Austenitic stainless steel

Abstract

Austenitic stainless steel utilizing the TRIP effect has high corrosion resistance and a balanced pitting corrosion resistance equivalent. Austenitic stainless steel contains 0-0.04 wt% C, 0.2-0.8 wt% Si, 0-2.0 wt% Mn, 16.0-19.0 wt% Cr, 4.0-6.5 wt% Ni, 1.0-4.0 wt% Mo, 0-4.0 wt% W, 0-2.0 wt% Cu, and 0.20-0.30 wt% N, with the remainder being iron and unavoidable impurities occurring in stainless steel. When quenched and heat treated at a temperature range of 900-1200°C, preferably 950-1150°C, the proportion of ferrite phase in the microstructure is 0-10.0 volume % and the remainder is austenite.

Description

[Technical Field]

[0001] The present invention relates to an austenitic stainless steel having high formability due to the TRIP (Transformation Induced Plasticity) effect, as well as high corrosion resistance and optimized pitting resistance equivalent (PRE).

[0002] The transformation-induced plasticity (TRIP) effect refers to the transformation of metastable retained austenite to martensite during plastic deformation as a result of applied stress or strain. This property allows stainless steels with the TRIP effect to have high formability while retaining excellent strength.

[0003] A method for producing a ferritic-austenitic stainless steel with good formability and high elongation is known from WO 2015 / 114222, which steel contains, by weight, 0.04% carbon, 0.2-0.8% silicon, 2.0% manganese, 16.5-19.5% chromium, 3.0-4.7% nickel, 1.0-4.0% molybdenum, less than 3.5% tungsten, less than 1% copper, and 0.13-0.26% nitrogen, with the remainder being iron and unavoidable impurities occurring in stainless steel. The sulfur content is limited to less than 0.010% by weight, preferably less than 0.005% by weight, the phosphorus content is less than 0.040% by weight, the sum of sulfur and phosphorus (S+P) is less than 0.04% by weight, and the total oxygen content is less than 100 ppm. The aluminum content is maximized to less than 0.04 wt.%, preferably less than 0.03 wt.%. Furthermore, small amounts of boron, calcium, and cerium may be optionally added, with the preferred boron and calcium contents being less than 0.004 wt.%, and the preferred cerium content being less than 0.1 wt.%. Optionally, up to 1 wt.% of cobalt may be added to partially replace nickel. The stainless steel of the present invention may also optionally contain one or more of the group consisting of niobium, titanium, and vanadium, with the niobium and titanium contents limited to a maximum of 0.1 wt.%, and the vanadium content limited to a maximum of 0.2 wt.%.

[0004] According to WO 2015 / 114222, the pitting resistance equivalent (PRE) is optimized to give good corrosion resistance in the range of 30-36. The critical pitting temperature (CPT) is in the range of 34-45°C. The heat treatment is carried out so that the microstructure of the stainless steel contains 45-80% austenite in the heat treated condition, with the remaining microstructure being ferrite. Furthermore, the measured M of the stainless steel d30 The temperature is adjusted to a range of -30 to 90°C, preferably 10 to 60°C, in order to utilize the TRIP effect to improve the formability of the stainless steel.

[0005] From WO 2011 / 135170, a method is known for producing a ferritic-austenitic stainless steel with good formability and high elongation, the steel containing, by weight, less than 0.05% C, 0.2-0.7% Si, 2-5% Mn, 19-20.5% Cr, 0.8-1.35% Ni, less than 0.6% Mo, less than 1% Cu, 0.16-0.24% N, and the balance being iron and unavoidable impurities. The stainless steel of WO 2011 / 135170 is heat treated so that the microstructure of the stainless steel contains 45-75% austenite in the heat treated condition, with the remaining microstructure being ferritic. Furthermore, in order to utilize the TRIP effect to improve the formability of the stainless steel, the measured M of the stainless steel is used. d30 The temperature is adjusted to 0 to 50°C.

