Austenitic steel material and method for producing same

Abstract

The present invention relates to an austenitic steel material suitable for use in various fields such as the energy industry and shipbuilding, and a method for manufacturing the same, and more particularly, to an austenitic steel material which can be used in a structure requiring high strength and excellent hydrogen embrittlement resistance, and a method for manufacturing the same, for example, a hull requiring high strength and excellent non-magnetic properties, a steel pipe or facility for transporting crude oil having a high hydrogen sulfide content, and the like.

Description

Technical Field

[0001] This disclosure relates to austenitic steels and methods for manufacturing the same, applicable to a variety of fields such as the energy industry, marine structures, and offshore structures. More specifically, this disclosure relates to austenitic steels and methods for manufacturing the same, applicable to ship hulls requiring excellent non-magnetic properties, and to steel pipes, facilities, etc., used for transporting crude oil containing large amounts of hydrogen sulfide. Background Technology

[0002] As crude oil extraction environments become increasingly extreme, such as acidic environments containing high levels of hydrogen sulfide, steel pipes and their structures used for transporting crude oil may require materials with excellent resistance to hydrogen embrittlement. Therefore, materials with sufficient toughness and strength for transporting crude oil, along with excellent resistance to hydrogen embrittlement and external pressure during transport, may be necessary.

[0003] In conventional high-strength carbon steel, the presence of microstructures known as hard spots, which are susceptible to hydrogen embrittlement, can limit its production and application. Various methods for manufacturing and detecting hard spots have been proposed to overcome these limitations. However, fundamentally, hard spots may be unavoidable for improving strength, making it difficult to form steels with excellent resistance to hydrogen embrittlement and high strength.

[0004] Furthermore, in structures such as warships and submarines that utilize large amounts of steel, the position of the other party can be tracked by detecting changes in the magnetic field caused by the interaction between the hull and the Earth's magnetic field. When such a structure is formed from austenitic steel that remains non-magnetic even after deformation, the probability of detection can be greatly reduced. Here, conventional carbon steel undergoes a periodic demagnetization process called demagnetization, which reduces ship operating time and leads to economic losses; therefore, the demand for steel with excellent non-magnetic properties is increasing. Summary of the Invention

[0005] Technical issues

[0006] One aspect of this disclosure is to provide austenitic steel with high strength properties, excellent impact toughness and resistance to hydrogen embrittlement, and low magnetic permeability even after deformation, and a method for manufacturing the same.

[0007] The purpose of this disclosure is not limited to this description. Other issues of this disclosure will not be difficult for those skilled in the art to which this disclosure pertains to, based on the entirety of this specification.

[0008] Technical solution

[0009] According to one aspect of this disclosure, the steel comprises, by weight, C: 0.050% to 1.70%, Mn: 15.0% to 40.0%, Cr: 3.00% or less, V: 1.00% to 3.00%, N: 1.000% or less but not exceeding 0%, Mo: 3.50% or less, Nb: 1.00% or less, and the remainder being iron (Fe) and unavoidable impurities, wherein the predominant microstructure is austenite, and the area fraction of grain boundary carbides formed at the grain boundaries of austenite is 5.0 area % or less, and the steel is expressed in units of 100 particles / mm². 2 The number of or more per unit area includes at least one fine precipitate of VC or VCN with a diameter of 50.0 nm or smaller.

[0010] The steel may also contain at least one of the following: Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less.

[0011] Steel can satisfy the following relational expression 1:

[0012] [Relational Expression 1] 23.6[C]+[Mn] ≥ 28.000, 33.5[C]-[Mn] ≤ 23.00

[0013] (In relational expression 1, [C] and [Mn] refer to the weight percentages of C and Mn contained in the steel, respectively.)

[0014] Grain boundary carbides may include at least one of Cr carbides, Mo carbides, or Nb carbides.

[0015] The area fraction of austenite can be 95% or greater.

[0016] The steel has a room temperature yield strength of 550 MPa or greater, a Charpy impact energy of 27 J or greater at -84°C, and a magnetic permeability of 1.200 or less after 20% cold plastic deformation at room temperature.

[0017] Furthermore, in steel, when the deformation rate during cold plastic deformation at room temperature is at least 2% or greater, the magnetic permeability can be 1.100 or less, and the crack length ratio (CLR) in the hydrogen-induced crack (HIC) test, as defined by the following relational expression 2, is 10% or less:

[0018] [Relational Expression 2]

[0019] CLR (Crack Length Ratio, %) = ∑(a / W) × 100

[0020] (In relational expression 2, a refers to the length of a single crack (μm), and w refers to the width of the specimen (μm))

[0021] Furthermore, in steel, after submerged arc welding with a heat input of 3.0 kJ / mm, the area fraction of grain boundary carbides in the weld heat-affected zone can be 5.0 area% or less.

[0022] A method for manufacturing steel according to another aspect of this disclosure includes: heating a slab containing, by weight, 0.050% to 1.70% C, 15.0% to 40.0% Mn, 3.00% or less Cr, 1.00% to 3.00% V, 1.000% or less N but not exceeding 0%, 3.50% or less Mo, 1.00% or less Nb, and the remainder being iron (Fe) and unavoidable impurities; hot-rolling the slab to obtain a hot-rolled steel sheet; performing a solution treatment and cooling the hot-rolled steel sheet to room temperature; and aging the hot-rolled steel sheet.

