JP2023540461A - austenitic stainless steel - Google Patents

Abstract

Austenitic stainless steels utilizing the TRIP effect have a balanced pitting resistance equivalent to high corrosion resistance. Austenitic stainless steel contains 0 to 0.04 wt% C, 0.2 to 0.8 wt% Si, 0 to 2.0 wt% Mn, 16.0 to 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, 0.20-0. Contains 30% by weight of N, the rest being unavoidable impurities that occur in iron and stainless steel, and when quenched and heat treated in the temperature range of 900-1200°C, preferably 950-1150°C, ferrite in the microstructure The phase proportion is 0 to 10.0% by volume, the remainder being austenite. [Selection diagram] None

Description

The present invention relates to an austenitic stainless steel with high formability due to TRIP (Transformation Induced Plasticity) effect and high corrosion resistance and optimized pitting resistance equivalent (PRE).

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 TRIP effect to have high formability while retaining good strength.

A method for producing a ferritic-austenitic stainless steel with good formability and high elongation is known from WO 2015/114222, which contains, in weight percent, 0.04 weight percent carbon, 0 .2-0.8 wt% silicon, 2.0 wt% manganese, 16.5-19.5 wt% chromium, 3.0-4.7 wt% nickel, 1.0-4.0 It contains by weight % molybdenum, less than 3.5% tungsten, less than 1% copper, 0.13-0.26% nitrogen, the remainder being unavoidable impurities that occur in iron and stainless steel. Sulfur is limited to less than 0.010% by weight, preferably less than 0.005% by weight, phosphorus content is less than 0.040% by weight, and 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% by weight, preferably the maximum is less than 0.03% by weight. Furthermore, boron, calcium and cerium may optionally be added in small amounts, with the preferred content of boron and calcium being less than 0.004% by weight, and the preferred content of cerium being less than 0.1% by weight. Optionally, up to 1% by weight of cobalt may be added for partial replacement of nickel. Furthermore, the stainless steel of the present invention can optionally be added with one or more of the group containing niobium, titanium, and vanadium, with the content of niobium and titanium being limited to a maximum of 0.1% by weight, and the content of vanadium The content is limited to a maximum of 0.2% by weight.

According to WO 2015/114222, the pitting resistance equivalent (PRE) is optimized to give good corrosion resistance in the range of 30-36. Critical pitting temperature (CPT) ranges from 34 to 45°C. The heat treatment is performed such that the microstructure of the stainless steel contains 45-80% austenite in the heat treated conditions, with the remainder of the microstructure being ferrite. Furthermore, the measured M _d30 temperature of the stainless steel is adjusted in the range of -30 to 90°C, preferably 10 to 60°C, in order to take advantage of the TRIP effect to improve the formability of the stainless steel.

From WO 2011/135170, a method is known for producing a ferritic-austenitic stainless steel with good formability and high elongation, which contains less than 0.05% C, in weight percent. 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% Contains Cu, 0.16 to 0.24% N, and the remainder is iron and unavoidable impurities. The stainless steel of WO 2011/135170 is heat treated such that the microstructure of the stainless steel contains 45-75% austenite under heat treated conditions, with the remaining microstructure being ferrite. Additionally, the measured M _d30 temperature of stainless steel is adjusted from 0 to 50° C. to take advantage of the TRIP effect to improve the formability of stainless steel.

Further, from WO 2013/034804, less than 0.04 wt% C, less than 0.7 wt% Si, less than 2.5 wt% Mn, 18.5 to 22.5 wt% Cr, Contains 0.8-4.5% by weight of Ni, 0.6-1.4% by weight of Mo, less than 1% by weight of Cu, 0.10-0.24% by weight of N, and the remainder is iron and Ferritic-austenitic stainless steels are known that utilize the TRIP effect, which is an unavoidable impurity that occurs in stainless steels. Sulfur is limited to less than 0.010% by weight, preferably less than 0.005% by weight, phosphorus content is less than 0.040% by weight, and 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. Stainless steel optionally contains one or more of the following additional elements. The aluminum content is at most less than 0.04% by weight, preferably less than at most 0.03% by weight. Furthermore, boron, calcium and cerium may optionally be added in small amounts, with the preferred content of boron and calcium being less than 0.003% by weight, and the preferred content of cerium being less than 0.1% by weight. Optionally, up to 1% by weight of cobalt may be added for partial replacement of nickel and up to 0.5% by weight of tungsten for partial replacement of molybdenum. Furthermore, the stainless steel of the present invention can optionally be added with one or more of the group containing niobium, titanium, and vanadium, with the content of niobium and titanium being limited to a maximum of 0.1% by weight, and the content of vanadium The content is limited to a maximum of 0.2% by weight.

