EP3088555A1

EP3088555A1 - Steel for low-temperature service having excellent surface processing quality

Info

Publication number: EP3088555A1
Application number: EP14873409.8A
Authority: EP
Inventors: Soon-Gi Lee; In-Shik Suh; Hak-Cheol Lee; In-Gyu Park
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2013-12-25
Filing date: 2014-12-19
Publication date: 2016-11-02
Also published as: WO2015099363A8; EP3088555A4; KR20150075315A; CN105849302A; CN109266959A; KR101543916B1; JP2017507249A; JP6615773B2; CN105849302B; CN109266959B; WO2015099363A1; US20160319407A1

Abstract

The present invention relates to steel for low-temperature service which can be used in a wide range of temperatures encompassing low temperatures as in liquefied gas storage tanks and transportation facilities, as well as room temperatures, and provides steel for low-temperature service having excellent surface processing quality even after a process for processing, such as tensioning, and a method for manufacturing the same.

Description

[Technical Field]

The present disclosure relates to steel for low-temperature service having a high degree of surface processing quality, and more particularly, to steel having a high degree of surface quality after being processed and usable for low-temperature service or structures such as liquefied gas storage tanks or transportation facilities in a wide temperature range from a low temperature to room temperature.

[Background Art]

Steels for manufacturing containers such as liquefied natural gas (LNG) containers or liquid nitrogen containers, marine structures, or structures for use in the Polar Regions are required to have sufficient toughness and strength at very low temperatures. To this end, steels for low-temperature service are used. Steels for low-temperature service are required to have low thermal expansion and thermal conductivity in addition to having a high degree of low-temperature toughness and strength. In addition, magnetic properties of such steels are factors to consider.
In the related art, for example, Cr-Ni-based stainless steel such as AISI304, 9% Ni steel, and 5000 series aluminum alloys have been used as materials for low-temperature service in liquefied gas environments. However, aluminum alloys are expensive, and since aluminum alloys have a low degree of strength, it is required to increase design thicknesses of structures to be formed of aluminum alloys. In addition, aluminum alloys have a low degree of weldability. Thus, aluminum alloys are used in limited applications. Cr-Ni-based stainless steel and 9% Ni steel may incur high manufacturing costs because of the use of expensive nickel (Ni) and the necessity of additional heat treatment processes. In addition, welding materials for Cr-Ni-based stainless steel and 9% Ni steel are also required to have a large amount of expensive nickel (Ni). Thus, the application of Cr-Ni-based stainless steel and 9% Ni steel is limited.
Examples of techniques for solving these problems by adding manganese (Mn) and chromium (Cr) instead of reducing the amount of expensive nickel (Ni) are disclosed in Patent Document 1 (Korean Patent Application Laid-open Publication No. 1998-0058369 ) and Patent Document 2 (International Patent Publication WO 2007/080646 ). In the technique disclosed in Patent Document 1, the content of nickel (Ni) is reduced to the range of 1.5% to 4%, and manganese (Mn) and chromium (Cr) are added in an amount of 16% to 22% and in an amount of 2% to 5.5%, respectively, so as to ensure the formation of austenite and improve cryogenic toughness. In the technique disclosed in Patent Document 2, the content of nickel (Ni) is reduced to about 5.5%, manganese (Mn) and chromium (Cr) are added in an amount of 2.0% or less and in an amount of 1.5% or less, respectively, and the size of ferrite grains is reduced by repeating a heat treatment process and a tempering process so as to guarantee cryogenic toughness. However, according to the techniques disclosed in Patent Documents 1 and 2, expensive nickel (Ni) is still used, and a heat treatment process and a tempering process are repeated in many steps to guarantee cryogenic toughness. That is, the techniques are not advantageous in terms of costs and the complexity of processes.
In another technique for structural steels for liquefied gas, Ni-free high-manganese steels which do not include any nickel (Ni) are proposed. Such high-manganese steels are classified into a ferritic type and an austenitic type according to the content of manganese (Mn). For example, in a technique for improving cryogenic toughness disclosed in Patent Document 3 ( US Patent No. 4257808 ), manganese (Mn) is added in an amount of 5% instead of adding nickel (Ni) in an amount of 9% (9% Ni), and a tempering process is performed after performing a heat treatment process four times within a austenite-ferrite coexistence temperature range to obtain the effect of grain refinement. Furthermore, in a technique for improving cryogenic toughness disclosed in Patent Document 4 (Korean Patent Application Laid-open Publication No. 1997-0043149 ), manganese (Mn) is added in an amount of 13%, and a tempering process is performed after performing heat treatment four times within an austenite-ferrite coexistence temperature range to obtain the effect of grain refinement. In the techniques disclosed in Patent Documents 3 and 4, ferrite is the main microstructure, and a heat treatment process is performed four or more times and then a tempering process is performed for ferrite grain refinement. However, these techniques may increase costs and significantly burden heat treatment equipment because a heat treatment process is repeated many times. Therefore, another technique for guaranteeing cryogenic toughness has been developed. According to the technique, instead of ferrite, austenite (or a mixed structure of austenite and ε-martensite) is the main microstructure.
In the case of steels for low-temperature service having austenite as the main microstructure, large amounts of carbon (C) and manganese (Mn) are added to stabilize austenite. However, this affects the recrystallization behavior of austenite, and thus partial recrystallization and non-uniform growth of grains are caused in a general rolling temperature range. As a result, some austenite grains grow excessively, and thus severe unevenness is observed in the size of austenite grains in the microstructure.