[0006] Furthermore, WO 2013 / 034804 discloses a ferritic-austenitic stainless steel utilizing the TRIP effect, containing less than 0.04 wt.% C, less than 0.7 wt.% Si, less than 2.5 wt.% Mn, 18.5-22.5 wt.% Cr, 0.8-4.5 wt.% Ni, 0.6-1.4 wt.% Mo, less than 1 wt.% Cu, and 0.10-0.24 wt.% N, with the remainder being iron and unavoidable impurities occurring in stainless steel. The sulfur content is limited to less than 0.010 wt.%, preferably less than 0.005 wt.%, the phosphorus content is less than 0.040 wt.%, the sum of sulfur and phosphorus (S+P) is less than 0.04 wt.%, and the total oxygen content is less than 100 ppm. The stainless steel may optionally contain one or more of the following additional elements: the aluminum content is less than a maximum of 0.04 wt.%, preferably less than a maximum of 0.03 wt.%. Additionally, small amounts of boron, calcium, and cerium may be optionally added, with the preferred boron and calcium contents being less than 0.003 wt. % and the preferred cerium content being less than 0.1 wt. Optionally, up to 1 wt. % of cobalt may be added to partially replace nickel, and up to 0.5 wt. % of tungsten may be added to partially replace molybdenum. The stainless steel of the present invention may also optionally contain one or more of the group consisting of niobium, titanium, and vanadium, with the niobium and titanium contents limited to a maximum of 0.1 wt. % and the vanadium content limited to a maximum of 0.2 wt. %.

[0007] According to WO 2013 / 034804, the Pitting Resistance Equivalent (PRE) is optimized to give good corrosion resistance in the range of 27-29.5. The Critical Pitting Temperature (CPT) is in the range of 20-33°C, preferably 23-31°C. The TRIP (Transformation Induced Plasticity) effect in the austenite phase is measured in the range of 0-90°C, preferably in the range of about 10-70°C, to ensure good formability. d30The austenite phase in the microstructure of the stainless steel of the present invention is maintained at 45-75% by volume, preferably 55-65% by volume, in the heat-treated condition to create favorable conditions for the TRIP effect, with the remainder being ferrite. Heat treatment can be carried out at temperatures in the range of 900-1200°C, preferably 950-1150°C, using various heat treatment methods such as solution annealing, high-frequency induction annealing, or local annealing.

[0008] Japanese Patent Application No. 2014001422(A) has a composition in mass percent of C: 0.02-0.30%, Cr: 10.0-25.0%, Ni: 3.5-10.0%, Si: 0.1-3.0%, Mn: 0.5%-5.0%, N: 0.10-0.40%, Mo: 0-3.0%, Cu: 0-3.0%, Ti: 0-0.10%, Nb: 0-0.50%, V: 0-1.0%, and M calculated based on a formula very similar to Nohara's. d30 The present invention shows austenitic stainless steels with a PRE of 0-40. Although the chemical range is very wide, the examples given in this invention show mainly low levels of Mo and N which give low PRE and corrosion resistance. The present invention shows that austenitic alloys with balanced Ni and N content can have low stability, but this is only accompanied by low PRE and corrosion resistance due to low Mo, and in reality, the actual M d30 is likely to be much higher than the claimed range.

[0009] The present invention describes a new austenitic stainless steel that utilizes the TRIP effect described in the prior art and adds a high Pitting Resistance Equivalent (PRE), thus providing excellent corrosion resistance combined with the TRIP effect through improved formability. Compared to other commercially available austenitic stainless steels, such as TRIP 301 and the slightly more stable 304, the new invention has much better PRE and corrosion resistance. Compared to 316, the TRIP effect and PRE are substantially higher. Compared to 904L, the PRE and corrosion resistance of the present invention are at most similar levels, but with a much better TRIP effect not observed in any other high-alloy austenitic stainless steel. In addition to all the reference alloys mentioned, the present invention has a much leaner composition, especially considering the Ni content. This gives the present invention a unique combination of high corrosion, TRIP, and cost-effective alternatives. Important features of the present invention are set forth in the accompanying claims.

[0010] According to the present invention, the austenitic stainless steel contains less than 0.04 wt% C, 0.2-0.8 wt% Si, 0-2.0 wt% Mn, 16.0-19.0 wt% Cr, 4.0-6.5 wt% Ni, 1.0-4.0 wt% Mo, 0-4.0 wt% W, 0-2.0 wt% Cu, and 0.20-0.30 wt% N, with the remainder being iron and unavoidable impurities present in stainless steel. The sulfur content is limited to less than 0.010 wt%, preferably less than 0.005 wt%, the phosphorus content is less than 0.040 wt%, the sum of sulfur and phosphorus (S+P) is less than 0.04 wt%, and the total oxygen content is less than 100 ppm.