[0023] The slab may also contain at least one of the following: Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less, and may satisfy the following relational expression 1:

[0024] [Relational Expression 1] 23.6[C]+[Mn] ≥ 28.000, 33.5[C]-[Mn] ≤ 23.00

[0025] (In relational expression 1, [C] and [Mn] refer to the weight percentages of C and Mn contained in the steel, respectively.)

[0026] Finally, heating can be carried out at 1000°C or higher and 1300°C or lower, hot rolling can be carried out at 700°C or higher and 1050°C or lower, solution treatment can be carried out at 900°C or higher and 1200°C or lower for 30 minutes or longer and 2 hours or less, and aging treatment can be carried out at 500°C or higher and 850°C or lower for 30 minutes or longer and 5 hours or less.

[0027] Beneficial effects

[0028] This disclosure provides steels that can be used in structures requiring high strength, excellent impact toughness, resistance to hydrogen embrittlement, and non-magnetic properties.

[0029] The various advantages and effects of this disclosure are not limited to this description, and can be more readily understood in the process of illustrating specific embodiments of this disclosure. Attached Figure Description

[0030] Figure 1 This is a diagram showing the range of carbon and manganese according to one aspect of this disclosure.

[0031] Figure 2 This is a transmission electron microscope image of a steel embodiment 1 according to one aspect of this disclosure. Detailed Implementation

[0032] Preferred embodiments of this disclosure will be described below. However, embodiments of this disclosure can be modified in various different forms, and the scope of this disclosure is not limited to the embodiments described below.

[0033] In this specification, unless specifically stated otherwise, the terms “comprising” or “including” may be used to indicate that other components may be further included, rather than excluding other components.

[0034] Furthermore, unless otherwise specified in this disclosure, % refers to weight.

[0035] Conventionally, chromium is added to austenitic steels with a high manganese content to improve corrosion resistance and strength, and there is also a tendency to add molybdenum and niobium, which have similar effects to chromium, to improve the strength of the steel.

[0036] However, in this case, it is difficult to ensure the steel’s impact toughness well because chromium, molybdenum or niobium combine with carbon to form carbides at the austenite grain boundaries.

[0037] Therefore, the inventors of this disclosure have discovered that the problem can be solved by reducing the amount of chromium, molybdenum or niobium added to steel and adding vanadium at a high concentration, and the strength of the steel can be improved by fine precipitates derived from vanadium.

[0038] From this perspective, the steel according to one embodiment of this disclosure may contain, by weight, C: 0.050% to 1.70%, Mn: 15.0% to 40.0%, Cr: 3.00% or less, V: 1.00% to 3.00%, N: 1.000% or less but excluding 0%, Mo: 3.50% or less, Nb: 1.00% or less, and the remainder being iron (Fe) and unavoidable impurities.

[0039] The configurations will be described in detail below.

[0040] C: 0.050% to 1.70%

[0041] C can be an element that stabilizes austenite and increases strength, and also serves to reduce Ms and Md, which can be the transformation points from austenite to ε-martensite or α-martensite through cooling processes or machining. Therefore, when C is insufficient, due to insufficient austenite stability, stable austenite cannot be obtained at low temperatures, and the steel may be prone to transformation to ε-martensite or α-martensite due to deformation caused by external stress, thereby reducing toughness and strength. Therefore, in this disclosure, the lower limit for C can be 0.050%. In another embodiment, this disclosure may contain 0.070% or more of C, and in another embodiment, it may contain 0.100% or more. Conversely, when the amount of C is excessive, toughness may deteriorate rapidly due to carbide precipitation, and machinability may deteriorate due to excessive strength increase; therefore, this disclosure may contain 1.70% or less of C. As another embodiment, the upper limit for C in this disclosure can be 1.50% or less, and as another embodiment, it can be 1.30%.

[0042] Mn: 15.0% to 40.0%

[0043] Mn can be an element that plays an important role in stabilizing austenite. In one embodiment of this steel, 15.0% or more of Mn may be included to stabilize austenite. When the amount of Mn is insufficient, a metastable phase of ε-martensite may form, and at ultra-low temperatures, it may easily transform into α-martensite through deformation-induced transformation, thus potentially failing to ensure high toughness. To suppress the formation of ε-martensite, there is a scheme to stabilize austenite by increasing the amount of C, but in this case, a large amount of carbides may be deposited, and the properties, more specifically the toughness, may deteriorate rapidly. Therefore, the amount of Mn is preferably 15.0% or more. In another embodiment, the amount of Mn may be 18.0% or more, and in yet another embodiment, the amount of Mn may be 20.0% or more.

[0044] When the amount of Mn is excessive, it can not only reduce the corrosion rate of the steel, but may also be undesirable from an economic perspective. Therefore, the amount of Mn contained in the steel according to one aspect of this disclosure may be 40.0% or less. In another embodiment, the amount of Mn may be 35.0% or less, and in yet another embodiment, it may be 30.0% or less.