[Disclosure]

[Technical Problem]

An aspect of the present disclosure may provide steel for low-temperature service having a high degree of surface quality even after a process such as a tensioning or bending process.

[Technical Solution]

According to an aspect of the present disclosure, steel for low-temperature service having a high degree of surface processing quality may include manganese (Mn): 15 wt% to 35 wt%, carbon (C)satisfying conditions of: 23.6C+Mn ≥28 and 33.5C-Mn≤23, copper (Cu): 5 wt% or less (excluding 0 wt%), nitrogen (N): 1 wt% or less (excluding 0 wt%), chromium (Cr) satisfying a condition of: 28.5C+4.4Cr ≤57, nickel (Ni): 5 wt% or less, molybdenum (Mo): 5 wt% or less, silicon (Si): 4 wt% or less, aluminum (Al): 5 wt% or less, and a balance of iron (Fe) and inevitable impurities,
wherein stacking fault energy (SFE) of the steel calculated by Formula 1 below may be 24 mJ/m² or greater, $SFE (mJ / m^{2}) = 1.6 Ni - 1.3 Mn + 0.06 M n^{2} - 1.7 Cr + 0.01 C r^{2} + 15 Mo - 5.6 Si + 1.6 Cu + 5.5 Al - 60 {(C + 1.2 N)}^{1 / 2} + 26.3 (C + 1.2 N) {(Cr + Mn + Mo)}^{1 / 2} + 0.6 {[Ni (Cr + Mn)]}^{1 / 2}$
where Mn, C, Cr, Si, Al, Ni, Mo, and N refer to contents in wt%.

[Advantageous Effects]

According to exemplary embodiments of the present disclosure, the stacking fault energy (SFE) of steel is increased by adjusting the composition of the steel and the ranges of alloying element contents of the steel, and thus the steel may have a high degree of surface processing quality regardless of the formation of abnormally coarse grains.

[Description of Drawings]

FIG. 1 is an image of the microstructure of steel of the related art in which abnormally coarse austenite grains are formed.
FIG. 2 is an image taken from the steel of FIG. 1 after a tensioning process, illustrating a non-uniform surface of the steel.
FIG. 3 is an image of the microstructure of steel of an exemplary embodiment of the present disclosure in which abnormally coarse austenite grains are formed.
FIG. 4 is an image taken from the steel of FIG. 3 after a tensioning process, illustrating a uniform surface of the steel.
FIG. 5 is a graph illustrating carbon and manganese content ranges according to an exemplary embodiment of the present disclosure.

[Best Mode]

