Austenite stainless steel including copper

Abstract

The present invention relates to austenitic stainless steel. According to the present invention, by adding a small amount of copper, the pitting potential is improved compared to conventional 304L STS, resulting in excellent corrosion resistance and the advantage of being price-competitive compared to conventional austenitic stainless steel. In addition, the shock absorption energy at approximately -196 ℃ can be 41 J or more.

Description

Technology Field

[0001] The present invention relates to an austenitic stainless steel containing copper. Background Technology

[0002] While existing 304L STS and 316L STS materials used in LNG vessels have stabilized the austenite phase down to temperatures below room temperature, the addition of expensive nickel (Ni) poses a cost burden. Therefore, there is a need to develop materials with superior price competitiveness compared to 304L STS and 316L STS. Austenitic materials must be able to stabilize the austenite phase even at room temperature by reducing the high-cost nickel content and adding low-cost elements such as manganese (Mn), carbon (C), and nitrogen (N). Furthermore, in accordance with International Maritime Organization (IMO) regulations, materials used in LNG vessels must have an impact absorption energy of at least 41J at -196°C, and it is necessary to develop the chemical composition of austenitic stainless steel materials with corrosion resistance equivalent to or greater than that of 304L STS and 316L STS. Prior art literature

[0003] Republic of Korea Registered Patent No. 10-1198486 (Published July 4, 2012) The problem to be solved

[0004] One objective of the present invention is to provide an austenitic stainless steel with improved corrosion resistance and low-temperature shock absorption energy by adding copper.

[0005] Another objective of the present invention is to provide an austenitic stainless steel containing nickel, manganese, carbon, and nitrogen in certain proportions to improve phase stability, shock absorption energy at cryogenic temperatures, processability, and corrosion resistance, and a method for manufacturing the same. means of solving the problem

[0006] To achieve the above objective, the present invention provides an austenitic stainless steel comprising chromium (Cr) in a weight ratio of 16 to 20, manganese (Mn) in a weight ratio of 4.5 to 9, nickel (Ni) in a weight ratio of 3 to 6, carbon (C) in a weight ratio of 0.001 to 0.3, nitrogen (N) in a weight ratio of 0.1 to 0.35, copper (Cu) in a weight ratio greater than 0 and less than or equal to 2.0, the remainder being iron (Fe) and other unavoidable impurities. Effects of the invention

[0007] According to the present invention, by adding a small amount of copper, the pitting potential is improved compared to conventional 304L STS, resulting in excellent corrosion resistance and the advantage of being price-competitive compared to conventional austenitic stainless steel. In addition, the shock absorption energy at approximately -196 ℃ can be 41 J or more.

[0008] In addition, the austenitic stainless steel according to the present invention is economical as it reduces or does not add the content of expensive nickel and molybdenum, stabilizes the austenite phase up to room temperature by adding low-cost manganese, carbon, and nitrogen, improves processability and corrosion resistance by adding carbon and nitrogen in a certain ratio, and can improve shock absorption energy at cryogenic temperatures compared to existing high-manganese stainless steel by controlling the compositional ratio of nickel, manganese, carbon, and nitrogen. Brief explanation of the drawing

[0009] Figure 1 is a microstructure image of austenitic stainless steel according to the copper content of an embodiment according to the present invention. Figure 2 is a graph of the formal potential evaluation results of an embodiment according to the present invention. Figure 3 is a graph of the shock absorption energy results of an embodiment according to the present invention. Figure 4 is a phase map according to the composition ratio of an austenitic stainless steel of an embodiment according to the present invention and an image of the austenitic stainless steel corresponding to the phase map taken with an optical microscope, with a scale bar of 75 μm. Figure 5 is an image showing the phase map and impact absorption energy according to the composition ratio of the austenitic stainless steel of an embodiment according to the present invention. Figure 6 is the result of a machinability test of specimens prepared according to the composition ratio of austenitic stainless steel of an embodiment according to the present invention. Figure 7 shows the results of an anodic polarization experiment of specimens prepared according to the composition ratio of austenitic stainless steel of an embodiment according to the present invention. Figure 8 is an image showing the results of a salt spray test on a specimen prepared according to the composition ratio of the austenitic stainless steel of an embodiment according to the present invention. Figure 9 is an image showing the results of an intergranular corrosion test of a specimen prepared according to the composition ratio of the austenitic stainless steel of an embodiment according to the present invention. Specific details for implementing the invention

[0010] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. Since the present invention is susceptible to various modifications and may take various forms, specific embodiments are illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to the specific disclosed forms, and it should be understood that the invention includes all modifications, equivalents, and substitutions that fall within the spirit and scope of the invention. Similar reference numerals have been used for similar components in the description of each drawing. In the attached drawings, the dimensions of the structures are shown enlarged compared to the actual dimensions for the clarity of the present invention.