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) ranges from 20 to 33°C, preferably from 23 to 31°C. The TRIP (Transformation Induced Plasticity) effect in the austenitic phase is important for the measured M _d30 in the range of 0 to 90 °C, preferably in the range of 10 to 70 °C, to ensure good formability. Maintained according to temperature. The proportion of austenitic phase in the microstructure of the stainless steel of the invention is between 45 and 75% by volume, advantageously between 55 and 65% by volume, in heat-treated conditions, in order to create favorable conditions for the TRIP effect. , the rest is ferrite. The heat treatment can be carried out at a temperature range of 900-1200°C, preferably 950-1150°C using various heat treatment methods such as solution annealing, radio frequency induced annealing or localized annealing.

Japanese Patent Application No. 2014001422 (A), in mass %, C: 0.02 to 0.30%, Cr: 10.0 to 25.0%, Ni: 3.5 to 10.0%, Si: 0.1 to 3.0%, Mn: 0.5% to 5.0%, N: 0.10 to 0.40%, Mo: 0 to 3.0%, Cu: 0 to 3.0%, It has a composition of Ti: 0 to 0.10%, Nb: 0 to 0.50%, V: 0 to 1.0%, and has an M _d30 of 0 to 0, calculated based on a formula very similar to Nohara. 40 austenitic stainless steel is shown. Although the chemical range is very wide, the examples given in this invention mainly show low levels of Mo and N giving low PRE and corrosion resistance. The present invention shows that austenitic alloys with balanced Ni and N contents can have low stability, but only with low PRE and corrosion resistance due to low Mo, In reality, the actual M _d30 is likely to be much higher than the claimed area.

The present invention utilizes the TRIP effect described in the prior art and adds a high pitting resistance equivalent (PRE), thus providing a new austenite with excellent corrosion resistance combined with the TRIP effect due to improved formability. Describes stainless steel. 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 is up to similar levels, but with much better TRIP effects 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 invention are set out in the appended claims.

According to the invention, the austenitic stainless steel comprises less than 0.04 wt.% C, 0.2-0.8 wt.% Si, 0-2.0 wt.% Mn, 16.0-19.0 wt.% wt% Cr, 4.0-6.5 wt% Ni, 1.0-4.0 wt% Mo, 0-4.0 wt% W, 0-2.0 wt% Cu, 0 Contains .20 to 0.30% by weight of N, with the remainder being unavoidable impurities present in iron and stainless steel. Sulfur is limited to less than 0.010% by weight, preferably less than 0.005% by weight, phosphorus content is less than 0.040% by weight, and 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 austenitic stainless steel of the present invention optionally contains one or more of the following additional elements. The aluminum content is at most less than 0.04% by weight, preferably less than at most 0.03% by weight. Furthermore, boron, calcium and cerium may optionally be added in small amounts, with the preferred content of boron and calcium being less than 0.004% by weight, and the preferred content of cerium being less than 0.1% by weight. Optionally, up to 1% by weight of cobalt may be added for partial replacement of nickel. Furthermore, the austenitic stainless steel of the present invention may optionally contain one or more of the group containing niobium, titanium, and vanadium, and the content of niobium and titanium is limited to a maximum of 0.1% by weight. , the vanadium content is limited to a maximum of 0.2% by weight.