The present disclosure relates to steel for low-temperature service having a high degree of surface quality even after a processing process such as a tensioning or bending process regardless of the formation of abnormally coarse grains in the steel. In addition, the present disclosure relates to a method of manufacturing the steel.
Unlike general carbon steel, austenite generally including large amounts of carbon (C) and manganese (Mn) undergoes deformation by slip and twinning: initial deformation occurs mainly by slip (uniform deformation), followed by twinning (non-uniform deformation). Main variables describing stress causing the occurrence of twinning are the size of grains and stacking fault energy having a functional relationship with alloying elements. In particular, as the size of grains increases, the value of stress causing the occurrence of twinning decreases. That is, twinning easily occurs even by a small amount of deformation. If a small number of coarse grains exist in the microstructure of steel, twinning occurs in the coarse grains at the initial stage of deformation, and thus non-uniform deformation occurs. This worsens surface quality and results in non-uniformity in the thickness of a final structure. IN particular, this may cause significant problems in design and use of pressure structures such as low-temperature pressure containers that are required to have a uniform steel thickness for pressure resistance. In other words, the surface processing quality of steels in which an austenitic microstructure is formed by the addition of carbon (C) and manganese (Mn) may be improved by removing surface non-uniformity caused by initial twinning deformation.
In a general rolling temperature range, austenite of steel containing large amounts of carbon (C) and manganese (Mn) may undergo partial recrystallization and grain growth, and thus abnormally coarse austenite may be formed. In general, a critical value of stress causing twinning is higher than a critical value of stress causing slip. However, if grains are coarse because of the above-described reason, the value of stress causing twinning decreases, and thus twinning may occur at the initial stage of deformation. This leads to discontinuous deformation and worsens surface quality. However, according to the present disclosure, even though abnormally coarse austenite grains are formed, twinning deformation may be prevented by increasing a critical value of stress causing twinning deformation.
Hereinafter, steel for low-temperature service having a high degree of surface processing quality will be described in detail according to an exemplary embodiment of the present disclosure.
According to the exemplary embodiment of the present disclosure, the steel for low-temperature service having a high degree of surface processing quality includes manganese (Mn): 15 wt% to 35 wt%, carbon (C) satisfying the conditions of: 23.6C+Mn≥28 and 33.5C-Mn≤23, copper (Cu): 5 wt% or less (excluding 0 wt%), nitrogen (N): 1 wt% or less (excluding 0 wt%), chromium (Cr) satisfying the condition of: 28.5C+4.4Cr≤57, nickel (Ni): 5 wt% or less, molybdenum (Mo): 5 wt% or less, silicon (Si): 4 wt% or less, aluminum (Al): 5 wt% or less, and a balance of iron (Fe) and inevitable impurities, wherein stacking fault energy (SFE) of the steel calculated by Formula 1 below is within the range of 24 mJ/m² or greater. $SFE (mJ / m^{2}) = 1.6 Ni - 1.3 Mn + 0.06 M n^{2} - 1.7 Cr + 0.01 C r^{2} + 15 Mo - 5.6 Si + 1.6 Cu + 5.5 Al - 60 {(C + 1.2 N)}^{1 / 2} + 26.3 (C + 1.2 N) {(Cr + Mn + Mo)}^{1 / 2} + 0.6 {[Ni (Cr + Mn)]}^{1 / 2}$
where Mn, C, Cr, Si, Al, Ni, Mo, and N refer to contents in wt%.
Compared to general carbon steel, high-manganese steel has a relatively low degree of SFE, and thus partial dislocations easily occur in the high-manganese steel. A high density of such partial dislocations leads to variations in the deformation behavior of steel. Therefore, the deformation behavior of steel may be varied by controlling the SFE of the steel, and the SFE of steel has a functional relationship with alloying elements. That is, different alloying elements increase or decrease the SFE of steel to different degrees. Formula 1 above describes variations of SFE according to the contents of alloying elements. Formula 1 is obtained based on values calculated according to the existing theory and various experiments conducted by the inventors.
FIG. 3 is an image of the microstructure of the steel of the exemplary embodiment having the above-described composition and satisfying Formula 1, and FIG. 1 is an image of the microstructure of steel of the related art. Abnormally coarse grains are observed in both of the microstructures illustrated in FIGS. 1 and 3.
The related-art steel having the microstructure illustrated in FIG. 1 was tensioned, and then an image of a surface of the related-art steel was taken as illustrated in FIG. 2. Referring to FIG. 2, the surface of the related-art steel is non-uniform. The steel of the exemplary embodiment having the microstructure illustrated in FIG. 3 was tensioned, and then an image of a surface of the steel was taken as illustrated in FIG. 4. Referring to FIG. 4, the surface of the steel is uniform, unlike the surface of the related-art steel illustrated in FIG. 2. $Stress causing twining = \frac{SFE}{Magnitude of Burger' s vector of partial dislocation} + \frac{Hall - Petch constant}{\sqrt{Grain size}}$
The reason that the steel of the exemplary embodiment has a uniform surface as illustrated in FIG. 4 even after a processing process may be explained by Formula 2 above. If steel is deformed by external force, slip occurs because of dislocation movement. Along with this, if the steel is austenitic steel having a high carbon content and a high manganese content, twinning deformation additionally occurs due to low SFE of the steel. That is, although slip deformation mainly occurs at the initial stage of deformation, if stress increases to a critical value or higher, twinning deformation also occurs. In general, slip deformation caused by dislocation is uniform, and twinning deformation is non-uniform. In particular, if twinning deformation occurs locally in a region of coarse grains of steel, the microstructure of the steel becomes non-uniform after the twinning deformation. This may cause problems when the steel is used.
In general, a critical value of stress causing twinning is higher than a critical value of stress causing slip. However, as described in Formula 2, if the size of grains increases, the value of stress causing twinning decreases, and thus twinning occurs locally along coarse grains at the initial stage of deformation. As a result, discontinuous deformation occurs, and surface quality deteriorates.
However, as illustrated in Formula 2, if SFE is increased, the value of stress causing twinning may be increased regardless of the size of grains. That is, even after a processing process, a high degree of surface quality may be obtained regardless of coarse grains.
If SFE expressed by Formula 1 above is maintained to be a certain value or higher, twinning may be suppressed. That is, if the composition of steel is adjusted such that the SFE of the steel may be maintained to be a certain value or higher, the steel may have a high degree of surface quality and may be used for low-temperature service.
Hereinafter, reasons for limiting the contents of alloying elements of the steel of the exemplary embodiment will be described.

