Austenitic stainless steel containing niobium

Abstract

An austenitic stainless steel is provided. The austenitic stainless steel includes chromium (Cr), manganese (Mn), nickel (Ni), carbon (C), nitrogen (N), and niobium (Nb), with iron (Fe) as a balance. By incorporating niobium in a specified range, the austenitic stainless steel exhibits an improved pitting corrosion potential compared to conventional STS 304L, thereby providing enhanced corrosion resistance and cost competitiveness compared to existing austenitic stainless steels. The austenitic stainless steel further demonstrates high cryogenic toughness, with an impact absorption energy of at least 41 J at a temperature of approximately −196° C.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to Korean Patent Application Nos. 10-2025-0024358, filed on Feb. 25, 2025 and 10-2025-0092254, filed on Jul. 9, 2025, the disclosures of which are incorporated herein by reference in their entireties.BACKGROUNDTechnical Field

[0002] Exemplary embodiments relate to an austenitic stainless steel containing niobium.Description of the Related Art

[0003] The conventional STS 304L and STS 316L materials used for liquefied natural gas (LNG) carriers have an austenite phase that remains stabilized even at temperatures below room temperature. However, the use of high-cost nickel (Ni) in such materials results in a significant cost burden. Accordingly, there is a need to develop materials that have improved cost competitiveness relative to STS 304L and STS 316L. To stabilize the austenite phase at room temperature, austenitic materials require a reduction in the content of high-cost nickel and the addition of relatively low-cost elements such as manganese (Mn), carbon (C), and nitrogen (N). In addition, in accordance with the regulations of the International Maritime Organization (IMO), materials used for LNG carriers are required to exhibit an impact absorption energy of at least 41 J at a temperature of −196° C. Accordingly, there is a need to develop an austenitic stainless-steel material that has corrosion resistance comparable to or greater than that of STS 304L and STS 316L while satisfying the foregoing impact performance requirements.SUMMARY

[0004] Aspects of one or more exemplary embodiments provide an austenitic stainless steel having improved corrosion resistance and enhanced low-temperature impact absorption energy through the optimized addition of niobium.

[0005] Aspects of one or more exemplary embodiments also provide an austenitic stainless steel including nickel, manganese, carbon, and nitrogen in predetermined proportions to improve phase stability, impact absorption energy at a cryogenic temperature, machinability, and corrosion resistance, and a method of manufacturing the same.

[0006] Additional aspects will be set forth in part in the description which follows and, in part, will become apparent from the description, or may be learned by practice of the exemplary embodiments.

[0007] According to an aspect of an exemplary embodiment, there is provided an austenitic stainless steel including 16 to 20 wt % of chromium (Cr), 4.5 to 9 wt % of manganese (Mn), 3 to 6 wt % of nickel (Ni), 0.001 to 0.3 wt % of carbon (C), 0.1 to 0.35 wt % of nitrogen (N), 0.01 to 0.5 wt % of niobium (Nb), and the balance being iron (Fe) and other unavoidable impurities.

[0008] A combined content of carbon and nitrogen may be in a range of 0.15 to 0.45 wt %.

[0009] The austenitic stainless steel may have a pitting corrosion potential in a range of 300 to 400 mV in a 3.5% NaCl solution at a temperature in a range of 16 to 20° C.

[0010] The austenitic stainless steel may have a pitting corrosion potential in a range of 350 to 400 mV in a 3.5% NaCl solution at a temperature in a range of 16 to 20° C.

[0011] The austenitic stainless steel may have an impact absorption energy in a range of 41 to 100 J as measured at a temperature of −196° C.

[0012] The austenitic stainless steel may satisfy a pitting corrosion potential represented by Equation 1:Ep⁢i⁢t=0.8⁢833×Nb+0.3416[Equation⁢ 1]in Equation 1, Nb denotes the content of niobium in wt %.

[0014] The niobium content may be in a range of 0.0289 to 0.5 wt %.

[0015] A combined content of nickel and manganese may be in a range of 8 to 15 wt %.

[0016] The combined content of nickel and manganese may be in a range of 8.5 to 15 wt %.

[0017] The combined content of carbon and nitrogen may be in a range of 0.225 to 0.33 wt %.

[0018] The austenitic stainless steel may exhibit a tool wear in a range of greater than 0 mm and less than or equal to 0.15 mm as measured using an end mill under dry conditions.

[0019] A weight ratio of the carbon content to the combined content of carbon and nitrogen, defined by the formula [C / (C+N)], may be in a range of 0.1 to 0.7.

[0020] At least one condition selected from Expressions 2 to 5 may be satisfied:(Ni+Mn)+165.94(C+N)≥49.47[Equation⁢ 2](Ni+Mn)-27.28(C+N)≥2.1[Equation⁢ 3](Ni+Mn)≥12.3[Equation⁢ 4](Ni+Mn)+13.133(C+N)≥1.73[Equation⁢ 5]in Expressions 2 to 5, Ni, Mn, C, and N denote the content of nickel, manganese, carbon, and nitrogen in wt %, respectively.

[0022] The conditions of Expression 2 and Expression 5 may be satisfied.

[0023] The conditions of Expressions 2, 3, and 4 may be satisfied.