According to the stainless steel of the present invention, the pitting resistance equivalent (PRE) is optimized to give good corrosion resistance and ranges from 27 to 35. The critical pitting temperature (CPT) ranges from 30 to 50 °C, and the TRIP (Transformation Induced Plasticity) effect in the austenitic phase follows the calculated M _d30 temperature, which ranges from -70 °C to +60 °C. maintained. The M _d30 temperature, a measure of austenite stability against the TRIP effect, is defined as the temperature at which a true strain of 0.3 results in 50% transformation of 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 steel, which is 10-16 mJ/m ² according to equation (3). The proportion of ferrite phase in the microstructure of the austenitic stainless steel of the present invention is less than 10% by volume under heat treated conditions. The heat treatment can be carried out using different heat treatment methods such as solution annealing, radio frequency induced annealing, localized annealing or any other type of heat treatment at a temperature range of 900-1200°C, preferably 950-1150°C. can.

The effect of different elements in the microstructure is described below, and the elemental content is stated in weight %.

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

Nitrogen (N) is an important austenite stabilizer in stainless steels and, like carbon, increases stability to martensite. Nitrogen also increases strength, strain hardening and corrosion resistance. The general empirical expression for M _d30 temperature shows that nitrogen and carbon have the same strong influence on austenite stability, but this is lower than previously reported in other M _d30 expressions. Nitrogen can be added to stainless steel in higher amounts than carbon without adversely affecting corrosion resistance, so nitrogen contents of 0.20 to 0.30% are effective in the present stainless steel.

Silicon (Si) is usually added to stainless steel 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 indicated by the current expression. For this reason, the silicon content is at most 0.8%, preferably 0.5%.

Manganese (Mn) is an important additive to stabilize the austenite phase and increase the solubility of nitrogen in stainless steel. Manganese can partially replace expensive nickel and bring stainless steel to the correct phase balance. If the content is too high, corrosion resistance will decrease. Manganese has a stronger effect on austenite stability versus deformed martensite, so manganese content must be carefully addressed. The range of manganese shall be 0-2.0%, preferably 0-1.5%.

Chromium (Cr) is the main additive to make steel corrosion resistant. Furthermore, chromium strongly increases the resistance to martensite formation and thus reduces the TRIP effect. Also, being a strong ferrite stabilizer, the level of Cr needs to be limited in austenitic stainless steels. To provide these functions, the chromium level should be at least 16.0% and the maximum level should be 19.0%. Preferably the chromium content is between 16.5 and 18.7%.

Nickel (Ni) stabilizes the austenitic phase and is an essential alloying element for good ductility and must be added to the stainless steel of the present invention in an amount of at least 4.5%. Nickel, which has a major influence on austenite stability against martensite formation, must be present in 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 to 6.5%, preferably 6.2% in the stainless steel of the present invention.

Copper (Cu) is typically present as a 0.1-0.5% residue in most stainless steels, where the majority of the raw material is in the form of stainless steel scrap containing this element. Although copper is a weak stabilizer of the austenite phase, it has a strong influence on the resistance to martensite formation and must be considered in the evaluation of the formability of the present stainless steel. Preferably the copper content is at most 1.6%, although intentional additions of up to 2.0% can be made.

Molybdenum (Mo) is added to significantly increase the PRE and corrosion resistance, so it shall have a content of at least 1.0%, preferably at least 1.5%. Furthermore, molybdenum, like chromium, has been found to strongly increase the resistance to martensite formation and reduce the TRIP effect more significantly than previously expected. Therefore, molybdenum cannot be added in an amount exceeding 4.0.

Tungsten (W) has similar properties to molybdenum and can sometimes replace molybdenum. However, tungsten and molybdenum promote sigma phase precipitation, and the sum of molybdenum and tungsten content according to the formula (Mo+0.5W) should be between 0 and 4.0%, preferably between 2.0 and 4.0%. , where the promotion of sigma and chi phases can be handled in technically relevant processes. Since the stacking fault energy controls the deformation response with respect to dislocation glide, twinning or martensite formation, the most important influence of tungsten is the surprisingly positive influence on the TRIP effect, which in turn influences the stacking faults of the alloy. May be related to effects on energy. For this purpose, tungsten should be limited to a maximum of 3.8%, but preferably at least 1.0% if tungsten is used instead of molybdenum.

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

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

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 therefore the sum of sulfur and phosphorus (S+P) content is less than 0.04%.

