Steel material and method for manufacturing same

Abstract

The present invention relates to a steel material which can be used in various fields such as the energy industry and shipbuilding, and a method for manufacturing same.

Description

Steel and its manufacturing method

[0001] The present invention relates to steel that can be used in various fields such as energy industry and shipbuilding, and a method for manufacturing the same.

[0002] Crude oil mining environments are becoming increasingly harsh, with high levels of hydrogen sulfide and other gases. Consequently, structures such as steel pipes and storage tanks used to transport crude oil require materials with superior hydrogen embrittlement resistance. Therefore, mining and transporting crude oil requires materials with superior hydrogen embrittlement resistance and sufficient toughness and strength to withstand the external pressures encountered during transportation.

[0003] Conventional high-strength carbon steels are difficult to produce and have limited use due to the presence of internal structures vulnerable to hydrogen embrittlement called hard spots. While various production methods and hard spot detection methods have significantly improved this issue, the fundamental requirement for high strength is a large amount of high-strength structures, making it difficult to avoid hard spots and thus difficult to manufacture above X80 grade. This is undesirable.

[0004] Furthermore, in structures containing large amounts of steel, such as military vessels like submarines, the location of an adversary can be tracked by detecting changes in the magnetic field generated by the interaction between the vessel and the Earth's magnetic field. Therefore, if a structure is manufactured using non-magnetic steel, the probability of detection can be drastically reduced.

[0005] To manufacture non-magnetic steel, conventional carbon steel is periodically demagnetized. However, this process is undesirable because it reduces the operational life of vessels and other vessels by the amount of time required for demagnetization, and also incurs economic losses.

[0006] One aspect of the present invention is to provide a steel material having excellent non-magnetic properties and excellent hydrogen embrittlement resistance, and a method for manufacturing the same.

[0007] The objectives of the present invention are not limited to the above-described matters. Additional objectives of the present invention are described throughout the specification, and those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the contents described in the specification.

[0008] An example of the present invention includes manganese (Mn): 13.0 to 25.0%, carbon (C): a range satisfying 10×[C]+[Mn]≥20.0, chromium (Cr): 5.0% or less (excluding 0%), vanadium (V): 1.5% or less (excluding 0%), the remainder iron (Fe) and unavoidable impurities,

[0009] The microstructure provides a steel comprising, in area %, 95% or more (including 100%) of austenite and 5% or less (including 0%) of carbides.

[0010] The above steel may contain at least one of niobium (Nb): 1% or less, silicon (Si): 5% or less, and nitrogen (N): 1% or less.

[0011] The above steel may have an average grain size of austenite of 5 to 200 μm.

[0012] The above steel may have 10 or more carbides having a size of 100 nm or less observed per 100 nm*100 nm unit area.

[0013] The above steel may have a crack length ratio (CLR) of 10 or less in a hydrogen-induced cracking (HIC) test defined by Equation 1 below.

[0014] [Formula 1]

[0015] CLR(Crack Length Ratio, %) = ∑(a / W) * 100

[0016] In Equation 1, a represents the length of a single crack, and W represents the width of the specimen.

[0017] The above steel may have an investment ratio of less than 1.01 after 30% of plastic deformation has occurred.

[0018] Another example of the present invention is a step of heating a steel slab at 1000 to 1300°C, which includes manganese (Mn): 13.0 to 25.0% by weight, carbon (C): a range satisfying 10×[C]+[Mn]≥20.0, chromium (Cr): 5.0% or less (excluding 0%), vanadium (V): 1.5% or less (excluding 0%), the remainder iron (Fe) and unavoidable impurities;

[0019] A step of finishing hot rolling the above-mentioned heated steel slab at 800 to 1050°C; and

[0020] A method for manufacturing steel is provided, which includes a step of cooling to a temperature range of 500°C or less at a cooling rate of 10°C / s or more after hot rolling.

[0021] The above steel slab may contain at least one of niobium (Nb): 1% or less, silicon (Si): 5% or less, and nitrogen (N): 1% or less.

[0022] The heating of the above steel slab can satisfy the heating time of Equation 2 below.

[0023] [Formula 2]

[0024] Heating time (min) ≥ 1 / 4 × slab thickness (mm) + 30 min

[0025] According to one aspect of the present invention, it is possible to provide a steel material that can be used in structures that require excellent hydrogen embrittlement resistance properties, such as a ship hull that requires excellent non-magnetic properties, or steel pipes or equipment that transport crude oil with a high hydrogen sulfide content.

