EP4640893A1

EP4640893A1 - Austenitic steel material and manufacturing method therefor

Info

Publication number: EP4640893A1
Application number: EP23907502.1A
Authority: EP
Inventors: Soon-Gi Lee; Sang-Deok Kang
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-20
Filing date: 2023-12-04
Publication date: 2025-10-29
Also published as: WO2024136216A1; CN120457233A; KR20240097540A; EP4640893A4; JP2025542024A

Abstract

The present invention relates to an austenitic steel material, and a manufacturing method therefor, and, more specifically, to an austenitic steel material and a manufacturing method therefor, the material being preferably applicable as a structural material used in a cryogenic environment such as that of a liquefied gas storage tank and liquefied gas transportation equipment.

Description

Technical Field

The present disclosure relates to an austenitic steel material, and a manufacturing method therefor, and, more specifically, to an austenitic steel material and a method for manufacturing the same, the material preferably being applicable as a structural material used in a cryogenic environment such as that of a liquefied gas storage tank, liquefied gas transportation equipment, or the like.

Background Art

Liquefied gases such as liquefied hydrogen (boiling point: -253°C), liquefied natural gas (LNG, boiling point: - 164°C), liquefied oxygen (boiling point: -183°C), liquefied nitrogen (boiling point: -196°C) and the like require ultra-low temperature storage. Therefore, to store these gases, a structure such as a pressure vessel or the like made of a material with sufficient toughness and strength at ultra-low temperatures is required.
As materials that may be used at low temperatures in a liquefied gas atmosphere, Cr-Ni stainless steel alloys such as AISI 304 or the like, 9% Ni steel, 5000 series aluminum alloys, or the like have been used. However, in the case of aluminum alloys, the alloy cost is high, the design thickness of the structure increases due to low strength, and the weldability is also poor, so their use is limited. Cr-Ni stainless steel, 9% nickel (Ni) steel and the like have greatly improved the physical properties of aluminum, but it is not desirable from an economic perspective because it contains a large amount of expensive nickel (Ni).

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide an austenitic steel material and a method for manufacturing the same.
A preferred aspect of the present disclosure is to provide an austenitic steel material having excellent ultra-low temperature toughness and a method for manufacturing the same.

Solution to Problem

According to an aspect of the present disclosure, an austenitic steel material includes, in weight%, manganese (Mn): 10 to 45%, carbon (C): within a range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and a remainder of iron (Fe) and unavoidable impurities, wherein a microstructure in a welded heat-affected zone thereof includes austenite of 95% or more (including 100%) and carbides of 5% or less (including 0%) in area%, and a content of C solid-solubilized in the austenite is 60% or more of an average C content of the steel material.
The austenite may have an average grain size of 10 to 200 µm.
The welded heat-affected zone may have a transverse expansion of 0.32 mm or more after a Charpy impact test at - 253°C.
The welded heat-affected zone may have a Charpy impact energy of 27 J or more at -253°C.
According to another aspect of the present disclosure, a method for manufacturing an austenitic steel material includes an operation of heating a slab at 1000 to 1300°C, the slab containing, in weight%, manganese (Mn): 10 to 45%, carbon (C): within a range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and a remainder of iron (Fe) and unavoidable impurities; an operation of obtaining a hot-rolled steel sheet by finishing hot-rolling the heated slab at 800 to 1050°C; and an operation of welding the hot-rolled steel sheet.

Advantageous Effects of Invention

According to an aspect of the present disclosure, an austenitic steel material and a method for manufacturing the same may be provided.
According to a preferred aspect of the present disclosure, an austenitic steel material having excellent ultra-low temperature toughness and a method for manufacturing the same may be provided.

Brief Description of Drawings

FIG. 1 is a drawing illustrating the relationship between the carbon content and the manganese content of an austenitic steel material according to an aspect of the present disclosure.
FIG. 2 is a drawing schematically illustrating a method for measuring a transverse expansion value of an austenitic steel material according to an aspect of the present disclosure.