[0011] Terms such as "first," "second," etc., may be used to describe various components, but said components should not be limited by said terms. These terms are used solely for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the first component may be named the second component, and similarly, the second component may be named the first component.

[0012] The terms used in this application are used merely to describe specific embodiments and are not intended to limit the invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, terms such as "comprising" or "having" are intended to specify the existence of the features, numbers, steps, actions, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, actions, components, parts, or combinations thereof.

[0013] Meanwhile, unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains. Terms such as those defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and should not be interpreted in an ideal or overly formal sense unless explicitly defined in this application.

[0015] According to an embodiment of the present invention, it may include chromium (Cr) in a weight ratio of 16 to 20, manganese (Mn) in a weight ratio of 4.5 to 9, nickel (Ni) in a weight ratio of 3 to 6, carbon (C) in a weight ratio of 0.001 to 0.3, nitrogen (N) in a weight ratio of 0.1 to 0.35, copper (Cu) in a weight ratio greater than 0 and less than or equal to 2.0, the remainder being iron (Fe) and other unavoidable impurities.

[0016] In one embodiment, the austenitic stainless steel may contain copper in a weight ratio of greater than 0 and less than or equal to 1.657 or 0.0222 to 1.5932.

[0017] In one embodiment, the austenitic stainless steel may contain the carbon and the nitrogen in a weight ratio of 0.15 to 0.45.

[0018] The reasons for the numerical limitations of the present invention will be explained below.

[0019] (1) Chromium (Cr): Approximately 16 to 20 by weight

[0020] Chromium is added because it can impart corrosion resistance by forming a passivation film on the surface of stainless steel. Therefore, to ensure corrosion resistance and stabilize the austenite phase of austenitic stainless steel, a chromium content of at least 16 weight percent is required. If the chromium content exceeds approximately 20 weight percent, a sigma phase that weakens corrosion resistance may be formed. Therefore, the austenitic stainless steel may contain chromium in a weight ratio of approximately 16 to 20.

[0021] (2) Manganese: about 4.5 to 9 by weight

[0022] Manganese can stabilize the austenite phase and increase the solubility of nitrogen. The above effects can be obtained by adding manganese in a weight ratio of about 4.5 or more. However, if the manganese exceeds about 9 weight ratio, it combines with impurities such as sulfur (S) or oxygen (O) to form manganese sulfides or manganese oxides, which may degrade the corrosion resistance and mechanical properties of the austenitic stainless steel. Therefore, the austenitic stainless steel may contain manganese in a weight ratio of about 4.5 to 9.

[0023] (3) Nickel: about 3 to 6 weight ratio

[0024] Nickel can stabilize the austenite phase and increase toughness and formability. In addition, it improves shock absorption energy at approximately -196°C by minimizing the increase in pitting dislocations and the amount of interstitial elements. However, since it is an expensive element, adding an excess amount of nickel increases manufacturing costs. Therefore, the above-mentioned austenitic stainless steel may contain nickel in a weight ratio of approximately 3 to 6.

[0025] (4) Carbon: Approximately 0.001 to 0.3 weight ratio

[0026] Carbon can stabilize the austenite phase and suppress martensite transformation. However, carbon causes carbides to form, which reduces corrosion resistance. Therefore, the austenitic stainless steel may contain carbon in a weight ratio of about 0.05 to 0.3.

[0027] (5) Nitrogen: about 0.1 to 0.35 by weight

[0028] Nitrogen can stabilize the austenite phase and partially replace nickel. However, nitrogen causes the formation of nitrides, which reduces corrosion resistance and toughness. Therefore, the austenitic stainless steel may contain nitrogen in a weight ratio of about 0.1 to 0.35.