[0045] Cr: 3.00% or less

[0046] Cr can be an austenite stabilizing element, which can increase the strength of steel or help improve corrosion resistance, provided it is not exceeded in an appropriate amount. However, as mentioned above, Cr can be a carbide-forming element, and when added in excess to steel, it may reduce the low-temperature impact toughness of the steel by forming carbides at the austenite grain boundaries. Furthermore, when the amount of Cr added exceeds a certain level, excess carbides may deposit in the heat-affected zone (HAZ) of the weld, and thus the low-temperature toughness may deteriorate. Therefore, in this disclosure, the upper limit for Cr is 3.00%. In another embodiment, the upper limit for the amount of Cr is 2.80%, and in yet another embodiment, the upper limit for the amount of Cr is 2.50%.

[0047] V: 1.00% to 3.00%

[0048] V can be an element that combines with C to form at least one of VC precipitates or VCN precipitates, and can be an element that inhibits grain growth of austenite and delays recrystallization, thereby contributing to increased strength. In particular, it can more effectively contribute to increased strength of steel when a certain fraction or more of fine VC precipitates or fine VCN precipitates can be formed. In this disclosure, for this effect, the lower limit of the amount of V can be 1.00%. In another embodiment, this disclosure may contain 1.10% or more of V, and in another embodiment, it may contain 1.20% or more.

[0049] Conversely, when an excessive amount of V is added, it may be virtually impossible to completely dissolve the coarse carbides formed during steelmaking during reheating, and the properties may deteriorate as the coarse carbides remain in subsequent processes. Furthermore, V can be an expensive element, and adding excessive amounts of V may not be economically desirable. Therefore, in this disclosure, the upper limit for V content is 3.00%, and in another embodiment, the upper limit for V content is 2.80%.

[0050] N: 1.000% or less, but excluding 0%.

[0051] Nitrogen (N) can be an element that, along with carbon, improves toughness by stabilizing austenite, and can be particularly beneficial in improving strength through a strengthening effect similar to that of carbon. In particular, it is known to be an element that effectively promotes slip by increasing the packing defect energy.

[0052] When nitrogen is added in amounts exceeding 1.000%, there is a possibility of forming coarse nitrides, which could degrade the surface quality and properties of the steel. Therefore, the upper limit is limited to 1.000%. A preferred upper limit for the nitrogen (N) content is 0.500%, and a more preferred upper limit is 0.200%. Although this disclosure does not specify a lower limit for the nitrogen (N) content separately, considering the unavoidable addition of N, the lower limit for the nitrogen (N) content may be 0.005% or 0.007%.

[0053] Mo: 3.50% or less

[0054] Mo can be an element that can be used in the matrix to improve strength. However, similar to Cr, when Mo is added in excess to steel, carbides may form at the austenite grain boundaries, thereby reducing the low-temperature impact toughness of the steel. Furthermore, when the amount of Mo exceeds a certain level, excessive grain boundary carbides may deposit in the heat-affected zone (HAZ), thus potentially degrading the ultra-low temperature toughness. Therefore, this disclosure may include Mo in an amount of 3.50% or less. As another embodiment, the upper limit of Mo in this disclosure may be 3.40%, and as yet another embodiment, the upper limit of Mo may be 3.20%. Since the purposes of this disclosure can be achieved even when Mo is not added at all, this disclosure does not specifically limit the lower limit of Mo, but as an example, the lower limit of Mo may be 0.01%.

[0055] Nb: 1.00% or less

[0056] Nb can be an element that combines with C to form NbC precipitates, and it can also be an element that helps increase strength by increasing the amount of non-recrystallized zone rolled during steel manufacturing through inhibiting grain growth in the austenite structure and raising the recrystallization temperature. Furthermore, even when Nb can be completely omitted without causing problems in achieving the objectives of this disclosure, the formation of a certain fraction or more of fine NbC precipitates can more effectively contribute to increasing the strength of the steel. This disclosure takes into account the precipitation strengthening phenomenon due to precipitate formation, which limits the preferred lower limit of the Nb content to 0.01%.

[0057] When an excessive amount of Nb is added, the coarse carbides formed during steelmaking may become susceptible to external forces and cause cracks during continuous casting, thereby degrading the quality of the cast steel. Furthermore, when a large amount of Nb is added, a large amount of carbides may be coarsely deposited in the weld heat-affected zone, thus potentially reducing impact toughness, which may be undesirable. Therefore, this disclosure limits the upper limit of the Nb content to 1.00%, and preferably to 0.10%.

[0058] Additionally, the steel according to one embodiment of this disclosure may also contain at least one of the following: Ti: 1.0% or less, Al: 5.0% or less, or Si: 5.0% or less.

[0059] Ti: 1.00% or less

[0060] Ti can be an element that inhibits austenite grain growth by forming carbonitrides and can contribute to increased strength. Even when Ti is completely absent, it does not affect the purpose of this disclosure; therefore, this disclosure may be completely devoid of Ti. On the other hand, when excessive Ti is added, the corresponding Ti may crystallize or coarsen, thereby deteriorating the quality of the casting. Therefore, this disclosure may include Ti in an amount of 1.00% or less. In another embodiment, this disclosure may include 0.005% to 0.90% Ti, and in another embodiment, it may include 0.01% to 0.80%.