[0026] The various advantageous and beneficial advantages and effects of the present invention are not limited to the above-described contents, and will be more easily understood in the course of explaining specific embodiments of the present invention.

[0027] Figure 1 schematically illustrates a specimen for calculating the crack length ratio (CLR) during a hydrogen-induced cracking (HIC) test.

[0028] Figure 2 is an EPMA photograph (a) and a transmission electron microscope (TEM) photograph (b) observing the microstructure of Invention Example 1 in Example 1.

[0029] Figure 3 is a transmission electron microscope (TEM) photograph of Comparative Example 2 in Example 1.

[0030] Figure 4 is an optical microscope photograph of the internal structure of a steel slab subjected to different heating times in Example 2.

[0031] The terminology used herein is for the purpose of describing the present invention and is not intended to limit the present invention. Furthermore, the singular forms used herein also include the plural forms, unless the context clearly dictates otherwise.

[0032] The meaning of "comprising" as used in the specification is to specify a configuration and not to exclude the presence or addition of other configurations.

[0033] Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by a person of ordinary skill in the art to which this invention pertains. Terms defined in the dictionary are to be interpreted to have meanings consistent with the relevant technical literature and the present disclosure.

[0034] First, an example of the steel material of the present invention will be described in detail. Below, the alloy composition and range of the steel material will be described. Unless otherwise specified, the alloy composition content described below refers to weight percent.

[0035] Manganese (Mn): 13.0~25.0%

[0036] The above Mn is an element that plays an important role in stabilizing austenite. In order to stabilize austenite in this steel, 13.0% or more of Mn may be included. If the Mn content is insufficient, epsilon (ε) martensite, a metastable phase, is formed, which easily transforms into alpha martensite through processing-induced transformation at ultra-low temperatures, making it difficult to secure toughness. There is a method of increasing the carbon (C) content to stabilize austenite in order to suppress the formation of epsilon (ε) martensite, but in this case, a large amount of carbides may be precipitated, which may rapidly deteriorate the physical properties, more specifically, the toughness. Therefore, the Mn content may be included at 13.0% or more. The preferable Mn content may be 15% or more, and a more preferable Mn content may be 18% or more. If the Mn content is excessive, not only may it slow down the corrosion rate of the steel, but it is also undesirable from an economic perspective. Therefore, the Mn content is preferably 22% or less. A more preferable Mn content may be 20% or less.

[0037] Carbon (C): 10×[C]+[Mn]≥20.0

[0038] The above C is an element that stabilizes austenite and increases its strength. In particular, C plays a role in lowering the transformation point Ms or Md from austenite to epsilon or alpha martensite during the cooling process or processing. Therefore, C is an element that effectively contributes to the stabilization of austenite. If the C content is insufficient, the stability of austenite is insufficient, so stable austenite cannot be obtained at ultra-low temperatures. In addition, external stress can easily cause a working-induced transformation into epsilon or alpha martensite, which can reduce the toughness of the steel or lower the strength of the steel. Therefore, the C content may be 0.01% or more. On the other hand, if the C content is excessive, the toughness of the steel may deteriorate rapidly due to carbide precipitation, and the strength of the steel may increase excessively, lowering the workability.

[0039] The inventors of the present invention have conducted in-depth research on the relative behavior between the C and Mn contents in relation to carbide formation, and as a result, have reached the conclusion that determining the relative content relationship between C and Mn can effectively promote austenite stabilization while effectively controlling the amount of carbide precipitation. Although carbides are formed by C, C does not independently affect the formation of carbides, but rather interacts in a complex manner with Mn and other alloying elements to affect the formation of carbides.

[0040] In order to promote the stabilization of austenite, it is preferable to manage the value of 10×[C]+[Mn] (wherein, [C] and [Mn] mean the contents of each component expressed in weight%) to 20.0 or more, provided that other components meet the ranges specified in the present invention. When 10×[C]+[Mn] is less than 20.0, the stability of austenite decreases, causing a working-induced transformation due to deformation, which may lower the impact toughness of the steel. In addition, when a working-induced transformation occurs, alpha prime (α') martensite with a BCC structure is formed in addition to non-magnetic austenite, which may increase the permeability, so it is preferable that it is 20.0 or more.