[0029] (6) Copper: Weight ratio greater than 0 and less than or equal to 1.657

[0030] Copper acts in conjunction with chromium to form a corrosion-resistant surface. As the copper content increases, the pitting potential gradually increases, but a decreasing trend is observed when added at a weight ratio of 1.0 or higher; up to a weight ratio of 1.657, the pitting potential is higher than that of 304 STS, and from a weight ratio of 0.0222 to 1.5932, it is higher than that of 316L STS. Based on this, it can be concluded that copper interacts with chromium to contribute to the improvement of the corrosion resistance properties of stainless steel.

[0031] If the weight ratio of copper exceeds 1.657, copper oxide or copper chloride may form, which can cause corrosion of stainless steel.

[0033] In one embodiment, the pitting potential of the austenitic stainless steel with a 3.5% NaCl solution at 16 to 20°C may be 300 to 420 mV and may be 350 to 400 mV.

[0034] In one embodiment, the austenitic stainless steel may have an impact absorption energy of 41 to 120 J at about -196 ℃, or 80 to 120 J. If the impact absorption energy is less than about 41 J, it may not be suitable for LNG vessels.

[0035] In one embodiment, the austenitic stainless steel can satisfy the pitting potential expressed by the following formula (1) in a weight ratio range of copper content greater than 0 and less than or equal to 1.657:

[0036] [Equation 1]

[0037] E pit =-0.0481×Cu 2 + 0.0777×Cu + 0.3654

[0038] In the above Equation 1, Cu represents the weight percent of the element.

[0040] In one embodiment, the austenitic stainless steel may contain nickel and manganese in a weight ratio of 8 to 15 or 8.5 to 15. When the total content of nickel and manganese and the total content of carbon and nitrogen are satisfied as described above, the austenitic stainless steel can stabilize the austenite phase even if the amount of nickel added is reduced.

[0041] For example, in order to stabilize the austenite phase while reducing the amount of nickel added, low-cost manganese is added. Since manganese is 0.5 times more effective than nickel in stabilizing the austenite phase, carbon and nitrogen are added to compensate for this. Carbon and nitrogen have 15 times and 30 times the effect, respectively, on stabilizing the austenite phase compared to nickel. However, if the amount of carbon and nitrogen added increases excessively, there is a disadvantage that the shock absorption energy decreases at low temperatures. Therefore, the above range must be satisfied to achieve austenite phase stabilization and excellent shock absorption energy at cryogenic temperatures.

[0042] In addition, the total content of carbon and nitrogen in the above-mentioned austenitic stainless steel may be 0.33 wt% or less, 0.3 wt% or less, 0.295 wt% or less, 0.285 wt% or less, 0.225 wt% to 0.33 wt%, 0.255 wt% to 0.3 wt%, or 0.25 wt% to 0.295 wt%. By satisfying the above-mentioned total content of carbon and nitrogen, excellent processability and cryogenic shock absorption energy can be exhibited even when the manganese content is reduced.

[0043] Specifically, in the austenitic stainless steel, the ratio of the total content of carbon and nitrogen to carbon [C / (C+N)] may be 0.1 to 0.7 or 0.2 to 0.5. By satisfying the total content of carbon and nitrogen to carbon as described above, corrosion resistance at the level of existing 304STS material can be exhibited.

[0044] In addition, when the wear of the tool is measured using an end mill under dry conditions, the machinability of the austenitic stainless steel may be greater than 0 and less than or equal to 0.15 mm or greater than or equal to 0 and less than or equal to 0.125 mm.

[0045] In addition, the above austenitic stainless steel may satisfy any one or more selected from Formulas 2 to 5 below:

[0046] [Equation 2]

[0047] (Ni+Mn)+165.94(C+N) ≥ 49.47

[0048] [Equation 3]

[0049] (Ni+Mn)-27.28(C+N) ≥ 2.10

[0050] [Equation 4]

[0051] (Ni+Mn) ≥ 12.3

[0052] [Equation 5]

[0053] (Ni+Mn)+13.133(C+N) ≥ 1.73

[0054] In the above formulas 2 to 5, Ni, Mn, C, and N represent the weight percent of nickel, manganese, carbon, and nitrogen, respectively.

