EP4603618A1

EP4603618A1 - Austenitic stainless steel with improved low-temperature impact toughness and strength and method for manufacturing same

Info

Publication number: EP4603618A1
Application number: EP23900827.9A
Authority: EP
Inventors: Kwangmin Kim; Seokweon SONG; Hongju Lee; Inho Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-06
Filing date: 2023-09-04
Publication date: 2025-08-20
Also published as: WO2024122806A1; KR102819597B1; KR20240084347A; CN120435584A; EP4603618A4; JP2025539894A

Abstract

An embodiment of the present invention may provide an austenitic stainless steel with improved low-temperature impact toughness and strength, the austenitic stainless steel containing, in wt%, 0.1% or less (excluding 0) of C, 1.5% or less (excluding 0) of Si, 16.0-23.0% of Cr, 5.0-12.0% of Ni, 10.0% or less (excluding 0) of Mn, 0 to 1.5% or less of Mo, 0 to 1.6% or less of Cu, and 0.1-0.3% of N, with the remainder comprising Fe and impurities, wherein the austenitic stainless steel has a composition that, when substituted into Expressions (1) and (2), results in values that satisfy the specified ranges. Expression (1): 1.7*[Ni]+410*[C]-0.9*[Cr]-100*[N]-13*[Si]-1.2*[Mn]+2*[Cu]-5.5*[Mo]+40, where the value of Expression (1) is from 10 to 35; Expression (2): ([Ni]+0.52*[Cu]+40*([C]+[N])+0.7*[Mn]+45)-(0.48*([Cr]+[Mo]+1.7*[Si])) -51, where the value of Expression (2) is 0 or greater. The elements in brackets represent the content of each element in weight percent.

Description

[Technical Field]

The present invention relates to an austenitic stainless steel having excellent low-temperature toughness in a low-temperature environment, such as liquefied hydrogen, and more specifically, to an austenitic stainless steel having improved low-temperature impact toughness and strength by controlling a composition relationship and an alloy composition, and a method of manufacturing the same.

[Background Art]

With the growing demand for eco-friendly energy such as hydrogen energy, the development of containers and parts for hydrogen storage has become increasingly necessary.
In general, hydrogen storage containers may be divided into a liquefied hydrogen storage method and a gaseous hydrogen storage method depending on the form of hydrogen. In particular, the liquefied hydrogen storage method is expected to be used in various fields in the future due to its relatively high storage efficiency. For example, a liquefied hydrogen storage type container may be used as a method of long-distance transportation of hydrogen from overseas to domestic markets and for storing large amounts of hydrogen at hydrogen charging stations or hydrogen production plants.
Steel materials for such hydrogen storage containers are exposed to a temperature ranging from -253°C to room temperature, and therefore require no degradation in properties at various temperatures. Considering this, hydrogen storage tanks and peripheral devices are required to have resistance to deterioration of toughness caused by hydrogen and extremely low temperature s, as well as high mechanical strength and corrosion resistance.
Therefore, in order to utilize hydrogen resources such as liquefied hydrogen and gaseous hydrogen, there is an increasing need for technologies that improve the properties of hydrogen storage containers such as hydrogen storage tanks.

[Disclosure]

[Technical Problem]

To resolve the above-described issues, the present invention is directed to providing an austenitic stainless steel with improved low-temperature impact toughness and strength for hydrogen and a method of manufacturing the same, in which high impact toughness at low temperatures is ensured by controlling the relationship of alloy elements that has a significant effect on low-temperature toughness, and high strength is ensured by sufficiently adding nitrogen (N).
The technical objectives of the present invention are not limited to the above, and other objectives that are not described above will be clearly understood by those skilled in the art from the above detailed description.

