WO2017111251A1 - Austenitic stainless steel with improved creep-resistant properties and tensile strength and method for producing same - Google Patents

Abstract

Disclosed are austenitic stainless steel with improved creep-resistant properties and tensile strength and a method for producing the same. The austenitic stainless steel with improved creep-resistant properties and tensile strength according to an embodiment of the present invention includes, by weight%, 0.1% or less (excluding 0) of carbon (C), 1.0% or less (excluding 0) of silicon (Si), 2.0% or less (excluding 0) of manganese (Mn), 18 to 24% of chromium (Cr), 9 to 12% of nickel (Ni), 0.05 to 0.5% of niobium (Nb), 0.1 to 0.25% of nitrogen (N), 0.001 to 0.005% of boron (B), the remainder being iron (Fe) and other unavoidable impurities, wherein the number of carbonitrides is 10/100 ㎛² or more. Therefore, the composition of the austenitic stainless steel is controlled to refine the grains thereof, which improves the tensile strength and creep-resistant properties at high temperature, thereby reducing the production cost.

Description

Austenitic stainless steel with improved creep resistance and tensile strength and method of manufacturing the same

The present invention relates to an austenitic stainless steel with improved creep resistance and tensile strength, and more particularly, to fine carbonitride or nitride by controlling the components of Nb, N, and B of the austenitic stainless steel composition. Austenitic stainless steel that can reduce the cost by increasing the tensile strength and remarkably reducing the creep strain, and reducing the expensive element Ni by reducing the crystal grains by minimizing the crystal grains. It relates to a production method thereof.

In general, austenitic stainless steel is widely used in various applications because of its excellent corrosion resistance, processability, and weldability.

Stainless steel used in high temperature environment under constant load should have excellent creep characteristics as well as heat resistance.

A creep refers to a phenomenon in which an object is deformed with time due to a constant stress under a high temperature environment. The creep deformation is known to be greatly affected by temperature, time, grain size, and stress.

Therefore, when deformation due to creep with high temperature corrosion occurs in a high temperature environment, it is impossible to support the original function by failing to support stress or causing numerical deformation.

In general, as the separator of molten carbonate fuel cell used in high temperature environment, material of STS 310S is used, and such material is creep deformation due to stress caused by fastening of fuel cell and environment of high temperature of 650 ℃. Due to the problem of shortening the life of the separator.

In order to solve this problem, there have been attempts to control grain size and to form precipitates by controlling annealing temperature and atmosphere, but it is difficult to change the conditions in the steel manufacturing process and maintain mechanical strength other than creep characteristics.

 (Patent Document 0001) Korean Laid-Open Patent Publication No. 10-2013-0121981

Embodiments of the present invention are intended to provide austenitic stainless steels with improved tensile strength, excellent creep resistance at high temperatures, and cost reduction by controlling austenitic stainless steel compositions and component ratios.

In addition, according to the present invention, to provide a method for producing the austenitic stainless steel.

Austenitic stainless steel with improved creep resistance and tensile strength according to an embodiment of the present invention is a weight percent, carbon (C) 0.1% or less (excluding 0), silicon (Si) 1.0% or less (excluding 0), Manganese (Mn) 2.0% or less (excluding 0), Chromium (Cr) 18 to 24%, Nickel (Ni) 9 to 12%, Niobium (Nb) 0.05 to 0.5%, Nitrogen (N) 0.1 to 0.25%, Boron (B) 0.001 to 0.005%, balance iron (Fe) and other unavoidable impurities, the number of carbonitrides is 10 / 100㎛ ² or more.

In addition, according to an embodiment of the present invention, titanium (Ti) may include 0.05% or less.

In addition, according to an embodiment of the present invention, sulfur (S) may be included in 0.003% or less.

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

In addition, according to an embodiment of the present invention, the creep strain of the stainless steel may be 0.20% or less.

In addition, according to an embodiment of the present invention, the tensile strength of the stainless steel may be 700MPa or more.

Method for producing austenitic stainless steel with improved creep resistance and tensile strength according to an embodiment of the present invention, the step of hot rolling and cold rolling the austenitic stainless steel slab and cold rolled steel sheet of 900 to 1,200 ℃ Annealing at temperature.

According to an embodiment of the present invention, by controlling the components of Nb, N, B of the austenitic stainless steel composition to precipitate fine carbonitride or nitride having a composition of Cr-Nb-CN to refine the crystal grains compared to the existing heat-resistant stainless steel Tensile strength is improved, creep strain is significantly reduced, creep resistance at high temperature is improved, and cost can be reduced by reducing expensive element Ni.

1 is a photograph of the annealing structure of the STS 310S steel, which is a conventional austenitic stainless steel.