[0055] Specifically, the above-mentioned austenitic stainless steel can stabilize the austenite phase down to room temperature and exhibit excellent shock absorption energy at cryogenic temperatures (-196℃) when satisfying at least one of the selected formulas 2 to 5.

[0056] Equations 2 and 5 above were derived based on the phase map of austenitic stainless steel to correspond to the γ-phase stable region, satisfying the region where the austenite structure is stable at room temperature.

[0057] Equations 2 to 4 above were derived based on data showing the phase map and impact absorption energy of austenitic stainless steel, which simultaneously satisfy the γ-phase stable region and an impact absorption energy of 41J or more.

[0059] Hereinafter, the present invention will be described in detail with reference to examples to aid in understanding. However, the following examples are merely illustrative of the content of the present invention and the scope of the present invention is not limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those with average knowledge in the art.

[0061] <Example 1>

[0062] Nitrogen gas was injected into a vacuum induction furnace (VIM), and commercial electrolytic iron, chromium, manganese, nickel, molybdenum, and carbon powder (purity 95% or higher) were melted using a master alloy and produced into small steel ingots. The small steel ingots were heated to approximately 1200°C and hot-rolled to produce a plate shape with a thickness of approximately 11.5 mm. The nitrogen content was controlled by controlling the amount injected into the VIM and the reaction time. The copper content was set to a weight ratio of 0.501.

[0064] <Example 2>

[0065] Small steel ingots were manufactured in the same manner as in Example 1, except that the copper content in Example 1 was set to 0.874 by weight.

[0067] <Example 3>

[0068] Small steel ingots were manufactured in the same manner as in Example 1, except that the copper content in Example 1 was set to about 1.272 by weight.

[0070] <Example 4>

[0071] Small steel ingots were manufactured in the same manner as in Example 1, except that the copper content in Example 1 was set to 1.657 by weight.

[0073] <Comparative Example 1>

[0074] Commercial 304L STS was used.

[0076] <Comparative Example 2>

[0077] Commercial 316L STS was used.

[0079] <Experimental Example 1>

[0080] Figure 1 is an image showing the result of observing the microstructure of an embodiment according to the present invention, and the microstructure was observed using an optical microscope. Referring to Figure 1, it was confirmed that even when copper was added up to 1.657 wt%, only an austenite structure was observed and no ferrite structure was observed. Through this, it was confirmed that the addition of copper has no effect on the microstructure. Annealing twins were observed within the grains, and the grain boundaries appeared as straight lines, and no precipitates formed during cooling were observed at the grain boundaries.

[0081] Figure 2 is a graph showing the pitting potential of the embodiments according to the present invention. Figure 2 shows the results of measuring the pitting potential of Examples 1 to 4 and Comparative Examples 1 and 2 under conditions of 3.5 wt% NaCl, 16 to 20°C, 0.2 mV / s (potential sweep rate), and SiC 1500 grit (surface finish). Referring to Figure 2, it was observed that the pitting potential increases as the copper content increases, but tends to decrease when 1.0 wt% or more is added. However, it was confirmed that even with the addition of 1.657 wt% copper, the material exhibits excellent corrosion resistance, having a superior pitting potential compared to conventional 304 STS. It was also confirmed that when the copper content is added at 0.0222 to 1.5932 wt%, the material exhibits a higher pitting potential than 316L STS.

[0082] Figure 3 is a graph of the shock absorption energy results of an embodiment according to the present invention. The shock absorption energy was measured using a Charpy impact tester (Tinius Olsen, IT542E) in accordance with ISO 148-1: Metal Charpy Impact Test. For LNG vessels, the shock absorption energy must be at least approximately 41 J at approximately -196 ℃. All of Examples 1 to 3 recorded a shock absorption energy of at least approximately 80 J.

[0084] <Example 5>

[0085] 1) Manufacture of small steel ingots

[0086] After loading the raw materials Fe, Cr, Mn, Ni, and C into a crucible, the crucible was placed inside a chamber and a vacuum atmosphere was created to remove water vapor, oxygen, and nitrogen present inside the chamber.

