components. In order to reduce the Fe and Cr oxides and replace the surface with an oxide rich in Si components, it is necessary to set the dew point at -40 캜 or lower in the atmosphere heat treatment temperature range of 1,000 to 1,200 ° C. In the region where the temperature of the dew point is lower than -70 占 폚, the Si oxide is reduced and the corrosion resistance is lowered. Therefore, it is necessary to maintain the dew point within the range of -70 占 폚 to -40 占 폚 in the heat treatment temperature range of 1,000 to 1,200 占 폚.

In general, the higher the annealing temperature and the lower the annealing temperature, the better the annealing temperature is, and the better the annealing temperature is, the annealing temperature 5 should be 1,000 to 1,200 ° C. If the annealing temperature is less than 1,000 ° C, the ferrite and austenite are not recovered and recrystallized during the heat treatment of the atmosphere, and the elongation tends to be lowered. When the atmospheric heat treatment temperature exceeds 1,200 占 폚, it is impossible to obtain a tensile strength of 800 MPa or more with the crystal grains of the ferrite and the austenite being too coarse.

At the atmosphere heat treatment (5), the heat treatment temperature should be maintained at 1,000 to 1,200 ° C for at least 10 seconds. A holding time of at least 10 seconds or more is required in order to sufficiently reduce Fe and Cr oxides in the surface oxide film and to form a Si-rich oxide to improve the corrosion resistance.

Thus, a lean duplex stainless steel having improved corrosion resistance can be produced.

The lean duplex stainless steel produced according to the above may include austenite phase and ferrite phase in a microstructure.

For example, the lean duplex stainless steel may comprise 45 to 75% by volume of the austenite phase and the remainder of the ferrite phase. The austenite phase may be 45% to 75% by volume fraction and the ferrite phase may be 25% to 55% by volume fraction.

When the volume fraction of the austenite phase is less than 45%, excessive austenitization of the austenite forming element occurs in the austenite phase, thereby suppressing the amount of fired organic martensite transformation and increasing the tensile strength of the two-phase stainless steel by increasing the austenite strength However, the ductility of the two-phase stainless steel is also lowered, so that the elongation at the STS 304 steel level of 50% or more can not be secured.

On the other hand, when the volume fraction of the austenite phase is more than 75%, surface cracking occurs during rolling, which deteriorates hot workability, and the phase fraction of the ferrite phase is not balanced, so that the physical properties of the duplex stainless steel may be lost . Therefore, it is preferable that the austenite phase has a volume fraction of 45% to 75%.

In addition, it is most preferable that the duplex stainless steel is composed of only an austenite phase and a ferrite phase. For example, if the duplex stainless steel consists only of an austenite phase and a ferrite phase, the ferrite phase may be remnant of the austenite phase, and thus the ferrite phase may be 25% to 55% by volume fraction.

On the other hand, in the duplex stainless steel, a part of the austenite phase may be transformed into sintered organic martensite during cold working.

For example, in the duplex stainless steel, the amount of fired organic martensite formed in the cold working may be 5% or less. The calcined organic martensite is a phase formed when the unstable austenite is deformed, and can induce work hardening to increase the elongation of the two-phase stainless steel.

The lean duplex stainless steel according to the present invention may contain silicon (Si) in an amount of 10% by weight or more based on the total weight of the oxides contained in the region within 0.02 탆 in the depth direction from the surface of the lean duplex stainless steel.

In order to form an oxide having excellent corrosion resistance on the surface of the stainless steel of lean duplex stainless steel, it is necessary to reduce the Fe and Cr oxides formed after the cold rolling and replace the surface with oxides rich in Si components. The distribution of the silicon content increases in the stainless steel adjacent to the surface portion of the steel sheet, and the distribution of the silicon content decreases toward the inside of the steel sheet.

Therefore, when evaluating the area ratio of the lean duplex stainless steel according to the ASTM D610 standard, the area ratio of the leached area may be less than 1%.

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

Invention river

Lin-duplex stainless steels of 21Cr-3Mn-1.0Cu-0.25N were prepared according to the composition shown in Table 1 below.

Specifically, a casting plate having a thickness of 3 mm was manufactured through strip casting (PoStrip casting process), hot-annealing was performed for 3 minutes at a hot-rolling annealing temperature of 1,080 ° C, and pickling was performed. Thereafter, the cold-rolled steel was subjected to a thickness of 1 mm to prepare a duplex stainless steel specimen. This process was performed in common to the embodiments and the comparative examples described below.