[Technical Solution]

To resolve the above-described issues, an austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention includes, in percent by weight (wt%), 0.1% or less (excluding 0) of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 16.0 to 23.0% of chromium (Cr), 5.0 to 12.0% of nickel (N), 10.0% or less (excluding 0) of manganese (Mn), 0 to 1.5% or less of molybdenum (Mo), 0 to 1.6% or less of Copper (Cu), 0.1 to 0.3% of nitrogen (N), the remainder of Iron (Fe), and impurities, wherein Equation (1): 1.7Ni+410C-0.9Cr-100N-13Si-1.2Mn+2Cu-5.5Mo+40 is within a range of 10 to 35, and Equation (2): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51 is 0 or greater.
In the austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention, a Charpy impact energy at - 196°C may be 100J or more.
In the austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention, a yield strength may be 280MPa or more.
A method of manufacturing an austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention includes: performing hot-rolling on a slab comprising, in percent by weight (wt%), 0.1% or less (excluding 0) of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 16.0 to 23.0% of chromium (Cr), 5.0 to 12.0% of nickel (N), 10.0% or less (excluding 0) of manganese (Mn), 0 to 1.5% or less of molybdenum (Mo), 0 to 1.6% or less of Copper (Cu), 0.1 to 0.3% of nitrogen (N), the remainder of Iron (Fe), and impurities, and satisfying Equation (1) being from 10 to 35 and Equation (2) being 0 or more, wherein Equation (1) is defined as 1.7Ni+410C-0.9Cr-100N-13Si-1.2Mn+2Cu-5.5Mo+40, and Equation (2) is defined as (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51; and performing hot annealing at 900 to 1200°C
In the method of manufacturing an austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention, a Charpy impact energy at -196°C may be 100J or more.
In the method of manufacturing an austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention, a yield strength may be 280MPa or more.

[Advantageous Effects]

According to an embodiment of the present invention, it is possible to provide an austenitic stainless steel having improved low-temperature impact toughness and strength and a method of manufacturing the same that are capable of ensuring a Charpy impact toughness value of 100 J or more at -196°C and a yield strength of 280 MPa or more by adjusting alloying elements effective for low-temperature impact toughness and strength to control a relationship related to low-temperature toughness, a relationship related to strength, and a N content to 0.1% or more.

[Modes of the Invention]