[0087] After injecting high-purity Ar and high-purity nitrogen (N) gases in a vacuum, an electric current was passed through an induction furnace to heat the inside of the crucible to 1530°C, causing the raw materials to melt and the introduced elements to be uniformly distributed. At this time, the nitrogen concentration in the molten steel was controlled by adjusting the nitrogen gas pressure. After melting, the molten steel was poured into a mold to produce a rectangular steel ingot (44-45mm × 28-30mm × 67-70mm).

[0088] 2) Rolling

[0089] A rolling process was used to produce a small steel ingot into a plate shape. The small steel ingot was held at 1200°C for 2 hours and then plastically deformed through a rolling process to produce a flat, long plate shape. At this time, the small steel ingot was heated to maintain low processing stress in order to process it into a plate shape.

[0090] 3) Heat treatment

[0091] The flattened plate was subjected to normalizing heat treatment, and the normalizing was performed by heating to 1130 ℃, maintaining it for 1 hour, and then water cooling.

[0093] <Experimental Example 2>

[0094] To confirm the phase stability of the austenitic stainless steel according to the present invention, austenitic stainless steels prepared with each component were observed using an optical microscope to show the phase map of the austenitic stainless steel, and the results are shown in Figure 4 and Table 1.

[0095]

[0096]

[0097] Figure 4 is a phase map of austenitic stainless steel according to compositional ratios, prepared based on the results of Table 1, which shows the results of observing the microstructure of each sample according to compositional ratios using an optical microscope. Looking at Figure 4, it was confirmed that the austenite phase is stabilized down to room temperature when the content of nickel and manganese (Ni+Mn) is 6 wt% or more and the content of carbon and nitrogen (C+N) is 0.225 wt% or more. Specifically, based on the above phase map, a mathematical formula corresponding to the γ-phase stability region was derived, which is represented by the following Equations 1 and 2. When Equations 1 and 2 are satisfied, the austenite phase can be stabilized down to room temperature.

[0098] [Mathematical Formula 1]

[0099] (Ni+Mn) + 165.94(C+N)≥49.47

[0100] [Mathematical Formula 2]

[0101] (Ni+Mn) + 13.133(C+N)≥1.73

[0102] In mathematical formulas 1 and 2, Ni represents the weight% of nickel, Mn represents the weight% of manganese, C represents the weight% of carbon, and N represents the weight% of nitrogen.

[0104] <Experimental Example 3>

[0105] To confirm the shock absorption properties of the austenitic stainless steel according to the present invention, the shock absorption energy was measured, and based on the shock absorption energy and the phase map obtained in Experimental Example 2, the composition ratio of the austenitic stainless steel having phase stability at room temperature and a shock absorption energy above a certain level was confirmed, and the results are shown in Fig. 5 and Table 2.

[0106] The shock absorption energy was obtained through the following shock test.

[0107] Specimens were taken from the sound portion of the plate (where a shrinkage cavity exists in the top portion of the steel ingot), and specimens (10x10x55 mm) of standard specimen size were taken in the rolling direction. A notch (2 mm V-notch) was machined perpendicular to the rolling surface, and six impact specimens were machined from each plate.

[0108] The impact test was performed at -196℃. Specimens were immersed at -196℃ for 30 minutes and then tested in an impact tester according to ASTM A370-20. Two specimens were tested at each test temperature.

[0109]

[0111] Figure 5 is an image showing the phase map and impact absorption energy according to the composition ratio of austenitic stainless steel. Looking at Figure 5, it was confirmed that stability is improved when the content of nickel and manganese (Ni+Mn) is 8 wt% or more and the content of carbon and nitrogen (C+N) is 0.225 wt% or more.

[0112] In addition, Table 2 shows the results of measuring the impact absorption energy of specimens prepared according to the composition ratio of austenitic stainless steel.

[0113] Looking at Table 2, it was confirmed that the austenitic stainless steel has a certain level of microstructure stability at room temperature and impact absorption energy at low temperature (-196℃), and that when the content of nickel and manganese (Ni+Mn) is 8.5 wt% or more and the content of carbon and nitrogen (C+N) is 0.225 wt% or more, the austenitic microstructure stability and impact absorption energy of 41 J or more are satisfied.

[0115] <Experimental Example 4>

[0116] To confirm the machinability of the austenitic stainless steel according to the present invention, a machinability test was performed on specimens according to the composition ratio of the austenitic stainless steel and 316 stainless steel (STS), and the results are shown in Fig. 6.

