EP3088555A1 - Acier lingot basse température présentant une excellente qualité de traitement de surface - Google Patents

Acier lingot basse température présentant une excellente qualité de traitement de surface Download PDF

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Publication number
EP3088555A1
EP3088555A1 EP14873409.8A EP14873409A EP3088555A1 EP 3088555 A1 EP3088555 A1 EP 3088555A1 EP 14873409 A EP14873409 A EP 14873409A EP 3088555 A1 EP3088555 A1 EP 3088555A1
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Prior art keywords
steel
low
austenite
less
content
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German (de)
English (en)
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EP3088555A4 (fr
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Soon-Gi Lee
In-Shik Suh
Hak-Cheol Lee
In-Gyu Park
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Posco Holdings Inc
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Posco Co Ltd
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite

Definitions

  • 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.
  • 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.
  • 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.
  • magnetic properties of such steels are factors to consider.
  • 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.
  • 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.
  • aluminum alloys have a low degree of weldability.
  • 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.
  • welding materials for Cr-Ni-based stainless steel and 9% Ni steel are also required to have a large amount of expensive nickel (Ni).
  • the application of Cr-Ni-based stainless steel and 9% Ni steel is limited.
  • Patent Document 1 Korean Patent Application Laid-open Publication No. 1998-0058369
  • Patent Document 2 International Patent Publication WO 2007/080646
  • the content of nickel (Ni) is reduced to the range of 1.5% to 4%
  • 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.
  • 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.
  • 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.
  • 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).
  • 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.
  • 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.
  • 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/m 2 or greater, SFE mJ
  • 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.
  • 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.
  • the present disclosure relates to a method of manufacturing the 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.
  • 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.
  • a critical value of stress causing twinning is higher than a critical value of stress causing slip.
  • 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.
  • twinning deformation may be prevented by increasing a critical value of stress causing twinning deformation.
  • 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/m 2 or greater.
  • SFE stacking fault energy
  • high-manganese steel 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
  • 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 .
  • 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 .
  • the surface of the steel is uniform, unlike the surface of the related-art steel illustrated in FIG. 2 .
  • Stress causing twining SFE Magnitude of Burger ′ s vector of partial dislocation + Hall ⁇ Petch constant 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.
  • a critical value of stress causing twinning is higher than a critical value of stress causing slip.
  • 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.
  • 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.
  • manganese (Mn) is an element added to stabilize austenite.
  • 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, ⁇ -martensite being a metastable phase may be formed. The ⁇ -martensite may be easily transformed into ⁇ -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.
  • 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) is an element stabilizing austenite and increasing strength.
  • carbon (C) decreases transformation points M s and M d at which austenite transforms into ⁇ -martensite or ⁇ -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.
  • transformation from austenite to ⁇ -martensite or ⁇ -martensite may be easily mechanically induced by external stress, and thus the toughness and strength of the steel may decrease.
  • 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.
  • 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 .
  • FIG. 5 illustrates a proper content of carbon (C).
  • 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.
  • 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) 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.
  • the upper limit of the content of copper (Cu) may be set to be 5 wt%.
  • the content of copper (Cu) 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%)
  • nitrogen (N) is an element stabilizing austenite and improving toughness.
  • nitrogen (N) is very effective in improving strength by the effect of solid solution strengthening.
  • nitrogen (N) is known as an element effectively increasing SFE and thus promoting slip.
  • the upper limit of the content of nitrogen (N) be set to be 1 wt%.
  • the steel (austenitic steel) of the exemplary embodiment may further include chromium (Cr), nickel (Ni), molybdenum (Mo), silicon (Si), and aluminum (Al).
  • 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.
  • chromium (Cr) dissolves in austenite and thus increases the strength of the steel.
  • chromium (Cr) improves the corrosion resistance of the steel.
  • chromium (Cr) is a carbide forming element.
  • chromium (Cr) leads to the formation of carbides along grain boundaries of austenite and thus decreases the low-temperature impact toughness of the steel.
  • 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.
  • Nickel (Ni) is effective in stabilizing austenite.
  • nickel (Ni) decreases transformation points M s and M d at which austenite transforms into ⁇ -martensite or ⁇ -martensite during a cooling or processing process, and thus nickel (Ni) improves the toughness of the steel.
  • nickel (Ni) is known as a very effective element in increasing SFE and thus promoting slip.
  • 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) stabilizes austenite and improves the toughness of the steel by decreasing transformation points M s and M d at which austenite transforms into ⁇ -martensite or ⁇ -martensite during a cooling or processing process.
  • molybdenum (Mo) dissolves in the steel and improves the strength of the steel.
  • 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.
  • molybdenum (Mo) is known as an element effectively increasing SFE and thus promoting slip.
  • 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.
  • 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) improves casting properties of molten steel.
  • silicon (Si) added to austenitic steel dissolves in the austenitic steel and effectively increases the strength of the austenitic steel.
  • 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.
  • 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) stabilizes austenite and improves the toughness of the steel by decreasing transformation points M s and M d at which austenite transforms into ⁇ -martensite or ⁇ -martensite during a cooling or processing process.
  • aluminum (Al) dissolves in the steel and increases the strength of the steel.
  • 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.
  • aluminum (Al) is known as an element effectively increasing SFE and thus promoting slip.
  • 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%.
  • 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.
  • 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%.
  • the area fraction of carbides existing along grain boundaries of austenite may be 5% or less.
  • 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%.
  • 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%.
  • the value of stress causing twinning refers to a value calculated by Formula 2
  • the tensile strain of 5% refers to a tensile strain of 5% in a uniaxial tensile test.
  • 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.
  • 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/m 2 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.
  • a steel slab having the above-described composition and a degree of SFE calculated by Formula 1 within the range of 24 mJ/m 2 or greater is prepared.
  • 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.
  • 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.
  • 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.
  • the SFE of each of Inventive Examples 1 to 8 calculated by Formula 1 was 24 mJ/m 2 or higher, and thus steel materials free of surface non-uniformity could be manufactured.
  • 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.
  • 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.
  • C carbon
  • 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/m 2 .
  • 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.