[0041] Chromium (Cr): 5.0% or less (excluding 0%)

[0042] The above Cr is also an austenite stabilizing element, and when added in an appropriate amount, it stabilizes austenite, thereby improving the low-temperature impact toughness of steel, and acts to increase the strength of steel by being dissolved in austenite. In addition, Cr is also an element that effectively contributes to improving the corrosion resistance of steel. Therefore, the present invention adds Cr as an essential element. The lower limit of the preferable Cr content may be 1%, and the lower limit of the more preferable Cr content may be 2%.

[0043] However, Cr is a carbide-forming element, and can reduce the low-temperature impact toughness of steel by forming carbides, especially at austenite grain boundaries. In addition, if the amount of Cr added exceeds a certain level, excessive carbides may precipitate in the heat-affected zone (HAZ) of the weld, which may result in poor ultra-low-temperature toughness. Therefore, the upper limit of Cr may be 5.0%. The upper limit of the preferable Cr content may be 4%.

[0044] Vanadium (V): 1.5% or less (excluding 0%)

[0045] The above V is an element that forms VC carbide by combining with C, and is an element that contributes to improving strength by inhibiting grain growth of the austenite structure and delaying recrystallization. Since V can achieve the effect of grain growth of the austenite structure even when added in a small amount, the lower limit of the V content in the present invention may be limited to a range excluding 0%. Since it can more effectively contribute to improving the strength of the steel when a certain fraction or more of fine VC carbide is formed, the present invention may limit the preferable lower limit of the V content to 0.1% in consideration of the precipitation strengthening phenomenon due to the formation of carbide. If the content of V is excessively added, it is practically impossible to completely dissolve coarse carbide formed in the steelmaking process during the reheating process, and since coarse carbide remains in the subsequent process, it may deteriorate the physical properties. In addition, since V is an expensive element, it may not be economically desirable to add it in excess. Therefore, the present invention can limit the upper limit of the V content to 1.5%, and the preferred upper limit of the V content can be 1%.

[0046] The above steel may additionally include at least one of Nb: 1% or less, Si: 5% or less, and N: 1% or less.

[0047] Niobium (Nb): less than 1%

[0048] The above Nb is an element that forms NbC carbide by combining with C, which suppresses grain growth of the austenite structure and increases the recrystallization temperature, thereby contributing to strength improvement by increasing the amount of rolling in the non-recrystallized region during the steel manufacturing process. Even when Nb is added in a small amount, it can additionally achieve the effect of grain growth of the austenite structure, but Nb is not essential in the present invention. However, when a certain fraction or more of fine NbC carbide is formed, it can more effectively contribute to improving the strength of the steel. Therefore, in the present invention, considering the precipitation strengthening phenomenon due to carbide formation, the preferable lower limit of the Nb content may be 0.01%. If the content of Nb is excessively added, coarse carbides formed in the steelmaking process may act vulnerablely to external force in the continuous tank process, which may induce cracks, thereby deteriorating the physical properties. In addition, if a large amount of Nb is added, a large amount of coarse carbides may be precipitated in the weld heat-affected zone, which may lower the impact toughness, which is not preferable. Therefore, the present invention can limit the upper limit of the Nb content to 1%.

[0049] Silicon (Si): 5% or less

[0050] The above-mentioned Si is an element that improves the castability of molten steel and, especially, when added to austenitic steel, effectively increases the strength by being dissolved in the steel. In addition, it is an element that affects the activity of carbon in the steel, effectively suppresses the formation of carbides, and increases toughness. However, when added in excess of 5%, there is a problem that the stacking fault energy is reduced, promoting twinning, and a decrease in toughness may occur due to high strength, so the upper limit is preferably limited to 5%. The upper limit of the preferable silicon (Si) content may be 3%, and the upper limit of the more preferable silicon (Si) content may be 2.5%. The lower limit of the preferable silicon (Si) content may be 0.1%, and the lower limit of the more preferable silicon (Si) content may be 0.3%.

[0051] Nitrogen (N): 1% or less

[0052] The above N, together with carbon, is an element that stabilizes austenite and improves toughness, and is particularly advantageous in improving strength through solid solution strengthening, like carbon. In particular, it is well known as an element that effectively increases stacking fault energy and promotes slip. However, when added in excess of 1%, coarse nitrides are formed, which causes a problem of deteriorating the surface quality and physical properties of the steel, so it is preferable that the upper limit is limited to 1 wt%. The upper limit of the preferable nitrogen (N) content may be 0.5%, and the upper limit of the more preferable nitrogen (N) content may be 0.2%. The lower limit of the preferable nitrogen (N) content may be 0.005%, and the lower limit of the more preferable nitrogen (N) content may be 0.007%.