[0011] The austenitic stainless steel of the present invention may optionally contain one or more of the following additional elements: The maximum aluminum content is less than 0.04 wt.%, preferably less than 0.03 wt.%. Furthermore, small amounts of boron, calcium, and cerium may optionally be added, with the preferred contents of boron and calcium being less than 0.004 wt.%, and the preferred content of cerium being less than 0.1 wt.%. Optionally, up to 1 wt.% of cobalt may be added to partially replace nickel. The austenitic stainless steel of the present invention may also optionally contain one or more of the following elements: niobium, titanium, and vanadium, with the niobium and titanium contents limited to a maximum of 0.1 wt.%, and the vanadium content limited to a maximum of 0.2 wt.%.

[0012] According to the stainless steel of the present invention, the Pitting Resistance Equivalent (PRE) is optimized to provide good corrosion resistance and is in the range of 27 to 35. The Critical Pitting Temperature (CPT) is in the range of 30 to 50°C, and the TRIP (Transformation Induced Plasticity) effect in the austenite phase is measured by the calculated M in the range of -70°C to +60°C. d30 The M, a measure of austenite stability against the TRIP effect, is maintained according to the temperature. d30 The temperature is defined as the temperature at which a true strain of 0.3 results in a 50% transformation from austenite to martensite. Furthermore, the stacking fault energy of the stainless steel of the present invention is much lower than that of commercially available stainless steels, ranging from 10 to 16 mJ / m according to equation (3). 2 The proportion of ferrite phase in the microstructure of the austenitic stainless steel of the present invention is less than 10% by volume in the heat-treated condition. The heat treatment can be carried out using different heat treatment methods such as solution annealing, high-frequency induction annealing, local annealing, or any other type of heat treatment in the temperature range of 900-1200°C, preferably 950-1150°C.

[0013] The effect of different elements in the microstructure is described below, with element contents given in wt %.

[0014] Carbon (C) partitions into the austenite phase and has a strong effect on austenite stability. Carbon can be added up to 0.04%, but higher levels have a detrimental effect on corrosion resistance.

[0015] Nitrogen (N) is an important austenite stabilizer in stainless steels, and like carbon, it increases the stability of martensite. Nitrogen also increases strength, strain hardening, and corrosion resistance. d30 The general empirical expression for temperature is that nitrogen and carbon have the same strong effect on austenite stability, but this is different from other M d30 This indicates that the nitrogen content is lower than that previously reported. Nitrogen can be added to stainless steels in greater amounts than carbon without adversely affecting corrosion resistance, so a nitrogen content of 0.20 to 0.30% is effective for this stainless steel.

[0016] Silicon (Si) is usually added to stainless steels for deoxidation purposes in the melt shop and should not be less than 0.2%. Silicon has a stronger stabilizing effect on austenite stability against martensite formation than current expressions suggest. For this reason, silicon is limited to a maximum of 0.8%, preferably 0.5%.

[0017] Manganese (Mn) is an important additive for stabilizing the austenite phase and increasing the solubility of nitrogen in stainless steel. Manganese can partially replace expensive nickel and ensure the correct phase balance in stainless steel. If the content is too high, corrosion resistance will decrease. Manganese has a stronger effect on austenite stability than deformed martensite, so the manganese content must be carefully considered. The manganese content range should be 0-2.0%, preferably 0-1.5%.

[0018] Chromium (Cr) is the primary additive for making steel corrosion resistant. Furthermore, Cr strongly increases resistance to martensite formation, thus reducing the TRIP effect. Because Cr is also a strong ferrite stabilizer, the Cr level must be limited in austenitic stainless steels. To provide these functions, the Cr level should be at least 16.0%, with a maximum level of 19.0%. Preferably, the Cr content is between 16.5 and 18.7%.

[0019] Nickel (Ni) is an essential alloying element for stabilizing the austenite phase and providing good ductility, and at least 4.5% should be added to the stainless steel of the present invention. Nickel, which has a significant effect on austenite stability relative to martensite formation, must be present within a narrow range. Ni also has a significant effect on increasing the stacking fault energy of stainless steel. Furthermore, due to the high cost and price fluctuations of nickel, nickel should be maximized in the stainless steel of the present invention to 6.5%, preferably 6.2%.

