Austenitic steel material and manufacturing method therefor

Abstract

The present invention relates to an austenitic steel material applicable to various fields such as the energy industry and shipbuilding, and a manufacturing method therefor and, more specifically, to an austenitic steel material and a manufacturing method therefor wherein the austenitic steel material can be used in a structure requiring high strength and excellent hydrogen embrittlement resistance such as a hull requiring high strength and excellent non-magnetic properties, a steel pipe or facilities transporting crude oil having high hydrogen sulfide content, and the like.

Description

Technical Field

[0001] The present disclosure relates to an austenitic steel material applicable to various fields such as the energy industry, marine structures, offshore structures, or the like, and a method for manufacturing the same. More specifically, the present disclosure relates to an austenitic steel material applicable to a hull requiring excellent non-magnetic properties, a steel pipe, a facility, or the like for transporting crude oil or the like having a large amount of a hydrogen sulfide, and a method for manufacturing the same.Background Art

[0002] As a mining environment of crude oil may be increasingly extreme, such as a sour environment containing a large amount of hydrogen sulfide or the like, a steel pipe, a structure thereof, or the like, transporting the same may require a material having excellent hydrogen embrittlement resistance. Therefore, a material having sufficient toughness and strength, to transport the crude oil, and having excellent hydrogen embrittlement resistance and resistance, to external pressure in a transport process, may be required.

[0003] In conventional high-strength carbon steel, there may be limitations in that it may be difficult to produce the same and use thereof may be limited because a microstructure vulnerable to hydrogen embrittlement, called a hard spot, may exist inside a steel material. Various methods for manufacturing the same and methods of detecting the hard spot have been proposed to overcome these limitations, but basically, the hard spot may be inevitable to increase strength, making it difficult to form steel having excellent hydrogen embrittlement resistance and high strength properties.

[0004] In addition, in a structure in which a large amount of steel is used, such as military ships, like submarines, or the like, a location of another party may be tracked by detecting changes in magnetic fields caused by interaction between a body of a ship and the magnetic field of the earth. When such a structure is generated and is formed of austenitic steel capable of maintaining non-magnetic properties even after deformation, the probability of being detected may be extremely reduced. Here, conventional carbon steel undergoes a process, called degaussing, of periodically removing magnetism, which reduces an operating time of the ship by this time and results in economic losses, so demand for steel having excellent non-magnetic properties is increasing.Disclosure of InventionTechnical Problem

[0005] An aspect of the present disclosure is to provide an austenitic steel material having high strength characteristics, excellent impact toughness and hydrogen embrittlement resistance, and having low magnetic permeability even after deformation, and a method for manufacturing the same.

[0006] An object of the present disclosure is not limited to the description. Those of ordinary skill in the technical field to which the present disclosure pertains will have no difficulty in understanding additional problems of the present disclosure from the overall contents of the present specification.Solution to Problem

[0007] A steel material according to an aspect of the present disclosure includes, by weight, C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities, wherein a main structure of a microstructure is austenite, an area fraction of a grain boundary carbide formed at a grain boundary of the austenite is 5.0 area% or less, and the steel material includes at least one fine precipitate of VC or VCN having a diameter of 50.0 nm or less in the number per unit area of 100 / mm 2< or more.

[0008] The steel material may further include at least one of Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less.

[0009] The steel material may satisfy Relational expression 1 below:

[0010] (In Relational expression 1, [C] and [Mn] refer to weight percentages of C and Mn included in the steel material, respectively.)

[0011] The grain boundary carbide may include at least one of Cr carbide, Mo carbide, or Nb carbide.

[0012] The area fraction of the austenite may be 95 area% or more.

[0013] Room-temperature yield strength of the steel material at room-temperature may be 550 MPa or more, a Charpy impact energy value at -84°C may be 27J or more, and magnetic permeability may be 1.200 or less, after cold plastic deformation of 20% at room-temperature is performed.

[0014] In addition, in the steel material, magnetic permeability may be 1.100 or less, when a deformation rate is at least 2% or more during cold plastic deformation at room temperature, and a crack length ratio (CLR) during a Hydrogen-Induced Crack (HIC) test, defined by Relational expression 2 below, is 10% or less: CLR Crack Length Ratio , % = ∑ a / W * 100

[0015] (In Relational expression 2, a refers to a length of a single crack (µm), and w refers to a width of a specimen (µm))

[0016] In addition, in the steel material, after submerged arc welding with a heat input of 3.0 kJ / mm, an area fraction of grain boundary carbide in a heat affected zone of welding may be 5.0 area% or less.

[0017] A method for manufacturing a steel material, according to another aspect of the present disclosure, includes heating a slab including, by weight, C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities; finish hot-rolling the slab to obtain a hot-rolled steel sheet; performing solution treatment and cooling the hot-rolled steel sheet to room temperature; and performing aging treatment of the hot-rolled steel sheet.

[0018] The slab may further include at least one of Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less, and may satisfy Relational expression 1 below:

[0019] (In Relational expression 1, [C] and [Mn] refer to weight percentages of C and Mn included in the steel material, respectively.)