[0024] The combined content of carbon and nitrogen may be less than or equal to 0.33 wt %.

[0025] According to the present disclosure, an austenitic stainless steel provides several technical advantages. By incorporating a specified amount of niobium, the pitting corrosion potential can be improved compared to conventional STS 304L, resulting in enhanced corrosion resistance and cost competitiveness over existing austenitic stainless steels. Further, the austenitic stainless steel maintains high cryogenic toughness, exhibiting an impact absorption energy of at least 41 J at a temperature of approximately −196° C.

[0026] In addition, the austenitic stainless steel according to the present disclosure provides superior compositional efficiency by reducing or eliminating the requirement for high-cost elements such as nickel and molybdenum. Furthermore, the austenitic stainless steel maintains a stabilized austenite phase at room temperature by adding low-cost elements such as manganese, carbon, and nitrogen. By controlling the ratios of carbon and nitrogen in predetermined proportions, the austenitic stainless steel exhibits machinability and corrosion resistance. Moreover, by precisely regulating the compositional ratios of nickel, manganese, carbon, and nitrogen, the austenitic stainless steel demonstrates enhanced cryogenic impact absorption energy compared to conventional high-manganese stainless steels.BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The above and other aspects will become more apparent from the following description of the exemplary embodiments with reference to the accompanying drawings, in which:

[0028] FIG. 1 shows microstructure images of austenitic stainless steels with varying amounts of niobium according to exemplary examples;

[0029] FIG. 2 is a graph showing pitting corrosion potential evaluation results according to exemplary examples;

[0030] FIG. 3 is a graph showing impact absorption energy results according to exemplary examples;

[0031] FIG. 4 shows a phase map of austenitic stainless steels with varying composition ratios according to an exemplary example, and optical microscope images of the corresponding austenitic stainless steels, with a scale bar of 75 μm;

[0032] FIGS. 5A and 5B show images illustrating both a phase map and impact absorption energy of austenitic stainless steels having varying composition ratios according to an exemplary example;

[0033] FIG. 6 shows machinability test results for specimens manufactured from austenitic stainless steels with varying composition ratios according to an exemplary example;

[0034] FIG. 7 shows anodic polarization test results for specimens manufactured from austenitic stainless steels having varying composition ratios according to an exemplary example;

[0035] FIGS. 8A and 8B show images illustrating salt spray test results for specimens manufactured from austenitic stainless steels with varying composition ratios according to an exemplary example; and

[0036] FIG. 9 shows images illustrating intergranular corrosion test results for specimens manufactured from austenitic stainless steels with varying composition ratios according to an exemplary example.DETAILED DESCRIPTION

[0037] Various modifications and various embodiments will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the disclosure. However, it should be understood that the various embodiments are not limited to the scope of the disclosure to the specific embodiments, but they should be interpreted to include all modifications, equivalents, and alternatives of the embodiments included within the idea and technical scope disclosed herein. In the following description, like reference numerals refer to like components, even in different drawings. In certain embodiments, detailed descriptions of known functions and configurations which are deemed to make the gist of the present disclosure obscure will be omitted. For the same reason, some components in the accompanying drawings may be exaggerated, omitted, or simplified.

[0038] Terms such as “first” and “second” may be used to describe various components, but these components should not be limited by these terms. These terms are used only for the purpose of distinguishing one component from another. The use of such ordinal numbers should not be construed as limiting the meaning of the term. For example, the components associated with such an ordinal number should not be limited in the order of use, placement order, or the like. If necessary, each ordinal number may be used interchangeably.

[0039] Terms used herein are only for illustrative purposes of specific embodiments and are not intended to limit the present disclosure. The singular expressions include the plural expressions where not contrary to the context. It will be further understood that the terms such as “comprises”, “includes”, and “have / has” specify the presence of stated features, integers, steps, operations, components, parts, and / or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts, and / or combinations thereof.

[0040] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by those skilled in the art to which the present disclosure pertains. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having meanings consistent with their ordinary usage in the context of the related art, and should not be interpreted in an idealized or overly formal manner unless expressly defined otherwise herein.

[0041] An austenitic stainless steel according to an exemplary embodiment may include (1) 16 to 20 wt % of chromium (Cr), (2) 4.5 to 9 wt % of manganese (Mn), (3) 3 to 6 wt % of nickel (Ni), (4) 0.001 to 0.3 wt % of carbon (C), (5) 0.1 to 0.35 wt % of nitrogen (N), (6) 0.01 to 0.5 wt % of niobium (Nb), and a balance of iron (Fe) and unavoidable impurities.

[0042] In an exemplary embodiment, the austenitic stainless steel may include carbon and nitrogen in a total amount in the range of 0.15 to 0.45 wt %.

[0043] In an exemplary embodiment, the austenitic stainless steel may include niobium in an amount in the range of 0.01 to 0.5 wt %, in the range of 0.0289 to 0.5 wt %, or in the range of 0.0289 to 0.05 wt %.