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The embodiments of the present disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art.
Also, the terms used herein are merely used to describe particular embodiments. An expression used in the singular encompasses the expression of the plural, unless otherwise indicated. Throughout the specification, the terms such as "including" or "having" are intended to indicate the existence of features, operations, functions, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, operations, functions, components, or combinations thereof may exist or may be added.
Meanwhile, unless otherwise defined, all terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Thus, these terms should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise.
The terms "about", "substantially", etc. used throughout the specification means that when a natural manufacturing and a substance allowable error are suggested, such an allowable error corresponds the value or is similar to the value, and such values are intended for the sake of clear understanding of the present invention or to prevent an unconscious infringer from illegally using the disclosure of the present invention.
According to embodiments of the present invention, an austenitic stainless steel with improved low-temperature impact toughness and strength for hydrogen and a method of manufacturing the same, in which high impact toughness at low temperatures is ensured by controlling the relationship of alloy elements that has a significant effect on low-temperature toughness, and high strength is ensured by sufficiently adding nitrogen (N) may be provided. Specifically, the possibility of use in an extremely low-temperature environment may be predicted based on the relationship between contents of main elements constituting an alloy and a low-temperature toughness. In addition, the strength may be improved while increasing the austenite stability by adding nitrogen (N).
According to the relationship related to low-temperature toughness, excessive Mn is undesirable in terms of low-temperature toughness. Therefore, when a large amount of Mn is added, the content of Ni, which has a strong effect on increasing low-temperature toughness, needs to be increased. Since higher additions of Ni to an alloy is beneficial in terms of low-temperature toughness and austenite stabilization, Ni may be added in large amounts when designing an alloy. However, from an economic perspective, it may be considered to replace Ni with Mn and N, which are also austenite stabilizing elements.
The precipitation strengthening form has a negative influence on the low-temperature toughness effect. Therefore, according to an embodiment of the present invention, the strength may be improved by adding N to achieve a solid solution strengthening form rather than a precipitation strengthening form. However, given that N has a negative coefficient in the relationship related to low-temperature toughness, it is required to design an alloy that may ensure the effect of sufficient strength improvement while minimizing the decrease in low-temperature toughness. Accordingly, the present invention designs the steel grade by adjusting the amount of N added based on a value of the relationship related to low-temperature toughness expressed by Equation (1) being 10 to 35.
In addition, given that Mn also has a negative coefficient in the relationship related to low-temperature toughness, Mn and N need to be added in an appropriate ratio. When designing a steel grade that ensures sufficient low-temperature toughness in an extremely low-temperature environment, the steel grade composition needs to be designed such that a value calculated by the relationship related to low-temperature toughness suggested by the present invention is 10 to 35. In the present invention, the amounts of Ni, Mn, and N added are adjusted based on a point in which the value of the relationship related to low-temperature toughness expressed by Equation (1) is 10 to 35.
The present invention may provide an austenitic stainless steel having improved low-temperature impact toughness and strength by satisfying both the relationship related to low-temperature toughness and the relationship related to strength while simultaneously controlling the compositional ranges of alloy elements.
An austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention may include, in percent by weight (wt%), 0.1% or less (excluding 0) of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 16.0 to 23.0% of chromium (Cr), 5.0 to 12.0% of nickel (N), 10.0% or less (excluding 0) of manganese (Mn), 0 to 1.5% or less of molybdenum (Mo), 0 to 1.6% or less of Copper (Cu), 0.1 to 0.3% of nitrogen (N), the remainder of Iron (Fe), and impurities.
The reason for limiting the composition range of each alloy element is described in more detail below.
The content of C may be 0.1 wt% or less (excluding 0).
C is an element effective in stabilizing austenite, suppressing delta-ferrite, and increasing strength through solid solution strengthening. However, when the content of C is excessive, C may easily combine with carbide-forming elements such as Cr, Ti, and Nb, thereby reducing the corrosion resistance, ductility, and toughness of the base material. Therefore, it is preferable to set the content of C to 0.1% or less. More preferably, the content of C may be 0.019% to 0.23%.
The content of Si may be 1.5 wt% or less (excluding 0).
Si is an element effective in improving corrosion resistance and solid solution strengthening. However, since Si is a ferrite stabilizing element, when the content of Si is excessive, intermetallic compounds such as sigma phases may be formed, thereby reducing the ductility and toughness of the base material. Therefore, it is preferable to set the content of Si to 1.5% or less. More preferably, the Si content may be 0.38% to 0.46%.
The content of Cr may be 16.0 to 23.0 wt%.
Cr is an alloy element that needs to be added to improve corrosion resistance in stainless steel, and it is preferable to add Cr in an amount of 16.0% or more to ensure corrosion resistance. However, when the Cr content is excessive, Cr serves as a ferrite-forming element, resulting in retention of excessive delta-ferrite, which lowers hot workability. In addition, austenite becomes unstable, a large amount of Ni needs to be included for phase stability, which may cause an increase in cost. Therefore, it is preferable to set the Cr content to 23.0% or less. More preferably, the Cr content may be 16.8% to 21.5%.
The content of Ni may be 5.0 to 12.0 wt%.
Ni is an austenite stabilizing element. In terms of the austenite stabilizing effect and low-temperature toughness, the more Ni is added, the more beneficial it is. It is preferable to add Ni in an amount of 5.0% or more to suppress the formation of delta-ferrite in the manufacturing process. However, when the content of Ni is excessive, the probability of surface defects in the manufacturing process may increase and the cost may increase. Therefore, it is preferable to set the Ni content to 12.0% or less. More preferably, the Ni content may be 5.0% to 10.4%.
The content of Mn may be 10.0 wt% or less (excluding 0).
Mn is an austenite stabilizing element. Mn is an element that may serve as a substitute for expensive Ni. In addition, since Mn is an important element in terms of low-temperature toughness, Mn and Ni need to be added in an appropriate ratio to ensure low-temperature toughness. However, when the content of Mn is excessive, additional Ni needs to be added to obtain a value of 0 or higher in a relationship related to low-temperature toughness suggested by the present invention. However, the addition of expensive Ni may be undesirable in terms of cost. Therefore, it is preferable to set the content of Mn to be 10.0% or less. More preferably, the Mn content may be 0.8% to 7.9%.
