KR101819697B1 - Austenitic stainless steel having excellent corrosion resistance at welded part and high-temperature creep resistance and method of manufacturing the same - Google Patents

Abstract

The present invention relates to an austenitic stainless steel having excellent corrosion resistance and high temperature creep resistance in a welding portion, and a manufacturing method thereof. According to one embodiment of the present invention, the austenitic stainless steel having an improved creep resistant property and tensile strength comprises: 0.05 wt% or less (0 wt% is excluded) of carbon (C); 0.5 wt% or less (0 wt% is excluded) of silicon (Si); 2-5 wt% of manganese (Mn); 20-25 wt% of chromium (Cr); 9-12 wt% of nickel (Ni); 0.25 wt% or less (0 wt% is excluded) of nitrogen (N); and the remaining including iron (Fe) and other inevitable impurities, wherein the number of carbonitride distributed within crystal grain is at least 10 per 100 m^2.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to austenitic stainless steel having excellent corrosion resistance and high temperature creep resistance at welds 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 for fuel cells excellent in corrosion resistance at a weld and high temperature creep resistance, and a method for manufacturing the same.

Generally, austenitic stainless steels are excellent in corrosion resistance, workability and weldability, and are widely used in various applications.

The Molten Carbonate Fuel Cell (MCFC) is a highly efficient and environmentally friendly power generation device. The main components of the molten carbonate fuel cell are an anode and a cathode where an electrochemical reaction takes place, a matrix containing a carbonate electrolyte, a current collector for electric current collection and gas distribution, and a separator plate for electric conduction and gas inflow and outflow . The anode collector mainly uses nickel, nickel alloy, STS 310S and the like which are stable in the anode environment. The cathode current collector mainly uses 316L and the separator plate uses 316L or 310S.

The fuel cell separator is combined with each current collector and electrode to form one assembly. In this process, the welding technique is applied to the portion where bonding and gas sealing are required.

In the welded part of the stainless steel separator plate, heat-affected parts due to high-temperature heat are generated, and remarkable intergranular corrosion may occur at this part. And such that a screen sensitive to intergranular corrosion (sensitization), it is caused by the Cr carbide of Cr ₂₃ C ₆ type precipitates along the grain boundaries which the Cr-depleted zone is lower Cr concentration than the base material around the grain boundaries generated.

In the case of local corrosion of stainless steel material collectors and separators of MCFC components, resistance may increase due to the nonconductive corrosion products generated at the corrosion site and the structure may lose its support role. Which may lead to a reduction in performance of the entire fuel cell.

In addition, stainless steel which is subjected to a constant load and used in a high temperature environment should not only have heat resistance but also excellent high-temperature creep characteristics. Creep refers to a phenomenon in which an object undergoes deformation over time due to a certain stress under a high temperature environment. It is known that deformation due to creep is greatly affected by temperature, time, grain size and stress.

Generally, the separation plate material of a molten carbonate fuel cell used in a high temperature environment is subject to creep deformation due to high temperature environment at 650 ° C and stress due to fuel cell fastening, thereby shortening the life of the separator plate there was.

Therefore, it is inevitable to add nitrogen (N) because a high temperature creep resistance is essential for application as a separator for a molten carbonate fuel cell.

In order to prevent such corrosion of welds, there is a method of solving the problem by controlling the content of C + N or solubilizing the welds in order to prevent the formation of Cr carbonitride in the grain boundary. However, it is difficult to apply heat treatment after welding because of the additional cost due to heat treatment and process limitations, and it is difficult to apply general C + N content control according to addition of nitrogen (N) for ensuring high temperature creep resistance .

Patent Document 1 discloses a stainless steel which increases the twin grain boundary ratio of the material structure by 30% or more and additionally contains 0.05 to 3.0% Mo in order to improve grain boundary corrosion resistance. In order to increase the twin grain boundary ratio by more than 30%, an additional heat treatment step is required, and stainless steel containing Mo is not an optimal method considering productivity and economical efficiency.

Patent Document 2 discloses a ferritic stainless steel in which Cr carbonitride is suppressed. However, in order to suppress intergranular corrosion of high-N austenitic stainless steel for use as a material of a fuel cell separator requiring high-temperature creep resistance, No reviews have been made.

Japanese Patent Application Laid-Open No. 2005-015896 Korean Patent Publication No. 10-2016-0014685

Embodiments of the present invention are to provide austenitic stainless steel having an excellent austenitic stainless steel composition and composition ratio to secure intergranular corrosion resistance of a welded portion and at the same time to exhibit excellent creep resistance at a high temperature.

Further, according to the present invention, there is provided a method of manufacturing the austenitic stainless steel.