[0020] Finally, the heating may be performed at 1000°C or more and 1300°C or less, the finish hot-rolling may be performed at 700°C or more and 1050°C or less, the performing solution treatment may be performed at 900°C or more and 1200°C or less for 30 minutes or more and 2 hours or less, and the performing aging treatment may be performed at 500°C or more and 850°C or less for 30 minutes or more and 5 hours or less.Advantageous Effects of Invention

[0021] The present disclosure may provide a steel material that may be used in a structure requiring high strength characteristics, excellent impact toughness and hydrogen embrittlement resistance, and non-magnetic properties.

[0022] Various advantages and effects of the present disclosure are not limited to the description, and can be more easily understood in the process of explaining the specific embodiment of the present disclosure.Brief Description of Drawings

[0023] FIG. 1 is a graph showing ranges of carbon and manganese according to an aspect of the present disclosure. FIG. 2 is a transmission electron microscope photograph of Example 1, a steel material, according to an aspect of the present disclosure. Best Mode for the Invention

[0024] Hereinafter, preferred embodiments of the present disclosure will be described. However, embodiments of the present disclosure may be modified into various different forms, and the scope of the present disclosure is not limited to embodiments described below.

[0025] In the present specification, the term "comprise" or "include" may be used to indicate that other components may be further included, rather than excluding other components unless specifically opposed.

[0026] In addition, unless otherwise specified in the specification of the present disclosure, a unit of % means wt%.

[0027] Conventionally, chromium was added to an austenitic steel material having a large amount of manganese to improve corrosion resistance and strength, and molybdenum and niobium, playing a similar role to chromium described above, also tended to be added to improve the strength of the steel.

[0028] However, in this case, as chromium, molybdenum, or niobium combine with carbon to form a carbide at an austenite grain boundary, it was difficult to secure impact toughness of the steel material excellently.

[0029] Accordingly, the inventors of the present disclosure found that when an amount of chromium, molybdenum, or niobium, added to the steel material, is reduced and vanadium is added in a high concentration, the problem may be solved, and the strength of the steel material may be improved by a vanadium-derived fine precipitates.

[0030] From this point of view, a steel material according to an embodiment of the present disclosure may include, by weight, C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities.

[0031] Hereinafter, each configuration will be described in detail.C: 0.050%~1.70%

[0032] C may be an element that stabilizes austenite and increases strength, and plays a role in lowering Ms and Md, which may be transformation points from austenite by cooling process or processing to epsilon or alpha martensite. Therefore, when C is insufficiently added, stable austenite cannot be obtained at cryogenic temperatures due to insufficient stability of austenite, and may easily cause deformation-induced transformation to epsilon or alpha martensite by external stress, thereby reducing toughness and reducing strength of steel. Therefore, in the present disclosure, a lower limit of C may be 0.050%. In another embodiment, the present disclosure may include 0.070% or more of C, and in another embodiment, may include 0.100% or more. Conversely, when an amount of C is excessive, toughness may rapidly deteriorate due to carbide precipitation, and workability may deteriorate due to an excessive increase in strength, so the present disclosure may include C in 1.70% or less. As another embodiment, an upper limit of C in the present disclosure may be 1.50% or less, and as another embodiment, 1.30%.Mn: 15.0~40.0%

[0033] Mn may be an element that plays an important role in stabilizing austenite. In an embodiment of the present steel material, 15.0% or more of Mn may be included to stabilize austenite. When an amount of Mn does not reach this, a metastable phase of epsilon martensite may be formed and may be easily transformed into alpha martensite by deformation-induced transformation at ultra-low temperatures, so high toughness may not be secured. In order to suppress the formation of epsilon martensite, there may be a plan to stabilize austenite by increasing the amount of C, but in this case, a large amount of carbide may be deposited and properties, more specifically, toughness, may be rapidly deteriorated. Therefore, the amount of Mn may be preferably 15.0% or more. In another embodiment, the amount of Mn may be 18.0% or more, and in another embodiment, the amount of Mn may be 20.0% or more.

[0034] When the amount of Mn is excessive, not only the corrosion rate of the steel material may be reduced, but it may not be also preferable in terms of economic feasibility. Therefore, the amount of Mn included in the steel material according to an aspect of the present disclosure may be 40.0% or less. In another embodiment, the amount of Mn may be 35.0% or less, and in another example, 30.0% or less.Cr: 3.00% or less

[0035] Cr may be an austenite stabilizing element, which may increase strength of a steel material or contribute to improvement of corrosion resistance up to an appropriate amount, but as described above, Cr may be a carbide-forming element, and when excessively added to the steel material, low-temperature impact toughness of the steel material may be reduced by forming carbides at the austenite grain boundaries. In addition, when the added amount of Cr exceeds a certain level, excessive carbides may be deposited at a heat-affected zone (HAZ) in welding, and thus the cryogenic toughness may deteriorate. Accordingly, in the present disclosure, an upper limit of Cr may be 3.00%. In another embodiment, the upper limit of the amount of Cr may be 2.80%, and in another embodiment, the upper limit of the amount of Cr may be 2.50%.V: 1.00~3.00%

[0036] V may be an element that forms at least one precipitate of a VC precipitate or a VCN precipitate by combining with C, and may be an element that suppresses grain growth of austenite structures and delays recrystallization, thereby contributing to strength improvement. In particular, when at least one precipitate of a fine VC precipitate or a fine VCN precipitate of a certain fraction or more may be formed, it may more effectively contribute to strength improvement of the steel material. In the present disclosure, a lower limit of an amount of V may be 1.00% in order to achieve effects. In another embodiment, the present disclosure may include 1.10% or more of V, and in another embodiment, may include 1.20% or more.