Manganese (Mn): 15 wt% to 35 wt%

In the exemplary embodiment of the present disclosure, manganese (Mn) is an element added to stabilize austenite. According to the exemplary embodiment, to obtain the austenite stabilizing effect at a very low temperature, it may be preferable that the content of manganese (Mn) be within the range of 15 wt% or greater. If the content of manganese (Mn) is less than 15 wt% and the content of carbon (C) is low, ε-martensite being a metastable phase may be formed. The ε-martensite may be easily transformed into α-martensite at a very low temperature by strain induced transformation, and thus it may be difficult to ensure toughness. This may be prevented by increasing the content of carbon (C) and thus stabilizing austenite. In this case, however, the properties of the steel may be markedly worsened because of the precipitation of carbides. Therefore, it may be preferable that the content of manganese (Mn) be within the range of 15 wt% or greater. However, if the content of manganese (Mn) is greater than 35 wt%, the corrosion rate of the steel decreases, and the value of the steel may decrease in terms of economical aspects. Therefore, it may be preferable that the content of manganese (Mn) be within the range of 15 wt% to 35 wt%.

Carbon (C) satisfying the conditions of: 23.6C+Mn≥28 and 33.5C-Mn≤23

Carbon (C) is an element stabilizing austenite and increasing strength. In particular, carbon (C) decreases transformation points M_s and M_d at which austenite transforms into ε-martensite or α-martensite during a cooling or processing process. Therefore, if the content of carbon (C) is insufficient, the stability of austenite is low, and thus stable austenite may be not obtained at a very low temperature. In addition, transformation from austenite to ε-martensite or α-martensite may be easily mechanically induced by external stress, and thus the toughness and strength of the steel may decrease. Conversely, if the content of carbon (C) is excessively high, the toughness of the steel may markedly decrease because of the precipitation of carbides, and the workability of the steel may decrease because the strength of the steel excessively increases.
In particular, according to the exemplary embodiment of the present disclosure, the content of carbon (C) may be determined according to the contents of other elements. The inventors found a relationship between carbon (C) and manganese (Mn) in the formation of carbides, and the relationship is shown in FIG. 5. As illustrated in FIG. 5, although carbides are formed from carbon (C), the formation of carbides is not affected only by carbon (C) but is affected by carbon (C) and manganese (Mn). FIG. 5 illustrates a proper content of carbon (C). Referring to FIG. 5, if it is assumed that the contents of the other elements of the steel are within the ranges proposed in the exemplary embodiment, it may be preferable that 23.6C+Mn be adjusted to be 28 or greater (where C and Mn respectively refer to the content of carbon (C) and the content of manganese (Mn) in wt%), so as to prevent the formation of carbides. This corresponds to the left boundary of the parallelogram region in FIG. 5. If 23.6C+Mn is less than 28, the stability of austenite may decrease. Thus, if the steel is impacted at a very low temperature, strain induced transformation may occur in the steel, and the impact toughness of the steel may decrease. If the content of carbon (C) is excessively high (that is, if 33.5C-Mn is greater than 23), the low-temperature impact toughness of the steel may be decreased by the precipitation of carbides. That is, it may be preferable that the content of carbon (C) satisfies 23.6C+Mn≥28 and 33.5C-Mn≤23. As illustrated in FIG. 5, the lower limit of the content of carbon (C) satisfying the conditions is 0 wt%.

Copper (Cu): 5 wt% or less (excluding 0 wt%)

Since copper (Cu) has low solid solubility in carbides and diffuses slowly in austenite, copper (Cu) concentrates on boundaries of carbide nuclei formed in austenite, thereby suppressing the diffusion of carbon (C) and effectively retarding the growth of carbides. That is, copper (Cu) suppresses the formation of carbides. Parent metals to be welded together by a welding process may be subjected to an accelerated cooling process to suppress the precipitation of carbides. However, during a welding process, it is not easy to adjust the cooling rate of heat affected zones. Therefore, copper (Cu) which is very effective in suppressing the precipitation of carbides is added to the steel of the exemplary embodiment of the present disclosure. In addition, copper (Cu) stabilizes austenite and thus improves cryogenic toughness. However, if the content of copper (Cu) in the steel is greater than 5 wt%, the hot workability of the steel may deteriorate. Therefore, preferably, the upper limit of the content of copper (Cu) may be set to be 5 wt%. To obtain the above-described carbide suppressing effect, it may be more preferable that the content of copper (Cu) be 0.5 wt% or greater.