The austenitic stainless steel according to one embodiment of the present invention is excellent in corrosion resistance and resistance to creep resistance at high temperatures. It contains 0.05% or less of carbon (C), 0.5% or less (excluding 0) of silicon (Si) , Iron (Fe) and other unavoidable impurities in an amount of 2 to 5% of manganese (Mn), 20 to 25% of chromium (Cr), 9 to 12% of nickel (Ni) And the number of carbonitrides distributed in the crystal grains is 10/100 占 퐉 ² or more.

Further, according to an embodiment of the present invention, at least one selected from the group consisting of 0.05 to 0.8% of niobium (Nb), 0.05 to 1.0% of vanadium (V) and 0.05 to 1.0% of titanium can do.

According to an embodiment of the present invention, the sum of niobium (Nb), vanadium (V), and titanium (Ti) may be 0.30% or more.

According to an embodiment of the present invention, sulfur (S) can be contained in an amount of 0.003% or less (excluding 0).

In addition, according to an embodiment of the present invention, the grain number (ASTM E 112) of the stainless steel may be 8 or more.

Also, according to an embodiment of the present invention, the carbonitride may have a size of 30 to 1,000 nm.

Further, according to one embodiment of the present invention, the high temperature creep strain rate may be 0.20% or less.

Also, according to an embodiment of the present invention, the number of the carbonitride may be 40 pieces / 100 탆 ^{2 or} more.

Further, according to one embodiment of the present invention, the high temperature creep strain may be 0.15% or less.

A method for manufacturing an austenitic stainless steel having excellent corrosion resistance and high temperature creep resistance according to an embodiment of the present invention is characterized by containing 0.05% or less (excluding 0) of carbon (C), 0.5% or less of silicon (Si) (Excluding 0), the balance iron (Fe), and other unavoidable elements (excluding Fe) and the like. Hot-rolling and cold-rolling an austenitic stainless steel slab containing impurities; and annealing the cold-rolled steel sheet at a temperature of 900 to 1,200 占 폚.

According to the embodiment of the present invention, corrosion resistance and electric conductivity in a molten carbonate environment are ensured by a high Cr and Mn content of austenitic stainless steel, and the contents of C, Nb, V and Ti are controlled and Nb, V (Nb, V, Ti) -Cr-CN carbonitride present in the crystal grains by precipitating Cr ₂₃ C ₆ precipitates around the grain boundaries by precipitating Ti, Cr, And the creep resistance at high temperature can be simultaneously improved through the effect and grain refinement.

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 according to one embodiment of the present invention is excellent in corrosion resistance and resistance to creep resistance at high temperatures, and contains 0.05% or less (excluding 0) carbon (C), 0.5% or less silicon (Si) (Excluding 0), the remainder iron (Fe) and the balance iron (Fe), and the balance iron (Fe) and And other unavoidable impurities.

The content of carbon (C) is 0.05% or less (excluding 0).

Carbon (C) is easily combined with carbide-forming elements such as chromium (Cr) which are effective for increasing the strength of materials by solid solution strengthening and corrosion-resistant corrosion resistance at welding, thereby lowering the chromium (Cr) Therefore, in order to maximize the corrosion resistance, the carbon (C) content is preferably 0.05% or less.

The content of silicon (Si) is 0.5% or less (excluding 0).

When silicon (Si) is formed of an oxide, the corrosion resistance can be improved, but the electrical conductivity may be lowered. Therefore, it is preferable to limit the silicon (Si) content to 0.5% or less.

The content of manganese (Mn) is 2 to 5%.

Manganese (Mn) is an element that replaces nickel (Ni) as an austenite stabilizing element like nitrogen (N), and manganese (Mn) oxides present in the scale are iron (Fe), chromium (Cr) ) Phase to improve electrical conductivity. However, when it is added in an excessive amount, it is preferable to limit the content of manganese (Mn) to 2 to 5% since the corrosion resistance is lowered.

The content of chromium (Cr) is 20 to 25%.

Chromium (Cr) is an alloy element that must be added to improve corrosion resistance and oxidation resistance in stainless steel. In order to secure corrosion resistance, it is necessary to add at least 20%, and if it is added excessively, it is an element of ferrite phase, It is preferable to limit the upper limit to 25%.

The content of nickel (Ni) is 9 to 12%.

Nickel (Ni) is an austenite stabilizing element together with manganese (Mn) and nitrogen (N), and manganese (Mn) and nitrogen (N) can be added in place of expensive nickel for cost reduction (Ni) content is limited to 9 to 12% because of the excessive amount of nitrogen (N) content, rather it is difficult to secure corrosion resistance due to reduction of corrosion resistance and hot workability or reduction of Cr content.