Oxygen (O), together with other residual elements, has a negative effect on hot ductility. The presence of oxide inclusions may reduce corrosion resistance (pitting corrosion) depending on the type of inclusions. High oxygen content also reduces impact toughness. Like sulfur, oxygen improves penetration by changing the surface energy of the weld pool. For the stainless steel of the present invention, the maximum recommended oxygen level is less than 100 ppm. For metal powders, the maximum oxygen content can be up to 250 ppm.

Aluminum (Al) is kept at low levels in our austenitic stainless steels with high nitrogen content because these two elements can combine to form aluminum nitride, which degrades impact toughness. Should. The aluminum content is limited to less than 0.04%, preferably less than 0.03%.

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 considerably improves hardness and hot strength retention. Cobalt improves cavitation erosion resistance and strain hardening. Cobalt reduces the risk of sigma phase formation in stainless steel. Cobalt content is limited to a maximum of 1.0%.

The "microalloying" elements titanium (Ti), vanadium (V) and niobium (Nb) significantly change the properties of steel at low concentrations, often with beneficial effects in carbon steels, but in austenitic stainless steels. In some cases, they belong to a group of additives so named because they also contribute to undesirable property changes such as a reduction in impact properties, an increase in surface defect levels and a reduction in ductility during casting and hot rolling. Many of these effects depend, in the case of modern austenitic stainless steels, on their strong affinity for carbon and especially nitrogen. In the present invention, niobium and titanium should be limited to a maximum level of 0.1%, while vanadium is less harmful and should be less than 0.2%.

FIG. 1 shows the dependence of the minimum and maximum M _d30 temperatures and PRE values between the elemental contents Si+Cr and Cu+Mo+0.5W in the tested alloys of the invention. Figure 2 shows the dependence of the minimum and maximum M _d30 temperature and PRE values between the elemental contents Si + Cr and Cu + Mo + 0.5W in the tested alloys of the invention according to Figure 1 with constant values of C + N and Mn + Ni. show. Figure 3 shows the dependence of the minimum and maximum M _d30 temperature, PRE and SFE values between the elemental contents C+N and Mn+Ni in the tested alloys of the invention. FIG. 4 shows the dependence of the minimum and maximum M _d30 temperature, PRE and SFE values between the elemental 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. Give an example. Figure 5 shows the microstructure of several alloys after annealing at 1100 °C followed by water quenching and how the observed martensite levels relate to the calculated M _d30 values of the present invention. Show that. 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 shows the microstructure of several alloys after annealing at 1100 °C followed by water quenching and how the observed martensite levels relate to the calculated M _d30 values of the present invention. Show that.

The invention will be explained in more detail with reference to the drawings.

FIG. 1 shows the dependence of the minimum and maximum M _d30 temperatures and PRE values between the elemental contents Si+Cr and Cu+Mo+0.5W in the tested alloys of the invention.

Figure 2 shows the dependence of the minimum and maximum M _d30 temperature and PRE values between the elemental contents Si + Cr and Cu + Mo + 0.5W in the tested alloys of the invention according to Figure 1 with constant values of C + N and Mn + Ni. show.

Figure 3 shows the dependence of the minimum and maximum M _d30 temperature, PRE and SFE values between the elemental contents C+N and Mn+Ni in the tested alloys of the invention. FIG. 4 shows the dependence of the minimum and maximum M _d30 temperature, PRE and SFE values between the elemental 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. Give an example.

In all of FIGS. 1-4, the M _d30 limits given are the preferred limits of the present invention, calculated according to the optimization mathematical constraints used in the present invention. These calculated M _d30 values are shown in Table 2 for all alloys. In addition, limit Nohara M _d30 values are also given in the figure for reference and comparison.

Figures 5-7 show the microstructure of several alloys after annealing at 1100°C followed by water quenching. Figures 5 and 7 show the microstructures of two alloys of the invention and show how the observed martensite levels relate to the calculated M _d30 values of the invention. A low ferrite content is also shown indicating that the alloy of the present invention is considered fully austenitic. Figure 6 is from reference alloy UNS S30403 for comparison purposes.

Based on the effects of the elements, the austenitic stainless steels according to the invention have chemical compositions A to S as shown in Table 1. Table 1 also includes typical chemical compositions of commercially available austenitic stainless steels for reference, designated T to X, and all contents in Table 1 are in weight percent.