Nitrogen (N) : 1 wt% or less (excluding 0%)

Like carbon (C), nitrogen (N) is an element stabilizing austenite and improving toughness. In particular, like carbon (C), nitrogen (N) is very effective in improving strength by the effect of solid solution strengthening. Moreover, as illustrated in Formula 1, nitrogen (N) is known as an element effectively increasing SFE and thus promoting slip. However, if the content of nitrogen (N) in the steel is greater than 1 wt%, the content of nitrogen (N) is unnecessarily high because the value of stress causing twinning becomes greater than a value of stress corresponding to a general amount of work in a steel processing process, and the surface quality and properties of the steel are worsened because coarse nitrides are formed. Therefore, it may be preferable that the upper limit of the content of nitrogen (N) be set to be 1 wt%.
In addition to the above-described elements, the steel (austenitic steel) of the exemplary embodiment may further include chromium (Cr), nickel (Ni), molybdenum (Mo), silicon (Si), and aluminum (Al).

Chromium (Cr): 28.5C+4.4Cr ≤ 57

If chromium (Cr) is added to the steel in an appropriate amount, chromium (Cr) stabilizes austenite and thus improves the low-temperature impact toughness of the steel. In addition, chromium (Cr) dissolves in austenite and thus increases the strength of the steel. Furthermore, chromium (Cr) improves the corrosion resistance of the steel. However, chromium (Cr) is a carbide forming element. In particular, chromium (Cr) leads to the formation of carbides along grain boundaries of austenite and thus decreases the low-temperature impact toughness of the steel. Therefore, according to the exemplary embodiment, the content of chromium (Cr) may be determined according to the content of carbon (C) and the contents of the other elements. If it is assumed that the contents of the other elements are within the ranges proposed in the exemplary embodiment of the present disclosure, it may be preferable that 28.5C+4.4Cr be 57 or less (where C and Cr respectively refer to the content of carbon (C) and the content of chromium (Cr) in wt%), so as to prevent the formation of carbides. If 28.5C+4.4Cr is greater than 57, it is difficult to effectively suppress the formation of carbides along grain boundaries of austenite because of excessive amounts of chromium (Cr) and carbon (C), and thus the low-temperature impact toughness of the steel may decrease. Therefore, according to the exemplary embodiment of the present disclosure, it may be preferable that the content of chromium (Cr) satisfies 28.5C+4.4Cr ≤ 57.

Nickel (Ni): 5 wt% or less

Nickel (Ni) is effective in stabilizing austenite. In addition, nickel (Ni) decreases transformation points M_s and M_d at which austenite transforms into ε-martensite or α-martensite during a cooling or processing process, and thus nickel (Ni) improves the toughness of the steel. In particular, as illustrated in Formula 1, nickel (Ni) is known as a very effective element in increasing SFE and thus promoting slip. However, if the content of nickel (Ni) in the steel is greater than 5 wt%, the content of nickel (Ni) is unnecessarily high because the value of stress causing twinning becomes greater than a value of stress corresponding to a general amount of work in a steel processing process, and the value of the steel may decrease in terms of economical aspects because nickel (Ni) is an expensive element. Therefore, it may be preferable that the upper limit of the content of nickel (Ni) be set to be 5 wt%.

Molybdenum (Mo): 5 wt% or less

If molybdenum (Mo) is added to the steel in an appropriate amount, molybdenum (Mo) stabilizes austenite and improves the toughness of the steel by decreasing transformation points M_s and M_d at which austenite transforms into ε-martensite or α-martensite during a cooling or processing process. In addition, molybdenum (Mo) dissolves in the steel and improves the strength of the steel. In particular, molybdenum (Mo) segregates along grain boundaries of austenite, thereby improving the stability of grain boundaries and decreasing the energy of grain boundaries. Therefore, molybdenum (Mo) suppresses the precipitation of carbides along grain boundaries. Moreover, as illustrated in Formula 1, molybdenum (Mo) is known as an element effectively increasing SFE and thus promoting slip. However, if the content of molybdenum (Mo) is greater than 5 wt%, the content of molybdenum (Mo) is unnecessarily high because the value of stress causing twinning becomes greater than a value of stress corresponding to a general amount of work in a steel processing process, and the effect of improving the stability of grain boundaries is not further increased. In addition, since molybdenum (Mo) is expensive, the value of the steel may decrease in terms of economical aspects, and the toughness of the steel may decrease because the strength of the steel increases excessively. Therefore, it may be preferable that the upper limit of the content of molybdenum (Mo) be set to be 5 wt%.