[0061] A1: 5.00% or less

[0062] Al can be used in the matrix to increase the strength of the steel and increase the stacking defect energy, thereby increasing the strength to control the deformation mode, which in turn can slip. To achieve this effect, the steel of this disclosure may contain Al, but even when Al is completely absent, it may not affect the achievement of the purpose of this disclosure; therefore, this disclosure may contain no Al at all. On the other hand, when excessive Al is added, there may be problems that could degrade the quality of the casting due to the crystallization or precipitation of coarse AlN; therefore, this disclosure may contain 5.00% or less Al. In another embodiment, this disclosure may contain 0.01% to 4.50% Al, and in another embodiment, it may contain 0.02% to 4.00%.

[0063] Si: 5.00% or less

[0064] Si can be an element that improves the casting properties of molten steel, and in particular, when added to austenitic steel, it can dissolve within the steel to effectively increase strength. Furthermore, it can be an element that effectively inhibits carbide formation by influencing the activity of carbon in the steel, thereby improving toughness. When added at a concentration exceeding 5.00%, it can reduce stacking defect energy to promote the formation of twins, and there is a possibility that the high strength may lead to a decrease in toughness; therefore, it is preferable to limit the upper limit to 5.00%. As another example, the upper limit of the Si content can be 3.00%, and as yet another example, the upper limit of the Si content can be 2.50%. Furthermore, according to another example, the lower limit of the Si content can be 0.10%, and as yet another example, the lower limit of the Si content can be 0.30%.

[0065] Furthermore, according to another aspect of this disclosure, the steel of this disclosure can satisfy the following relational expression 1, which can be a relational expression between carbon (C) and manganese (Mn).

[0066] [Relational Expression 1] 23.6[C]+[Mn] ≥ 28.000, 33.5[C]-[Mn] ≤ 23.00

[0067] (In relational expression 1, [C] and [Mn] refer to the weight percentages of C and Mn contained in the steel, respectively.)

[0068] This disclosure provides an in-depth study of the relative behavior between C and Mn amounts related to carbide formation, and as a result, such as Figure 1 As shown, determining the relative quantitative relationship between C and Mn can be an important factor in effectively controlling the amount of carbide precipitation while promoting austenite stabilization.

[0069] To promote austenite stabilization, provided that other components meet the ranges specified in this disclosure, the value of 23.6[C]+[Mn] can preferably be controlled to 28.000 or greater. When the value of 23.6[C]+[Mn] is less than 28.000, the stability of austenite decreases, and deformation-induced transformation due to deformation may occur, thus potentially reducing the impact toughness of the steel. Furthermore, when deformation-induced transformation occurs, α′ martensite with a BCC structure may form in addition to non-magnetic austenite, potentially deteriorating the non-magnetic properties of the steel and increasing its magnetic permeability.

[0070] Furthermore, in one embodiment of this disclosure, the value of 33.5[C]-[Mn] can be 23.00 or less. This is to prevent property degradation due to the formation of carbides from excess carbon. In another embodiment, the value of 33.5[C]-[Mn] can be 20.00 or less, and in yet another embodiment, it can be 18.00 or less.

[0071] In addition to the components described herein, the steel according to one aspect of this disclosure may also contain additional Fe and other unavoidable impurities. These cannot be completely eliminated during normal manufacturing processes due to the unavoidable mixing of raw materials or unintended impurities from the surrounding environment. Since these impurities are well known to anyone skilled in the art, their full amounts are not specifically mentioned in the specification. Furthermore, the addition of additional active ingredients besides the components described herein is not entirely excluded.

[0072] According to an example of the steel disclosed herein, the main microstructure may be austenite.

[0073] This can be to ensure the desired properties of the steel according to one aspect of this disclosure. According to one embodiment, the austenite area fraction can be 95% or more. In another embodiment, the austenite area fraction can be 97% or more. In particular, preferably, for the purpose of ensuring non-magnetic properties, the austenite area fraction can be 100%, but is not necessarily limited to this. In this disclosure, the method used to measure the austenite area fraction is not particularly limited and can be readily confirmed by measurement methods commonly used by those skilled in the art for measuring microstructure and carbides.

[0074] In the case of a steel according to one example of this disclosure, the area fraction of grain boundary carbides formed at the grain boundaries of austenite can be 5.0 area % or less. In this case, as a non-limiting example, the area used as a reference when measuring the area fraction of grain boundary carbides can be the entire measurement area.

[0075] In other words, one example of this disclosure can reduce the amount of Cr, Mo, and Nb in the steel, thereby reducing the carbides formed at the austenite grain boundaries to prevent deterioration of low-temperature impact toughness and cryogenic toughness in the weld heat-affected zone. Therefore, the carbides formed at the austenite grain boundaries can include at least one of the aforementioned Cr carbides, Mo carbides, or Nb carbides, and the area fraction of the grain boundary carbides can be 5.0 area % or less. In another embodiment, the area fraction of the grain boundary carbides can be 4.8 area % or less, and in yet another embodiment, it can be 4.5 area % or less. Furthermore, according to a non-limiting embodiment, the aforementioned carbides formed at the austenite grain boundaries can be measured using a scanning electron microscope or an optical microscope, and with this in mind, the minimum observable diameter of the grain boundary carbides in their original state can be 100 nm. The grain boundary diameter can refer to the diameter of a virtual circle when a virtual circle with the same area as the grain boundary carbides exposed on the surface is defined.