[0117] The above machinability test was performed by evaluating the tool wear after 5 passes using an end mill under dry conditions, and the machining conditions are as shown in Table 3 below. In addition, the composition ratio of each specimen is shown in Table 4, and the unit is weight%.

[0118] item condition Cutting speed (m / min) 100 Feed rate (mm / min) 636 Z-axis depth of cut (mm) 3.0 Y-axis depth of cut (mm) 0.5 Processing form Downward processing Cutting tool (Dia. 10.0 end mill) Taegutec TSE 4100M TT5525 Workpiece 75 mm x 50 mm x 100 mm cutting fluid deflation

[0119] alloy# Cr Mn Ni C N C+N L1 18.45 8.8 4.43 0.057 0.156 0.213 L3 18.3 7.1 4.76 0.098 0.2 0.298 L5 20.2 4.88 5.16 0.107 0.23 0.337 L7 20.36 5.59 5.15 0.08 0.2019 0.282

[0120] Figure 6 shows the results of machinability tests on specimens prepared according to the composition ratio of austenitic stainless steel. Looking at Figure 6, it was confirmed that machinability is excellent when the total amount of carbon and nitrogen (C+N) is 0.33 wt% or less.

[0122] <Experimental Example 5>

[0123] To confirm the corrosion resistance of the austenitic stainless steel according to the present invention, anodic polarization tests, salt spray tests, and intergranular corrosion tests were performed on specimens according to the composition ratio of the austenitic stainless steel and on existing commercial materials such as 304 stainless steel (STS) and 316 stainless steel (STS), and the results are shown in FIGS. 7 to 9 and Table 5.

[0124] The above anodic polarization test was conducted in accordance with ASTM G5, and 3.5 wt% NaCl (seawater) was used as the solution, and the temperature was maintained at 25 ℃.

[0125] The above salt spray test was conducted in accordance with KS D 9502:2020, and the test was performed by KTR (accredited testing institution). A solution of (50±5) g / L NaCl was used, and the temperature was maintained at 35±2 ℃ for 500 hours. The specimen dimensions were W24 × L(40-50) × T(2-3) mm. 3 And, three were tested per alloy.

[0126] The above intergranular corrosion test evaluated the degree of intergranular sensitization in accordance with ASTM A262-15 (2021), and 10% oxalic acid (1 mA·cm² after immersion at 25 ℃) -2 After corroding the sample by applying 90 seconds, the presence of intergranular corrosion was determined by observing the surface.

[0127] Chemical composition (wt%) Official potential (Epit) Salt spray test alloy# Cr Mn Ni C N Mo C+N C / N C / (C+N) N / (C+N) Mn+Ni Average St Dev. Changes in appearance rate of change of mass 1 18.20 8.99 5.50 0.007 0.315 -　 0.322 0.022 0.022 0.978 14.490 0.1805 0.0311 - - - - 2 18.15 8.96 5.51 0.094 0.232 - 0.326 0.405 0.288 0.712 14.470 0.3563 0.0219 - - - - 3 19.02 5.78 6.11 0.074 0.203 - 0.277 0.365 0.267 0.733 11.890 0.3549 0.0182 No issues 0.03 0.03 0.03 4 18.79 5.71 6.02 0.080 0.196 - 0.276 0.408 0.290 0.710 11.730 0.336 0.0174 No issues 0.00 0.00 0.00 5 19.16 7.66 3.96 0.076 0.208 - 0.284 0.365 0.268 0.732 11.620 0.262 0.0169 No issues 0.01 0.01 0.01 6 19.58 5.78 6.04 0.075 0.197 - 0.272 0.381 0.276 0.724 11.820 0.4082 0.0321 No issues 0.00 0.00 0.00 SS304 18.43 0.99 7.30 0.042 -　 0.115 0.042 　 1.000 0.000 8.290 0.2814 0.0115 No issues 0.00 0.00 0.00 SS316 16.58 0.97 9.17 0.026 - 1.973 0.026 　 1.000 0.000 10.140 0.3476 0.0142 - - - -