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

Comparing the values in Table 1, the content of nickel, nitrogen and tungsten in the austenitic stainless steels of the invention is significantly different from the reference stainless steels TX and R.

The properties, values of M _d30 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. show.

The predicted M _d30 temperatures (M _d30 Nohara) for the steels in Table 2 are based on the Nohara equation (1) established for austenitic stainless steels when annealed at a temperature of 1050°C.
Calculated using M _d30 =551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo-68Nb (1).

when annealed at the temperature of 1050℃.

The calculated M _d30 temperatures in Table 2 were achieved following optimization mathematical constraints.

Critical pitting temperature (CPT) is measured in 1M sodium chloride (NaCl) solution according to the ASTM G150 test, below which pitting is not possible and only passive behavior is observed. It will be done.

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

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

Comparing the values in Table 2, the PRE values of alloys A to S with a range of 27 to 35 are higher than the PRE values of reference stainless steels T to V, which indicates that the corrosion resistance of alloys A to S is more It means high. The critical pitting temperature CPT ranges from 31 to 48°C, which is a high level of corrosion resistance and is much higher than the CPT of the mentioned austenitic stainless steels T to V.

The M _d30 temperatures predicted using the Nohara equation (1) are in good agreement with the known M _d30 for reference alloys T to V, but are not believed to be accurate for the invention alloys A to S. do not have. This is because the alloys A to S of the present invention have a unique chemical composition from the austenitic steel from which the Nohara formula was originally derived. In particular, the influence of N on austenite stability and M _d30 , although still high, is found to be considerably lower in these inventive steels. The development of Nohara was originally obtained for austenitic steels with low N, so the overall effect of nitrogen in these steels is small. In the present invention, the N content is much higher than in the alloy in order to increase the austenite content given the low Ni and have a high corrosion resistance stainless steel, i.e. increase the PRE. According to Nohara, this makes the steel of the invention very stable. The majority of steels A-S have a M d30 of -50 or less when using Nohara's M _d30 formula. The evaluation of the microstructure of steels A to S in the present invention, together with previous prior art knowledge of measured M _d30 values of alloys with similar composition levels in stainless steels as in WO 2015/114222, The M _d30 of alloys A to S was shown to be higher than predicted by Nohara. Thus, a new M _d30 equation was derived for the alloy of the present invention. As can be seen in Table 2, the M _d30 values calculated for the reference allow very good agreement with the predicted values using Nohara, which is also consistent with the known values for these commercial grades. consistent with the actual measurements. Therefore, the new formula is a good measure of M _d30 even for low N content stainless steel grades. In FIG. 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 consistent with the M _d30 of +56 given by the calculated M _d30 for the present invention. , 2 Nohara M _d30 . An alloy with a M _d30 of 2 predicted by Nohara for Alloy E would be expected by one skilled in the art to be essentially free of martensite at the annealing and quenching conditions described above. For example, FIG. 6 shows the as-quenched microstructure of reference alloy T (UNS S30403), which is remelted to give an equivalent cast microstructure and alloy E. This microstructure exhibits no martensite, or at least very little martensite, which is consistent with the M _d30 of 0-10 predicted by both Nohara and our calculated M _d30 . As expected for commercially available grades. These two examples show that the new M _d30 calculation gives a better description of the true stability and M _d30 values of all alloys than the Nohara equation, and therefore the claimed calculated This shows that it is the basis of the M _d30 limit.

Figure 7 shows the microstructure of alloy P with a calculated M _d30 of -35 and no evidence of martensite in the as-quenched state. In addition, observation of the microstructures in Figures 5 and 7 shows high austenite content and 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 FIG. 6 for Alloy T. These examples show that alloys with higher and lower calculated M _d30 (-70°C to +60°C) have an austenitic microstructure comparable to reference alloy T.

For austenitic stainless steel, the stacking fault energy is 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. It is calculated using Equation (3) of 492.
SFE (mJ/m ² )=2.2+1.9Ni-2.9Si+0.77Mo+0.5Mn+40C-0.016Cr-3.6N (3)

The stacking fault energy (SFE) of the austenitic stainless steel of the present invention is lower than that of commercially available austenitic stainless steels. The SFE of relatively underalloyed reference alloys T and X with low PRE (<27) and therefore high M _d30 >0 is greater than 16 mJ/m ² . In addition, with the low M _d30 (<-70) grade 904L, the SFE for higher alloying and higher PRE (>35) is also greater than 16 mJ/m ² . Alloys A to S of the present invention are unique in the sense that they all have SFEs of less than 16 mJ/m ² but PREs of 27 to 35 and M _d30 of −70 to +60.