[0053] The remaining components are iron (Fe), and may contain some unintended and unavoidable impurities introduced during the manufacturing process. Since these impurities are readily apparent to anyone skilled in the art of manufacturing, their full details are not specifically mentioned in this specification.

[0054] The above steel may contain a microstructure of 95% or more austenite by area %. Preferably, it may contain 97% or more, and may include a case where the austenite phase fraction is 100%.

[0055] Meanwhile, the steel may contain carbides to prevent a decrease in impact toughness. The carbides include vanadium carbide (VC). The carbides exist within the steel grains or at grain boundaries and may be formed to have a size of tens of nm or less. When observed using a transmission electron microscope (TEM), the carbides may have 10 or more carbides each having a size of 100 nm or less per 100 nm x 100 nm unit area. The upper limit is not particularly limited, but may be 20,000 or less.

[0056] In steel, austenite is an unstable structure at room temperature. To maintain the austenite, which is stable at high temperatures, at room temperature, austenite-stabilizing elements are usually added. Even if austenite is obtained at room temperature, it can transform into epsilon martensite or alpha martensite through further cooling or processing. This transformation varies depending on the alloying composition, cooling temperature, and processing amount. Since austenitic steels typically undergo cold working for structural forming, even if the microstructure is austenite when rolled, the transformation can occur during the deformation process during the fabrication of the structure. If weak epsilon or alpha martensite is formed through this transformation, impact toughness is deteriorated and non-magnetic properties are degraded.

[0057] The austenite of the steel of the present invention may have an average grain size of 5 to 200 μm. If the average grain size of the austenite exceeds 200 μm, the strength decreases due to coarsening of the austenite. In addition, there may be a disadvantage that the work-induced transformation temperature increases, easily causing transformation into epsilon martensite or martensite during working, thereby reducing impact toughness. On the other hand, if the average grain size of the austenite is less than 5 μm, the strength may increase excessively, resulting in a disadvantage that the toughness decreases. The lower limit of the average grain size of the austenite is more preferably 7 μm, and more preferably 10 μm. The upper limit of the average grain size of the austenite is more preferably 180 μm, and more preferably 150 μm.

[0058] The steel of the present invention may have an impact toughness of 30J or more after a Charpy impact test at -100℃.

[0059] The inventors of the present invention have determined that hydrogen embrittlement resistance is a key factor in ensuring safety when applied to sour environments. A steel material with excellent sour resistance according to one embodiment of the present invention may have a yield strength of 500 MPa or greater and an elongation of 20% or greater. As the strength of a steel material increases, its impact toughness decreases. In particular, for steel materials used in applications such as the present invention, excessively high yield strengths are likely to prevent the desired impact toughness from being achieved. Therefore, the upper limit of the room temperature yield strength of the present invention may be 900 MPa.

[0060] In addition, the steel having excellent sour resistance according to one embodiment of the present invention may have a crack length ratio (CLR) of 10 or less in a hydrogen induced cracking (HIC) test, a preferable crack length ratio (CLR) of 5 or less, and a more preferable crack length ratio (CLR) of 2 or less.

[0061] Below, the crack length ratio (CLR) during the hydrogen-induced cracking (HIC) test will be explained in more detail.

[0062] The hydrogen-induced cracking (HIC) test is performed by immersing the specimen in an acidic solution (5% NaCl + 0.5% CH3COOH) saturated with H2S gas, maintaining it for 96 hours, and then observing the location of crack occurrence. That is, as shown in Fig. 1, the cross-section of a specimen having a constant width (w) and thickness (T) is observed, the crack length (a) is measured at the point of crack occurrence, and the crack length ratio (CLR) is derived from the average value of these values. The formula for calculating the crack length ratio (CLR) is as shown in Equation 1 below. In Fig. 2, the symbol b represents the crack thickness.

[0063] [Formula 1]

[0064] CLR(Crack Length Ratio, %) = ∑(a / W) * 100

[0065] In Equation 1, a represents the length of a single crack, and W represents the width of the specimen.