[0076] Furthermore, according to one embodiment of the present disclosure, the steel can form fine precipitates derived from V within the grains through aging treatment, and these fine precipitates derived from V can refer to at least one of VCN fine precipitates or VC fine precipitates. Additionally, the fine precipitates can refer to precipitates with a diameter of 50.0 nm or less. In this case, the diameter can refer to the diameter described above in its original state. As another example, the fine precipitates can refer to precipitates with a diameter of 10.0 nm or less, and as yet another example, precipitates with a diameter of 5.0 nm or less. Since smaller diameters of the fine precipitates are more advantageous, a lower limit is not necessarily imposed individually; however, when the diameter is measured using a transmission electron microscope according to a non-limiting example, the lower limit for the diameter of the fine precipitates can be 0.2 nm. According to the present disclosure, by forming fine precipitates derived from V at a certain level or above, high strength properties of the steel can be ensured even with a reduction in Cr. In particular, according to one embodiment of this disclosure, fine precipitates derived from V can be formed at a certain level within the grains, and in this case, the high strength properties of the steel can be ensured more effectively.

[0077] More specifically, to ensure the aforementioned strength-enhancing effect, the steel according to one aspect of this disclosure may contain 100 mm² / mm². 2 Or more VC and VCN precipitates with a diameter of 50.0 nm or smaller.

[0078] The method for measuring the number of at least one fine precipitate per unit area of ​​at least one of VC or VCN is not limited individually, as it can be readily employed by those skilled in the art according to the purpose; however, as an example, transmission electron microscopy can be used for measurement. In another embodiment, this disclosure may include at least one fine precipitate of at least one of VC or VCN with a diameter of 50.0 nm or less, comprising 120 precipitates / mm. 2 Or 150 pieces / mm 2 More or more precipitates, and in another embodiment, may contain 500 precipitates / mm. 2 More or 1000 pieces / mm 2 More or more precipitates. According to one aspect of this disclosure, a greater number of fine precipitates of at least one of VC or VCN is more advantageous, and therefore there is no upper limit on the number per unit area. However, in practice, the upper limit on the number of fine precipitates of at least one of VC or VCN with a diameter of 50.0 nm or less can be 10,000 per mm. 2 .

[0079] Furthermore, as mentioned above, reducing grain boundary carbides has the advantage of preventing excessive carbides from precipitating in the weld heat-affected zone (HAZ) after steel welding, which could lead to deterioration of cryogenic toughness. More specifically, in a steel according to one embodiment of this disclosure, at 3.0 kJ / mm 2 After submerged arc welding with the specified heat input, the area fraction of grain boundary carbides in the weld heat-affected zone can be 5.0 area % or less. As another example, the area fraction of grain boundary carbides in the weld heat-affected zone can be 4.8 area % or less, and as yet another example, it can be 4.5 area % or less. The description of grain boundary carbides in the weld heat-affected zone according to the lower limit of grain boundary diameter can also be applied to the description of carbides generated in austenite grain boundaries prior to welding, and therefore can be omitted.

[0080] As described above, the steel according to one aspect of this disclosure can ensure high strength properties and excellent impact toughness.

[0081] Specifically, the steel according to one aspect of this disclosure may have a room temperature yield strength of 550 MPa or greater, and according to another example, it may have a room temperature yield strength of 690 MPa or greater, and according to yet another example, the lower limit of the room temperature yield strength may be 750 MPa or 900 MPa. Furthermore, the Charpy impact energy measured at -84°C may be 27 J or greater.

[0082] Furthermore, the steel according to one example of this disclosure can excellently ensure non-magnetic properties. Specifically, the steel according to this disclosure can have a magnetic permeability of 1.200 or less after undergoing 20% ​​cold plastic deformation at room temperature.

[0083] As described above, the magnetic permeability after 20% cold plastic deformation at room temperature can be 1.200 or less, meaning that the austenitic structure can be stably maintained even after cold forming. Specifically, since austenite can be an unstable structure, even when austenite is obtained at room temperature, it may transform into ε-martensite or α-martensite due to further cooling or processing. In this case, since impact toughness tends to deteriorate and nonmagnetic properties tend to deteriorate, the steel according to an example of this disclosure can maintain a stable austenitic structure even after cold forming by controlling the relative amounts of C and Mn via the above-described relational expression 1, thereby ensuring excellent nonmagnetic properties.

[0084] As another example, steel according to one embodiment of this disclosure may have a magnetic permeability of 1.100 or less when the strain is at least 2% or greater during cold plastic deformation at room temperature.

[0085] Finally, since the steel according to one embodiment of this disclosure has excellent resistance to hydrogen embrittlement, a high level of safety can be ensured when applied to acidic environments.