The content of nitrogen (N) is 0.25% or less (excluding 0).

Nitrogen (N) is an element that stabilizes the austenite and improves both the high temperature strength and the corrosion resistance. If it is added excessively, it reduces the hot workability and limits the content of nitrogen (N) to 0.25% or less.

In addition, the austenitic stainless steel having excellent weld corrosion resistance and high temperature creep resistance according to an embodiment of the present invention is characterized in that it contains 0.05 to 0.8% of niobium (Nb), 0.05 to 1.0% of vanadium (V) and 0.05 to 1.0% 1.0%, and further contains 0.003% or less (excluding 0) of sulfur (S).

The content of niobium (Nb) is 0.05 to 0.8%.

Niobium (Nb) is an effective element for improving high-temperature strength and creep strength, but if it is excessive, crystal grain size is reduced and hot workability is reduced. Therefore, the content of niobium (Nb) is preferably limited to 0.05 to 0.8%.

The content of vanadium (V) is 0.05 to 1.0%.

Vanadium (V) is an effective element to improve creep strength and strength by forming carbonitride like niobium (Nb). When added excessively, vanadium (V) content is 0.05 To 1.0%.

The content of titanium (Ti) is 0.05 to 1.0%.

Titanium (Ti) is an element effective in improving creep strength and strength by forming carbonitride as in the case of niobium (Nb). When added excessively, titanium clogging may occur at the time of playing, so the content of titanium (Ti) is 0.05 To 1.0%.

For example, the total sum of niobium (Nb), vanadium (V) and titanium (Ti) may be 0.30% or more.

Niobium (Nb), vanadium (V) and titanium (Ti) are effective elements for improving high-temperature strength and creep resistance, as well as carbonitride of niobium (Nb), vanadium (V) and titanium (Ti) , V, Ti) -Cr-CN precipitates in the crystal grains to reduce Cr carbide formation in the form of Cr ₂₃ C ₆ along the grain boundary around the welded portion, thereby inhibiting intergranular corrosion.

However, excessive amounts of these elements may cause surface defects or a decrease in hot workability, resulting in clogging of nozzles during performance, and even if they are contained in excess, grain boundary corrosion resistance and high temperature creep strain reduction are saturated, .

(Nb), vanadium (V) and titanium (Ti) can be contained singly or in combination, and the effect of improving the grain boundary corrosion resistance around the welded portion by precipitation of (Nb, V, Ti) In order to obtain a high temperature creep resistance improving effect, it is preferable that the lower limit of each of niobium (Nb), vanadium (V) and titanium (Ti) is 0.05 when each element is contained singly.

For example, at least one selected from the group consisting of 0.05 to 0.8% of niobium (Nb), 0.05 to 1.0% of vanadium (V) and 0.05 to 1.0% of titanium (Ti) , And the total sum thereof is preferably 0.30% or more.

The content of sulfur (S) is 0.003% or less (excluding 0).

Sulfur (S) is a trace amount of impurity element and is segregated at crystal grain boundaries and is a main element causing cracks during hot rolling. Therefore, it is preferable to limit the content to 0.003% or less as low as possible.

The austenitic stainless steel having improved creep characteristics and tensile strength according to an embodiment of the present invention has a number of carbonitride distributed in the crystal grains of 10/100 mu m ² or more.

When the number of the carbonitride is less than 10/100 탆 ² , it is difficult to obtain a grain refinement effect due to a small deposition amount of carbonitride, and the creep strain rate is more than 0.20%. For example, the carbonitride may have a size of 30 to 1,000 nm. If the size of the carbonitride is larger than 1,000 nm, the structure may be uneven. If the size of the carbonitride is less than 30 nm, the size of the carbonitride is too small to obtain the grain refinement effect.

More preferably, the number of the carbonitride distributed in the crystal grains may be 40/100 탆 ^{2 or} more. For example, when the sum of niobium (Nb), vanadium (V) and titanium (Ti) is 0.30% or more, the number of carbonitride is 40/100 탆 ² or more.

For example, the creep strain of the stainless steel may be 0.20% or less.

More preferably, the creep strain rate may be 0.15% or less. For example, when the total sum of niobium (Nb), vanadium (V) and titanium (Ti) is 0.30% or more, the creep strain can be ensured to 0.15% or less.

For example, the grain number (ASTM E 112) of the stainless steel may be 8 or more.