Best Mode for Invention

Hereinafter, an austenitic steel material according to an embodiment of the present disclosure will be described. First, the alloy composition will be described. The content of the alloy composition described below refers to weight% unless otherwise specified.

Manganese (Mn): 10 to 45%

Manganese is an element that plays an important role in stabilizing austenite. It is preferable to include 10% or more of manganese (Mn) to stabilize austenite at ultra-low temperatures. If the manganese (Mn) content is less than this, epsilon martensite that is a metastable phase is formed and may be easily transformed into alpha martensite by processing-induced transformation at ultra-low temperatures, so that toughness cannot be secured. There is a method to increase the carbon (C) content to stabilize austenite to suppress the formation of epsilon martensite, but in this case, a large amount of carbides may be precipitated, rapidly deteriorating the physical properties. Therefore, the manganese (Mn) content is preferable to be 10% or more. The preferable manganese (Mn) content may be 15% or more, and the more preferable manganese (Mn) content may be 18% or more. If the manganese (Mn) content is excessive, it may not only reduce the corrosion rate of the steel material, but is also undesirable from an economical perspective. Therefore, the manganese (Mn) content is preferably 45% or less. The preferable manganese (Mn) content may be 40% or less, and the more preferable manganese (Mn) content may be 35% or less.

Carbon (C): A range satisfying 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18

Carbon (C) is an element that stabilizes austenite and increases strength. In particular, carbon (C) plays a role in lowering Ms or Md, which is the transformation point from austenite to epsilon or alpha martensite, during the process of cooling, processing or the like. Therefore, carbon (C) is a component that effectively contributes to the stabilization of austenite. If the carbon (C) content is insufficient, the stability of austenite is insufficient, so stable austenite cannot be obtained at ultra-low temperatures, and external stress may easily cause a processing-induced transformation into epsilon or alpha martensite, which may reduce the toughness of the steel material or reduce the strength of the steel material. On the other hand, if the carbon (C) content is excessive, the toughness of the steel material may rapidly deteriorate due to carbide precipitation, and the strength of the steel material may excessively increase, thereby reducing the workability.
The inventor of the present disclosure conducted an in-depth study on the relative behavior between the carbon (C) and manganese (Mn) contents in relation to the formation of carbides, and as a result, as illustrated in FIG. 1, it was concluded that determining the relative content relationship between carbon (C) and manganese (Mn) may effectively promote the stabilization of austenite while effectively controlling the amount of carbide precipitation. Carbide is formed by carbon (C), but carbon (C) does not independently affect the formation of carbide, but acts in combination with manganese (Mn) to affect the formation of carbide.
To stabilize austenite, it is preferable to control the value of 24×[C]+[Mn] (wherein, [C] and [Mn] mean the contents of respective components expressed in weight%) to 25 or more, assuming that other components meet the range specified in the present disclosure. The boundary means the inclined left boundary of the parallelogram region illustrated in FIG. 1. When 24×[C]+[Mn] is less than 25, the stability of austenite decreases, causing processing-induced transformation by impact at ultra-low temperatures, and thus the impact toughness of the steel material may deteriorate. On the other hand, to suppress the formation of carbides, it is preferable to control the value of 33.5×[C]-[Mn] (wherein, [C] and [Mn] mean the contents of respective components expressed in weight%) to 18 or less, provided that the other components satisfy the ranges stipulated in the present disclosure. When 33.5×[C]-[Mn] exceeds 18, carbides may be precipitated due to the excessive addition of carbon (C), which may deteriorate the low-temperature impact toughness of the steel material. Therefore, in the present disclosure, it is preferable that carbon (C) is added to satisfy 24×[C]+[Mn] ≥ 25 and 33.5×[C]-[Mn]≤18. As can be seen in FIG. 1, the lowest limit of the carbon (C) content is 0% within the range satisfying the above-mentioned formula.