[0044] Hereinafter, reasons for the numerical limitations of the present disclosure will be described.(1) Chromium (Cr): In the Range of 16 to 20 wt %

[0045] Chromium can form a passive film on a surface of the stainless steel, thereby providing corrosion resistance, and is therefore added. Therefore, to ensure corrosion resistance and stabilize an austenite phase of the austenitic stainless steel, chromium is included in an amount of at least 16 wt %. When the amount of chromium exceeds approximately 20 wt %, a sigma phase that degrades corrosion resistance may be formed. Therefore, the austenitic stainless steel may include chromium in an amount of 16 to 20 wt %.(2) Manganese (Mn): In the Range of 4.5 to 9 wt %

[0046] Manganese can help stabilize the austenite phase and increase the solubility of nitrogen. When the amount of manganese added is greater than or equal to 4.5 wt %, the effects described above can be achieved. However, if the amount of manganese exceeds approximately 9 wt %, manganese combines with impurities such as sulfur (S) or oxygen (O) to form manganese sulfide or manganese oxide, which may degrade the corrosion resistance and mechanical properties of the austenitic stainless steel. Therefore, the austenitic stainless steel may include 4.5 to 9 wt % of manganese.(3) Nickel (Ni): In the Range of 3 to 6 wt %

[0047] Nickel can help stabilize the austenite phase and increase toughness and formability. It can also improve impact absorption energy at a temperature of approximately −196° C., by increasing the pitting corrosion potential and minimizing the amount of interstitial elements. However, nickel is a high-cost element, so excessive addition increases manufacturing costs. Therefore, the austenitic stainless steel may include 3 to 6 wt % of nickel.(4) Carbon (C): In the Range of 0.001 to 0.3 wt %

[0048] Carbon can help stabilize the austenite phase and suppress martensitic transformation. However, carbon may form carbides which reduce corrosion resistance. Therefore, the austenitic stainless steel may include 0.001 to 0.3 wt % of carbon. Specifically, carbon may be present in an amount in the range of 0.05 to 0.3 wt %.(5) Nitrogen (N): In the Range of 0.1 to 0.35 wt %

[0049] Nitrogen can help stabilize the austenite phase and partially replace nickel. However, nitrogen may form nitrides which reduce corrosion resistance and toughness. Therefore, the austenitic stainless steel may include 0.1 to 0.35 wt % of nitrogen.(6) Niobium (Nb): In the Range of 0.01 to 0.5 wt %

[0050] Niobium modifies the microstructure, enhances the stability of the austenite phase, and improves corrosion resistance. The pitting corrosion potential increases exhibits an increase in correlation with increasing niobium content. Specifically, when the niobium content is greater than or equal to 0.0289 wt %, it is possible to achieve pitting corrosion potential comparable to or exceeding that of STS 316L. Accordingly, it can be seen that niobium contributes to improving the corrosion resistance of stainless steel.

[0051] However, when the amount of niobium exceeds 0.05 wt %, a carbide may be formed, which may adversely affect the physical properties of stainless steel.

[0052] In an exemplary embodiment, the austenitic stainless steel may have a pitting corrosion potential in the range of 300 to 400 mV or 350 to 400 mV when immersed in a 3.5% NaCl solution at a temperature in the range of 16 to 20° C.

[0053] In an exemplary embodiment, the austenitic stainless steel may have an impact absorption energy ranging from 41 to 100 J, or more preferably, from 41 to 85 J, at a temperature of approximately −196° C. If the impact absorption energy is less than approximately 41 J, the austenitic stainless steel may be unsuitable for use in liquefied petroleum gas (LPG) carriers.

[0054] In an exemplary embodiment, the austenitic stainless steel may satisfy a pitting corrosion potential represented by Equation 1 below when the amount of niobium is greater than 0 wt % and less than or equal to 0.5 wt %.Ep⁢i⁢t=0.8⁢833×Nb+0.3416[Equation⁢ 1]in Equation 1, Nb denotes the wt % of niobium.

[0056] In an exemplary embodiment, the austenitic stainless may include nickel and manganese in a total amount in the range of 8 to 15 wt % or 8.5 to 15 wt %. When the total amount of nickel and manganese and the total amount of carbon and nitrogen fall within the numerical ranges described above, the austenitic stainless steel may maintain a stabilized austenite phase even with a reduced amount of added nickel.

[0057] For example, manganese, which is a relatively low-cost element, may be added to stabilize the austenite phase while reducing the amount of nickel added. However, the effect of manganese in stabilizing the austenite phase is only about half that of nickel. Thus, carbon and nitrogen may be added to compensate for the reduced stabilizing effect. Although carbon and nitrogen exhibit approximately 15-fold and 30-fold greater effects in stabilizing the austenite phase, respectively, compared to nickel, excessive additions of carbon and nitrogen may reduce impact absorption energy at a low temperature, which is undesirable. Therefore, the above numerical ranges are selected to stabilize the austenite phase while achieving excellent impact absorption energy at a cryogenic temperature.

[0058] In addition, carbon and nitrogen in the austenitic stainless steel may be present in a total amount less than or equal to 0.33 wt %, less than or equal to 0.3 wt %, less than or equal to 0.295 wt %, less than or equal to 0.285 wt %, in the range of 0.225 to 0.33 wt %, in the range of 0.255 to 0.3 wt %, or in the range of 0.25 to 0.295 wt %. When the total amount of carbon and nitrogen falls within the above-described numerical range, excellent machinability and impact absorption energy at a cryogenic temperature can be achieved even when the amount of manganese is reduced.