[0037] On the contrary, when the amount of V is excessively added, it may be substantially impossible to completely dissolve the coarse carbide formed in the steelmaking process in the reheating process, and since the coarse carbide remains in the subsequent process, properties may be deteriorated. In addition, V may be an expensive element and may be undesirable economically when an excessive amount of V is added. Accordingly, in the present disclosure, an upper limit of the V content may be 3.00%, and in another embodiment, the upper limit of the V content may be 2.80%.N: 1.000% or less (excluding 0%)

[0038] N may be an element that improves toughness by stabilizing austenite with carbon, and may be particularly advantageous for improving strength through strengthening employment, such as carbon. In particular, it may be well known as an element that effectively promotes slip by increasing energy of stacking defects.

[0039] When nitrogen is added in an amount exceeding 1.000%, there may be a problem in that a coarse nitride may be formed to deteriorate surface quality and properties of the steel material, and thus an upper limit may be limited to 1.000%. An upper limit of a preferable amount of nitrogen (N) may be 0.500%, and an upper limit of a more preferable amount of nitrogen (N) may be 0.200%. Although the present disclosure does not separately stipulate a lower limit of an amount of nitrogen (N), the lower limit of the amount of nitrogen (N) may be 0.005% or 0.007% in consideration of a case in which N is inevitably added.Mo: 3.50% or less

[0040] Mo may be an element that may be employed in a matrix to increase strength. However, similar to Cr, when Mo is excessively added to the steel material, carbides may be formed at the austenite grain boundaries to reduce low-temperature impact toughness of the steel material. In addition, when an amount of Mo exceeds a certain level, excessive grain boundaries carbide may be deposited in a heat-affected zone (HAZ), and thus ultra-low temperature toughness may deteriorate, and thus the present disclosure may include Mo in an amount of 3.50% or less. As another embodiment, an upper limit of Mo in the present disclosure may be 3.40%, and as another embodiment, the upper limit of Mo may be 3.20%. Since the object of the present disclosure may be achieved even when Mo may not be added at all, the present disclosure does not separately limit a lower limit of Mo, but as an example, the lower limit of Mo may be 0.01%.Nb: 1.00% or less

[0041] Nb may be an element that forms an NbC precipitate by combining with C, and may be an element that contributes to improvement of strength by increasing an amount of unfiltered crystal ball rolling in a manufacturing process of the steel material by suppressing growth of grains in the austenite structure and increasing a recrystallization temperature. In addition, even when the Nb may not be added at all, there may be no problem in achieving the object of the present disclosure, but when fine NbC precipitates of a certain fraction or more are formed, it may more effectively contribute to improvement of strength of the steel material. The present disclosure may limit a preferred lower limit of an amount of Nb to 0.01%, taking into account a phenomenon of precipitation enhancement due to precipitate formation.

[0042] When the amount of Nb is excessively added, the coarse carbide formed in the steelmaking process may act vulnerably to external force in a continuous casting process and cause cracks, thereby deteriorating quality of the cast steel. In addition, when a large amount of Nb is added, a large amount of carbide may be coarsely deposited in the welding heat-affected zone, and thus impact toughness may decrease, which may not be preferable. Accordingly, the present disclosure may limit an upper limit of an amount of Nb to 1.00%, and the upper limit of a preferred amount of Nb may be 0.10%.

[0043] Meanwhile, a steel material according to an embodiment of the present disclosure may further include at least one of Ti: 1.0% or less, Al: 5.0% or less, or Si: 5.0% or less.Ti: 1.00% or less

[0044] Ti may be an element that inhibits austenitic grain growth by forming carbonitride and may help increase strength. Even when Ti is not included at all, there may be no effect on achievement of the object of the present disclosure, and thus the present disclosure may not include the Ti at all. On the other hand, when Ti is excessively added, the corresponding Ti may be purified or coarse, thereby deteriorating quality of the main piece, and thus the present disclosure may include Ti in an amount of 1.00% or less. In another embodiment, the present disclosure may include 0.005% to 0.90% of Ti, and in another embodiment, may include 0.01% to 0.80%.Al: 5.00% or less

[0045] Al may be employed in a matrix to increase strength of the steel material and increase stacking defect energy, thereby increasing the strength to control a deformation mode, and as a result, the deformation mode may be slipped. Such Al may be included in the steel material of the present disclosure to obtain the effect, but even when Al is not included at all, there may be no effect on the achievement of the object of the present disclosure, and thus the present disclosure may not include Al at all. On the other hand, when Al is excessively added, there may be a problem that quality of the cast pieces may be deteriorated due to purification or precipitation of coarse AlN, so the present disclosure may include 5.00% or less of Al. In another embodiment, the present disclosure may include 0.01% to 4.50% of the Al, and in another embodiment, 0.02% to 4.00%.Si: 5.00% or less

[0046] Si may be an element that improves casting properties of molten steel and, in particular, when added to austenite steel, may be dissolved inside the steel material to effectively increase strength. In addition, it may be an element that effectively suppresses formation of carbides by affecting activity of carbon in the steel material, thereby increasing toughness. When it is added in excess of 5.00%, stacking defect energy may be reduced to encourage occurrence of double crystals, and there may be a problem that toughness may decrease due to high strength, so it may be preferable to limit an upper limit to 5.00%. As another example, an upper limit of an amount of Si may be 3.00%, and as another example, the upper limit of the amount of Si may be 2.50%. In addition, a lower limit of an amount of Si according to another example may be 0.10%, and as another example, the lower limit of the amount of Si may be 0.30%.