Chromium (Cr): 10% or less (excluding 0%)

Chromium (Cr) is also an austenite stabilizing element, and stabilizes austenite up to the range of an appropriate amount of addition, thereby improving the low-temperature impact toughness of steel materials, and acts to increase the strength of steel materials by being dissolved in austenite. In addition, chromium (Cr) is also an element that effectively contributes to improving the corrosion resistance of steel materials. Therefore, in the present disclosure, chromium (Cr) is added as an essential element. The preferable lower limit of the chromium (Cr) content may be 1%, and the more preferable lower limit of the chromium (Cr) content may be 2%. However, chromium (Cr) is a carbide-forming element, and in particular, may form carbides at austenite grain boundaries to reduce the low-temperature impact toughness of steel materials. In addition, in the case in which the addition amount of chromium (Cr) exceeds a certain level, excessive carbides may be precipitated in the welded heat-affected zone (HAZ), which may result in poor ultra-low-temperature toughness. Therefore, the present disclosure may limit the upper limit of chromium (Cr) to 10%. The preferable upper limit of the chromium (Cr) content may be 8%, and the more preferable upper limit of the chromium (Cr) content may be 7%.

At least one of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%)

Cu: 5% or less (excluding 0%)

Cu has a very low solubility in carbides and is slow in diffusion in austenite, and thus is concentrated at the interface between austenite and nucleated carbides. Accordingly, it effectively slows down the growth of carbides by hindering the diffusion of carbon, and ultimately has the effect of suppressing the formation of carbides. In addition, copper has the effect of stabilizing austenite and improving ultra-low temperature toughness. However, if the content of Cu exceeds 5%, there is a problem of lowering the hot workability of the steel material, so it is preferable to limit the upper limit to 5 wt%. The preferable upper limit of the copper (Cu) content may be 3%, and the preferable upper limit of the copper (Cu) content may be 2%. The preferable lower limit of the copper (Cu) content may be 0.1%, and the more preferable lower limit of the copper (Cu) content may be 0.3%.

Si: 5% or less (excluding 0%)

Silicon (Si) is an element that improves the castability of molten steel and, in particular, when added to an austenitic steel material, effectively increases the strength by being dissolved inside the steel material. It is also an element that affects the activity of carbon in the steel material and effectively suppresses the formation of carbides, thereby increasing the toughness. However, if added in excess of 5%, it reduces the stacking fault energy, promotes twinning, and there is a problem that the toughness may be lowered due to high strength, so it is preferable to limit the upper limit to 5%. The preferable upper limit of the silicon (Si) content may be 3%, and the more preferable upper limit of the silicon (Si) content may be 2.5%. The preferable lower limit of the silicon (Si) content may be 0.1%, and the more preferable lower limit of the silicon (Si) content may be 0.3%.

Al: 5% or less (excluding 0%)

Al is an element that stabilizes austenite within an appropriate addition range and lowers the Ms and Md, which are transformation points from austenite to epsilon or alpha martensite by cooling or processing, thereby improving the toughness of steel materials. It is also an element that is dissolved in the steel material to increase strength, and in particular, it is an element that effectively suppresses the formation of carbides by affecting the activity of carbon in the steel material, thereby increasing toughness. In particular, it is well known as an element that effectively increases the stacking fault energy and promotes slip. However, if it is added in excess of 5%, there is a problem of deteriorating the castability and surface quality of the steel through the formation of oxides and nitrides, so it is preferable to limit the upper limit to 5 wt%. The preferable upper limit of the aluminum (Al) content may be 3%, and the more preferable upper limit of the aluminum (Al) content may be 2.5%. The preferable lower limit of the aluminum (Al) content may be 0.2%, and the more preferable lower limit of the aluminum (Al) content may be 0.3%.