C Cr Mn Ni Si Cu N Inventive Steel 1 0.054 20.93 3.03 0.55 0.62 1.0 0.252 Invention river 2 0.047 21.33 3.04 1.02 1.16 1.0 0.250

Example  One

The cold-rolled steel sheet according to Inventive Steel 1 was subjected to the atmosphere heat treatment under the conditions of Example 1 shown in Table 2 below.

Example  2

The cold-rolled steel sheet according to Inventive Steel 1 was subjected to the atmosphere heat treatment under the conditions of Example 2 shown in Table 2 below.

Example  3

The cold-rolled steel sheet according to Inventive Steel 1 was subjected to the atmosphere heat treatment under the conditions of Example 3 shown in Table 2 below.

Comparative Example  One

The cold-rolled steel sheet according to Inventive Steel 1 was subjected to cold rolling and annealing at 1,080 ° C for 30 seconds or less.

Comparative Example  2

The cold-rolled steel sheet according to the invention steel 1 was performed in the same manner as in Example 1, except that the atmosphere heat treatment was performed under the conditions of Comparative Example 2 shown in Table 2 below.

Comparative Example  3

The cold-rolled steel sheet according to the invention steel 1 was carried out in the same manner as in Example 1, except that the atmosphere heat treatment was performed under the conditions of Comparative Example 3 shown in Table 2 below.

Comparative Example  4

The cold-rolled steel sheet according to Inventive Steel 1 was subjected to the atmosphere heat treatment under the conditions according to Comparative Example 4 in Table 2, and the same procedure as in Example 1 was conducted.

Comparative Example  5

The cold-rolled steel sheet according to the invention steel 1 was performed in the same manner as in Example 1, except that the atmosphere heat treatment was performed under the conditions of Comparative Example 5 of Table 2 below.

Dew point (° C) Heat treatment temperature (캜) Holding time (sec) Example 1 -45 1,080 10 Example 2 -55 1,080 10 Example 3 -70 1,080 15 Comparative Example 1 - - - Comparative Example 2 -30 1,080 10 Comparative Example 3 -35 1,080 10 Comparative Example 4 -45 1,080 5 Comparative Example 5 -80 1,080 15

The corrosion resistance evaluation results of the specimens of the examples and comparative examples made according to the conditions of Table 2 are shown in Table 3 below. The corrosion resistance evaluation was carried out by using a combined cycle corrosion test apparatus in which a 5% NaCl aqueous solution was sprayed for 2 hours at 30 ° C., dried at a temperature of 60 ° C. and a relative humidity of 20% for 4 hours, And maintained in a wet state for 1 cycle. The corrosion resistance was evaluated by repeating a total of 60 cycles.

In order to measure the surface area ratio, first, the salt present on the surface of the specimen was washed with running water, and the photograph was taken by minimizing the reflection by the light source during the photographing. Thereafter, the percentage of perforation was measured using an image analyzer. The evaluation of the surface area ratio of the test piece was evaluated according to ASTM D610 standard.

Percentage of area exposed (%) ASTM D610 (Grade) Suitability Example 1 Less than 1 6 fitness Example 2 Less than 1 6 fitness Example 3 Less than 1 6 fitness Comparative Example 1 35.45 One incongruity Comparative Example 2 16.5 3 incongruity Comparative Example 3 12.5 3 incongruity Comparative Example 4 10.1 3 incongruity Comparative Example 5 5.5 4 incongruity

FIGS. 3 and 4 are photographs of a surface of a lean duplex stainless steel specimen according to Examples and Comparative Examples of the present invention after the evaluation of surface area ratio according to ASTM D610. FIG.

The results are shown in Table 3. Referring to Table 3, in the case of Examples 1 to 3 of the present invention, the percent area of bare area was measured to be less than 1%, and in Comparative Examples 2 and 3 in which the conditions of the atmosphere heat treatment were different, , And the Cr oxide is not sufficiently reduced. Therefore, it can be seen that the blowing rate is measured somewhat higher to the extent that the blowing rate exceeds 10%. In the case of Comparative Example 4, in the case of the dew point, a temperature of -40 ° C or less is sufficient. However, since the holding time of the heat treatment temperature is not sufficient, the Fe and Cr oxides remaining on the surface are not sufficiently reduced. . In the case of Comparative Example 5, the Fe and Cr oxides were sufficiently reduced at the dew point of -80 ° C, but the Si oxide was not sufficiently formed, so that the surface area ratio was measured to be about 5%.

FIG. 3 is a photograph of a surface of a duplex stainless steel specimen manufactured according to Comparative Example 1, and FIG. 4 is a photograph of a surface of a duplex stainless steel specimen manufactured according to Example 1. FIG.