Silicon (Si): 4 wt% or less

Silicon (Si) improves casting properties of molten steel. In particular, silicon (Si) added to austenitic steel dissolves in the austenitic steel and effectively increases the strength of the austenitic steel. However, if the content of silicon (Si) in the steel is greater than 4 wt%, the SFE of the steel decreases and thus promotes the occurrence of twinning. In addition, the toughness of the steel may decrease because of solid solution strengthening. Therefore, it may be preferable that the upper limit of the content of silicon (Si) be set to be 4 wt%.

Aluminum (Al): 5 wt% or less

If aluminum (Al) is added to the steel in an appropriate amount, aluminum (Al) stabilizes austenite and improves the toughness of the steel by decreasing transformation points M_s and M_d at which austenite transforms into ε-martensite or α-martensite during a cooling or processing process. In addition, aluminum (Al) dissolves in the steel and increases the strength of the steel. In particular, aluminum (Al) affects the mobility of carbon (C) in the steel and effectively suppresses the formation of carbides, thereby increasing the toughness of the steel. Moreover, as illustrated in Formula 1, aluminum (Al) is known as an element effectively increasing SFE and thus promoting slip. However, if the content of aluminum (Al) in the steel is greater than 5 wt%, the content of aluminum (Al) is unnecessarily high because the value of stress causing of twinning becomes greater than a value of stress corresponding to a general amount of work in a steel processing process, and the casting properties and surface quality of the steel may be worsened because of the formation of oxides and nitrides. Therefore, it may be preferable that the upper limit of the content of aluminum be set to be 5 wt%.
In the exemplary embodiment of the present disclosure, the other components of the steel sheet are iron (Fe) and inevitable impurities. Impurities of raw materials or manufacturing environments may be inevitably included in the steel, and such impurities may not be removed from the steel. Such impurities are well-known to those of ordinary skill in the steel manufacturing industry, and thus descriptions thereof will not be provided in the present disclosure.
Preferably, the steel for low-temperature service may include austenite in an area fraction of 95% or greater. Austenite being a typical soft microstructure undergoing ductile fracture even at a low temperature is required to ensure low-temperature toughness, and thus it may be preferable that the steel includes austenite in an area fraction of 95% or greater. If the area fraction of austenite in the steel is less than 95%, the steel may not have sufficient low-temperature toughness. That is, the steel may not have an impact toughness of 41 J or greater at -196°C. Therefore, it may be preferable that the lower limit of the area fraction of austenite may be set to be 95%.
Preferably, the area fraction of carbides existing along grain boundaries of austenite may be 5% or less. In the exemplary embodiment of the present disclosure, for example, carbides may exist in the steel in addition to austenite, and such carbides may precipitate along grain boundaries of the austenite of the steel. This may cause grain boundary fracture and may thus decrease the low-temperature toughness and ductility of the steel. Therefore, it may be preferable that the upper limit of the area fraction of carbides be set to be 5%.
Preferably, the value of stress causing twinning in the steel for low-temperature service may be equal to or greater than a value of stress corresponding to a tensile strain of 5%. Here, the value of stress causing twinning refers to a value calculated by Formula 2, and the tensile strain of 5% refers to a tensile strain of 5% in a uniaxial tensile test. In general, when a processing process is performed on a sheet material to manufacture a low-temperature structure such as a low-temperature container, the deformation of the steel material is within the range of 5% or less in tensile strain. Therefore, if the value of stress causing twinning is adjusted to be equal to or greater than a value of stress corresponding to a strain of 5% caused by uniaxial tension, non-uniform deformation (twinning) may be suppressed.
Hereinafter, a method of manufacturing steel for low-temperature service having a high degree of surface processing quality will be described in detail according to an exemplary embodiment of the present disclosure.
The method of the exemplary embodiment includes: preparing a steel slab having the above-described composition and a degree of SFE calculated by Formula 1 within the range of 24 mJ/m² or greater; heating the steel slab to a temperature range of 1050°C to 1250°C; and performing a finish rolling process on the heated steel slab within a temperature range of 700°C to 950°C.
According to the method of the exemplary embodiment, first, a steel slab having the above-described composition and a degree of SFE calculated by Formula 1 within the range of 24 mJ/m² or greater is prepared.
Next, preferably, the steel slab is heated to a temperature range of 1050°C to 1250°C. Owing to the heating process, cast structures, segregates, and secondary phases generated during manufacturing processes of the steel slab may undergo solid solution and homogenization. If the steel slab is heated to a temperature lower than 1050°C, homogenization may occur insufficiently, or due to an insufficiently low temperature of a heating furnace, the steel slab may have a high degree of resistance to deformation when being hot rolled. Conversely, if the steel slab is heated to a temperature higher than 1250°C, partial melting may occur in segregation regions of cast structures, and the surface quality of the steel slab may be worsened. Therefore, it may be preferable that the reheating temperature of the steel slab be within the range of 1050°C to 1250°C.
The hot rolling process may preferably be performed within a finish rolling temperature of 700°C to 950°C. If the finish rolling temperature is lower than 700°C, carbides may precipitate along grain boundaries of austenite, thereby decreasing elongation and low-temperature toughness. In addition, an anisotropic microstructure may be formed, and thus anisotropic mechanical properties may be present. Conversely, if the finish rolling temperature is greater than 950°C, austenite grains may become coarse, and thus strength and elongation may be decreased. Therefore, it may be preferable that the finish rolling temperature be within the range of 700°C to 950°C.