The sum of the elemental contents for C+N, Si+Cr, Mn+Ni and Cu+Mo+0.5W of the austenitic stainless steels of the present invention is used in the optimization mathematical constraints between C+N and Mn+Ni on the one hand and Si+Cr and Cu+Mo+0.5W on the other hand. A dependence between 5W was established. According to this mathematical constraint of optimization, the sum of Cu+Mo+0.5W and Si+Cr, respectively the sum of Mn+Ni and C+N, has a minimum and maximum PRE value (27<PRE<35) and a minimum and maximum M _d30 temperature value (-30<M A linear dependence of _d30 <60) forms the x- and y-axes of the coordinates of FIGS. 1-4.

According to FIG. 1, chemical composition windows for Si+Cr and Cu+Mo+0.5W are established in the preferred ranges of 0.21-0.33 for C+N and 4.0-7.7 for Mn+Ni. According to the stainless steel of the present invention, the sum of Si+Cr is limited to 16.2<Si+Cr<19.8, and the sum of Cu+Mo+0.5W is limited to 1.0<Cu+Mo+0.5W<6.0. 1.
The chemical composition windows within the boundaries of regions a', b', c', d', e', f' and g' in FIG. 1 are defined by the following labeled positions of the coordinates in Table 3.

FIG. 1 shows that the composition range of the present invention for Si+C and Cu+Mo+0.5W is 27<PRE>35 and the calculated M _d30 >−30 when the composition limits for C+N and Ni+Mn are within the preferred limits of the present invention. (or Nohara M _d30 <-70).

FIG. 2 shows one chemical composition of FIG. 1 if, instead of the preferred ranges of C+N and Mn+Ni in FIG. 1, constant values of 0.295 for C+N and 6.0 for Mn+Ni are used at all points. An example window is shown. The same inventive limitations as in FIG. 1 are given for the sum of Si+Cr and Cu+Mo+0.5W in FIG. The chemical composition windows within regions a, b, c, d, e, f and g of FIG. 2 are defined by the following labeled positions of the coordinates of Table 4.

FIG. 2 shows that the composition range of the present invention for Si+C and Cu+Mo+0.5W is further restricted given certain fixed levels of C+N and Mn+Ni. In addition to the constraints of FIG. 1, the composition window is also limited by the calculated line M _d30 <60 (or Nohara <10).

Figure 3 shows the chemical composition window for C+N and Mn+Ni when austenitic stainless steel is annealed at a temperature of 1050°C, with the preferred composition ranges being 16.5-19.5 for Cr+Si, Cu+Mo+0.5W The ratio is 2.2 to 5.7. Furthermore, according to the invention, the sum C+N is limited to 0.20<C+N<0.34, and the sum Mn+Ni is limited to 4.0<Mn+Ni<8.5. In FIG. 3, possible constraints for SFE are also added. The chemical composition windows within the regions p', q', r' and s' of FIG. 3 are defined by the following labeled positions of the coordinates of Table 5.

The effect of limiting C+N and Mn+Ni with the preferred range of elemental content of the present invention is that if the chemical composition window of FIG. It is limited only by the minimum and maximum sum limits of C+N and Mn+Ni. This is because none of the limiting constraints of M _d30 , PRE or SFE are within the composition limits of the sum of Si+Cr and Cu+Mo+0.5W.

FIG. 4 has constant values of 17.6 for Cr+Si and 3.5 for Cu+Mo+0.5W, with further limits of 0.20<C+N<0.34 and 4.0<Mn+Ni. 4 shows one chemical composition example window of FIG. 3; FIG. The chemical composition windows within regions p, q, r and s of FIG. 4 are defined by the following labeled positions of the coordinates of Table 6.