[0047] In addition, according to another aspect of the present disclosure, the steel material of the present disclosure may satisfy the following Relational expression 1, which may be the Relational expression between carbon (C) and manganese (Mn).

[0048] (In Relational expression 1, [C] and [Mn] refer to weight percentages of C and Mn included in the steel material, respectively.)

[0049] The present disclosure conducted an in-depth study on the relative behavior between the amount of C and the amount of Mn in relation to carbide formation, and as a result, as shown in FIG. 1, determining a relative amount Relational expression between C and Mn may be an important factor in effectively controlling an amount of carbide precipitation while promoting stabilization of austenite.

[0050] In order to promote the stabilization of austenite, it may be preferable to control the value of 23.6[C]+[Mn] to be 28.000 or more on the premise that other components meet the range specified in the present disclosure. When a value of 23.6[C]+[Mn] is less than 28.000, stability of austenite decreases, causing deformation-induced transformation due to deformation, and accordingly, impact toughness of the steel material may decrease. In addition, when the deformation-induced transformation occurs, non-magnetic properties of steel may deteriorate because BCC-structured alpha prime martensite may be formed in addition to non-magnetic austenite, increasing magnetic permeability.

[0051] In addition, in an embodiment of the present disclosure, a value of 33.5[C]-[Mn] may be 23.00 or less. This may be to prevent deterioration of properties due to formation of carbides due to an excessive amount of carbon. In another embodiment, the value of 33.5[C]-[Mn] may be 20.00 or less, and in another embodiment, may be 18.00 or less.

[0052] The steel material according to an aspect of the present disclosure may include other Fe and other inevitable impurities, in addition to the components. In a general manufacturing process, since raw materials or unintended impurities from the surrounding environment may inevitably be mixed, it cannot be completely excluded. Since these impurities may be known to anyone skilled in the art, all of the amounts may not be specifically mentioned in the specification. In addition, additional addition of effective ingredients other than the ingredients may not be completely excluded.

[0053] The microstructure of the steel material according to an example of the present disclosure may be austenite in its main structure.

[0054] This may be to secure desired properties of the steel material according to an aspect of the present disclosure. According to an embodiment, an area fraction of the austenite may be 95 area% or more. In another embodiment, the area fraction of the austenite may be 97 area% or more. In particular, preferably, for the purpose of securing non-magnetic properties, the area fraction of the austenite may be 100 area%, but may not be necessarily limited thereto. In the present disclosure, the method for measuring the area fraction of austenite may not be particularly limited, and may be easily confirmed through a measurement method commonly used by those skilled in the art for measuring microstructure and carbides.

[0055] In the case of the steel material according to an example of the present disclosure, an area fraction of the grain boundary carbide formed at the grain boundary of the austenite may be 5.0 area% or less. In this case, as a non-limiting example, the area as a reference when measuring the area fraction of the grain boundary carbide may be the entire measurement area.

[0056] In other words, an example of the present disclosure may reduce amounts of Cr, Mo, and Nb included in the steel material, thereby reducing carbides generated at austenite grain boundaries to prevent low-temperature impact toughness and ultra-cold toughness in welding heat affected zones from deteriorating. Therefore, the carbide produced at the austenite grain boundary may include at least one of Cr carbide, Mo carbide, or Nb carbide, described above, and the area fraction of the grain boundary carbide may be 5.0 area% or less. In another embodiment, the area fraction of the grain boundary carbide may be 4.8 area% or less, and in another embodiment, 4.5 area% or less. In addition, according to a non-limiting embodiment, the carbide produced at the austenite grain boundary described above may be measured using a scanning electron microscope or an optical microscope, and in consideration of this, the minimum observable diameter per original state of the grain boundary carbide may be 100 nm. The grain boundary diameter may mean the diameter of the virtual circle when a virtual circle having the same area as the grain boundary carbide exposed on the surface may be set.

[0057] In addition, the steel material according to an example of the present disclosure may form a V-derived fine precipitate in the grain through aging treatment, and the V-derived fine precipitate may mean at least one of a VCN fine precipitate or a VC fine precipitate. In addition, the fine precipitate may mean a precipitate with a diameter of 50.0 nm or less. In this case, the diameter may refer to the diameter per original state described above. As another example, the fine precipitate may refer to a precipitate having a diameter of 10.0 nm or less, and as another example, may refer to a precipitate having a diameter of 5.0 nm or less. Meanwhile, since the smaller the diameter of the fine precipitate is, the smaller the diameter, a lower limit may not be limited separately, but when the diameter is measured using a transmission electron microscope according to a non-limiting example, the lower limit of the fine precipitate diameter may be 0.2 nm. According to the present disclosure, high strength characteristics of the steel material may be secured despite a decrease in Cr by forming the V-derived fine precipitate above a certain level. In particular, according to an example of the present disclosure, the present disclosure may form the V-derived fine precipitate above a certain level in the grain, and in this case, high strength characteristics of the steel material may be secured more efficiently.

[0058] More specifically, in order to secure the effect of improving the strength described above, the steel material according to an aspect of the present disclosure may include 100 / mm 2< or more of precipitates of VC and VCN having a diameter of 50.0 nm or less.