[Mode for Invention]

Hereinafter, the present disclosure will be described more specifically according to examples. However, the following examples should be considered in a descriptive sense only and not for purpose of limitation.

Slabs having compositions as illustrated in Table 1 below were processed under the conditions illustrated in Table 2 below so as to manufacture steel materials. Thereafter, the stacking fault energy (SFE), microstructures, yield strength, and carbide fractions of the steel materials were measured. In addition, physical properties of the steel materials such as elongation and Charpy impact toughness were measured as illustrated in Table 3. Referring to Table 3, the column "surface non-uniformity" shows evaluation results obtained by observing the steel materials by the naked eye.

[Table 1]

No.	C	Mn	Cu	Cr	Ni	Mo	Si	Al	N
CE1	0.62	18.12	0.12	0.2					0.012
CE2	0.37	25.4	1.12	3.85					0.018
CE3	0.61	18.13	1.5	1.25					0.012
CE4	0.26	17.03							0.009
CE5	1.36	18.25							0.011
CE6	0.42	10.51							0.009
CE7	0.94	14.6							0.012
CE8	0.38	24.6	0.8	3.4					0.015
IE1	0.65	18.2	0.2	0.5				1.5	0.008
IE2	0.4	24.8	0.3	2.7		0.6		0.8	0.012
IE3	0.56	21.5	0.7		1.2			0.8	0.005
IE4	0.31	30.5	0.3	1.2		0.2	0.3		0.015
IE5	1.2	18.6	0.52	0.8	0.5			1.2	0.007
IE6	0.83	22.6	0.7	1.9		0.5			0.01
IE7	0.35	27	1.2	2.2				0.7	0.03
IE8	0.2	32	0.42	1.9		0.5	0.5	0.7	0.014

CE: Comparative Example,
IE: Inventive Example

[Table 2]

No.	23.6C+Mn	33.5C-Mn	28.5C +4.4Cr	HFT (°C)	RFT (°C)	SFE (mJ/m²)	AF (%)	CF (%)
CE1	32.8	2.5	18.6	1160	920	19.4	99.1	0.9
CE2	34.1	-13.1	27.5	1160	875	18.7	99.6	0.4
CE3	32.5	2.1	22.9	1160	912	21.0	99	1
CE4	23.2	-8.4	7.4	1160	859	-6.8	52
CE5	50.3	26.9	38.8	1140	921	80.0	85	15
CE6	20.4	3.4	12.0	1160	875	-9.8	82
CE7	36.8	16.6	26.8	1160	907	30.9	94	6
CE8	33.6	-12.0	25.8	1170	695	17.1	94	6
IE1	33.5	3.4	20.7	1190	915	30.0	99	1
IE2	34.2	-11.5	23.3	1190	890	32.8	100	0
IE3	34.7	-2.9	16.0	1190	875	34.0	100	0
IE4	37.8	-20.2	14.1	1190	920	29.9	100	0
IE5	46.9	21.2	37.7	1190	880	79.2	100	0
IE6	42.2	5.0	32.0	1190	905	62.1	100	0
IE7	35.3	-15.4	19.7	1190	915	27.4	100	0
IE8	36.7	-25.4	14.1	1190	826	30.9	100	0

CE: Comparative Example,
IE: Inventive Example,
HFT: Heating Furnace Temperature,
RFT: Rolling Finish Temperature,
AF: Austenite Fraction,
CF: Carbide Fraction

Referring to Table 2 above, each of Inventive Examples 1 to 8 satisfying the alloying element content ranges proposed in the exemplary embodiment of the present disclosure had an austenite fraction of 95% or greater and a carbide fraction of less than 5% in the microstructure thereof. That is, stable austenite was formed, and thus each of Inventive Examples 1 to 8 had a high degree of cryogenic toughness.