[0086] In detail, the steel according to one embodiment of the present disclosure may have a crack length ratio (CLR) of 10% or less, preferably 5% or less, and more preferably 2% or less during hydrogen-induced cracking (HIC) testing.

[0087] The hydrogen-induced cracking (HIC) test can be performed by observing the number of cracks that appear after immersing steel in an acidic solution saturated with H2S gas (5% NaCl + 0.5% CH3COOH) for 96 hours. Specifically, the crack length (a) can be measured at the crack initiation point by observing a cross-section of a specimen with a constant width (W) and thickness (T), and the crack length ratio (CLR) can be derived from these average values. The formula for calculating the crack length ratio (CLR) can be shown in the following relational expression 2.

[0088] [Relational Expression 2]

[0089] CLR (Crack Length Ratio, %) = ∑(a / W) × 100

[0090] (In relational expression 2, a refers to the length of a single crack (μm), and w refers to the width of the specimen (μm))

[0091] Below, a method for manufacturing steel according to one embodiment of the present disclosure will be described. However, the following method for manufacturing steel may be merely an example, and it is important to note that the steel of the present disclosure does not necessarily have to be manufactured by this method, and any manufacturing method satisfying the claims of the present disclosure is acceptable for implementing the various embodiments of the present disclosure.

[0092] According to one embodiment of this disclosure, a method for manufacturing steel includes: heating a slab containing, by weight, 0.050% to 1.70% C, 15.0% to 40.0% Mn, 3.00% or less Cr, 1.00% to 3.00% V, 1.000% or less N but not exceeding 0%, 3.50% or less Mo, 1.00% or less Nb, and the remainder being iron (Fe) and unavoidable impurities; hot-rolling the slab to obtain a hot-rolled steel sheet; performing a solution treatment and cooling the hot-rolled steel sheet to room temperature; and aging the hot-rolled steel sheet.

[0093] The steps will be described in detail below.

[0094] Heating the slab

[0095] In a method for manufacturing steel according to one embodiment of the present disclosure, firstly, a slab may be prepared and heated, the slab comprising C: 0.050% to 1.70%, Mn: 15.0% to 40.0%, Cr: 3.00% or less, V: 1.00% to 3.00%, N: 1.000% or less but not exceeding 0%, Mo: 3.50% or less, Nb: 1.00% or less, and the remainder being iron (Fe) and unavoidable impurities.

[0096] In addition, the slab may contain at least one of the following: Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less, and may satisfy the following relational expression 1. Since the composition of the slab is the same as described above, its description will be omitted.

[0097] [Relational Expression 1] 23.6[C]+[Mn] ≥ 28.000, 33.5[C]-[Mn] ≤ 23.00

[0098] (In relational expression 1, [C] and [Mn] refer to the weight percentages of C and Mn contained in the steel, respectively.)

[0099] The heating can be performed in a temperature range of 1000°C or higher and 1300°C or lower. When the slab heating temperature is below 1000°C, a potential disadvantage is that the alloy components may not be able to be redissolved and homogenized, or it may take a long time to reach the target temperature in the center of the slab. In another embodiment, the lower limit of the slab heating temperature can be 1050°C, and in yet another embodiment, the lower limit of the slab heating temperature can be 1100°C or 1150°C.

[0100] When the slab heating temperature exceeds 1300°C, localized melting or surface oxidation may occur in the segregated portions of the alloy composition of the slab. Furthermore, in another embodiment, the upper limit of the slab heating temperature can be 1250°C, and in yet another embodiment, the upper limit of the slab heating temperature can be 1230°C or 1200°C.

[0101] Obtain hot-rolled steel plates

[0102] After heating, the slab can be hot-rolled according to one aspect of the present disclosure for manufacturing steel to obtain a hot-rolled steel plate.

[0103] Furthermore, as a non-limiting example, finishing hot rolling can be performed at 700°C or higher and 1050°C or lower. When the finishing hot rolling temperature is below 700°C, rolling may be difficult due to the high-temperature strength of the material, and a potential disadvantage is that the strength of the material may increase excessively due to over-rolling of the non-recrystallized regions, thereby reducing impact toughness. In another embodiment, the lower limit of the finishing hot rolling temperature may be 750°C or 800°C.

[0104] When the finishing hot rolling temperature exceeds 1050°C, there may be disadvantages such as coarsening of austenite and reduced strength. In another embodiment, the upper limit of the finishing hot rolling temperature of the slab can be 1000°C or 950°C. The reduction rate during hot rolling can be applied within an appropriate range according to the desired plate thickness, and as a non-limiting example, the final thickness of the hot-rolled steel plate can be from 5 mm to 50 mm.

[0105] Solution treatment and cooling of hot-rolled steel sheets to room temperature.

[0106] Next, in one example of this disclosure, the hot-rolled steel sheet obtained by the method can be cooled to room temperature after solution treatment.