Mo: 5% or less (excluding 0%)

Molybdenum (Mo) is an element that stabilizes austenite within an appropriate addition range and lowers the transformation points Ms and Md from austenite to epsilon or alpha martensite by the cooling process or processing, thereby improving the toughness of steel materials. In addition, it is an element that is dissolved in the steel material to increase strength, and in particular, it is an element that segregates in austenite grain boundaries to increase the stability of grain boundaries, thereby reducing energy and thus suppressing the precipitation of carbonitride at grain boundaries. In addition, molybdenum (Mo) is well known as an element that effectively increases stacking fault energy and promotes slip. However, if it is added in excess of 5 wt%, it is an expensive element, and thus there is a problem that the economy decreases and the toughness may decrease due to the high strength. Thus it is preferable to limit the upper limit to 5 wt%. The preferable upper limit of the molybdenum (Mo) content may be 4.5%, and the more preferable upper limit of the molybdenum (Mo) content may be 4%. The preferable lower limit of the molybdenum (Mo) content may be 0.1%, and the more preferable lower limit of the molybdenum (Mo) content may be 0.3%.

B: 0.5% or less (excluding 0%)

Boron (B) is a representative element that improves the hardenability of steel materials. It is preferentially segregated at the grain boundaries of austenite at high temperatures, lowering the energy of the austenite grain boundaries and stabilizing the same. Therefore, it improves the hardenability so that undesirable structures such as ferrite, pearlite, or the like are not generated at the austenite grain boundaries during quenching of carbon steel. Boron (B) is also segregated in the austenite grain boundary of high manganese steel, which lowers the energy of the austenite grain boundary, thereby preventing carbon from diffusing, and is an effective element in suppressing the formation of carbides. However, if it is added in excess of 0.5 wt%, there is a problem of coarse boron nitrides being formed, which reduces the physical properties, so it is preferable to limit the upper limit to 0.5 wt%. The preferable upper limit of the boron (B) content may be 0.4%, and the more preferable upper limit of the boron (B) content may be 0.3%. The preferable lower limit of the boron (B) content may be 0.0005%, and the more preferable lower limit of the boron (B) content may be 0.001%.
Since structures are generally provided by processing and welding steel materials, even if the ultra-low temperature impact toughness of the base material itself is secured, if the ultra-low temperature impact toughness of the weld is not secured, the safety of the structure itself may be significantly reduced. Therefore, the austenitic steel material according to an aspect of the present disclosure is provided to secure not only the ultra-low temperature impact toughness of the base material itself but also the ultra-low temperature impact toughness of the welded heat-affected zone (HAZ). Accordingly, in the present disclosure, not only the microstructure of the base material but also the microstructure of the welded heat-affected zone is controlled within a specific range.
According to an aspect of the present disclosure, an austenitic steel material may include austenite of 95 area% or more in the microstructure of not only the base material but also the welded heat-affected zone (HAZ) in terms of securing the desired physical properties. The preferred austenite fraction may be 97 area% or more, and may include a case where the austenite fraction is 100 area%. Meanwhile, the austenitic steel material according to an aspect of the present disclosure may actively suppress the carbide fraction to 5 area% or less to prevent a decrease in ultra-low temperature impact toughness. The preferred carbide fraction may be 3 area% or less, and may include a case in which the carbide fraction is 0 area%. In the present disclosure, a method for measuring the austenite fraction and the carbide fraction is not particularly limited, and may be easily confirmed through a measuring method commonly used by a person skilled in the art to which the present disclosure pertains for measuring microstructure and carbide.
It is preferable that the content of C dissolved in the austenite is 60% or more of the average C content of the steel material. When all of the C contained in the steel material is dissolved in austenite, the content of C dissolved in the austenite is 100%, and when the C contained in the steel material is precipitated as carbide, the content of C dissolved in the austenite is less than 100%. By dissolving a large amount of C in the austenite in this way, the precipitation of carbide may be suppressed, and at the same time, the deterioration of impact toughness may be prevented. It is more preferable that the content of C dissolved in the austenite is 70% or more, more preferably 85%, and most preferably 100%, compared to the average C content of the steel material. The method for measuring the content of C dissolved in austenite relative to the average C content of the steel material is not particularly limited, and may be easily confirmed through a measurement method commonly used by a person skilled in the art to which the present disclosure pertains for measuring microstructure and carbides.
The austenite preferably has an average grain size of 10 to 200 µm. If the average grain size of austenite in the welded heat-affected zone (HAZ) is excessively small, the strength of the weld is improved, but local ultra-low temperature impact toughness may deteriorate in the welded heat-affected zone (HAZ). In addition, since the main precipitation location of carbides is the austenite grain boundary, if the grain size is excessively fine, carbides may precipitate excessively, resulting in a decrease in impact toughness. Therefore, the average grain size of austenite in the welded heat-affected zone may be limited to 10 µm or more. Meanwhile, the larger the average grain size of austenite in the welded heat-affected zone is advantageous for securing ultra-low temperature impact toughness of the weld, but local strength degradation may occur in the welded heat-affected zone. Therefore, the average grain size of austenite may be limited to 200 µm or less. The lower limit of the average grain size of austenite is more preferably 15 µm, and more preferably 20 µm. The upper limit of the average grain size of austenite is more preferably 180 µm, and more preferably 150 µm.