[0059] A method of measuring the number of at least one fine precipitate per unit area of at least one of VC or VCN may not be limited separately because it may be easily adopted by a person skilled in the art according to the purpose, but may be measured using a transmission electron microscope as an example. In another embodiment, the present disclosure may include at least one fine precipitate of at least one of VC or VCN having a diameter of 50.0 nm or less, 120 / mm 2< or 150 / mm 2< or more of precipitates, and in another embodiment, may include 500 / mm 2< or more, or 1000 / mm 2< or more of precipitates. According to an aspect of the present disclosure, the larger the number of fine precipitates of at least one of VC or VCN is, and thus an upper limit in the number per unit area of is not limited, but in reality, the upper limit of the number of fine precipitates of at least one of VC or VCN having a diameter of 50.0 nm or less may be 10000 / mm 2< .

[0060] In addition, as described above, when reducing grain-based carbides, there may be an advantage in that excessive carbides may be deposited on the welding heat affected zone (HAZ) after welding steel, thereby preventing the ultra-low temperature toughness from deteriorating. More specifically, in a steel material according to an embodiment of the present disclosure, after submerged arc welding may be performed with a heat input of 3.0 kJ / mm 2< , the area fraction of the grain boundaries carbide in the welding heat affected zone may be 5.0 area% or less. As another example, the area fraction of the grain boundaries carbide in the welding heat affected zone may be 4.8 area% or less, and as another example, 4.5 area% or less. Regarding a lower limit of the diameter per grain boundaries of the grain boundaries carbide in the welding heat affected zone, the contents described in relation to the carbides generated in the austenite grain boundaries before welding may be equally applied, and thus, may be omitted.

[0061] As described above, the steel material according to an aspect of the present disclosure may secure high strength characteristics and excellent impact toughness.

[0062] Specifically, the steel material according to an aspect of the present disclosure may have a yield strength at room-temperature of 550 MPa or more, and according to another example, may be 690 MPa or more, and according to another example, a lower limit of the yield strength at room-temperature may be 750 MPa or 900 MPa. In addition, the Charpy impact energy value measured at -84°C may be 27J or more.

[0063] In addition, the steel material according to one example of the present disclosure may excellently secure non-magnetic properties. Specifically, the steel material according to the present disclosure may have a magnetic permeability of 1.200 or less after performing 20% cold plastic deformation at room temperature.

[0064] As described above, the magnetic permeability after performing 20% cold plastic deformation at room-temperature may be 1.200 or less, which means that the austenite structure may be stably maintained even after cold molding. Specifically, since austenite may be an unstable structure, even when austenite is obtained at room temperature, austenite may be transformed into epsilon martensite or alpha martensite by further cooling or processing. In this case, since impact toughness tends to deteriorate and non-magnetic properties tend to deteriorate, the steel according to an example of the present disclosure may maintain stable austenite structure even after cold molding by controlling a relative amount between C and Mn through Relational expression 1 described above, thereby securing excellent non-magnetic properties.

[0065] As another example, a steel material according to an embodiment of the present disclosure may have a magnetic permeability of 1.100 or less when strain is at least 2% or more during cold plastic deformation at room temperature.

[0066] Finally, since a steel material according to an embodiment of the present disclosure has excellent hydrogen embrittlement resistance, a high level of safety may be ensured when applied to a sour environment.

[0067] In detail, a steel material according to an embodiment of the present disclosure may have a crack length ratio (CLR) of 10% or less, a preferable crack length ratio (CLR) of 5% or less, and a more preferable crack length ratio (CLR) of 2% or less during a Hydrogen-Induced Crack (HIC) test.

[0068] This Hydrogen-Induced Crack (HIC) test may be performed by observing the number of cracks generated after immersing the steel material in an acidic solution (5% NaCl + 0.5% CH 3 COOH) saturated with H 2 S gas and maintaining it for 96 hours. That is, the length (a) of the crack may be measured at the point of occurrence of the crack by observing the cross section of the specimen provided with a constant width (W) and thickness (T), and the crack length ratio (CLR) may be derived from these average values. The formula for calculating the crack length ratio (CLR) may be shown in Relational expression 2 below. CLR Crack Length Ratio , % = ∑ a / w * 100

[0069] (In Relational expression 2, a refers to a length of a single crack (µm), and w refers to a width of a specimen (µm))

[0070] Hereinafter, a method for manufacturing a steel material according to an embodiment of the present disclosure will be described. However, the following method for manufacturing the steel material may be only an example, and it may be important to note that the steel material of the present disclosure does not necessarily have to be manufactured by the present manufacturing method, and that any manufacturing method satisfies the claims of the present disclosure has no problem in using it to implement each embodiment of the present disclosure.

[0071] According to an embodiment of the present disclosure, the method for manufacturing a steel material includes heating a slab including, by weight, C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities; finish hot-rolling the slab to obtain a hot-rolled steel sheet; performing solution treatment and cooling the hot-rolled steel sheet to room temperature; and performing aging treatment of the hot-rolled steel sheet.

[0072] Hereinafter, each step will be described in detail.Heating Slab

[0073] In a method for manufacturing a steel material according to an embodiment of the present disclosure, first, a slab including C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities may be prepared and then heated.

[0074] In addition, the slab may further include at least one of Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less, and may satisfy Relational expression 1 below. Since a composition of the slab is the same as described above, description thereof will be omitted.