[0020] Copper (Cu) is typically present as a 0.1-0.5% residual in most stainless steels when the raw material is in the form of stainless steel scrap, which contains a significant proportion of this element. Although copper is a weak stabilizer of the austenite phase, it has a strong effect on the resistance to martensite formation and must be considered in assessing the formability of this stainless steel. Intentional additions of up to 2.0% are permitted, but the preferred copper content is a maximum of 1.6%.

[0021] Molybdenum (Mo) is added to significantly increase PRE and corrosion resistance, and therefore, the molybdenum content should be at least 1.0%, preferably at least 1.5%. Furthermore, it has been found that, like chromium, it strongly increases resistance to martensite formation and reduces the TRIP effect more significantly than previously expected. Therefore, the molybdenum content cannot exceed 4.0%.

[0022] Tungsten (W) has similar properties to molybdenum and can sometimes replace it. However, tungsten and molybdenum promote sigma phase precipitation, and the sum of the molybdenum and tungsten content according to the formula (Mo + 0.5W) should be 0-4.0%, preferably 2.0-4.0%, where promotion of sigma and chi phases can be managed in technologically relevant processes. The most significant effect of tungsten is its surprisingly positive influence on the TRIP effect, which can then be related to its effect on the stacking fault energy of the alloy, since stacking fault energy controls the deformation response in terms of dislocation slip, twinning, or martensite formation. For this purpose, tungsten should be limited to a maximum of 3.8%, but if tungsten is used in place of molybdenum, it is preferably at least 1.0%.

[0023] Boron (B), calcium (Ca) and cerium (Ce) are added in small amounts to austenitic steels to improve hot workability, but not in too high a content, as this may degrade other properties. The preferred contents of boron and calcium in the stainless steel of the present invention are less than 0.004%, and the preferred content of cerium is less than 0.1%.

[0024] Sulfur (S) in austenitic steels can form sulfide inclusions that degrade hot workability and adversely affect corrosion resistance, so the sulfur content should be limited to less than 0.010%, preferably less than 0.005%.

[0025] Phosphorus (P) can form phosphide particles or films that degrade hot workability and adversely affect corrosion resistance. Therefore, the phosphorus content should be limited to less than 0.040%, and the combined sulfur and phosphorus (S+P) content is therefore less than 0.04%.

[0026] Oxygen (O), along with other residual elements, adversely affects hot ductility. The presence of oxide inclusions can reduce corrosion resistance (pitting corrosion) depending on the type of inclusion. High oxygen content also reduces impact toughness. Like sulfur, oxygen improves penetration by altering the surface energy of the weld pool. In the stainless steels of this invention, the recommended maximum oxygen level is less than 100 ppm. For metal powders, the maximum oxygen content can be up to 250 ppm.

[0027] Aluminum (Al) should be kept at low levels in the austenitic stainless steels of the present invention with high nitrogen content because these two elements can combine to form aluminum nitride, which degrades impact toughness. The aluminum content is limited to less than 0.04%, preferably less than 0.03%.

[0028] Cobalt (Co) has similar metallurgical behavior to its sister element, nickel, and cobalt can be processed in much the same way in steel and alloy production. Cobalt inhibits grain growth at high temperatures and significantly improves hardness and hot strength retention. Cobalt improves cavitation erosion resistance and strain hardening. Cobalt reduces the risk of sigma phase formation in stainless steels. The cobalt content is limited to a maximum of 1.0%.

[0029] The "microalloying" elements titanium (Ti), vanadium (V), and niobium (Nb) belong to a group of additives so named because they significantly alter the properties of steel at low concentrations, often providing beneficial effects in carbon steels, but also contributing to undesirable property changes in austenitic stainless steels, such as reduced impact properties during casting and hot rolling, increased surface defect levels, and reduced ductility. Many of these effects depend on their strong affinity for carbon, and especially nitrogen, in modern austenitic stainless steels. In the present invention, niobium and titanium should be limited to a maximum level of 0.1%, while vanadium is less detrimental and should be less than 0.2%. [Brief explanation of the drawings]