The content of Mo may be 0 wt% or more than 0 wt% and 1.5 wt% or less.
Since Mo is an element effective in improving corrosion resistance in stainless steel, Mo may be optionally added in the present invention. However, when the content of Mo is excessive, the ferrite fraction may increase, which may cause a decrease in low-temperature toughness and may be undesirable in terms of cost. Therefore, preferably, the content of Mo may be 1.5% or less. More preferably, the Mo content may be 0% to 0.6%.
The content of Cu may be 0 wt% or more than 0 wt% and 1.6 wt% or less.
Cu is an element for suppressing martensite formation and increasing austenite stabilization during forming. In the present invention, Cu may be optionally added. However, when the content of Cu is excessive, a low melting point phase may be formed, which may reduce hot workability and degrade surface quality. Therefore, it is preferable that the content of Cu be 1.6% or less.
Specifically, the content of Cu may be 0.1 to 1.6%. Within the above range, the effect of suppressing martensite formation, increasing austenite stabilization, and preventing a decrease in hot workability may be further improved. In this case, the stainless steel of the present invention may further improve surface quality. Preferably, the Cu content may be 0.36% to 1.6%.
The content of N may be 0.1 to 0.3 wt%.
N is an austenite stabilizing element and is also an element effective in increasing strength through solid solution strengthening. In the present invention, the strength may be improved in the form of solid solution strengthening, not in the form of precipitation strengthening that has a negative effect on low-temperature impact toughness, by adding N. It is preferable to adjust the amount of N added based on a point in which the relationship related to low-temperature toughness is 0 such that the N content is 0.1% or more. However, when the N content is excessive, it is difficult to ensure a value of 10~35 in the relationship related to low-temperature toughness, which makes it difficult to ensure both strength and low-temperature impact toughness. Therefore, it is preferable to limit the N content to 0.3% or less. More preferably, the N content may be 0.11% to 0.2%.
The remainder of the present invention is iron (Fe). However, since unintended impurities may inevitably be introduced from raw materials or the surrounding environment during a typical manufacturing process, this may not be excluded. Since such impurities may be well known to those skilled in the art during a typical manufacturing process, details thereof are not described in this specification.
The austenitic stainless steel having improved low-temperature impact toughness and strength according to an example of the present invention may have a delta ferrite fraction of 4.0 vol% or less. Specifically, the austenitic stainless steel according to the present invention may have a delta ferrite fraction of 0% or more than 0% and 4.0% or less. Within the range, the austenitic stainless steel according to the present invention may have further improved impact toughness.
In the present invention, the relationship related to low-temperature toughness expressed by Equation (1) is defined as 1.7Ni+410C-0.9Cr-100N-13Si-1.2Mn+2Cu-5.5Mo+40. When Equation (1) is from 10 to 35, a Charpy energy value at -196°C may be 100J or more.
The present invention may ensure low-temperature impact toughness by controlling the content of Ni corresponding to a positive coefficient in Equation (1). However, the present invention needs to ensure both low-temperature impact toughness and strength. N is a component required for improving strength, but has a negative coefficient in Equation (1). Accordingly, there is a need to control N.
That is, the present invention controls the contents of Mn and N, which have negative coefficients in Equation (1), in order to ensure both low-temperature impact toughness and strength. In particular, the present invention may ensure low-temperature impact toughness while achieving a yield strength of 280 MPa or more by controlling the N content to 0.1% or more.
The present invention ensures an austenitic stainless steel with improved low-temperature impact toughness and strength by controlling a value of the relationship Equation related to low-temperature toughness expressed by Equation (1) to 10 to 35, thereby ensuring a Charpy energy value of -196°C of 100J or more and a yield strength of 280MPa or more.
In the present invention, the relationship related to strength expressed by Equation (2) is set to 0 or more, and Equation (2) is defined as (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51. When Equation (2) is 0 or more, the yield strength may be 280MPa or more.
That is, as described above, N is a component that is required for improving strength, but has a negative coefficient in Equation (1). Therefore, it is required to design the composition such that Equation (1) is in a range of 10 to 35 and Equation (2) is 0 or more. In the present invention, considering the above, the content of N may be controlled to 0.1 to 0.3.
The present invention controls Equation (1) to 10 to 35 and Equation (2) to 0 or more, such that the Charpy energy value at -196°C is 100J or more, and the yield strength is ensured to 280MP or more, thereby ensuring an austenitic stainless steel with improved low-temperature impact toughness and strength.
According to an embodiment of the present invention, a method of manufacturing an austenitic stainless steel having improved low-temperature impact toughness and strength comprises: performing hot-rolling on a slab comprising, in percent by weight (wt%), 0.1% or less carbon (C) (excluding 0), 1.5% or less (excluding 0) of silicon (Si), 16.0 to 23.0% of chromium (Cr), 5.0 to 12.0% of nickel (N), 10.0% or less (excluding 0) of manganese (Mn), 0 to 1.5% or less of molybdenum (Mo), 0 to 1.6% or less of Copper (Cu), 0.1 to 0.3% of nitrogen (N), the remainder of Iron (Fe), and impurities, and satisfying Equation (1) being 10 to 35 and Equation (2) being 0 or more, wherein Equation (1) is defined as 1.7Ni+410C-0.9Cr-100N-13Si-1.2Mn+2Cu-5.5Mo+40, and Equation (2) is defined as (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51; and performing hot annealing at 900 to 1200°C.
The alloy composition, Equation (1) and Equation (2) are as described above with respect to the austenitic stainless steel of the present invention.
The temperature of the hot annealing performed after the hot-rolling is 900 to 1200°C. The annealing temperature affects the release of residual stress and the microstructure. When the annealing temperature is less than 900°C, coarse carbides may be generated, which may make the structure uneven. In addition, Cr23C6 precipitates may be formed around grain boundaries, which may cause intergranular corrosion. When the annealing temperature exceeds 1200°C, the grains may become extremely coarsened. Considering this, it is preferable that the temperature of the hot-annealing be 900 to 1200°C.
The austenitic stainless steel manufactured by the method of manufacturing austenitic stainless steel with improved low-temperature impact toughness and strength of the present invention may have a Charpy impact energy of -196°C of 100J or more. In addition, the yield strength may be 280MPa or more at room temperature.
Hereinafter, the present invention will be described in more detail through embodiments. However, the descriptions of the embodiments are only for illustrating the implementation of the present invention, and the present invention is not limited by the descriptions of the embodiments. This is because the scope of the rights of the present invention is determined by matters described in the scope of claims and matters reasonably inferred therefrom.