When the number of the carbonitride is less than 10/100 μm ² , the grain number (ASTM E 112) of the stainless steel is about 7. When the number of carbonitrides is 10/100 μm ² or more, the stainless steel has a grain number (ASTM E 112) of 8 or more. That is, it is possible to obtain fine grains having a grain index number (ASTM E 112) of 8 or more of the stainless steel, thereby reducing the creep strain rate.

According to the method of manufacturing an austenitic stainless steel excellent in corrosion resistance and high temperature creep resistance of a weld part according to an embodiment of the present invention, (Except for 0), manganese (Mn) 2 to 5%, chromium (Cr) 20 to 25%, nickel (Ni) 9 to 12% An austenitic stainless steel slab containing unavoidable impurities is hot-rolled and cold-rolled, and the cold-rolled steel sheet is annealed at a temperature of 900 to 1,200 占 폚 to prepare the austenitic stainless steel.

In the process of annealing after cold rolling, the annealing temperature greatly influences residual stress relief, grain refinement and fine carbonitride precipitation. The annealing temperature is 900 to 1,200 占 폚.

When the annealing temperature is lower than 900 ° C, coarse carbides are generated and the structure becomes uneven or Cr ₂₃ C ₆ precipitates are formed on the grain boundaries, so that the grain boundary phase may occur. When the annealing temperature is higher than 1,200 ° C, It is preferable to restrict the annealing temperature to 900 to 1,200 캜.

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

In the present invention, in order to obtain an austenitic stainless steels for fuel cells having improved intergranular corrosion resistance and high temperature creep resistance at welded portions, corrosion resistance and creep strain rate at high temperature were measured by varying the amount of various alloying elements added.

Invention river  And Comparative steel

Ingots containing the respective composition components according to inventive steels 1 to 10 and comparative steels 1 to 5 in Table 1 were cast and hot-rolled and cold-rolled, annealed at a temperature of 1,100 ° C, To prepare a cold-rolled sheet having a thickness of 3 mm.

(weight%) C Si Mn Cr Ni N Nb V Ti Inventive Steel 1 0.029 0.23 2.11 21.5 11.2 0.17 0.06 - Invention river 2 0.035 0.33 2.43 21.2 10.7 0.16 0.51 - Invention steel 3 0.028 0.27 3.15 21.7 11.6 0.13 - 0.07 Inventive Steel 4 0.039 0.24 3.51 22.2 10.3 0.14 0.11 0.28 Invention steel 5 0.045 0.30 2.80 22.6 10.5 0.16 - 0.63 Invention steel 6 0.025 0.34 3.04 22.1 10.9 0.16 0.21 - 0.29 Invention steel 7 0.034 0.26 2.97 22.3 10.6 0.15 - - 0.06 Inventive Steel 8 0.037 0.31 2.89 21.8 11.1 0.15 - - 0.59 Invention river 9 0.025 0.33 2.96 21.6 11.3 0.16 - 0.24 0.29 Invented Steel 10 0.031 0.28 3.04 22.2 10.9 0.15 0.16 0.14 0.17 Comparative River 1 0.063 0.32 2.26 18.9 10.3 0.12 0.23 - Comparative River 2 0.091 0.28 3.12 21.6 10.9 0.16 - 0.19 - Comparative Steel 3 0.043 0.30 2.75 22.1 11.2 0.11 - - - Comparative Steel 4 0.11 0.25 2.92 21.7 10.6 0.14 0.08 0.16 - Comparative Steel 5 0.038 0.35 2.89 22.1 11.2 0.15 0.02

Thus, the physical properties of the cold-rolled steel sheets of the manufactured steels 1 to 10 and the comparative steels 1 to 5 were measured and are shown in Table 2 below.

Weld water spray test cycle test Intergranular corrosion Number of carbonitride (pieces / 100 ㎛ ² ) High temperature creep strain (%) Inventive Steel 1 ○ ○ 24 0.19 Invention river 2 ○ ○ 65 0.14 Invention steel 3 ○ ○ 33 0.18 Inventive Steel 4 ○ ○ 57 0.15 Invention steel 5 ○ ○ 81 0.13 Invention steel 6 ○ ○ 63 0.14 Invention steel 7 ○ ○ 13 0.20 Inventive Steel 8 ○ ○ 74 0.14 Invention river 9 ○ ○ 68 0.14 Invented Steel 10 ○ ○ 62 0.14 Comparative River 1 × × 36 0.16 Comparative River 2 × × 32 0.17 Comparative Steel 3 × ○ <1 0.38 Comparative Steel 4 × × 38 0.16 Comparative Steel 5 × ○ 3 0.27

Table 2 shows the measurement results of the corrosion resistance of the base material or welded part, the number of carbonitrides per ² 탆 of 100 탆, and the high temperature creep strain rate at 650 캜.