[0128] Figure 7 shows the results of anodic polarization tests on specimens prepared according to the compositional ratio of austenitic stainless steel. Looking at Figure 7 and Table 5, the pitting potential of the economical austenitic stainless steel according to the present invention was found to be equivalent to or greater than that of 304 stainless steel, and a pitting potential almost similar to that of 316 stainless steel was confirmed. Figure 8 is an image showing the results of salt spray tests on specimens prepared according to the compositional ratio of austenitic stainless steel. Looking at Figure 8 and Table 5, it was confirmed that the specimens did not suffer damage or change in mass even after being exposed to salt water for 500 hours. In particular, as a result of evaluating and comparing the corrosion resistance of existing commercial materials, STS 304 and STS 316, it was confirmed that the austenitic stainless steel of the present invention secured corrosion resistance equivalent to or greater than that of STS 316. Figure 9 is an image showing the results of intergranular corrosion tests on specimens prepared according to the compositional ratio of austenitic stainless steel. Looking at Fig. 9, it was confirmed that intergranular corrosion did not occur in the economical austenitic stainless steel according to the present invention.

[0129] Through this, it can be seen that the austenitic stainless steel of the present invention has strong corrosion resistance.

[0131] Although the present invention has been described above with reference to preferred embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as set forth in the following claims.

Claims

Claim 1 Austenitic stainless steel comprising chromium (Cr) in a weight ratio of 16 to 20, manganese (Mn) in a weight ratio of 4.5 to 9, nickel (Ni) in a weight ratio of 3 to 6, carbon (C) in a weight ratio of 0.001 to 0.3, nitrogen (N) in a weight ratio of 0.1 to 0.35, copper (Cu) in a weight ratio greater than 0 and less than or equal to 2.0, the remainder being iron (Fe) and other unavoidable impurities. Claim 2 In claim 1, the austenitic stainless steel comprises copper in a weight ratio of greater than 0 and less than or equal to 1.

657. Claim 3 In claim 1, the austenitic stainless steel comprises carbon and nitrogen in a weight ratio of 0.15 to 0.

45. Claim 4 In claim 1, the austenitic stainless steel has a pitting potential of 300 to 420 mV in a 3.5% NaCl solution at 16 to 20°C. Claim 5 In claim 4, the austenitic stainless steel has a pitting potential of 350 to 400 mV in a 3.5% NaCl solution at 16 to 20°C. Claim 6 In claim 1, the austenitic stainless steel has an impact absorption energy of 80 to 120 J at -196 ℃. Claim 7 In paragraph 2, the austenitic stainless steel satisfies the following formula (1), [Formula 1]E pit = -0.0481×Cu 2 + 0.0777×Cu + 0.3654 In the above Formula 1, Cu represents the weight% of copper. Claim 8 In paragraph 3, the austenitic stainless steel is an austenitic stainless steel comprising nickel and manganese in a weight ratio of 8 to 15. Claim 9 In claim 8, the austenitic stainless steel comprises nickel and manganese in a weight ratio of 8.5 to 15 and carbon and nitrogen in a weight ratio of 0.225 to 0.

33. Claim 10 In claim 9, the austenitic stainless steel having a machinability greater than 0 and less than or equal to 0.15 mm when the wear of the tool is measured using an end mill under dry conditions. Claim 11 An austenitic stainless steel according to claim 1, wherein the ratio of the carbon content [C / (C+N)] to the total content of carbon and nitrogen is 0.1 to 0.

7. Claim 12 In claim 8, an austenitic stainless steel satisfying one or more conditions selected from the following formulas 2 to 5: [Formula 2] (Ni+Mn)+165.94(C+N) ≥ 49.47 [Formula 3] (Ni+Mn)-27.28(C+N) ≥ 2.10 [Formula 4] (Ni+Mn) ≥ 12.3 [Formula 5] (Ni+Mn)+13.133(C+N) ≥ 1.73 In the above formulas 2 to 5, Ni, Mn, C, and N represent the weight percent of nickel, manganese, carbon, and nitrogen, respectively. Claim 13 In Clause 12, an austenitic stainless steel satisfying the conditions of Formulas 2 and 5 above. Claim 14 In Clause 12, an austenitic stainless steel satisfying the conditions of Formulas 2 to 4 above. Claim 15 In paragraph 3, an austenitic stainless steel having a total carbon and nitrogen content of 0.33 weight% or less.

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