{Embodiment}

A slab having an alloy composition according to Table 1 below was hot rolled, and then hot-annealed at 900 to 1200°C to produce an austenitic stainless steel. In order to check the impact toughness value at extremely low temperatures, the Charpy impact energy value at -196°C and the yield strength were presented in Table 2 below.
The Charpy impact energy value was measured by an impact test at a temperature of -196°C using a specimen confirming to the ASTM E23 type A standard. The yield strength was measured in the air at room temperature.

Table 1 shows the alloy compositions of Inventive examples and Comparative examples.

[Table1]

Classification	C	Si	Mn	Cr	Ni	Mo	Cu	N
Inventive Example 1	0.019	0.38	6.96	20.2	10	-	0.4	0.19
Inventive Example2	0.02	0.4	6.89	18.3	8.5	-	0.4	0.18
Inventive Example3	0.022	0.43	7.2	17.9	7.4	-	0.4	0.17
Inventive Example4	0.019	0.41	5.6	17.4	7.2	-	0.41	0.17
Inventive Example5	0.019	0.41	3.44	18.9	8.9	-	0.38	0.17
Inventive Example6	0.019	0.4	4.1	19.0	8.5	-	0.43	0.2
Inventive Example7	0.021	0.4	4.45	18.3	8.0	-	0.4	0.17
Inventive Example8	0.019	0.41	2.49	20.4	8.57	-	0.81	0.19
Inventive Example9	0.019	0.41	3.6	20.2	8.38	-	0.8	0.19
Inventive Example 10	0.021	0.42	0.8	21.3	9.31	0.6	0.81	0.2
Inventive Example 11	0.019	0.46	1.01	18.3	7.97		0.4	0.16
Inventive Example12	0.023	0.41	0.81	21.5	10.4	0.6	0.79	0.2
Inventive Example 13	0.021	0.38	7.63	17.4	5.98	-	0.39	0.19
Inventive Example14	0.022	0.39	7.9	16.8	5.0	-	0.39	0.18
Inventive Example15	0.02	0.39	5.65	17.3	5.6	-	0.36	0.18
Inventive Example16	0.02	0.41	4.6	17.9	7.83	-	1.07	0.2
Inventive Example17	0.019	0.41	4.51	18	8.4	-	1.17	0.15
Inventive Example18	0.019	0.4	1.22	19.6	9.25	-	0.81	0.15
Inventive Example19	0.019	0.4	1.18	19.4	10.3	-	0.41	0.11
Inventive Example20	0.02	0.42	6.6	17.9	5.6	-	1.6	0.16
Inventive Example21	0.018	0.4	0.87	21.2	9.16	0.6	0.79	0.15
Comparative Example 1	0.02	0.61	1.1	16.1	10.2	2.11	0.31	0.016
Comparative Example2	0.022	0.39	9.8	17.6	5.4	-	0.39	0.19
Comparative Example3	0.021	0.38	7.8	18.2	5.9	-	0.4	0.21
Comparative Example4	0.02	0.54	8.9	16.8	2.8	-	1.89	0.22
Comparative Example5	0.02	0.41	1.18	18.8	10.2	-	0.42	0.015
Comparative Example6	0.019	0.45	0.92	18.2	8.0	0.13	0.3	0.017
Comparative Example7	0.021	0.47	1.28	16.7	10.09	2.05	0.27	0.071