In this case, the salt water spraying cycle test of the welded part is performed by using Acidified Salt Mist Condition (5% NaCl solution spraying, 35 ℃ ± 1 ℃, 2Hr) → Dry Condition (60 ℃ ± 1 ℃, relative humidity <30%, 4Hr) → Wet Condition ℃ ± 1 ℃, relative humidity> 95%, 2Hr) is 1 cycle. After 21 cycles, it passes the case where there is no corrosion of the base material or welding part.

In the modified Strauss test method, the test solution was prepared by adding distilled water to CuSO ₄ · H ₂ O (100 g) + distilled water (700 cc) + H ₂ SO ₄ (95%, 100 ml) Size (50 ⅹ 20mm), specimens were completely immersed, and after continuous 24-hour immersion in the boiling point, U-band test was performed to investigate the presence or absence of cracks (R = 2t). The cases where cracks did not occur and those where cracks occurred were denoted by ◯ and ×, respectively.

The number of carbonitride was measured by the replica extraction method and then the number of carbonitride was measured by transmission electron microscope (TEM). The number of the carbonitrides was measured for each 100 mu m &lt; ² &gt; of precipitates observed.

The high temperature creep strain was expressed by the measured strain (%) after holding at 650 DEG C under 70 MPa for 500 hours. Since the creep strain rate shows a degree of deformation by a constant load at 650 ° C, the lower the value, the better the creep characteristics.

Referring to Tables 1 and 2, it can be seen that the austenitic stainless steel sheet according to the embodiments of the present invention satisfies the corrosion resistance and creep resistance at high temperatures.

Particularly, in the case of Examples 4, 6, 9, and 10 in which Nb, V, and Ti were not combined with each other, the result of creep strain when the two elements were mixed together and the case where the three elements were mixed The creep strain rate and the number of carbonitrides are not significantly different from each other.

In the case of Examples 2, 6, 9 and 10 in which the difference in the content of Nb, V and Ti between the single content and the two or three composite contents was not large, the difference in the high temperature creep strain rate and the difference in the number of the carbonitrides were not large.

Therefore, it is preferable to contain at least one of Nb, V and Ti or to contain them in combination in order to secure an excellent weld portion corrosion resistance and a low high temperature creep strain rate.

More preferably, as in Examples 2, 4 to 6 and 8 to 10, when the total amount of Nb, V and Ti is 0.30% or more, the number of carbonitrides distributed in the crystal grains is set to 40/100 탆 ² or more And it can be seen that the high temperature creep strain can be ensured at 0.15% or less.

In the case of Comparative Examples 1, 2 and 4, the high-temperature creep strain showed excellent results by the addition of Nb and V, but in Comparative Example 1, due to the low Cr content and the high C content, Corrosion and cracking occurred in the salt water spray cycle test and intergranular corrosion evaluation. The content of Nb and V satisfies the range of the present invention. However, in Comparative Example 4 in which the C content exceeds the range of the present invention, excellent high-temperature creep strain was obtained, but the welded portion corrosion resistance was not obtained.

In the case of Comparative Examples 3 and 5, the C content satisfied the range of the present invention, but the content of at least one of Nb, V, and Ti was insufficient, so corrosion was observed in the welding water spray cycle test, and the high temperature creep strain measurement , And the value of over 0.2% was high.

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 (10)

(C) not more than 0.05% (excluding 0), silicon (Si) not more than 0.5% (excluding 0), manganese (Mn) 2 to 5%, chromium (Cr) 20 to 25%, nickel (Ni) (N) of not more than 0.25% (excluding 0) and niobium (Nb) of 0.05 to 0.8%, vanadium (V) of 0.05 to 1.0%, and titanium (Ti) of 0.05 to 1.0% And the remainder is made of iron (Fe) and other unavoidable impurities,
The total sum of niobium (Nb), vanadium (V) and titanium (Ti) is 0.30% or more,
The number of carbonitrides distributed in the crystal grains is 40/100 탆 ² or more,
Austenitic stainless steels with excellent creep resistance of 0.15% or less at welded parts and high temperature creep resistance. delete delete The method according to claim 1,
Austenitic stainless steels superior in weld corrosion resistance and high temperature creep resistance including sulfur (S) up to 0.003% (excluding 0). The method according to claim 1,
Wherein the stainless steel has a grain number (ASTM E 112) of at least 8, wherein the austenitic stainless steel has excellent corrosion resistance and high temperature creep resistance. The method according to claim 1,
The carbonitride is austenitic stainless steel having a size of 30 to 1,000 nm and excellent in corrosion resistance and high temperature creep resistance. delete delete delete delete