[0075] (In Relational expression 1, [C] and [Mn] refer to weight percentages of C and Mn included in the steel material, respectively.)

[0076] The heating may be performed in a temperature range of 1000°C or more and 1300°C or less. When a slab heating temperature is less than 1000°C, there may be a disadvantage in that redissolution and homogenization of an alloy component may not be performed, or it takes a long time to reach the target temperature to a central portion of the slab. In another embodiment, a lower limit of the slab heating temperature may be 1050°C, and in another embodiment, the lower limit of the slab heating temperature may be 1100°C or 1150°C.

[0077] When the slab heating temperature exceeds 1300°C, partial melting or surface oxidation may occur in a segregation portion of the alloy component of the slab. In addition, in another embodiment, an upper limit of the slab heating temperature may be 1250°C, and in another embodiment, the upper limit of the slab heating temperature may be 1230°C or 1200°C.Obtaining Hot-Rolled Steel Sheet

[0078] After the heating, a method for manufacturing a steel material according to an aspect of the present disclosure may finish hot-roll the slab, thereby obtaining a hot-rolled steel sheet.

[0079] In addition, as a non-limiting example, the finishing hot-rolling may be performed at 700°C or more and 1050°C or less. When a finishing hot-rolling temperature is less than 700°C, rolling may not be easy due to high-temperature strength of the material, and there may be a disadvantage in that the strength of the material increases excessively due to excessive rolling of a non-recrystallization region, thereby reducing impact toughness. In another embodiment, a lower limit of the finishing hot-rolling temperature may be 750°C or 800°C.

[0080] When the finishing hot-rolling temperature exceeds 1050°C, there may be a disadvantage in that the austenite is coarsely adjusted and the strength decreases. In another embodiment, an upper limit of the hot-rolling temperature of the slab finish may be 1000°C or 950°C. A reduction rate during the hot-rolling may be applied in an appropriate range according to the desired sheet thickness, and as a non-limiting example, the final thickness of the hot-rolled steel sheet may be 5 to 50 mm.Performing Solution Treatment and Cooling Hot-Rolled Steel Sheet to Room Temperature

[0081] Next, an example of the present disclosure may be cooled to room-temperature after performing solution treatment of the hot-rolled steel sheet obtained by the method.

[0082] In addition, as a non-limiting example, the solution treatment may be performed at 900°C or more and 1200°C or less for 30 minutes or more and 2 hours or less. The solution treatment may be to employ grain boundaries and intra-grain coarse carbides generated during hot-rolling as a base material and to lower internal energy due to potential generated excessively by rolling. In order to achieve the object, in the method for manufacturing a steel material according to an aspect of the present disclosure, the temperature of the solution treatment may be 900°C or more, and a time of the solution treatment may be 30 minutes or more. According to another embodiment, a lower limit of a solution treatment temperature may be 950°C or 1000°C.

[0083] On the other hand, when the solution treatment temperature exceeds 1200°C or the treatment time exceeds 2 hours, there may be a disadvantage in that the austenite is excessively coarsened and the strength decreases. In addition, according to another embodiment, an upper limit of the solution treatment temperature may be 1180°C, and according to another embodiment, the upper limit of the solution treatment temperature may be 1150°C.Performing Aging Treatment

[0084] A method for manufacturing a steel material according to an embodiment of the present disclosure may include performing aging treatment of the hot-rolled steel sheet after the solution treatment. This aging treatment may be for improving strength of austenitic steel materials by depositing at least one fine precipitate of VC or VCN in the grain.

[0085] As a non-limiting example, such aging treatment may be performed at 500°C or more and 850°C or less for 30 minutes or more and 5 hours or less. When an aging treatment temperature is less than 500°C, diffusion of the precipitating elements may not be easy, and thus there may be a disadvantage that the time required for precipitation may be excessively long. In another embodiment, a lower limit of the aging treatment temperature may be 550°C or 600°C. When the aging treatment temperature exceeds 850°C, there may be a disadvantage in that the austenite is excessively coarse and the strength thereof decreases. In addition, as another embodiment, an upper limit of the aging treatment temperature may be 830°C or 800°C.

[0086] When the aging treatment time is less than 30 minutes, there may be a disadvantage in that a sufficient time for precipitation cannot be secured. In another embodiment, a lower limit of the aging treatment time may be 36 minutes or 42 minutes. However, an upper limit of the aging treatment time may be preferably 5 hours or less. When it exceeds 5 hours, intensity may be rather reduced due to over-aging and there may be a problem that it is uneconomical. In another embodiment, an upper limit of the aging treatment time may be 4.8 hours or 4.5 hours.

[0087] Hereinafter, a steel material and a manufacturing method thereof according to an aspect of the present disclosure will be described in more detail through specific embodiments. It may be important to note that the following embodiments may be for understanding of the present disclosure and may not be intended to specify the scope of the present disclosure. The scope of the present disclosure may be determined by matters described in the claims and matters reasonably inferred therefrom.Mode for the Invention

[0088] After preparing a slab having a thickness of 250 mm having an alloy composition shown in the following Table 1, heating of the slab, hot-rolling, and solution treatment were performed under conditions shown in the following Table 2, and after cooling to room temperature, aging treatment was performed to prepare a steel material. In Comparative Example 7, the solution treatment and the aging treatment were not performed. After measuring a microstructure and properties of the steel material thus prepared, results were shown in the following Tables 3 and 4.