Table 2 below shows values of the Equations (1), (2), Charpy impact energy values at -196°C, and yield strengths at room temperature according to the Inventive Examples and Comparative Examples.

[Table 2]

Classification	Equation (1)	Equation (2)	-196°C impact toughness (J)	Yield strength (MPa)
Inventive Example 1	15.1	7.4	128.4	324.3
Inventive Example2	15.5	6.4	116.9	310.2
Inventive Example3	15.1	5.4	130.6	327.5
Inventive Example4	16.1	4.2	137.3	322
Inventive Example5	20.2	3.7	127.7	326.5
Inventive Example6	15.9	4.9	151.0	329.8
Inventive Example7	19.0	3.9	115.8	315
Inventive Example8	18.3	3.0	129.9	338.5
Inventive Example9	16.8	3.6	140.1	344
Inventive Example 10	17.2	2.3	126.3	350.8
Inventive Example 11	22.5	0.9	109.5	316.2
Inventive Example12	19.7	3.4	126.1	361.5
Inventive Example 13	10.8	5.3	112.7	313.8
Inventive Example14	10.6	4.4	102.8	337.7
Inventive Example15	13.0	3.1	106.3	334.7
Inventive Example16	16.7	5.5	121.3	341.5
Inventive Example17	22.5	4.0	121.3	318.8
Inventive Example18	25.8	1.6	133.2	305.3
Inventive Example19	31.0	0.9	187.5	282
Inventive Example20	15.4	3.3	124.6	323.3
Inventive Example21	20.9	0.1	120.8	341.6
Comparative Example 1	29.2	-2.7	164.5	237.7
Comparative Example2	7.3	6.2	96.7	346.2
Comparative Example3	7.8	5.8	85.0	359.2
Comparative Example4	1.9	5.1	34.9	374.1
Comparative Example5	41.2	-2.7	198.8	212.5
Comparative Example6	36.2	-4.9	135.6	264.5
Comparative Example7	25.3	-0.6	156.4	276.6