[0030] [Figure 1] FIG. 1 shows the dependence of the minimum and maximum Md30 temperature and PRE value between the element contents Si+Cr and Cu+Mo+0.5W in the tested alloys of the present invention. [Figure 2] FIG. 2 shows an example of the dependence of the minimum and maximum Md30 temperature and PRE value between the element contents Si+Cr and Cu+Mo+0.5W in the tested alloys of the invention according to FIG. 1 with constant values ​​of C+N and Mn+Ni. [Figure 3] FIG. 3 shows the dependence of the minimum and maximum Md30 temperature, PRE and SFE values ​​between the element contents C+N and Mn+Ni in the tested alloys of the present invention. [Figure 4] FIG. 4 shows an example of the dependence of the minimum and maximum Md30 temperature, PRE and SFE values ​​between the element contents C+N and Mn+Ni in the tested alloys of the invention according to FIG. 3, with constant values ​​of Si+Cr and Cu+Mo+0.5W. [Figure 5] FIG. 5 shows the microstructure of several alloys after annealing at 1100°C followed by water quenching, and illustrates how the observed levels of martensite relate to the calculated Md30 values ​​of the present invention. [Figure 6] Figure 6 shows the microstructure of the alloy after annealing at 1100°C followed by water quenching, which is from the reference alloy UNS S30403 for comparison purposes. [Figure 7] FIG. 7 shows the microstructures of several alloys after annealing at 1100°C followed by water quenching, and illustrates how the observed levels of martensite relate to the calculated Md30 values ​​of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] The invention will now be explained in more detail with reference to the drawings.

[0032] FIG. 1 shows the minimum and maximum M between the element contents Si+Cr and Cu+Mo+0.5W in the tested alloys of the present invention. d30 The dependence of temperature and PRE value is shown.

[0033] FIG. 2 shows the minimum and maximum M between the element contents Si+Cr and Cu+Mo+0.5W in the tested alloys of the present invention according to FIG. d30 The temperature and PRE value dependence is shown for an example with constant values ​​of C+N and Mn+Ni.

[0034] FIG. 3 shows the minimum and maximum M between the element contents C+N and Mn+Ni in the tested alloys of the present invention. d30 4 shows the dependence of the temperature, PRE and SFE values. d30 The dependence of temperature, PRE and SFE values ​​is shown with constant values ​​of Si+Cr and Cu+Mo+0.5W.

[0035] In all of Figures 1 to 4, given M d30 The limit values ​​are the preferred limit values ​​of the present invention calculated according to the mathematical constraints of the optimization used in the present invention. d30 The values ​​are shown in Table 2 for all alloys. In addition, the limit Nohara M d30 Values ​​are also given in the figures for reference and comparison.

[0036] Figures 5-7 show the microstructures of several alloys after annealing at 1100°C followed by water quenching. Figures 5 and 7 show the microstructures of two alloys of the present invention, and the observed martensite levels are consistent with the calculated M values ​​of the present invention. d30 The graph shows how the ferrite content correlates to the tensile strength of the alloys of the present invention. The low ferrite content is also shown, indicating that the alloys of the present invention can be considered fully austenitic. Figure 6 is from the reference alloy UNS S30403 for comparison purposes.

[0037] Based on the effects of elements, the austenitic stainless steel according to the present invention is represented by chemical compositions A to S shown in Table 1. Table 1 also includes typical chemical compositions of reference commercially available austenitic stainless steels named T to X, and all contents in Table 1 are in wt%.

[0038] [Table 1]

[0039] Alloys A-S were produced in small slabs in a 1 kg laboratory-scale induction furnace. The alloys T and X mentioned were produced on a 100 tonne production scale and subsequently hot and cold rolled into coil form with various final dimensions.

[0040] Comparing the values ​​in Table 1, the contents of nickel, nitrogen and tungsten in the austenitic stainless steel of the present invention are significantly different from those of the reference stainless steels T to X and R.

[0041] Characteristics, M d30 The values ​​of temperature, critical pitting temperature (CPT), pitting resistance equivalent PRE and stacking fault energy (SFE) were determined for the chemical compositions in Table 1 and the results are shown in Table 2 below.

[0042] Table 2 Steel Prediction M d30 Temperature (M d30 Nohara) has established the Nohara equation (1) for austenitic stainless steels when annealed at a temperature of 1050°C. M d30 =551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo-68Nb(1).

[0043] when annealed at the temperature of 1050℃.

[0044] Calculated M in Table 2d30 The temperature was achieved according to the mathematical constraints of the optimization.

[0045] The critical pitting temperature (CPT) is measured in 1 M sodium chloride (NaCl) solution according to the ASTM G150 test, below which pitting is not possible and only passive behavior is observed.