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WO2020152498A1 (fr) * 2019-01-22 2020-07-30 Aperam Alliage fer-manganèse à soudabilité améliorée
EP3699313A4 (fr) * 2017-10-18 2020-08-26 Posco Acier à haute teneur en manganèse pour basse température, ayant une excellente qualité de surface, et son procédé de fabrication
EP3730649A4 (fr) * 2017-12-22 2020-10-28 Posco Matériau d'acier présentant une excellente résistance à l'usure et son procédé de fabrication
EP3807428A4 (fr) * 2018-06-14 2022-03-09 The Nanosteel Company, Inc. Alliages d'acier à haute résistance présentant des caractéristiques de ductilité

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KR102004654B1 (ko) * 2018-03-23 2019-07-26 서울과학기술대학교 산학협력단 극저온 인성이 우수한 알루미늄 첨가 오스테나이트계 경량 고망간강 및 그의 제조방법
WO2020027211A1 (fr) 2018-08-03 2020-02-06 Ｊｆｅスチール株式会社 Acier à haute teneur en manganèse et son procédé de production
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KR102255827B1 (ko) * 2018-10-25 2021-05-26 주식회사 포스코 표면품질이 우수한 극저온용 오스테나이트계 고망간 강재 및 그 제조방법
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EP3561110A4 (fr) * 2016-12-22 2019-12-25 Posco Acier à teneur élevée en manganèse ayant une ténacité à basse température et une limite d'élasticité supérieures et procédé de fabrication
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EP3561120A4 (fr) * 2016-12-23 2019-11-13 Posco Matériau d'acier austénitique présentant une résistance à l'abrasion et une ténacité excellentes, et procédé pour le produire
US11566308B2 (en) 2016-12-23 2023-01-31 Posco Co., Ltd Austenitic steel material having excellent abrasion resistance and toughness and manufacturing method the same
EP3699313A4 (fr) * 2017-10-18 2020-08-26 Posco Acier à haute teneur en manganèse pour basse température, ayant une excellente qualité de surface, et son procédé de fabrication
EP3974556A1 (fr) * 2017-10-18 2022-03-30 Posco Acier à haute teneur en manganèse pour basse température, ayant une excellente qualitè de surfacé
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EP3730649A4 (fr) * 2017-12-22 2020-10-28 Posco Matériau d'acier présentant une excellente résistance à l'usure et son procédé de fabrication
EP3807428A4 (fr) * 2018-06-14 2022-03-09 The Nanosteel Company, Inc. Alliages d'acier à haute résistance présentant des caractéristiques de ductilité
WO2020152498A1 (fr) * 2019-01-22 2020-07-30 Aperam Alliage fer-manganèse à soudabilité améliorée
CN113383092A (zh) * 2019-01-22 2021-09-10 艾普伦 具有改善的焊接性的铁-锰合金

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WO2015099363A1 (fr) 2015-07-02
US20160319407A1 (en) 2016-11-03
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KR20150075315A (ko) 2015-07-03
CN109266959A (zh) 2019-01-25
CN109266959B (zh) 2022-05-24
KR101543916B1 (ko) 2015-08-11
JP2017507249A (ja) 2017-03-16
EP3088555A4 (fr) 2017-04-05
CN105849302A (zh) 2016-08-10
JP6615773B2 (ja) 2019-12-04

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