FIG. 4 shows that when the composition limits for Si+Cr and Cu+Mo+0.5W have constant values as given in Table 6, the composition range of the present invention for the sum of C+N and Mn+Ni is calculated as M _d30 >-30( or show that it is now limited by the constraints of Nohara M _d30 <−70) and SFE>10.

The ferritic-austenitic stainless steels of the invention can be used in ingots, slabs, blooms, billets and flat products such as plates, sheets, strips, coils, as well as long products such as bars, rods, wires, profiles and shapes, seamless and welded. It can be manufactured as a tube and/or pipe. Additionally, additional products such as metal powders, shaped shapes and profiles can be manufactured.

Claims (17)

Austenitic stainless steel utilizing TRIP effect with balanced pitting resistance equivalent to high corrosion resistance, comprising 0 to 0.04 wt% C, 0.2 to 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 Contains W by weight%, Cu by weight from 0 to 2.0%, and N from 0.20 to 0.30% by weight, the remainder being unavoidable impurities that occur in iron and stainless steel, preferably at 900 to 1200°C. is an austenitic stainless steel characterized in that when quenched and heat treated in a temperature range of 950 to 1150°C, the proportion of ferrite phase in the microstructure is 0 to 10.0% by volume, with the remainder being austenite. steel. Austenitic stainless steel according to claim 1, characterized in that the pitting corrosion resistance equivalent value (PRE) is in the range of 27-35. Austenitic stainless steel according to claim 1 or 2, characterized in that the calculated M _d30 temperature is in the range -70 to +60°C, preferably in the range -30 to +60°C. Austenitic stainless steel according to any one of claims 1 to 3, characterized in that the calculated SFE is in the range of 10.0 to 16.0 mJ/m ² . The austenitic stainless steel according to any one of claims 1 to 4, characterized in that the critical pitting temperature CPT is 30 to 50°C. Austenitic stainless steel according to any one of claims 1 to 5, characterized in that the chromium content is between 16.5 and 18.7% by weight. Austenitic stainless steel according to any one of claims 1 to 6, characterized in that the nickel content is between 4.5 and 6.2% by weight. Austenitic stainless steel according to any one of claims 1 to 7, characterized in that the manganese content is 0 to 1.5% by weight. Austenitic stainless steel according to any one of claims 1 to 8, characterized in that the copper content is from 0 to less than 1.5% by weight. Austenitic stainless steel according to any one of claims 1 to 9, characterized in that the tungsten content is 1.0 to 3.8% by weight. characterized in that 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, preferably 2.2 to 4.0% by weight , the austenitic stainless steel according to any one of claims 1 to 10. Austenitic stainless steel according to any one of claims 1 to 11, characterized in that the nitrogen content is 0.21 to 0.29% by weight. The stainless steel contains 0.0001 to 0.04% by weight of Al, preferably 0.0001 to 0.03% by weight of Al, 0.0001 to 0.004% by weight of B, and 0.0001 to 0.004% by weight of B. wt% Ca, 0.0001-0.1 wt% Ce, 0.0001-0.1 wt% Co, 0.0001-0.1 wt% Nb, 0.0001-0.1 wt% According to any one of claims 1 to 12, further comprising one or more additional elements selected from the group consisting of Ti, 0.0001 to 0.2% by weight of V. Austenitic stainless steel. The stainless steel contains 0.0001-0.010% by weight as inevitable impurities so that the total (S+P) is 0.0001-0.04% by weight and the total oxygen content is in the range of 0-100ppm. , preferably 0.0001 to 0.005% by weight of S, and 0.0001 to 0.040% by weight of P. stainless steel. The chemical composition windows within the frames of regions a', b', c', d', e', f' and g' in FIG. 1 are defined by the following labeled positions of the coordinates in weight percent The austenitic stainless steel according to claim 1, characterized in that:
Claim 1 characterized in that the chemical composition window lying within the frame of the regions p', q'r' and s' of FIG. 3 is defined by the following labeled positions of the coordinates in weight percent Austenitic stainless steel as described.
The steel may be ingots, flat plates, blooms, billets, plates, sheets, strips, coils, bars, rods, wires, profiles and shapes, seamless and welded tubes and/or pipes, metal powders, formed shapes and profiles. Austenitic stainless steel according to claim 1, characterized in that it is manufactured as.