[0046] The pitting resistance equivalent (PRE) is calculated using equation (2). PRE = %Cr + 3.3(%Mo + 0.5%W) + 16N(2)

[0047] The sum of the elemental contents (in wt.%) of C+N, Cr+Si, Cu+Mo+0.5W, and Mn+Ni are also calculated in Table 2 for the alloys of Table 1. The sum of C+N and Mn+Ni represents austenite stabilizers, the sum of Si+Cr represents ferrite stabilizers, and the sum of Cu+Mo+0.5W elements provides resistance to martensite formation.

[0048] [Table 2]

[0049] Comparing the values ​​in Table 2, the PRE values ​​of alloys A to S, which range from 27 to 35, are higher than those of the referenced stainless steels T to V, which means that alloys A to S have higher corrosion resistance. The critical pitting temperature CPT is in the range of 31 to 48°C, which is a high level of corrosion resistance and is much higher than the CPT of the referenced austenitic stainless steels T to V.

[0050] M predicted using Nohara equation (1) d30 The temperature is the known M for the reference alloy T~V. d30 While this equation agrees well with the formula, it is not believed to be accurate for alloys A to S of the present invention. This is because alloys A to S of the present invention have unique chemical compositions compared to the austenitic steels from which the Nohara formula was originally derived. In particular, the austenite stability and M d30The effect of N on the PRE is still high, but has been found to be much lower in these steels of the present invention. Nohara's results were originally obtained for austenitic steels with low N, and therefore the overall effect of nitrogen in these steels is small. In the present invention, the N content is much higher than in the alloys in order to increase the austenite content and have a highly corrosion-resistant stainless steel, i.e., increase the PRE, assuming low Ni. According to Nohara, this makes the steels of the present invention very stable. Most of the steels A to S are in accordance with Nohara's M d30 When the formula is used, the microstructure of steels A to S in the present invention is evaluated based on the measured M of alloys having similar composition levels in stainless steels such as those in WO 2015 / 114222. d30 M of alloys A to S, along with previous prior art knowledge of values d30 Thus, new M values ​​were obtained for the alloys of the present invention. d30 The formula was derived. As can be seen in Table 2, the calculated M d30 The values ​​allow for very good agreement with those predicted using Nohara, which also agrees with known actual measurements for these commercially available grades. Thus, the new formula allows for a very accurate calculation of M even for low N-containing stainless steel grades. d30 In Figure 5, a microstructural image of Alloy E is presented. The microstructure of this alloy shows a significant amount of martensite after annealing at 1100°C followed by water quenching, which is in line with the calculated M for the present invention. d30 +56 M given by d30 It matches with Nohara M of 2. d30 The M of 2 predicted by Nohara for alloy E does not agree with d30It would be expected by one skilled in the art that an alloy having the above annealing and quenching conditions would be essentially free of martensite. For example, Figure 6 shows the as-quenched microstructure of Reference Alloy T (UNS S30403), which has been remelted to give the equivalent cast microstructure and Alloy E. This microstructure exhibits no, or at least very little, martensite, which is consistent with the calculated M of Nohara and the present invention. d30 M between 0 and 10 predicted by both d30 These two examples are expected for this commercial grade with a new M d30 The calculation is more accurate than the Nohara formula for the true stability and M of all alloys. d30 This gives a better interpretation of the calculated M d30 Show that it is the basis of limits.

[0051] Figure 7 shows the calculated M d30 The microstructure of alloy P shows a microstructure of alloy P with no evidence of martensite in the as-quenched state. Additionally, observation of the microstructures in Figures 5 and 7 shows a high austenite content with a residual ferrite content of less than 10%. Fully austenitic stainless steels typically have a ferrite content of up to 10%, as shown by the microstructure in Figure 6 for alloy T. These examples demonstrate the higher and lower calculated M d30 This shows that the alloys with austenitic temperatures (-70°C to +60°C) have austenitic microstructures comparable to those of the reference alloy T.