[0066] The smaller the crack length ratio (CLR), the better the resistance to hydrogen-induced cracking. In particular, when the crack length ratio (CLR) is 10 or less, and more preferably, when the crack length ratio (CLR) is 5 or less, excellent resistance to hydrogen-induced cracking can be secured even in an extreme oxidizing environment.

[0067] The investment rate of the steel of the present invention can be maintained at 1.01 or less even when 30% of plastic deformation occurs.

[0068] Next, an example of the method for manufacturing the steel of the present invention will be described in detail.

[0069] The above manufacturing method may include a process of heating a steel slab having the above-described alloy composition, followed by hot rolling and cooling. This will be described in detail below.

[0070] First, a steel slab having the aforementioned alloy composition is heated at 1000 to 1300°C. If the slab heating temperature is lower than 1100°C, there are disadvantages in that re-dissolution and homogenization of the alloy components do not occur or it takes a long time to reach the target temperature to the center of the slab. If the slab heating temperature exceeds 1300°C, there are disadvantages in that partial melting occurs in the segregation of the slab alloy components or severe surface oxidation occurs. The lower limit of the slab heating temperature is more preferably 1030°C, more preferably 1070°C, and most preferably 1100°C. The upper limit of the slab heating temperature is more preferably 1250°C, more preferably 1230°C, and most preferably 1200°C.

[0071] In addition, during the above heating, it is preferable that the heating time satisfies Equation 2 below. Only when Equation 2, which is the heating time, is satisfied can sufficiently heat the steel slab. The upper limit of the heating time is not limited, but it may be preferably 5 hours or less, and more preferably 4 hours or less.

[0072] [Formula 2]

[0073] Heating time (min) ≥ 1 / 4 × slab thickness (mm) + 30 min

[0074] Thereafter, the heated slab is subjected to a finish hot rolling at 800 to 1050°C to obtain a hot-rolled steel sheet. If the finish hot rolling temperature is lower than 800°C, rolling is not easy due to the high high-temperature strength of the material, and excessive rolling in the non-recrystallized region occurs, which causes the strength of the material to increase excessively and reduce the impact toughness. If the finish hot rolling temperature exceeds 1050°C, there is a disadvantage in that the austenite coarsens and the strength decreases. The lower limit of the finish hot rolling temperature is more preferably 820°C, and even more preferably 850°C. Around the finish temperature, carbides, especially VC, are formed, which enables an increase in the yield strength through precipitation strengthening. At this time, VC can be formed in the grains and grain boundaries with a size of tens of nm or less, as can be seen in the photograph of Fig. 2 as an example. The upper limit of the above finishing hot rolling temperature is more preferably 1030°C, more preferably 1000°C, and most preferably 980°C. Meanwhile, the reduction ratio during the above hot rolling can be applied within an appropriate range depending on the desired plate thickness, and as a non-limiting example, the final thickness of the hot-rolled hot-rolled steel sheet can be 5 to 50 mm.

[0075] Thereafter, the hot-rolled steel sheet can be rapidly cooled to a temperature range of 500°C or lower at a cooling rate of 10°C / s or higher. If the cooling rate is less than 10°C / s or the cooling end temperature exceeds 500°C, excessive precipitation of cementite may occur, resulting in a decrease in impact toughness. Since most of the cementite formed during cooling is formed along grain boundaries, it is desirable to appropriately control the cooling rate and the cooling end temperature to suppress the formation of cementite as much as possible. Therefore, the cooling rate of the present invention may be 10°C / s or higher, and the cooling end temperature may be 500°C or lower, and a preferable cooling rate may be 15°C / s or higher, and a preferable cooling end temperature may be 450°C or lower. In addition, the present invention does not specifically limit the upper limit of the cooling rate, but in consideration of factors such as distortion of the steel plate shape and economy, the upper limit of the cooling rate may be limited to 120°C / s.

[0076] Hereinafter, embodiments of the present invention will be described. It should be apparent to those skilled in the art that various modifications to the following embodiments may be made without departing from the scope of the present invention. The following embodiments are intended to facilitate understanding of the present invention, and the scope of the present invention should not be limited to the following embodiments, but should be determined not only by the claims set forth below but also by their equivalents.

[0077] (Example 1)

[0078] After preparing a 250 mm thick slab having the alloy composition shown in Table 1 below (unit weight %, the remainder being Fe and unavoidable impurities), slab heating and hot rolling were performed as shown in Table 2 to manufacture steel.

[0079] For the steel manufactured above, the microstructure and physical properties were measured, and the results are shown in Table 2 below.