2 is an annealing structure photograph of the austenitic stainless steel according to an embodiment of the present invention.

Austenitic stainless steel with improved creep resistance and tensile strength according to an embodiment of the present invention is a weight percent, carbon (C) 0.1% or less (excluding 0), silicon (Si) 1.0% or less (excluding 0), Manganese (Mn) 2.0% or less (excluding 0), Chromium (Cr) 18 to 24%, Nickel (Ni) 9 to 12%, Niobium (Nb) 0.05 to 0.5%, Nitrogen (N) 0.1 to 0.25%, Boron (B) 0.001 to 0.005%, balance iron (Fe) and other unavoidable impurities, the number of carbonitrides is 10 / 100㎛ ² or more.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are presented to sufficiently convey the spirit of the present invention to those skilled in the art. The present invention is not limited to the embodiments presented herein but may be embodied in other forms. The drawings may omit illustrations of parts not related to the description in order to clarify the present invention, and may be exaggerated to some extent in order to facilitate understanding.

Austenitic stainless steel with improved creep resistance and tensile strength according to an embodiment of the present invention, in weight percent, carbon (C) 0.1% or less (excluding 0), silicon (Si) 1.0% or less (excluding 0) , Manganese (Mn) 2.0% or less (excluding 0), chromium (Cr) 18 to 24%, nickel (Ni) 9 to 12%, niobium (Nb) 0.05 to 0.5%, nitrogen (N) 0.1 to 0.25%, Boron (B) 0.001 to 0.005%, balance iron (Fe) and other unavoidable impurities.

The content of carbon (C) is 0.1% or less (excluding 0).

Carbon (C) is an element that is effective for increasing the strength of a material by strengthening solid solution, but it easily bonds with carbide forming elements such as chromium (Cr), which is effective in corrosion resistance when the content is excessive, thereby reducing the corrosion resistance by lowering the content of chromium (Cr) around grain boundaries. In order to maximize the corrosion resistance, it is preferable to limit the carbon (C) content to 0.1% or less.

The content of silicon (Si) is 1.0% or less (excluding 0).

When silicon (Si) is formed of an oxide, the corrosion resistance may be improved, but it is preferable to limit the silicon (Si) content to 1.0% or less since the content is an element that degrades weldability and hot workability.

The content of manganese (Mn) is 2.0% or less (excluding 0).

Manganese (Mn), like nitrogen (N), is an element that replaces nickel (Ni) as an austenite stabilizing element, and when added excessively, it lowers the corrosion resistance, so it is preferable to limit the manganese (Mn) content to 2.0% or less. .

The content of chromium (Cr) is 18 to 24%.

Chromium (Cr) is an element that promotes the formation of oxide film of stainless steel and needs more than 18% in order to secure heat resistance, and if it is added excessively, ferritic phase is generated, so excessive δ-ferrite phase remains, which degrades hot workability. It is desirable to limit 24% to the upper limit.

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

Nickel (Ni) is an austenite stabilizing element along with manganese (Mn) and nitrogen (N). Manganese (Mn) and nitrogen (N) can be added in place of expensive nickel (Ni) for cost reduction. When limiting the nickel content, it is difficult to secure the corrosion resistance due to the decrease of the corrosion resistance and hot workability or the decrease of the chromium content. It is preferable.

The content of niobium (Nb) is 0.05 to 0.5%.

Niobium (Nb) is an effective element to improve the high temperature strength and creep strength, but when excessive, the grain size becomes fine and the hot workability is reduced. Therefore, the content of niobium (Nb) is preferably limited to 0.05 to 0.5%.

The content of nitrogen (N) is 0.1 to 0.25%.

Nitrogen (N) is an austenite stabilizing element and is an element that improves high temperature strength and corrosion resistance at the same time, but when added excessively reduces the hot workability, it is preferable to limit the content of nitrogen (N) to 0.25% or less.

The content of boron (B) is 0.001 to 0.005%.

Boron (B) is an alloying element that improves hot workability at high temperature, and when added excessively, boron (B) is preferably limited to 0.001 to 0.005% because it inhibits ductility, toughness and workability.

The austenitic stainless steel having improved creep resistance and tensile strength according to an embodiment of the present invention may further include titanium (Ti) 0.05% or less and sulfur (S) 0.003% or less.

The content of titanium (Ti) is 0.05% or less.

Titanium (Ti), like niobium (Nb), is an effective element for improving creep strength and strength by forming carbonitrides. When excessively added, titanium (Ti) may cause a blockage of the nozzle when playing. It is preferable to limit it to% or less.

The content of sulfur (S) is less than 0.003%.