[0052] For austenitic stainless steels, the stacking fault energy is calculated using Equation (3) of G. Meric de Bellefon, J.C. van Duysen, and K. Sridharan, "Composition-dependence of stacking fault energy in austenitic stainless steels through linear regression with random intercepts," J. Nucl. Mater., vol. 492. SFE (mJ / m 2 ) = 2.2 + 1.9Ni - 2.9Si + 0.77Mo + 0.5Mn + 40C - 0.016Cr - 3.6N (3)

[0053] The stacking fault energy (SFE) of the austenitic stainless steels of the present invention is lower than that of commercially available austenitic stainless steels. The PRE is low (<27), and thus M d30 is high and greater than 0. The SFE of the relatively low-alloyed reference alloys T and X is greater than 16 mJ / m 2 . In addition, for Grade 904L with low M d30 (< -70), the SFE for higher alloying and high PRE (>35) is also greater than 16 mJ / m 2 . All of the alloys A - S of the present invention have an SFE of less than 16 mJ / m 2 but are unique in that they have a PRE of 27 - 35 and an M d30 of -70 to +60.

[0054] The total elemental contents of C + N, Si + Cr, Mn + Ni, and Cu + Mo + 0.5W of the austenitic stainless steels of the present invention were used in the mathematical constraints of optimization to establish the dependencies between C + N and Mn + Ni on the one hand and Si + Cr and Cu + Mo + 0.5W on the other hand. In accordance with this mathematical constraint of optimization, the sums of Cu + Mo + 0.5W and Si + Cr, and of Mn + Ni and C + N respectively, are within the minimum and maximum PRE values (27 < PRE < 35) and the minimum and maximum M d30 temperature values (-30 < M d30Form the x-axis and y-axis of the coordinates of FIGS. 1 to 4 in which the linear dependence of <60) is defined.

[0055] According to FIG. 1, the chemical composition windows of Si+Cr and Cu+Mo+0.5W are established in the preferred ranges of 0.21 to 0.33 for C+N and 4.0 to 7.7 for Mn+Ni. It is also shown in FIG. 1 that according to the stainless steel of the present invention, the total of Si+Cr is limited to 16.2 < Si+Cr < 19.8, and the total of Cu+Mo+0.5W is limited to 1.0 < Cu+Mo+0.5W < 6.0. The chemical composition windows within the frames of regions a', b', c', d', e', f' and g' in FIG. 1 are defined at the following marked positions of the coordinates in Table 3.

[0056] [Table 3]

[0057] FIG. 1 shows that when the composition ranges of the present invention for Si+C and Cu+Mo+0.5W are such that the composition limits for C+N and Ni+Mn are within the preferred limits of the present invention, 27 <pre>35 and the calculated M d30 >-30 (or Nohara M d30 <-70) is further restricted by the constraint.

[0058] Figure 2 shows one chemical composition example window of Figure 1 when constant values of 0.295 for C+N and 6.0 for Mn+Ni are used at all points instead of the preferred ranges of C+N and Mn+Ni in Figure 1. For the sum of Si+Cr and Cu+Mo+0.5W in Figure 2, the same invention constraints as in Figure 1 are given. The chemical composition windows within the frames of regions a, b, c, d, e, f, and g in Figure 2 are defined at the following labeled positions of the coordinates in Table 4.

[0059] [Table 4]

[0060] Figure 2 shows that the composition ranges of the present invention for Si+C and Cu+Mo+0.5W are further restricted when specific constant levels of C+N and Mn+Ni are given. In addition to the constraints of Figure 1, the composition window is also restricted by the line of the calculated M d30 <60 (or Nohara <10).

[0061] Figure 3 shows the chemical composition windows for C+N and Mn+Ni when the austenitic stainless steel is annealed at a temperature of 1050°C. The preferred composition ranges are 16.5 - 19.5 for Cr+Si and 2.2 - 5.7 for Cu+Mo+0.5W. Further, according to the present invention, the total C+N is restricted to 0.20 < C+N < 0.34, and the total Mn+Ni is restricted to 4.0 < Mn+Ni < 8.5. In Figure 3, possible constraints on SFE are also added. The chemical composition windows within the frames of regions p’, q’, r’, and s’ in Figure 3 are defined at the following labeled positions of the coordinates in Table 5.

[0062] [Table 5]

[0063] The effects of the restrictions on C+N and Mn+Ni, which have the preferred ranges of the element contents of the present invention, are such that the chemical composition window in FIG. 3 is limited only by the restrictions on the minimum and maximum total of C+N and Mn+Ni when the total of Si+Cr and Cu+Mo+0.5W is any value within the preferred range of the present invention. This is because none of the limited constraints of M d30 , PRE or SFE are within the compositional limits of the total of Si+Cr and Cu+Mo+0.5W.