[0080] The above microstructure was measured using an optical microscope at room temperature.

[0081] The average austenite grain size was measured by taking microstructural photographs using an optical microscope and then using image analysis.

[0082] The yield strength at room temperature was measured using a uniaxial tensile test method, and the Charpy impact energy value (CVN) was measured by taking a standard specimen based on ASTM A370 from the above steel material, cooling it to -100℃, and measuring the impact energy according to the ASTM standard.

[0083] Steel alloy composition (weight %)Mn10Х[C]+[Mn]≥20.0CrVNbSiNA18.0524.053.140.510.030.380.011B16.223.22.850.450.02--C1720.02.450.550.030.85-D21.122.13.570.320.01-0.15E19.324.34 .750.280.11.05-F18.518.63.110.010.038--G17.319.32.680.140.01--H13.114.611.20 .890.005--I24.528.30.9200.01--J14.417.51.080.180.0001--K15.2182.890.80.031--

[0084] ClassificationSteel gradeSlab heating temperature(℃)Finishing hot rolling temperature(℃)Steel thickness(mm)Microstructure(area%)Austenite average grain size(㎛)Yield strength at room temperature(MPa)Charpy impact energy(J, @-100℃)Invention example 1A118591318100%γ4362378Invention example 2B118091020100%γ4058084Invention example 3C120291730100%γ3755488Invention example 4D119589730100%γ5255189Invention example 5E120892817100%γ6464273Comparative example 1F118591512.718%ε+11%M+71%γ6134 726Comparative Example 2G1179942205%ε+7%M+88%γ5248022Comparative Example 3H11909182290%γ+10%Carbide5725427Comparative Example 4I12059161726%ε+17%M+57%γ4743014Comparative Example 5J11989202882%γ+18%Carbide8637320Comparative Example 6K118611863420%ε+13%M+67%γ21527716

[0085] (In Table 2 above, γ: austenite, M: α'-martensite, ε: ε-martensite, and carbide here means bulk carbide such as chromium carbide)

[0086] In the case of invention examples 1 to 5 that satisfy the alloy composition and manufacturing conditions of the present invention, it can be seen that excellent physical properties are secured by securing the microstructure and austenite average grain size that the present invention seeks to obtain.

[0087] Fig. 2 (a) is a photograph of carbides within the tissue of Invention Example 1 observed using EPMA, confirming VC carbides. From Fig. 2 (a), it can be confirmed that Invention Example 1 contains 10 or more VCs of 100 nm or less per unit area. (b) is a photograph observed using a transmission electron microscope (TEM), confirming VC carbides (circled) formed around grain boundaries.

[0088] Fig. 3 is a transmission electron microscope (TEM) photograph of Comparative Example 2, and grain boundaries indicated by dotted lines can be confirmed. In Fig. 3, carbides are thickly formed at grain boundaries, C is exhausted, and there is insufficient C for VC carbides to be precipitated within the grains, so VC carbides are not observed within the tissue.

[0089] In the case of Comparative Examples 1 to 6, which do not satisfy the alloy composition or microstructure of the present invention, it can be seen that the physical properties are at a low level because the microstructure or austenite average grain size that the present invention seeks to obtain is not secured.

[0090] Meanwhile, for some of the above specimens, the investment rate and CLR according to the plastic strain rate were measured and shown in Tables 3 and 4 below.

[0091] The investment rate is expressed as the relative investment rate, which is the ratio of the investment rate in a vacuum to the investment rate in a specific atmosphere. In the example, the relative investment rate (μ), which is the investment rate in a vacuum and the atmosphere, was measured using a paramagnetic measuring device.

[0092] The hydrogen-induced cracking (HIC) test is conducted by immersing a specimen in an acidic solution (5% NaCl + 0.5% CH3COOH) saturated with H2S gas, maintaining it for 96 hours, and observing the location of crack occurrence. The cross-section of a specimen with a constant width (W) and thickness (T) is observed, and the crack length (a) is measured at the point of crack occurrence, and the crack length ratio (CLR) is derived from the average value. The formula for calculating the crack length ratio (CLR) is as follows: Equation 1.

[0093] [Formula 1]

[0094] CLR(Crack Length Ratio, %) = ∑(a / W) * 100

[0095] In Equation 1, a represents the length of a single crack, and W represents the width of the specimen.