When welding is performed under normal welding conditions for welding ultra-low temperature structures using an austenitic steel material according to an aspect of the present disclosure as a base material, the welded heat-affected zone may have a transverse expansion value of 0.32 mm or more after a Charpy impact test at -253°C. Meanwhile, in the present disclosure, since the higher the transverse expansion value, the more advantageous it is, the upper limit thereof is not particularly limited. However, the upper limit of the transverse expansion value may be, for example, 2.30 mm.
The inventor of the present disclosure has found that in the case of steel materials applied to ultra-low temperature environments, plastic deformation characteristics are a major factor in terms of securing safety. That is, after in-depth research, the inventor of the present disclosure has confirmed that, in the case of steel materials satisfying the component system suggested by the present disclosure, the transverse expansion value (mm) in the welded heat-affected zone is a more important factor in terms of securing safety of the weld than the Charpy impact energy value (J) in the welded heat-affected zone.
The transverse expansion value in the welded heat-affected zone means the average value of the transverse plastic deformation amount of a specimen that has undergone a Charpy impact test at -253°C. FIG. 2 illustrates a photograph of a specimen that underwent a Charpy impact test at -253°C, and as illustrated in FIG. 2, the transverse length increase (ΔX1+ΔX2) near the fracture surface may be calculated to derive the transverse expansion value. If the transverse expansion value in the welded heat-affected zone is 0.32 mm or more, it may be determined that the minimum low-temperature safety required for ultra-low-temperature structures is provided.
According to the research results of the present inventors, it was confirmed that the Charpy impact energy (J) at -253°C and the transverse expansion value (mm) of the corresponding specimen generally showed a tendency similar to Relationship 1 below, and it was confirmed that the transverse expansion value (mm) is preferably 0.32 mm or more. It can be seen that the larger the transverse expansion value (mm), the better the low-temperature impact toughness, and 0.72 to 1.4 mm is more effective. Transverse expansion value (mm) = 0.0088 × Charpy impact energy value (J) + 0.0893
When welding is performed under normal welding conditions for welding ultra-low temperature structures using an austenitic steel material according to an aspect of the present disclosure as a base material, the transverse expansion value in the welded heat-affected zone of the specimen subjected to the Charpy impact test at -253°C is at a level of 0.32 mm or more, so that excellent structural safety may be secured when manufacturing an ultra-low temperature structure using the steel material.
When welding is performed under the normal welding conditions for welding ultra-low temperature structures using the austenitic steel material of the present disclosure as a base material, the welded heat-affected zone may have a Charpy impact energy of 27J or more at -253°C. By securing such a high level of ultra-low temperature Charpy impact energy, ductile fracture occurs when the structure is destroyed, and thus the destruction stability of the ultra-low temperature structure may be secured. Meanwhile, in the present disclosure, since the ultra-low temperature Charpy impact energy is higher, it is advantageous, and therefore the upper limit thereof is not particularly limited. However, the upper limit of the ultra-low temperature Charpy impact energy may be 250J as an example.
Hereinafter, a method for manufacturing an austenitic steel material according to an embodiment of the present disclosure will be described.
First, a slab having the above-mentioned alloy composition is heated at 1000 to 1300°C. If the slab heating temperature is less than 1100°C, there is a disadvantage that the alloy components are not re-dissolved and homogenized, or that it takes a long time to reach the target temperature to the center of the slab. If the slab heating temperature exceeds 1300°C, there is a disadvantage that partial melting occurs in the slab alloy component segregation area or that surface oxidation occurs severely. The lower limit of the slab heating temperature is more preferably 1030°C, even more preferably 1070°C, and most preferably 1100°C. The upper limit of the slab heating temperature is more preferably 1250°C, even more preferably 1230°C, and most preferably 1200°C.
Thereafter, the heated slab is finish hot-rolled at 800 to 1050°C to obtain a hot-rolled steel sheet. If the above-mentioned finish hot-rolling temperature is less than 800°C, rolling is not easy due to the high temperature strength of the material, and since non-recrystallized rolling occurs, there is a disadvantage in that the strength of the material increases excessively, reducing the impact toughness. If the finish hot-rolling temperature exceeds 1050°C, there is a disadvantage that austenite coarsens and the strength decreases. The lower limit of the finish hot rolling temperature is more preferably 820°C, more preferably 850°C, and most preferably 870°C. The upper limit of the slab finish hot rolling temperature is more preferably 1030°C, more preferably 1000°C, and most preferably 980°C. Meanwhile, the reduction ratio during the hot rolling may be applied within an appropriate range depending on the target plate thickness, and as a non-limiting example, the final thickness of the hot-rolled steel sheet may be 5 to 80 mm.
After that, the hot-rolled steel sheet may be air-cooled to room temperature.
After that, the hot-rolled steel sheet is welded. In the present disclosure, to form a welded heat-affected zone, welding may be performed under normal welding conditions for welding ultra-low temperature structures using a covered arc welding rod, a flux-cored arc welding wire, a TIG welding rod and wire, a submerged arc welding wire, a flux, and the like.