[0059] Specifically, for the austenitic stainless steel, the weight ratio of carbon content to a total content of carbon and nitrogen, expressed as [C / (C+N)], may be ranging from 0.1 to 0.7, or more preferably from 0.2 to 0.5. Maintaining the weight ratio [C / (C+N)] within these ranges ensures that the alloy achieves corrosion resistance comparable to that of the conventional STS 304 material.

[0060] Further, when tool wear is measured using an end mill under dry conditions, the austenitic stainless steel demonstrates excellent machinability, with tool wear values greater than 0 mm and less than or equal to 0.15 mm, or greater than 0 mm and less than or equal to 0.125 mm.

[0061] Furthermore, the austenitic stainless steel may satisfy at least one condition selected from Expressions 2 to 5 below.(Ni+Mn)+165.94(C+N)≥49.47[Equation⁢ 2](Ni+Mn)-27.28(C+N)≥2.1[Equation⁢ 3](Ni+Mn)≥12.3[Equation⁢ 4](Ni+Mn)+13.133(C+N)≥1.73[Equation⁢ 5]in Expressions 2 to 5, Ni, Mn, C, and N denote the wt % of nickel, manganese, carbon, and nitrogen, respectively.

[0063] Specifically, when at least one condition selected from Expressions 2 to 5 is satisfied, the austenitic stainless steel may maintain a stabilized austenite phase that to room temperature and may exhibit excellent impact absorption energy at a cryogenic temperature of approximately −196° C.

[0064] Expressions 2 and 5 are inequalities derived from the phase map of the austenitic stainless steel that defines the specific compositional ranges required to maintain a stable γ-phase austenite structure at room temperature.

[0065] Expressions 2 to 4 represent inequalities derived from the phase map and impact absorption energy data for the austenitic stainless steel. These expressions define the compositional region required to maintain a stable γ-phase while ensuring the impact absorption of 41 J or greater.

[0066] Hereinafter, to facilitate understanding the present disclosure, exemplary examples are provided and described in detail. However, the exemplary examples are disclosed for illustrated purposes only and are not intended to limit the scope of the present disclosure. The exemplary examples are provided to enable those skilled in the art to fully understand and practice the present disclosure.Example 1

[0067] Nitrogen gas was injected into a vacuum induction melting (VIM) furnace, and a small ingot was produced by adding niobium to an alloy steel containing 16 to 20 wt % of Cr, 4.5 to 9 wt % of Mn, 3 to 6 wt % of Ni, 0.05 to 0.3 wt % of C, and 0.1 to 0.35 wt % of N. The resulting small ingot was heated to approximately 1200° C. and then hot-rolled into a plate having a thickness of approximately 11.5 mm. The amount of nitrogen was controlled by adjusting the amount of nitrogen injected into the VIM furnace and the reaction time. The amount of niobium was set to 0.01 wt %.Example 2

[0068] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.02 wt %.Example 3

[0069] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.048 wt %.Example 4

[0070] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.054 wt %.Example 5

[0071] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.05 wt %.Comparative Example 1

[0072] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.08 wt %.Comparative Example 2

[0073] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.1 wt %.Comparative Example 3

[0074] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.2 wt %.Comparative Example 4

[0075] A small ingot was produced in the same manner as in Example 1, except that the amount of niobium was set to 0.3 wt %.Comparative Example 5

[0076] A commercially available STS 304L was used as a comparative example.Comparative Example 6

[0077] A commercially available STS 316L was used as a comparative example.Experimental Example 1

[0078] FIG. 1 shows microstructure images observed for the exemplary examples. These microstructures were observed using an optical microscope. Referring to FIG. 1, it was confirmed that the austenite structure was maintained without the formation of a delta-ferrite phase, even when niobium content was increased to 0.5 wt %. This analysis confirmed that the addition of niobium maintained a stable, single-phase austenitic microstructure. Microstructural characterization revealed the presence of annealing twins within the grains and linear grain boundaries. Furthermore, no precipitates formed during the cooling process were detected along the grain boundaries.

[0079] FIG. 2 is a graph showing pitting corrosion potential measurements for the exemplary examples. The pitting corrosion potential for Examples 1 to 5 and Comparative Examples 1 to 6 was measured in a 3.5 wt % NaCl solution under the following conditions: a temperature in the range of 16° C. to 20° C., a potential sweep rate of 0.2 mV / s, and a surface finish prepared with 1500-grit SiC. The results are illustrated in FIG. 2. Referring to FIG. 2, it was found that the pitting corrosion potential increased as the amount of niobium increased. Specifically, it was confirmed that when the amount of niobium was greater than or equal to 0.0289 wt %, the pitting corrosion potential comparable to or exceeding that of STS 316L was exhibited.

[0080] FIG. 3 is a graph showing impact absorption energy results according to the exemplary examples. The impact absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) in accordance with ISO 148-1 for metallic Charpy impact testing. For LNG carriers, the impact absorption energy is required to be at least 41 J at a temperature of approximately −196° C. Each of Examples 1 to 5 exhibited an impact absorption energy of at least 41 J, whereas each of Comparative Examples 1 to 4 exhibited an impact absorption energy of less than 41 J.Example 61) Manufacture of Small Ingot

[0081] A crucible was charged with raw materials including Fe, Cr, Mn, Ni, and C, and was then placed inside a chamber. The chamber was evacuated to establish a vacuum atmosphere, thereby removing moisture, oxygen, and nitrogen from the chamber.