[0089] In this case, an area fraction (area%) of the following austenite was measured five times at room-temperature using an optical microscope at a point 1 / 4t based on a thickness of a specimen after collecting the specimen at a size of about 2 cm in width and length of the steel sheet. In this case, an area of the austenite was calculated through image processing, and average values of measured values were shown in Table 3 below. In this case, measurement magnification of the optical microscope was 200 times.

[0090] In addition, in a case of the number ( / mm 2< ) per unit area of at least one fine precipitate of VC or VCN, the specimen was collected in the same manner as the above-described collected method, and measurements were performed five times using a transmission electron microscope at a 1 / 4t point based on the specimen thickness. The number of precipitates per unit area was calculated through image processing and the measured values were averaged. At this time, measurement magnification of the transmission electron microscope was 200,000×.

[0091] In addition, in a case of an area fraction (area%) of austenite grain boundary carbide, the specimen was collected in the same way as the collected method, and measured five times using the scanning electron microscope at 1 / 4t of the specimen thickness, and the area in fraction of the precipitate was calculated through image processing and averaged. The grain boundary carbide was at least one of Cr carbide, Mo carbide, or Nb carbide, and the measurement magnification of the scanning electron microscope was 2000 times.

[0092] The room-temperature yield strength of Table 3 was measured using a uniaxial tensile test method, and Charpy impact energy was measured using a Charpy impact tester after maintaining the specimen at -84°C for 15 minutes or more. In addition, a crack length ratio (CLR) during a Hydrogen-Induced Crack (HIC) test was derived by observing the number of cracks generated after immersing in an acidic solution (5% NaCl + 0.5% CH 3 COOH) saturated with H 2 S gas and maintaining it for 96 hours. CLR Crack Length Ratio , % = ∑ a / W * 100

[0093] (In Relational expression 2, a refers to a length of a single crack (µm), and W refers to a width of a specimen (µm))

[0094] Thereafter, the prepared steel material was subjected to cold plastic deformation to a strain of 20% at room temperature using a uniaxial tensile tester, and then the magnetic permeability of the steel material was measured. The results are shown in Table 3 below.

[0095] The magnetic permeability of the steel material was measured by cutting and collecting the cold-deformed specimen, preparing a sample having a size of about 2 cm in width and length, measuring the magnetic permeability three times using a magnetic permeability meter, and then obtaining the average value.

[0096] In addition, the prepared steel material was welded by a submerged arc welding with a heat input of 3.0 kJ / mm 2< , and then an area fraction (area%) of the grain boundary carbide in the welding heat-affected zone was measured and shown in Table 3 below. The grain boundary carbide was at least one of Cr, Mo, and Nb carbides. The surface area fraction of grain boundary carbide in the heat-affected zone of the welding was measured five times in the same manner as the base material, and the measured values were averaged. [Table 1]Alloy Composition (wt%)CMn23. 6C+ Mn33.5C-MnCrMoVNbTiAlSiNInventive Ex. 10.5826.940.588-7.47--2.07----0.030Inventive Ex. 20.6228.142.732-7.330.483.051.980.02---0.014Inventive Ex. 30.5230.242.472-12.782.010.042.31-0.02--0.016Inventive Ex. 40.7132.048.756-8.222.232.062.050.030.02-0.910.016Inventive Ex. 50.4825.837.128-9.721.521.021.28---1.120.017Comparative Ex. 10.2619.225.336 -10.491.36-----0.370.015Comparative Ex. 20.4928.540.064-12.093.52 -2.02----0.017Comparative Ex. 31.3518.350.1626.93 2.15--0.020.04--0.018Comparative Ex. 40.5527.340.28-8.88--2.15----0.015Comparative Ex. 50.4213.5 23.412 0.57-------0.009Comparative Ex. 60.5827.641.29-8.170.515.23 2.11----0.016Comparative Ex. 70.5527.039.98-8.58--2.11----0.018 [Table 2] Slab Heating Temp.(°C)Hot-Rolling Finish Temp.(°C)Solution Treatment Temp.(°C)Solution Treatment Time (hr)Aging Treatment Temp.(°C)Aging Treatment Time (hr)Steel Material Thickness (mm)Inventive Ex. 1115288611480.527630.5512Inventive Ex. 2114890511620.747240.8220Inventive Ex. 3117591511500.698120.7715Inventive Ex. 4118092211551.027200.6618Inventive Ex. 5118585911540.877491.0222Comparative Ex. 1118289511520.637480.5612Comparative Ex. 2114788911750.68864 1.3225Comparative Ex. 3120591011821.138370.8830Comparative Ex. 4114989411530.58950 6.7 35Comparative Ex. 5118490811630.767381.4210Comparative Ex. 6115589311650.997651.0324Comparative Ex. 71168892----12 [Table 3] Microstructure (area%)VC or VCN Fine Precipitate Number ( / mm 2< )Area Fraction (area%) of Grain boundary carbideArea Fraction (area%) of Grain boundary carbide in Welding Heat Affected ZoneInventive Ex. 195% or more γ8921.0% or less1.0% or lessInventive Ex. 295% or more γ10121.0% or less1.0% or lessInventive Ex. 395% or more γ11091.0% or less1.0% or lessInventive Ex. 495% or more γ15051.0% or less1.0% or lessInventive Ex. 595% or more γ5481.0% or less1.0% or lessComparative Ex. 195% or more γ-1.0% or less1.0% or lessComparative Ex. 295% or more γ85 3.0% or less 3.0% or less Comparative Ex. 394%γ -more than 5.0% more than 5.0% Comparative Ex. 495% or more γ-1.0% or less1.0% or lessComparative Ex. 584%γ -1.0% or less1.0% or lessComparative Ex. 693%γ 620more than 5 .0% more than 5 .0% Comparative Ex. 795% or more γ30 1.0% or less1.0% or less [Table 4] Relative magnetic PermeabilityRoom-Temperature Yield Strength (MPa)Charpy Impact Energy (-84°C, J)CLR(%)Inventive Ex. 11.002762690Inventive Ex. 21.002911720Inventive Ex. 31.002915680Inventive Ex. 41.0021117580Inventive Ex. 51.002778890Comparative Ex. 11.52 342 15 0Comparative Ex. 21.005489 12 0Comparative Ex. 31.003546 5 0Comparative Ex. 41.002423 1130Comparative Ex. 53.6 349 2 36 Comparative Ex. 61.00498615 0Comparative Ex. 71.002488 1120