[0096] Classification Steel grade Plastic strain (%) Permeability Invention example 1A01.001101.00317201.0045301.00433 Invention example 2B01.001101.0035201.0039301.0041 Comparative example 3H01.125101.1945201.3675302.18633 Comparative example 5J01.235101.314201.672302.363

[0097] Classification Steel Grade CLR (%) Invention Example 1A0.0 Invention Example 2B0.0 Comparative Example 1F11.3 Comparative Example 6K14.7

[0098] According to the above Tables 3 and 4, it was confirmed that Invention Examples 1 and 2 of the present invention can secure excellent non-magnetic properties and hydrogen embrittlement resistance with an investment ratio of 1.01 or less even when the plastic strain is 30% or more, but Comparative Examples 3 and 5 have poor non-magnetic properties, and Comparative Examples 1 and 6 have poor hydrogen embrittlement resistance.

[0099] (Example 2)

[0100] In the above Example 1, steel was manufactured under the same conditions as Inventive Example 1 for steel type A. However, during the heating process of the steel slab, the heating time was changed to 30 minutes, 1 hour, 2 hours, and 3 hours. At this time, optical microscope photographs of the steel slab after heating are shown in Fig. 4. Fig. 4 (a) and (b) show cases where the heating time was 30 minutes and 1 hour, respectively, which did not satisfy the heating time condition of Equation 2 below. As a result, a large amount of carbide was formed inside the steel slab, and due to the lack of re-dissolved C, the formation of VC carbide, etc. in the final steel cannot be expected. In addition, when coarse carbide (bulk carbide) is formed inside the steel slab and C is consumed in this coarse carbide, the austenite stability of the microstructure may decrease, and the impact toughness may decrease rapidly due to the large amount of coarse carbide contained in the final rolled steel.

[0101] [Formula 2]

[0102] Heating time (min) ≥ 1 / 4 × slab thickness (mm) + 30 min

[0103] On the other hand, (c) and (d) of Fig. 4 are photographs of steel slabs that have been sufficiently heated, satisfying the heating time condition of Equation 2, with heating times of 2 hours and 3 hours, respectively. Since sufficient reworking has been achieved, the formation of VC carbides, etc., can be expected in the final steel.

Claims

1. Manganese (Mn): 13.0~25.0%, Carbon (C): A range satisfying 10×[C]+[Mn]≥20.0, Chromium (Cr): 5.0% or less (excluding 0%), Vanadium (V): 1.5% or less (excluding 0%), the remainder including iron (Fe) and unavoidable impurities. Microstructure is a steel containing, in area %, 95% or more (including 100%) austenite and 5% or less (including 0%) carbides.

2. In claim 1, The above steel is a steel containing at least one of niobium (Nb): 1% or less, silicon (Si): 5% or less, and nitrogen (N): 1% or less.

3. In claim 1, Steel having an average grain size of the above austenite of 5 to 200 μm.

4. In claim 1, The above steel is a steel having 10 or more carbides each having a size of 100 nm or less observed per 100 nm*100 nm unit area.

5. In claim 1, The above steel is a steel having a crack length ratio (CLR) of 10 or less in a hydrogen-induced cracking (HIC) test defined by Equation 1 below. [Formula 1] CLR(Crack Length Ratio, %) = ∑(a / W) * 100 In Equation 1, a represents the length of a single crack, and W represents the width of the specimen.

6. In claim 1, The above steel is a steel with an investment ratio of less than 1.01 after 30% plastic deformation.

7. A step of heating a steel slab containing manganese (Mn): 13.0 to 25.0% by weight, carbon (C): a range satisfying 10×[C]+[Mn]≥20.0, chromium (Cr): 5.0% or less (excluding 0%), vanadium (V): 1.5% or less (excluding 0%), the remainder iron (Fe) and unavoidable impurities, at 1000 to 1300°C; A step of finishing hot rolling the above-mentioned heated steel slab at 800 to 1050°C; and A step of cooling to a temperature range of 500℃ or less at a cooling rate of 10℃ / s or more after hot rolling. A method for manufacturing steel including:

8. In claim 7, The above steel slab is a method for manufacturing a steel material containing at least one of niobium (Nb): 1% or less, silicon (Si): 5% or less, and nitrogen (N): 1% or less.

9. In claim 7, The above heating is a method for manufacturing steel that satisfies the heating time of the following formula 2. [Formula 2] Heating time (min) ≥ 1 / 4 × slab thickness (mm) + 30 min

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