[0107] Furthermore, as a non-limiting example, the solution treatment can be carried out at 900°C or higher and 1200°C or lower for 30 minutes or longer and 2 hours or less. The solution treatment can utilize grain boundaries and coarse intragranular carbides generated during hot rolling as a substrate and reduce internal energy due to excessive potential energy generated by rolling. To achieve this, in a method for manufacturing steel according to one aspect of this disclosure, the solution treatment temperature can be 900°C or higher, and the solution treatment time can be 30 minutes or longer. According to another embodiment, the lower limit of the solution treatment temperature can be 950°C or 1000°C.

[0108] On the other hand, when the solution treatment temperature exceeds 1200°C or the treatment time exceeds 2 hours, there may be disadvantages such as excessive coarsening of austenite and reduction in strength. Furthermore, according to another embodiment, the upper limit of the solution treatment temperature can be 1180°C, and according to another embodiment, the upper limit of the solution treatment temperature can be 1150°C.

[0109] Timeliness processing

[0110] A method for manufacturing steel according to one embodiment of this disclosure may include aging a hot-rolled steel sheet after solution treatment. This aging treatment can improve the strength of the austenitic steel by depositing at least one fine precipitate of VC or VCN within the grains.

[0111] As a non-limiting example, such aging treatment can be carried out at 500°C or higher and 850°C or lower for 30 minutes or longer and 5 hours or less. When the aging treatment temperature is below 500°C, the diffusion of precipitated elements may not be easy, thus potentially resulting in an excessively long precipitation time. In another embodiment, the lower limit of the aging treatment temperature can be 550°C or 600°C. When the aging treatment temperature exceeds 850°C, there may be disadvantages such as excessively coarse austenite and a decrease in its strength. Furthermore, as another embodiment, the upper limit of the aging treatment temperature can be 830°C or 800°C.

[0112] When the aging time is less than 30 minutes, there may be a drawback that sufficient precipitation time cannot be guaranteed. In another embodiment, the lower limit of the aging time can be 36 minutes or 42 minutes. However, the upper limit of the aging time can preferably be 5 hours or less. When it exceeds 5 hours, the intensity may actually decrease due to over-aging, and there may be uneconomical problems. In another embodiment, the upper limit of the aging time can be 4.8 hours or 4.5 hours.

[0113] In the following, steel products and methods of manufacturing the same according to one aspect of this disclosure will be described in more detail by way of specific embodiments. It is important to note that the following embodiments are intended for understanding this disclosure and are not intended to define the scope of this disclosure. The scope of this disclosure may be determined by the matters recited in the claims and those reasonably inferred therefrom.

[0114] Invention Embodiments

[0115] After preparing a slab with an alloy composition of 250 mm thickness as shown in Table 1, the slab was heated, hot-rolled, and solution-treated under the conditions shown in Table 2, and then aged after cooling to room temperature to prepare steel. In Comparative Example 7, no solution treatment or aging treatment was performed. The microstructure and properties of the steel thus prepared are shown in Tables 3 and 4.

[0116] In this case, after collecting a sample with a width and length of approximately 2 cm from the steel plate, the area fraction (area %) of austenite was measured five times at room temperature using an optical microscope at a point based on 1 / 4 t of the sample thickness. The area of ​​the austenite was calculated by image processing, and the average of the measurements is shown in Table 3 below. In this case, the magnification of the optical microscope was 200x.

[0117] In addition, the number of at least one fine precipitate per unit area (cells / mm) in VC or VCN. 2In the case of [specific condition], samples were collected in the same manner as described above, and measurements were taken five times using a transmission electron microscope at a point 1 / 4 t based on the sample thickness. The number of precipitates per unit area was calculated through image processing, and the measurements were averaged. The magnification of the transmission electron microscope was 200,000x at this time.

[0118] Furthermore, given the area fraction (area %) of austenitic grain boundary carbides, samples were collected in the same manner as described above, and measurements were taken five times at 1 / 4 t of the sample thickness using a scanning electron microscope. The area fraction of the precipitates was calculated and averaged through image processing. The grain boundary carbides were at least one of Cr carbides, Mo carbides, or Nb carbides, and the measurement magnification of the scanning electron microscope was 2000x.

[0119] The room temperature yield strength in Table 3 was measured using a uniaxial tensile test method, and the Charpy impact energy was measured using a Charpy impact testing machine after the specimens were held at -84°C for 15 minutes or longer. Furthermore, the crack length ratio (CLR) in the hydrogen-induced cracking (HIC) test was derived by observing the number of cracks generated after immersing the specimens in an acidic solution saturated with H2S gas (5% NaCl + 0.5% CH3COOH) for 96 hours.

[0120] [Relational Expression 2]

[0121] CLR (Crack Length Ratio, %) = ∑(a / W) × 100

[0122] (In relational expression 2, 'a' refers to the length of a single crack (μm), and 'W' refers to the width of the specimen (μm).)

[0123] Subsequently, the prepared steel was subjected to cold plastic deformation to 20% strain at room temperature using a uniaxial tensile testing machine, and then the magnetic permeability of the steel was measured. The results are shown in Table 3 below.

[0124] The permeability of the steel was measured by cutting and collecting cold-deformed specimens, preparing samples with a width and length of approximately 2 cm, measuring the permeability three times using a permeability meter, and then obtaining the average value.