Mode for Invention

Hereinafter, an austenitic steel material and a manufacturing method thereof according to an aspect of the present disclosure will be described in more detail through detailed examples. It should be noted that the examples below are only for understanding the present disclosure and are not intended to specify the scope of the rights of the present disclosure. The scope of the rights of the present disclosure may be determined by the matters described in the patent claims and matters reasonably inferred therefrom.

(Example)

After preparing a slab having a thickness of 250 mm with an alloy composition described in Table 1 below, the slab was heated and hot rolled under the conditions described in Table 2 below, and subjected to submerged arc welding under normal conditions to manufacture a steel material having a welded heat-affected zone.
The microstructure and physical properties of the welded heat-affected zone of the steel material manufactured in this manner were measured, and then the results are illustrated in Table 2 below.
The microstructure was measured using an optical microscope at room temperature.
The average grain size of austenite was measured by taking microscopic photographs using an optical microscope and then using image analysis.
The content of C dissolved in austenite compared to the average C content of the steel material was measured by point analysis of the matrix and carbides using an energy dispersive X-ray spectrometer of a scanning electron microscope.
The Charpy impact energy was measured using a Charpy impact tester after the specimen was maintained at -253°C for 15 minutes or more.

The above transverse expansion was calculated by calculating the average value of the transverse plastic deformation amount of the specimen subjected to the Charpy impact test at -253°C.

[Table 1]

Stee l Grad e	Alloy Composition (wt%)
Stee l Grad e	Mn	24×[C]+[Mn ]	33.5×[C]-[Mn]	Cr	Cu	Si	Al	Mo	B
A	23.85	29.13	-16.48	2.92	-	-	2.31	-	-
B	26.94	33.90	-17.23	3.14	0.83	-	-	-	-
C	21.09	30.45	-8.03	2.64	-	0.80	1.05	-	-
D	20.81	29.69	-8.42	3.44	0.41	-	-	-	-
E	27.26	37.34	-13.19	2.25	-	-	-	-	0.003
F	19.40	25.40	-11.03	6.35	-	-	-	1.32	-
G	22.50	30.90	-10.78	2.85	-	-	-	-	-
H	33.60	47.04	-14.84	11.70	-	-	-	-	-
I	18.06	25.02	-8.35	1.17	-	-	-	-	-
J	15.30	44.34	25.24	1.12	-	-	-	-	-