Sulfur (S) is a minor element that is segregated at the grain boundaries and is a main element that causes processing cracks during hot rolling. Therefore, the sulfur (S) is preferably limited to 0.003% or less, which is as low as possible.

The austenitic stainless steel having improved creep resistance and tensile strength according to an embodiment of the present invention has a carbonitride number of 10/100 µm ² or more. When the number of carbonitrides is less than 10/100 占 퐉 ² , the amount of precipitation of carbonitrides is so small that it is difficult to obtain grain refining effect. Properties and tensile strength cannot be obtained. For example, the carbonitride may have a size of 30 to 1,000 nm.

For example, the creep strain of the stainless steel may be 0.20% or less, and the tensile strength of the stainless steel may be 700 MPa or more.

When the carbonitride is less than 10/100 µm ² , the grain size number (ASTM E 112) of stainless steel is about 7 degrees. When the carbonitride is 10/100/100 μm ² or more, for example, the grain size number (ASTM E 112) of the stainless steel may be 8 or more. That is, the grain size number (ASTM E 112) of the stainless steel can be obtained a fine grain of 8 or more, through which the creep strain is reduced, tensile strength can be improved.

According to the manufacturing method of the austenitic stainless steel with improved creep resistance and tensile strength according to an embodiment of the present invention, the austenitic stainless steel slab having the above composition is hot rolled and cold rolled, the cold rolled steel sheet 900 to 1,200 Annealing treatment at a temperature of ℃ to produce the austenitic stainless steel.

In the annealing process after cold rolling, the annealing temperature greatly influences the relieving of residual stress, grain refinement and fine carbonitride precipitation. Annealing temperature may be 900 to 1,200 ℃.

If the annealing temperature is lower than 900 ° C, coarse carbides are produced and the tissue becomes non-uniform, and if the annealing temperature is higher than 1,200 ° C, it is important to limit the annealing temperature to 900 to 1200 ° C because the grains may be extremely coarse. desirable.

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

Invention steel and comparative steel

After casting the ingot (Ingot) containing each composition component according to the invention steels 1 to 9 and Comparative steels 1 to 3 of the following Table 1 to perform hot and cold rolling, after annealing at a temperature of 1,100 ℃, pickling Was performed to prepare a cold rolled plate of 3mm thickness.

(weight%)	C	Si	Mn	Cr	Ni	Mo	Nb	B	N	Ti
Inventive Steel 1	0.02	0.5	2.0	22	11	-	0.5	0.003	0.17	-
Inventive Steel 2	0.03	0.5	0.5	22	11	-	0.2	0.003	0.15	0.02
Invention Steel 3	0.03	1.0	1.0	18	9	-	0.5	0.003	0.15	-
Inventive Steel 4	0.02	1.0	1.0	18	9	-	0.2	0.003	0.14	-
Inventive Steel 5	0.02	0.5	1.0	22	9	-	0.5	0.003	0.12	-
Inventive Steel 6	0.03	0.5	0.5	22	11	-	0.05	0.003	0.13	-
Inventive Steel 7	0.02	0.5	1.0	22	11	-	0.1	0.003	0.14	0.05
Inventive Steel 8	0.02	0.5	1.0	22	11	-	0.2	0.003	0.14	-
Inventive Steel 9	0.02	0.5	1.0	22	10	-	0.2	0.003	0.10	-
Comparative Steel 1	0.03	0.5	1.0	18	9	2.0	-	-	0.05	-
Comparative Steel 2	0.02	0.5	1.0	22	11	-	0.2	0.003	0.06	-
Comparative Steel 3	0.04	0.5	1.4	25	20	-	-	-	0.03	-

Accordingly, the physical properties of each of the cold rolled sheets of the inventive steels 1 to 9 and comparative steels 1 to 3 were measured and shown in Table 2 below.

	Creep Strain (%)	Tensile Strength (MPa)	Grain size number	Number of carbonitrides (pcs / 100㎛ 2 )
Inventive Steel 1	0.15	755	11.8	74
Inventive Steel 2	0.17	728	10.8	46
Invention Steel 3	0.16	742	11.3	58
Inventive Steel 4	0.18	725	10.9	42
Inventive Steel 5	0.18	719	10.7	36
Inventive Steel 6	0.20	711	9.3	19
Inventive Steel 7	0.19	716	10.1	32
Inventive Steel 8	0.18	723	10.7	39
Inventive Steel 9	0.20	708	8.6	12
Comparative Steel 1	0.26	631	7.6	<0.1
Comparative Steel 2	0.24	659	7.8	7
Comparative Steel 3	0.31	556	7.2	<0.1

Table 2 shows the high temperature creep strain, tensile strength, grain size number, and number of carbonitrides per 100 μm ² at 650 ° C.