[0125] In addition, at 3.0 kJ / mm 2 The prepared steel was subjected to submerged arc welding with a heat input, and the area fraction (area %) of grain boundary carbides in the weld heat-affected zone was measured and shown in Table 3 below. The grain boundary carbides were at least one of Cr carbides, Mo carbides, and Nb carbides. The surface area fraction of grain boundary carbides in the weld heat-affected zone was measured five times in the same manner as the base material, and the measurements were averaged.

[0126] [Table 1]

[0127]

[0128] [Table 2]

[0129]

[0130] [Table 3]

[0131]

[0132] [Table 4]

[0133]

[0134] Referring to Tables 1 to 3, Comparative Examples 1, 3 and 5, which had no added V, did not have high strength properties because at least one fine precipitate of VC or VCN was not formed.

[0135] Furthermore, in Comparative Examples 2 and 4, where V was added but the aging treatment temperature was too high or the aging treatment time was too long, it can be seen that the strength actually decreased due to over-aging.

[0136] In Comparative Example 6, since Mo exceeds the range proposed in this disclosure, the carbides exceed 5% of the austenite grain boundaries. As a result, excellent toughness cannot be guaranteed.

[0137] Finally, in Comparative Example 7, since no solution treatment and aging treatment were performed, it was not possible to ensure at a certain level of at least one precipitate in fine VC or VCN, and as a result, the room temperature yield strength did not reach 550 MPa.

[0138] On the other hand, in Invention Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions of this disclosure, excellent properties are ensured by ensuring that the composition and microstructure of this disclosure are obtained.

Claims

1. A type of steel, comprising: By weight, C: 0.050% to 1.70%, Mn: 15.0% to 40.0%, Cr: 3.00% or less, V: 1.00% to 3.00%, N: 1.000% or less but excluding 0%, Mo: 3.50% or less, Nb: 1.00% or less, and the remainder being iron (Fe) and unavoidable impurities. The main microstructure is austenite. The area fraction of grain boundary carbides formed at the grain boundaries of the austenite is 5.0 area % or less, and The steel is 100 pieces / mm 2 The number of or more per unit area includes at least one fine precipitate of VC or VCN with a diameter of 50.0 nm or smaller.

2. The steel according to claim 1, further comprising at least one of the following: Ti: 1.00% or less, Al: 5.00% or less, or Si: 5.00% or less.

3. The steel according to claim 1, satisfying the following relational expression 1: [Relational Expression 1] 23.6[C]+[Mn] ≥ 28.000, 33.5[C]-[Mn] ≤ 23.00 In relational expression 1, [C] and [Mn] refer to the weight percentages of C and Mn contained in the steel, respectively.

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

5. The steel according to claim 1, wherein the area fraction of the austenite is 95% or greater.

6. The steel according to claim 1, wherein the room temperature yield strength is 550 MPa or greater.

7. The steel according to claim 1, wherein the Charpy impact energy at -84°C is 27 J or greater.

8. The steel according to claim 1, wherein the magnetic permeability is 1.2 or less after undergoing 20% ​​cold plastic deformation at room temperature.

9. The steel according to claim 1, wherein the magnetic permeability is 1.100 or less when the deformation rate is at least 2% or greater during cold plastic deformation at room temperature.

10. The steel according to claim 1, wherein the content is 3.0 kJ / mm 2 After submerged arc welding with the required heat input, the area fraction of grain boundary carbides in the heat-affected zone is 5.0% or less.

11. The steel according to claim 1, wherein the crack length ratio (CLR) in the hydrogen-induced cracking (HIC) test, as defined by the following relational expression 2, is 10% or less: [Relational Expression 2] CLR (Crack Length Ratio, %) = ∑(a / W) × 100 In relational expression 2, a refers to the length of a single crack (μm), and W refers to the width of the specimen (μm).

12. A method for manufacturing steel, comprising: The slab is heated, the slab comprising, by weight, C: 0.050% to 1.70%, Mn: 15.0% to 40.0%, Cr: 3.00% or less, V: 1.00% to 3.00%, N: 1.000% or less but excluding 0%, Mo: 3.50% or less, Nb: 1.00% or less, and the remainder of iron (Fe) and unavoidable impurities; The slab is subjected to precision hot rolling to obtain hot-rolled steel plate; The hot-rolled steel sheet is then subjected to solution treatment and cooled to room temperature. as well as The hot-rolled steel sheet is subjected to aging treatment.

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

14. The method according to claim 12, wherein, The slab satisfies the following relational expression 1: [Relational Expression 1] 23.6[C]+[Mn] ≥ 28.000, 33.5[C]-[Mn] ≤ 23.00 In relational expression 1, [C] and [Mn] refer to the weight percentages of C and Mn contained in the steel, respectively.

15. The method of claim 12, wherein the heating is performed in a temperature range of 1000°C or higher and 1300°C or lower. The finishing hot rolling is carried out in a temperature range of 700°C or higher and 1050°C or lower. The solution treatment is performed at a temperature range of 900°C or higher and 1200°C or lower for 30 minutes or longer and 2 hours or less. The aging process is carried out at a temperature range of 500°C or higher and 850°C or lower for 30 minutes or longer and 5 hours or less.

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