[0097] Referring to Tables 1 to 3, Comparative Examples 1, 3, and 5, to which V was not added at all, did not have high strength characteristics because at least one fine precipitate of VC or VCN was not formed at all.

[0098] In addition, in Comparative Examples 2 and 4, to which V was added, but as an aging treatment temperature was excessively high or an aging treatment time was excessively lengthened, it can be seen that the strength was rather lowered by the over-age.

[0099] In Comparative Example 6, as Mo exceeds the range suggested by the present disclosure, the carbide exceeds 5% of the austenite grain boundary. As a result, excellent toughness could not be ensured.

[0100] Finally, in Comparative Example 7, at least one precipitate among fine VC or VCN could not be secured at a certain level as solution treatment and aging treatment were not performed, and as a result, a room-temperature yield strength did not reach 550 MPa.

[0101] On the other hand, in Inventive Examples 1 to 5 satisfying the alloy composition and manufacturing conditions of the present disclosure, excellent properties were ensured by ensuring the components and microstructures of the present disclosure to be acquired.

Claims

1. A steel material comprising: by weight, C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities, wherein a main structure of a microstructure is austenite, an area fraction of a grain boundary carbide formed at a grain boundary of the austenite is 5.0 area% or less, and the steel material includes at least one fine precipitate of VC or VCN having a diameter of 50.0 nm or less in the number per unit area of 100 / mm2 or more.

2. The steel material of claim 1, further including at least one of Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less.

3. The steel material of claim 1, satisfying Relational expression 1 below: (In Relational expression 1, [C] and [Mn] refer to weight percentages of C and Mn included in the steel material, respectively.).

4. The steel material of claim 1, wherein the grain boundary carbide includes at least one of Cr carbide, Mo carbide, or Nb carbide.

5. The steel material of claim 1, wherein the area fraction of the austenite is 95 area% or more.

6. The steel material of claim 1, wherein room-temperature yield strength is 550 MPa or more.

7. The steel material of claim 1, wherein a Charpy impact energy value at -84°C is 27J or more.

8. The steel material of claim 1, wherein magnetic permeability is 1.2 or less, after 20% cold plastic deformation at room-temperature is performed.

9. The steel material of claim 1, wherein magnetic permeability is 1.100 or less, when a deformation rate is at least 2% or more during cold plastic deformation at room temperature.

10. The steel material of claim 1, wherein, after submerged arc welding with a heat input of 3.0 kJ / mm2, an area fraction of grain boundary carbide in a heat-affected zone is 5.0 area% or less.

11. The steel material of claim 1, wherein a crack length ratio (CLR) in a Hydrogen-Induced Crack (HIC) test, defined by Relational expression 2 below, is 10% or less: CLR Crack Length Ratio , % = ∑ a / W * 100 (In Relational expression 2, a refers to a length of a single crack (µm), and W refers to a width of a specimen (µm)).

12. A method for manufacturing a steel material, comprising: heating a slab including, by weight, C: 0.050%~1.70%, Mn: 15.0~40.0%, Cr: 3.00% or less, V: 1.00~3.00%, N: 1.000% or less (excluding 0%), Mo: 3.50% or less, Nb: 1.00% or less, with a remainder of iron (Fe) and inevitable impurities; finish hot-rolling the slab to obtain a hot-rolled steel sheet; performing solution treatment and cooling the hot-rolled steel sheet to room temperature; and performing aging treatment of the hot-rolled steel sheet.

13. The method of claim 12, wherein the slab further includes at least one of Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less.

14. The method of claim 12, wherein the slab satisfies Relational expression 1 below: (In Relational expression 1, [C] and [Mn] refer to weight percentages of C and Mn included in the steel material, respectively.).

15. The method of claim 12, wherein the heating is performed in the temperature range of 1000°C or more and 1300°C or less, the finish hot-rolling is performed in the temperature range of 700°C or more and 1050°C or less, the performing solution treatment is performed in the temperature range of 900°C or more and 1200°C or less for 30 minutes or more and 2 hours or less, and the performing aging treatment is performed in the temperature range of 500°C or more and 850°C or less for 30 minutes or more and 5 hours or less.

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