[0082] After evacuation, high-purity argon gas and nitrogen gas were injected into the chamber. Thereafter, the crucible was heated to a temperature of 1530° C. via an induction heating furnace to melt the raw materials, thereby ensuring uniform distribution of the alloying elements within the molten steel. During this process, the nitrogen concentration in the molten steel was controlled by adjusting the pressure of the nitrogen gas introduced into the chamber. Following the melting stage, the molten steel was cast into a mold to produce a rectangular ingot having dimensions of 44 to 45 mm×28 to 30 mm×67 to 70 mm.2) Rolling

[0083] A rolling process was used to form the small ingot into a plate. The small ingot was maintained at 1200° C. for a duration of 2 hours and subsequently subjected to the rolling process to achieve plastic deformation, thereby forming a flat and elongated plate. This thermal treatment was applied to reduce working stress during deformation and to facilitate forming of the small ingot into a plate shape.3) Heat Treatment

[0084] The flattened plate was subjected to normalizing heat treatment. For normalizing, the plate was heated to 1130° C., held at that temperature for 1 hour, and subsequently water-quenched.Experimental Example 2

[0085] To evaluate the phase stability of the austenitic stainless steel according to the present disclosure, specimens of each composition were prepared and characterized via an optical microscope. Based on these observations, a phase map for the austenitic stainless steels was constructed. The results are shown in FIG. 4 and Table 1.TABLE 1Microstructuralobservation(opticalmicroscope)Alloy #CrMnNiCNRemarksImpact-[1]18.389.0453.50.0150.2408γImpact-[2]18.768.9614.360.0120.2304γImpact-[3]18.169.0825.490.0070.2288γImpact-[4]18.259.0283.450.0120.3319γImpact-[5]18.219.0024.460.0170.3284γImpact-[6]18.29.0125.50.0070.3149γImpact-[7]18.569.2955.410.0170.2914γImpact-[8]18.229.043.450.0960.1441γImpact-[9]18.248.9334.510.0940.1342γImpact-18.189.0164.470.0930.1361γ