[Table 3]

No.	Twining stress (MPa)	YS (MPa)	Tensile stress for 5% deformation (MPa)	TS (MPa)	El (%)	IT (J, -196°C)	Surface non-uniformity
CE1	450	363	445	1006	70	81	Occurred
CE2	442	470	576	896	45	136	Occurred
CE3	471	405	497	1012	56	76	Occurred
CE4	129	342	419	826	35	24	Occurred
CE5	1194	403	494	692	5	6	Did not occur
CE6	92	352	432	765	12	8	Occurred
CE7	591	356	436	832	31	31	Did not occur
CE8	422	632	775	995	18	38	Occurred
IE1	581	431	528	816	67	102	Did not occur
IE2	615	468	574	842	51	146	Did not occur
IE3	630	472	579	763	62	162	Did not occur
IE4	579	460	564	823	64	153	Did not occur
IE5	1184	395	484	826	52	132	Did not occur
IE6	974	420	515	916	63	147	Did not occur
IE7	549	478	586	886	61	163	Did not occur
IE8	592	450	552	951	53	140	Did not occur

CE: Comparative Example,
IE: Inventive Example,
YS: Yield Strength,
TS: Tensile Strength,
El: Elongation,
IT: Impact Toughness

Referring to Table 3 above, the impact toughness of Inventive Examples 1 to 8 was markedly improved when compared to Comparative Examples 1 to 3. Owing to an appropriate content of carbon (C) and the addition of other elements, stable austenite was formed even though the content of manganese (Mn) was relatively low. Thus, the above-mentioned results could be obtained. In particular, when the content of carbon (C) was high, copper (Cu) was added to suppress the formation of carbides, and thus the stability of austenite could be improved.
In particular, the SFE of each of Inventive Examples 1 to 8 calculated by Formula 1 was 24 mJ/m² or higher, and thus steel materials free of surface non-uniformity could be manufactured. However, the SFE of Comparative Examples 1 to 3 calculated by Formula 1 was outside the range proposed in the exemplary embodiment of the present disclosure, and thus Comparative Examples 1 to 3 had non-uniform surfaces even though Comparative Examples 1 to 3 had high cryogenic toughness.
Comparative Examples 4 and 6 having carbon and manganese contents outside the ranges proposed in the exemplary embodiment of the present disclosure did not have an intended austenite fraction, and thus the cryogenic toughness of Comparative Examples 4 and 6 was low. In addition, the SFE of Comparative Examples 4 and 6 calculated by Formula 1 was outside the range proposed in the exemplary embodiment of the present disclosure, and thus Comparative Examples 4 and 6 had non-uniform surfaces.
Comparative Examples 5 and 7 not satisfying the alloying element content ranges proposed in the exemplary embodiment of the present disclosure had a low degree of impact toughness. In particular, due to a high content of carbon (C), carbides were excessively formed along grain boundaries of austenite, and thus the impact toughness of Comparative Examples 5 and 7 was low.
Comparative Example 8 did not satisfy the alloying element content ranges proposed in the exemplary embodiment of the present disclosure, and thus Comparative Example 8 had a non-uniform surface even though the SFE of Comparative Example 8 was higher than 24 mJ/m². In particular, the finish rolling temperature of Comparative Example 8 was lower than the range proposed in the exemplary embodiment of the present disclosure. Therefore, Comparative Example 8 had anisotropic physical properties and an excessive degree of strength, and thus the elongation and impact toughness of Comparative Example 8 were low.
While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and other embodiments could be made therefrom. That is, such modifications and other embodiments could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

Steel for low-temperature service having a high degree of surface processing quality, the steel comprising manganese (Mn): 15 wt% to 35 wt%, carbon (C) satisfying conditions of: 23.6C+Mn≥28 and 33.5C-Mn≤23, copper (Cu): 5 wt% or less (excluding 0 wt%), nitrogen (N): 1 wt% or less (excluding 0 wt%), chromium (Cr) satisfying a condition of: 28.5C+4.4Cr≤57, nickel (Ni) : 5 wt% or less, molybdenum (Mo): 5 wt% or less, silicon (Si): 4 wt% or less, aluminum (Al): 5 wt% or less, and a balance of iron (Fe) and inevitable impurities,
wherein stacking fault energy (SFE) of the steel calculated by Formula 1 below is 24 mJ/m² or greater, $SFE (mJ / m^{2}) = 1.6 Ni - 1.3 Mn + 0.06 M n^{2} - 1.7 Cr + 0.01 C r^{2} + 15 Mo - 5.6 Si + 1.6 Cu + 5.5 Al - 60 {(C + 1.2 N)}^{1 / 2} + 26.3 (C + 1.2 N) {(Cr + Mn + Mo)}^{1 / 2} + 0.6 {[Ni (Cr + Mn)]}^{1 / 2}$
where Mn, C, Cr, Si, Al, Ni, Mo, and N refer to contents in wt%.
The steel of claim 1, wherein the steel comprises austenite in an area fraction of 95% or greater.
The steel of claim 2, wherein the steel comprises carbides along austenite grain boundaries in an area fraction of 5% or less.
The steel of claim 1, wherein a value of stress causing twinning in the steel is equal to or a greater than a value of tensile stress corresponding to a tensile strain of 5% of the steel.