Referring to the results in Table 1 and Table 2, Inventive examples 1 to 21 satisfy the alloy composition, Equation (1) and Equation (2) of the present invention. In particular, by controlling the N content to 0.1% or more and Equation (1) to a value of 10 to 35, the Charpy impact energy of 100J or more at -196°C is ensured while achieving the yield strength of 280MPa or more. That is, it can be seen that an austenitic stainless steel with improved low-temperature impact toughness and strength is obtained. In contrast, Comparative Examples 1, 5, 6, and 7 fall within the range of Ni contents of the present invention, and have a Charpy impact energy -196°C of 100J or more. However, since the N content is less than 0.1% and the value of Equation (2) is less than 0, the strength is not ensured, with a yield strength of less than 280MPa. Through this, it can be seen that even when the alloy composition of the present invention excluding N is satisfied, when Equation (1) and Equation (2) are not satisfied, low-temperature impact toughness and strength may not be ensured at the same time.
Comparative Examples 2 to 4 correspond to the alloy composition according to the present invention, particularly, having an N content of 0.1% or more and satisfy Equation (2) with a value of 0 or more, ensuring a yield strength of 280 MPa or more. However, it can be seen that since Equation (1) does not satisfy the range of the present invention, the Charpy impact energy at -196°C is less than 100J, failing to ensure the low-temperature impact toughness. Through this, it can be seen that even when the alloy composition, particularly the content of N of 0.1% or more is ensured and Equation (2) is satisfied, when Equation (1) does not correspond to 10 to 35, low-temperature impact toughness may not be ensured, and thus strength and low-temperature impact toughness may not be ensured at the same time.
In addition, Comparative Example 4, despite having a high N content of 0.22% and ensuring a very high yield strength of 374.1 MPa, has a low Ni content, and therefore, does not satisfy the range of 10~35 in Equation (1), failing to ensure the low-temperature impact toughness, with a Charpy impact energy at -196°C being less than 100 J. Through this, it can be seen that even when the alloy composition, particularly the N content of 0.1% or more is ensured and Equation (2) is satisfied, when Equation (1) does not correspond to 10~35, the low-temperature impact toughness may not be ensured, and thus strength and low-temperature impact toughness may not be ensured at the same time.
Through the Inventive Examples and Comparative Examples of the present invention, it can be seen that although N is required for ensuring strength, in order to ensure low-temperature impact toughness at the same time as strength, there is a need to control not only the content of N but also the low-temperature toughness-related Equation (1) including Ni, Mn, and the like to a value of 10 to 35, and control the Equation (2) related to strength to a value of 0 or more, thereby obtaining an austenitic stainless steel with improved strength and low-temperature impact toughness at the same time.
The present invention may obtain an austenitic stainless steel with improved strength and low-temperature impact toughness at the same time through a compositional system that may control the relationship related to low-temperature toughness and the relationship related to strength. Through this, the austenitic stainless steel according to the present invention may allow reduction in the thickness of the material used, thereby reducing the amount of material used and contributing to cost reduction in the manufacturing of cryogenic tanks.
Conventional steels have relatively excellent low-temperature toughness, but have relatively low strength, which results in an increased material thickness when manufacturing cryogenic tanks or structures. However, the austenitic stainless steel according to the present invention may resolve this issue.
In addition, the austenitic stainless steel according to the present invention may be manufactured either in combination with, or independently of the conventional strength enhancement method through cold working and precipitation, while maintaining or further reducing hydrogen embrittlement or low-temperature toughness degradation and achieving a desired level of properties.
While exemplary embodiments of the present disclosure have been described above with reference to the accompanying drawings, it will be understood by those of ordinary skill in the art that various modifications and changes can be made without departing from the spirit and scope of the present disclosure as defined by the appended claims.

Claims

An austenitic stainless steel having improved low-temperature impact toughness and strength, comprising, in percent by weight (wt%), 0.1% or less (excluding 0) of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 16.0 to 23.0% of chromium (Cr), 5.0 to 12.0% of nickel (N), 10.0% or less (excluding 0) of manganese (Mn), 0 to 1.5% or less of molybdenum (Mo), 0 to 1.6% or less of Copper (Cu), 0.1 to 0.3% of nitrogen (N), the remainder of Iron (Fe), and impurities,
wherein Equation (1) is from 10 to 35 and Equation (2) is 0 or more,

Equation (1): 1.7Ni+410C-0.9Cr-100N-13Si-1.2Mn+2Cu-5.5Mo+40,

Equation (2): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51.
The austenitic stainless steel of claim 1, wherein a Charpy impact energy at -196°C is 100J or more.
The austenitic stainless steel of claim 1, wherein a yield strength is 280MPa or more.
A method of manufacturing an austenitic stainless steel having improved low-temperature impact toughness and strength, comprising:
performing hot-rolling on a slab comprising, in percent by weight (wt%), 0.1% or less (excluding 0) of carbon (C), 1.5% or less (excluding 0) of silicon (Si), 16.0 to 23.0% of chromium (Cr), 5.0 to 12.0% of nickel (N), 10.0% or less (excluding 0) of manganese (Mn), 0 to 1.5% or less of molybdenum (Mo), 0 to 1.6% or less of Copper (Cu), 0.1 to 0.3% of nitrogen (N), the remainder of Iron (Fe), and impurities, and satisfying Equation (1) being from 10 to 35 and Equation (2) being 0 or more; and

performing hot annealing at 900 to 1200°C,

Equation (1): 1.7Ni+410C-0.9Cr-100N-13Si-1.2Mn+2Cu-5.5Mo+40

Equation (2): (Ni+0.52Cu+40(C+N)+0.7Mn+45)-(0.48(Cr+Mo+1.7Si))-51.
The method of claim 4, wherein a Charpy impact energy at -196°C is 100J or more.
The method of claim 4, wherein a yield strength is 280MPa or more.