[10] Impact-18.169.0545.510.0820.1287γ

[11] Impact-18.29.0015.520.1070.1466γ

[12] Impact-18.28.9793.470.1000.19γ

[13] Impact-18.189.1034.480.1000.1901γ

[14] Impact-18.169.0024.490.0940.1903γ

[15] Impact-18.179.0245.540.0970.1859γ

[16] Impact-18.149.0175.470.0990.1877γ

[17] Impact-18.2919.0943.430.0950.223γ

[18] Impact-18.658.9714.550.1050.2267γ

[19] Impact-18.628.964.530.0980.2299γ

[20] Impact-18.158.9495.50.0940.2335γ

[21] Impact-18.279.0143.480.1920.1486γ

[22] Impact-18.138.9374.480.1900.1366γ

[23] Impact-18.18.9815.470.1840.1263γ

[24] Impact-18.178.9273.450.1880.1918γ

[25] Impact-18.128.9464.490.1900.1826γ

[26] Impact-18.088.9145.530.1900.1784γ

[27] Impact-18.188.7943.480.1940.1814γ

[28] Impact-18.688.923.510.2000.234γ

[29] Impact-18.138.9024.490.1920.2253γ

[30] Impact-18.18.9455.510.1880.2127γ

[31] Impact-18.668.9334.60.1070.2524γ

[32] Impact-18.648.9625.640.1100.2443γ

[33] Impact-18.218.974.520.2850.1477γ

[34] Impact-18.048.9325.490.2740.1429γ

[35] Impact-19.429.095.330.0250.276γ

[36] Impact-19.349.025.270.1020.206γ

[37] Impact-18.728.8884.260.0080.1486Partial α

[38] Impact-18.848.9344.310.0070.1914γ

[39] Impact-18.778.8314.310.0200.1844Partial α

[40] Impact-18.718.854.280.0100.2225γ

[41] Impact-18.778.8274.280.0120.2225γ

[42] Impact-17.720.0345.160.0010.3002Mart

[43] Impact-18.052.5815.070.0010.3064Partial mart (in trace

[44] amounts)Impact-182.5725.080.0760.2819γ

[45] Impact-18.334.8474.990.0010.3328γ

[46] Impact-18.544.7775.160.0850.2758γ

[47] Impact-18.142.5774.060.0020.3138Mart

[48] Impact-18.082.5774.060.0300.2556Partial mart

[49] Impact-18.542.6674.080.0840.2969γ

[50] Impact-182.5594.070.1200.257γ

[51] Impact-18.494.84.020.0010.3158γ

[52] Impact-17.80.0265.210.0880.1682Mart

[53] Impact-17.80.0365.210.0770.179Mart

[54] Impact-18.30.035.250.0950.1592Mart

[55] Impact-17.680.0255.150.0750.1809Mart

[56] Impact-17.780.0335.20.0710.1621Mart

[57] Impact-17.830.0325.240.0830.113Mart

[58] Impact-17.760.0355.240.0850.2569Partial mart

[59] Impact-18.162.5725.180.0790.2304γ

[60] Impact-18.082.5665.140.0910.1538Partial mart (in trace

[61] amounts)Impact-18.662.6545.170.0910.1604Partial mart (in trace

[62] amounts)Impact-18.242.5885.220.0890.2242γ

[63] Impact-18.092.5855.150.0670.146Partial mart (in trace

[64] amounts)Impact-18.062.5695.120.0850.1742Partial mart (in trace

[65] amounts)Impact-17.992.5635.120.0760.1876γ

[66] Impact-18.112.5625.170.0940.2437γ

[67] Impact-18.042.5635.110.0750.1105Partial mart

[68] Impact-18.395.0025.090.0770.1897γ

[69] Impact-18.95.0295.080.0800.1649γ

[70] Impact-18.344.7665.070.0900.2353γ

[71] Impact-18.12.5624.110.0930.144Mart

[72] Impact-18.242.6624.150.0830.1514Mart

[73] Impact-18.042.4514.040.0770.1919Partial mart

[74] Impact-18.052.6464.10.0880.2242Partial mart

[75] (in trace amounts)Impact-18.062.5614.050.0690.14Mart

[76] Impact-18.152.6684.150.0790.1475Partial mart

[77] Impact-18.252.6244.130.1030.0931Mart

[78] Impact-18.304.714.040.0760.1704γ

[79] Impact-18.354.7584.020.0840.0182γ

[80] Impact-18.814.9714.010.0910.1663γ

[81] Impact-18.334.7174.010.0810.2053γ

[82] Impact-17.670.0315.140.1310.1545Mart

[83] Impact-17.942.5485.080.1300.1495γ

[84] Impact-18.384.7325.130.1300.1589γ

[85] Impact-18.334.7594.000.1310.1585γ

[86] Impact-18.854.9885.090.1500.1291γ

[87] Impact-18.875.0025.080.0810.217γ

[88] Impact-18.472.67740.0180.2429Mart

[89] Impact-18.552.6484.10.0680.1827Partial mart

[90] Impact-18.082.574.050.0340.2870Partial mart

[91] (in small amounts)Impact-18.022.6524.120.0830.1421Partial mart

[92] Impact-18.350.0285.250.1170.1195Mart

[93] Impact-18.132.5565.170.1420.241γ

[94] Impact-18.022.5535.140.1140.2288γ

[95] Impact-18.632.6685.190.0940.1904γ

[96] Impact-18.915.0135.080.0770.2525γ

[97] Impact-18.985.7866.130.0780.2058γ

[98] Impact-18.815.7276.040.0750.2086γ

[99] Impact-19.235.8085.090.0950.2012γ

[100] Impact-196.7834.980.0730.2155γ

[101] Impact-19.267.7273.950.0790.2212γ

[102] Impact-18.645.7756.070.0810.1957γ

[103] Impact-19.65.6925.010.0770.1824γ

[104] Impact-19.696.6814.990.0780.2188γ

[105] Impact-19.797.6183.920.0790.2182γ

[106]

[0086] FIG. 4 shows the phase map of the austenitic stainless steels having varying composition ratios, prepared based on the results in Table 1. Table 1 shows the microstructural observation results obtained using the optical microscope for each sample with varying composition ratios. Referring to FIG. 4, when the total amount of nickel and manganese (Ni+Mn) was greater than or equal to 6 wt % and the total amount of carbon and nitrogen (C+N) was greater than or equal to 0.225 wt %, it was confirmed that the austenite phase was maintained in a stable state at room temperature. Specifically, mathematical expressions corresponding to the γ-phase stability region were derived from the phase map, as shown in Mathematical Expressions 1 and 2 below. When both Mathematical Expressions 1 and 2 are satisfied, the stable austenite phase can be consistently maintained at room temperature.(Ni+Mn)+165.94(C+N)≥49.47[Mathematical⁢ Expression⁢ 1](Ni+Mn)+13.133(C+N)≥1.73[Mathematical⁢ Expression⁢ 2]in Mathematical Expressions 1 and 2, Ni, Mn, C, and N denote the wt % of nickel, manganese, carbon, and nitrogen, respectively.Experimental Example 3

[0088] To confirm the impact absorption of the austenitic stainless steel according to the present disclosure, the impact absorption energy was measured. Based on the measured impact absorption energy and the phase map obtained in Experimental Example 2, the composition ratios of the austenitic stainless steel that provided phase stability at room temperature and an impact absorption energy above a predetermined level were identified. The results are shown in FIG. 5 and Table 2.

[0089] The impact absorption energy was obtained through the following impact test.

[0090] Specimens were extracted from the sound portion of the plate, with the top portion of the ingot being discarded due to the presence of shrinkage cavities. Standard-sized specimens (10 mm×10 mm×55 mm) were prepared with their longitudinal axes parallel to the rolling direction. A 2-mm V-notch was machined perpendicular to the rolling plane, and six impact specimens were prepared from each plate.