[0064] FIG. 4 shows one chemical composition example window of FIG. 3, having constant values of 17.6 for Cr+Si and 3.5 for Cu+Mo+0.5W, and further having the restrictions of 0.20 < C+N < 0.34 and 4.0 < Mn+Ni. The chemical composition windows within the frames of regions p, q, r and s in FIG. 4 are defined at the following labeled positions of the coordinates in Table 6.

[0065]

Table 6

[0066] FIG. 4 shows that when the compositional limits for Si+Cr and Cu+Mo+0.5W have constant values as given in Table 6, the compositional ranges of the present invention for the total of C+N and Mn+Ni are now limited by the constraints of M d30 > -30 (or Nohara M d30 < -70) and SFE > 10.

[0067] The ferritic-austenitic stainless steel of the present invention can be manufactured as ingots, slabs, blooms, billets and flat products such as plates, sheets, strips, coils, and also as long products such as bars, rods, wires, profiles and shapes, seamless and welded pipes and / or tubes. Further, additional products such as metal powders, formed shapes and profiles can be manufactured.< / pre>

Claims

1. % of C, 0.2-0.8 wt. % of Si, 0-2.0 wt. % of Mn, 16.0-19.0 wt. % of Cr, 4.0-6.5 wt. % of Ni, 1.0-4.0 wt. % of Mo, 0-4.0 wt. % of W, 0-2.0 wt. % of Cu, 0.20-0.30 wt. % of N, the remainder being iron and unavoidable impurities that occur in stainless steel; the proportion of ferrite phase in the microstructure being 0-10.0 vol. % and the remainder being austenite.

2. 2. The austenitic stainless steel according to claim 1, characterized in that it has a pitting resistance equivalent (PRE) in the range of 27 to 35.

3. Calculated SFE is 10.0 to 16.0 mJ / m ２ 3. The austenitic stainless steel according to claim 1, wherein the austenitic stainless steel has a hardness in the range of 0.1 to 1.

0.

4. 4. The austenitic stainless steel according to claim 1, wherein the critical pitting temperature CPT is 30 to 50°C.

5. 5. An austenitic stainless steel according to claim 1, characterized in that the chromium content is between 16.5 and 18.7% by weight.

6. 6. An austenitic stainless steel according to claim 1, characterized in that the nickel content is between 4.5 and 6.2% by weight.

7. 7. An austenitic stainless steel according to claim 1, characterized in that the manganese content is 0 to 1.5% by weight.

8. 8. An austenitic stainless steel according to claim 1, characterized in that the copper content is 0 to less than 1.5% by weight.

9. 9. An austenitic stainless steel according to claim 1, characterized in that the tungsten content is 1.0 to 3.8% by weight.

10. 10. The austenitic stainless steel according to claim 1, wherein the sum of the molybdenum (Mo) and tungsten (W) contents according to the formula (Mo+0.5W) is in the range of 0 to 4.0% by weight.

11. 11. An austenitic stainless steel according to claim 1, characterized in that the nitrogen content is 0.21 to 0.29% by weight.

12. 12. The austenitic stainless steel according to claim 1, further containing one or more additional elements selected from the group consisting of 0.0001 to 0.04 wt% Al, 0.0001 to 0.004 wt% B, 0.0001 to 0.004 wt% Ca, 0.0001 to 0.1 wt% Ce, 0.0001 to 0.1 wt% Co, 0.0001 to 0.1 wt% Nb, 0.0001 to 0.1 wt% Ti, and 0.0001 to 0.2 wt% V.

13. 13. The austenitic stainless steel according to claim 1, wherein the stainless steel contains, as inevitable impurities, 0.0001 to 0.010 wt. % S and 0.0001 to 0.040 wt. % P so that the sum of (S+P) is 0.0001 to 0.04 wt. % and the total oxygen content is in the range of 0 to 100 ppm.

14. 2. The austenitic stainless steel of claim 1, wherein the chemical composition windows framed within regions a, b, c, d, e, f, and g in FIG. 1 are defined, in weight percent, at the following labeled positions on the coordinate system: 【Table 1】

15. 2. The austenitic stainless steel of claim 1, wherein the chemical composition windows framed within regions p, q, r, and s in FIG. 3 are defined, in weight percent, at the following labeled positions on the coordinate system: 【Table 2】

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