[Table 2]

Class ifica tion	Steel Grade	Slab Heatin g Temper ature (°C)	Finish Hot Rolling Tempera ture (°C)	Stee l Mate rial Thic knes s (mm)	Microstructure (area%)	Austen ite Averag e Grain Size (µm)	Ratio of C conten t dissol ved in austen ite to averag e C conten t of steel materi al	Charpy Impact Energy (J, @-253°C)	Trans verse Expan sion (mm)
Examp le 1	A	1184	914	20	100%γ	43	100	134	1.27
Examp le 2	B	1181	910	25	100%γ	40	100	112	1.11
Examp le 3	C	1205	916	30	100%γ	37	100	86	0.85
Examp le 4	D	1190	896	32	98%γ, 2% Carbide	52	85	84	0.80
Examp le 5	E	1212	924	18	100%γ	75	100	92	0.89
Examp le 6	F	1204	927	22	99%γ, 1% Carbide	64	91	84	0.81
Compa rativ e Examp le 1	G	1180	836	20	94%γ, 6% Carbide	9.6	57	26	0.18
Compa rativ e Examp le 2	H	1193	925	35	92%γ, 8% Carbide	33	52	25	0.12
Compa rativ e Examp le 3	I	1200	915	18	80%γ, 20%ε	50	100	5	0.00
Compa rativ e Examp le 4	J	1205	862	30	89%γ, 11% Carbide	42	48	31	0.05
γ: austenite, ε: epsilon martensite

As can be seen from the above Tables 1 and 2, in the case of Examples 1 to 6 that satisfy the alloy composition and manufacturing conditions of the present disclosure, it can be seen that excellent physical properties are secured by securing the microstructure desired by the present disclosure and the content of C dissolved in austenite compared to the average C content of the steel material.
In the case of Comparative Examples 1 to 4 that do not satisfy the alloy composition of the present disclosure, it can be seen that the physical properties are at a low level because the microstructure desired by the present disclosure or the content of C dissolved in austenite compared to the average C content of the steel material are not secured.

Claims

An austenitic steel material comprising:
in weight%, manganese (Mn): 10 to 45%, carbon (C): within a range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and a remainder of iron (Fe) and unavoidable impurities,

wherein a microstructure in a welded heat-affected zone thereof includes austenite of 95% or more (including 100%) and carbides of 5% or less (including 0%) in area%, and

a content of C solid-solubilized in the austenite is 60% or more of an average C content of the steel material.
The austenitic steel material of claim 1, wherein the austenite has an average grain size of 10 to 200 µm.
The austenitic steel material of claim 1, wherein the welded heat-affected zone has a transverse expansion of 0.32 mm or more after a Charpy impact test at -253°C.
The austenitic steel material of claim 1, wherein the welded heat-affected zone has a Charpy impact energy of 27 J or more at -253°C.
A method for manufacturing an austenitic steel material, comprising:
an operation of heating a slab at 1000 to 1300°C, the slab containing, in weight%, manganese (Mn): 10 to 45%, carbon (C): within a range of 24×[C]+[Mn]≥25 and 33.5×[C]-[Mn]≤18, chromium (Cr): 10% or less (excluding 0%), and at least one of Cu: 5% or less (excluding 0%), Si: 5% or less (excluding 0%), Al: 5% or less (excluding 0%), Mo: 5% or less (excluding 0%), and B: 0.5% or less (excluding 0%), and a remainder of iron (Fe) and unavoidable impurities;

an operation of obtaining a hot-rolled steel sheet by finishing hot-rolling the heated slab at 800 to 1050°C; and

an operation of welding the hot-rolled steel sheet.