Here, the high temperature creep strain is represented as the strain (%) measured after holding at 650 ° C. for 70 hours under 70 MPa. Tensile strength was taken as ASTM sub-size tensile specimen in the rolling direction, the temperature during the tensile test was defined as room temperature and the strain rate was set to 20mm / min, and after performing the tensile test, it was expressed as tensile strength (MPa). Grain size number was sampled to the observation plane perpendicular to the rolling direction of each specimen, and then measured the grain size (ASTM E 112) through the electrolytic etching. The number of carbonitrides was collected by using a replica extraction method, and the number of carbonitrides was measured through a transmission electron microscope (TEM).

Since the creep strain indicates the degree of deformation by a constant load at 650 ° C., the lower the value, the better the creep resistance. In addition, the number of carbonitrides was measured for the precipitate observed per 100㎛ ² , the precipitate showed a size of 30 to 1,000nm. Precipitates observed are carbonitrides containing Cr, Nb, C, N and the like.

1 is a photograph of the annealing structure of the STS 310S steel, which is a conventional austenitic stainless steel. 2 is an annealing structure photograph of the austenitic stainless steel according to an embodiment of the present invention.

1 is a photograph of the annealing structure of the austenitic stainless steel STS 310S steel according to the comparative steel 3, Figure 2 is a photograph of the annealing structure of the austenitic stainless steel according to the invention steel 1.

1, 2 and Table 2, in Comparative Examples 1 to 3, the number of carbonitrides is insufficient, it can be seen that the crystal grains are coarsened to lower the tensile strength and creep resistance.

In the invention steels 1 to 9, it was found that Nb, N, and B were added as alloy elements to obtain a grain size number of 8 or more. In addition, it was found that a sufficient number of carbonitrides were precipitated with the number of carbonitrides of 10/100 µm ² or more.

Therefore, the tensile strength of the austenitic stainless steel according to an embodiment of the present invention is 700MPa or more, and exhibited excellent creep resistance of less than 0.2 creep strain.

In particular, referring to Figures 1 and 2, it can be seen that the inventive steel 1 is a fine grain compared to the comparative steel 3, which is due to the addition of niobium (Nb) and nitrogen (N) fine precipitates in the grains The grain growth was suppressed, and thus, tensile strength and creep resistance were improved.

Referring to the inventive steels 2 and 7 of Table 1 and Table 2, despite the relatively low niobium (Nb) content, it shows excellent creep resistance and tensile strength due to the addition of titanium (Ti). Titanium (Ti), like niobium (Nb), has the effect of depositing carbonitrides to refine grains, and thus may be used as necessary.

As described above, the exemplary embodiments of the present invention have been described, but the present invention is not limited thereto, and a person skilled in the art does not depart from the spirit and scope of the following claims. It will be understood that various changes and modifications are possible in the following.

The austenitic stainless steel and its manufacturing method having improved creep resistance and tensile strength according to embodiments of the present invention may be applied to a separator plate of a molten carbonate fuel cell.

Claims (7)

1. By weight, carbon (C) 0.1% or less (excluding 0), silicon (Si) 1.0% or less (excluding 0), manganese (Mn) 2.0% or less (excluding 0), chromium (Cr) 18 to 24%, nickel (Ni) 9 to 12%, niobium (Nb) 0.05 to 0.5%, nitrogen (N) 0.1 to 0.25%, boron (B) 0.001 to 0.005%, balance iron (Fe) and other unavoidable impurities Austenitic stainless steel with improved creep resistance and tensile strength of more than 10/100/100 μm ² cargoes.
2. The method of claim 1,

   Austenitic stainless steel with improved creep resistance and tensile strength, characterized by containing 0.05% or less of titanium (Ti).
3. The method of claim 1,

   Sulfur (S) austenitic stainless steel with improved creep resistance and tensile strength, characterized in that it comprises 0.003% or less.
4. The method of claim 1,

   Austenitic stainless steel with improved creep resistance and tensile strength, characterized in that the crystal grain number (ASTM E 112) of the stainless steel is 8 or more.
5. The method of claim 1,

   An austenitic stainless steel having improved creep resistance and tensile strength, wherein the creep strain of the stainless steel is 0.20% or less.
6. The method of claim 1,

   Austenitic stainless steel with improved creep resistance and tensile strength, characterized in that the tensile strength of the stainless steel is 700MPa or more.
7. Hot rolling and cold rolling the austenitic stainless steel slab according to any one of claims 1 to 6; And

   A method for producing austenitic stainless steel with improved creep resistance and tensile strength, comprising annealing the cold rolled steel sheet at a temperature of 900 to 1,200 ° C.

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