3, 4 and 3, in the case of the comparative example 1, the dusting rate on the surface after 60 cycles was measured to be about 35%, and the dusting rate according to ASTM D610 was 33.0 to 50.0% Grade 1, which is very low.

On the other hand, as in Example 1 of the present invention, in the case of the surface subjected to the atmosphere heat treatment after cold rolling, the flaking rate was measured to be 1% or less, and the ASTM D610 flammability rating grade was Grade 6, . Generally, in the case of Grade 6, it is considered that no glow occurs when observed with naked eyes.

5 to 8 are graphs showing the elemental distribution of oxides on the surface of a stainless steel specimen of a lean duplex stainless steel according to Examples and Comparative Examples of the present invention by GDS (Glow Discharge Spectrometer) analysis.

5 is a graph showing the distribution of oxide elements on the surface of a duplex stainless steel specimen produced according to Example 2, and FIG. 6 is a graph showing the distribution of oxide elements on the surface of a duplex stainless steel specimen produced according to Comparative Example 1 FIG. 7 is a graph showing the oxide element distribution on the surface of a duplex stainless steel specimen produced according to Comparative Example 3, and FIG. 8 is a graph showing the oxide element distribution on the surface of a duplex stainless steel specimen produced according to Comparative Example 5 Graph.

5, which is a result of measurement of the stainless steel produced according to the atmosphere heat treatment conditions according to the second embodiment of the present invention, the Fe and Cr oxides were sufficiently reduced and treated in the region of 0.02 占 퐉 in the depth direction from the stainless steel surface, (Si component > Fe, Cr component).

6, which is a result of measurement of stainless steel produced through the cold-rolling annealing and pickling process according to Comparative Example 1 of the present invention, the major components were Fe and Cr oxides (Fe Component > Cr component).

On the contrary, the stainless steel produced according to the atmosphere heat treatment conditions according to Comparative Examples 3 and 5 was measured, and the results are different from those of Comparative Example 1.

According to Comparative Example 3 of the present invention, in an atmosphere having a dew point of -35 占 폚, the Fe oxide is sufficiently reduced, but the Cr oxide is not sufficiently reduced. Also, since the oxide having a large amount of Si component on the surface is formed in a smaller amount than Example 1, it can be understood that the rate of sputtering is higher than that of Example 1.

According to Comparative Example 5 of the present invention, the Fe and Cr oxides were sufficiently reduced in an atmosphere having a dew point of -80 DEG C, but since Si oxide was reduced, no Si oxide was formed on the surface, It can be seen that it is measured.

As a result, the results of the GDS analysis and the composite cycle corrosion test of the surface oxide component of the atmosphere heat-treated specimen according to the embodiments of the present invention showed that the total oxide weight of the oxide included in the region within 0.02 m from the stainless steel surface in the depth direction It is understood that sufficient anticorrosion property of the present invention can be secured by containing silicon (Si) in an amount of 10 wt% or more.

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 (6)

(C) 0.08% or less (more than 0), silicon (Si) 0.2-3.0%, manganese (Mn) 2.0-3.0%, chromium (Cr) 19.0-23.0%, nickel (Ni) (Fe) and other unavoidable impurities in an amount of 0.1 to 1.0% by weight, nitrogen (N) in an amount of 0.2 to 0.3%, copper (Cu) in an amount of 0.5 to 2.5% and tungsten In this case,
Cold-rolling the lean-duplex stainless steel; And
A method for manufacturing a lean duplex stainless steel having improved corrosion resistance, comprising the steps of: controlling a cold-rolled steel sheet in a hydrogen atmosphere at a dew point of -70 to -40 占 폚 and a heat treatment temperature of 1,000 to 1,200 占 폚 for 10 seconds or longer; . The method according to claim 1, further comprising, prior to cold rolling the lean duplex stainless steel,
And heat-annealing the lean duplex stainless steel at a temperature of 1,000 to 1,250 ° C for 2 to 40 minutes. 4. A corrosion-resistant linseed duplex stainless steel produced according to claim 1 or 2. The method of claim 3,
A linseed duplex stainless steel improved in corrosion resistance comprising silicon (Si) in an amount of 10% by weight or more based on the total weight of oxides contained in a region within 0.02 占 퐉 in the depth direction from the surface of the lean duplex stainless steel. The method of claim 3,
Wherein the lean duplex stainless steel comprises 45 to 75% by volume of the austenite phase and the remainder of the ferrite phase. The method of claim 3,
The lean duplex stainless steel according to ASTM D610 of the lean duplex stainless steel has a chewable area ratio of less than 1%.

Images