[0091] The impact test was performed at −196° C. The specimens were immersed at −196° C. for 30 minutes and then tested using an impact tester in accordance with ASTM A370-20. Two specimens were tested at each temperature.TABLE 2CVN impact absorptionenergy, JAt −196° C.StandardAlloy #CrMnNiCNAveragedeviationImpact-18.259.0283.450.0120.331939.01.4[4]Impact-18.219.0024.460.0170.328460.52.1[5]Impact-18.299.0943.430.0950.22859.02.8

[18] Impact-18.279.0143.480.1920.148648.53.5

[22] Impact-18.178.9273.450.1880.191850.00.0

[25] Impact-18.588.923.510.2000.23441.02.8

[29] Impact-18.162.5725.180.0790.23047.00.0

[60] Impact-18.395.0025.090.0770.189740.57.8

[69] Impact-18.95.0295.080.0800.164967.53.5

[70]

[0092] FIGS. 5A and 5B show the correlation between the phase map and the impact absorption energy for the austenitic stainless steels at various composition ratios. Referring to FIGS. 5A and 5B, it was confirmed that the stability of the austenite phase was significantly improved when the total amount of nickel and manganese (Ni+Mn) was greater than or equal to 8 wt % and the total amount of carbon and nitrogen (C+N) was greater than or equal to 0.225 wt %.

[0093] Table 2 shows the impact absorption energy measurement results for the specimens prepared from the austenitic stainless steels having varying composition ratios.

[0094] As shown in Table 2, Experimental Example 3 was conducted to identify an austenitic stainless steel that exhibits structural stability at room temperature and provides an impact absorption energy above a predetermined level at a low temperature (−196° C.). When the total amount of nickel and manganese (Ni+Mn) was greater than or equal to 8.5 wt % and the total amount of carbon and nitrogen (C+N) was greater than or equal to 0.225 wt %, it was confirmed that both the structural stability of austenite and an impact absorption energy greater than or equal to 41 J were satisfied.Experimental Example 4

[0095] Machinability tests were performed on specimens of austenitic stainless steels having varying composition ratios and on STS 316 to evaluate the machinability of the austenitic stainless steel according to the present disclosure. The results are shown in FIG. 6.

[0096] The machinability test was performed by measuring and evaluating tool wear after five passes using an end mill under dry conditions. The machining conditions are shown in Table 3. The composition ratios of the respective specimens are shown in Table 4, with all units expressed in wt %.TABLE 3ItemConditionCutting speed (m / min)100Feed rate (mm / min)636Depth of cut in Z-axis (mm)3.0Depth of cut in Y-axis (mm)0.5Machining typeDown-millingCutting tool (Dia. 10.0 endTaegutec TSEmill)4100M TT5525Workpiece75 mm × 50 mm × 100 mmCutting fluidDryTABLE 4Alloy #CrMnNiCNC + NL118.458.84.430.0570.1560.213L318.37.14.760.0980.20.298L520.24.885.160.1070.230.337L720.365.595.150.080.20190.282FIG. 6 shows the machinability test results for the specimens prepared from the austenitic stainless steels having varying composition ratios. Referring to FIG. 6, when the total amount of carbon and nitrogen (C+N) was less than or equal to 0.33 wt %, excellent machinability was observed.Experimental Example 5

[0098] Anodic polarization tests, salt spray tests, and intergranular corrosion tests were performed on specimens of austenitic stainless steels having varying composition ratios, as well as on commercially available materials, STS 304 and STS 316, to evaluate the corrosion resistance of the austenitic stainless steel according to the present disclosure. The results are shown in FIGS. 7 to 9 and Table 5.

[0099] The anodic polarization test was performed in accordance with ASTM G5, using a 3.5 wt % NaCl (seawater) solution. The temperature was maintained at 25° C.

[0100] The salt spray test was performed in accordance with KS D 9502:2020 by the Korea Testing & Research Institute (KTR), a certified testing institute. A NaCl solution having a concentration of (50±5) g / L was used, the temperature was maintained at 35±2° C., and the test was performed for 500 hours. Test specimens were prepared with the following dimensions: a width (W) of 24 mm, a length (L) ranging from 40 to 50 mm, and a thickness (T) ranging from 2 to 3 mm. For each alloy composition, a total of three specimens were tested.

[0101] The intergranular corrosion test was performed in accordance with ASTM A262-15 (2021) to evaluate the degree of intergranular sensitization. Each specimen was immersed in 10% oxalic acid at 25° C. and polarized at 1 mA·cm-2 for 90 seconds to induce corrosion. Thereafter, the occurrence of intergranular corrosion was determined through surface observation.TABLE 5PittingcorrosionChemical composition (wt %)potentialSalt spray testC / N / (Epit)MassAlloyC +C / (C +(C +Mn +StandardAppearancechange#CrMnNiCNMoNNN)N)NiAveragedeviationchangerate118.208.995.500.0070.315—0.3220.0220.0220.97814.4900.18050.0311————218.158.965.510.0940.232—0.3260.4050.2880.71214.4700.35630.0219————319.025.786.110.0740.203—0.2770.3650.2670.73311.8900.35490.0182No0.030.030.03abnormality418.795.716.020.0800.196—0.2760.4080.2900.71011.7300.3360.0174No0.000.000.00abnormality519.167.663.960.0760.208—0.2840.3650.2680.73211.6200.2620.0169No0.010.010.01abnormality619.585.786.040.0750.197—0.2720.3810.2760.72411.8200.40820.0321No0.000.000.00abnormalitySS30418.430.997.300.042—0.1150.0421.0000.0008.2900.28140.0115No0.000.000.00abnormalitySS31616.580.979.170.026—1.9730.0261.0000.00010.1400.34760.0142————

[0102] FIG. 7 shows the anodic polarization test results for the specimens manufactured from the austenitic stainless steels having varying composition ratios. Referring to FIG. 7 and Table 5, it was found that the pitting corrosion potential of the cost-effective austenitic stainless steels according to the present disclosure was comparable to or greater than that of STS 304 and was nearly equivalent to that of STS 316.

[0103] FIGS. 8A and 8B show images illustrating the salt spray test results for the specimens manufactured from the austenitic stainless steels having varying composition ratios. Referring to FIG. 8 and Table 5, no damage or mass change was detected even after 500 hours of exposure to the salt spray environment. Specifically, upon evaluating and comparing the corrosion resistance of commercially available materials STS 304 and STS 316, it was confirmed that the austenitic stainless steels of the present disclosure provide corrosion resistance comparable to or exceeding that of STS 316.

[0104] FIG. 9 shows images illustrating the intergranular corrosion test results for the specimens manufactured from the austenitic stainless steels having varying composition ratios. Referring to FIG. 9, it was confirmed that no intergranular corrosion occurred in the cost-effective austenitic stainless steels according to the present disclosure.

[0105] This indicates that the austenitic stainless steel of the present disclosure exhibits excellent corrosion resistance.

[0106] While one or more exemplary embodiments have been described with reference to the accompanying drawings, it will be understood by those skilled in the art that various modifications in form and details may be made therein without departing from the idea and scope as defined in the appended claims.

Claims

1. An austenitic stainless steel comprising:16 to 20 wt % chromium (Cr);4.5 to 9 wt % manganese (Mn);3 to 6 wt % nickel (Ni);0.001 to 0.3 wt % carbon (C);0.1 to 0.35 wt % nitrogen (N);0.01 to 0.5 wt % niobium (Nb); anda balance of iron (Fe) and unavoidable impurities.

2. The austenitic stainless steel of claim 1, wherein a combined content of carbon and nitrogen is in a range of 0.15 to 0.45 wt %.

3. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has a pitting corrosion potential in a range of 300 to 400 mV in a 3.5% NaCl solution at a temperature in a range of 16 to 20° C.

4. The austenitic stainless steel of claim 3, wherein the pitting corrosion potential is in a range of 350 to 400 mV.

5. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel has an impact absorption energy in a range of 41 to 100 J as measured at a temperature of −196° C.

6. The austenitic stainless steel of claim 1, wherein the austenitic stainless steel satisfies a pitting corrosion potential represented by Equation 1 below,Ep⁢i⁢t=0.8⁢833×Nb+0.3416[Equation⁢ 1]in Equation 1, Nb denotes the content of niobium in wt %.

7. The austenitic stainless steel of claim 1, wherein the niobium content is in a range of 0.0289 to 0.5 wt %.

8. The austenitic stainless steel of claim 7, wherein the niobium content is in a range of 0.0289 to 0.05 wt %.

9. The austenitic stainless steel of claim 1, wherein a combined content of nickel and manganese is in a range of 8 to 15 wt %.

10. The austenitic stainless steel of claim 2, wherein a combined content of nickel and manganese is in a range of 8 to 15 wt %.

11. The austenitic stainless steel of claim 9, wherein the combined content of nickel and manganese is in a range of 8.5 to 15 wt %.

12. The austenitic stainless steel of claim 2, wherein the combined content of carbon and nitrogen is in a range of 0.225 to 0.33 wt %.

13. The austenitic stainless steel of claim 9, further comprising a combined content of carbon and nitrogen in a range of 0.225 to 0.33 wt %.

14. The austenitic stainless steel of claim 11, further comprising a combined content of carbon and nitrogen in a range of 0.225 to 0.33 wt %.

15. The austenitic stainless steel of claim 14, wherein the austenitic stainless steel exhibits a tool wear in a range of greater than 0 mm and less than or equal to 0.15 mm as measured using an end mill under dry conditions.

16. The austenitic stainless steel of claim 1, wherein a weight ratio of the carbon content to the combined content of carbon and nitrogen, defined by the formula [C / (C+N)], is in a range of 0.1 to 0.7.

17. The austenitic stainless steel of claim 10, wherein at least one condition selected from Expressions 2 to 5 is satisfied,(Ni+Mn)+165.94(C+N)≥49.47[Equation⁢ 2](Ni+Mn)-27.28(C+N)≥2.1[Equation⁢ 3](Ni+Mn)≥12.3[Equation⁢ 4](Ni+Mn)+13.133(C+N)≥1.73[Equation⁢ 5]in Expressions 2 to 5, Ni, Mn, C, and N denote the content of nickel, manganese, carbon, and nitrogen in wt %, respectively.

18. The austenitic stainless steel of claim 17, wherein the conditions of Expression 2 and Expression 5 are satisfied.

19. The austenitic stainless steel of claim 17, wherein the conditions of Expressions 2, 3, and 4 are satisfied.

20. The austenitic stainless steel of claim 2, wherein the combined content of carbon and nitrogen is less than or equal to 0.33 wt %.

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