Austenitic stainless steel, heat exchanger plates and chimney ducts made from this steel

Abstract

The composition, in mass percent, is: trace≦C≦0.03%; 1.0%≦Mn≦2.0%; 0.8%≦Si≦2.0%; preferentially 1.0%≦Si≦1.5%; trace≦Al≦0.06%; trace≦P≦0.045%; trace≦S≦0.015%; 8.0%≦Ni≦12.0%; 17.5%≦Cr<20.0%; 0.4%≦Mo≦0.8%; trace≦Sn≦0.05%; trace≦Nb≦0.08%; trace≦V≦0.15%; trace≦Ti≦0.08%; An austenitic stainless steel characterised in that it is made up of traces≦Zr≦0.08%; traces≦Co≦1.0%; traces≦B≦0.01%; traces≦W+Mo≦0.8%; traces≦Pb≦0.03%; traces≦N<0.1%; traces≦O≦0.01%, the balance being iron and impurities arising from manufacture. A heat exchanger plate and a chimney duct made of this steel are also disclosed.

Description

[Technical Field]

[0001] The present invention relates to the field of austenitic stainless steel. More specifically, the present invention relates to an austenitic stainless steel that exhibits a good balance between high resistance to various types of corrosion, good formability, and a reasonable cost achieved by limiting the presence of expensive alloying elements such as Ni and Mo as much as possible. [Background technology]

[0002] Preferred but non-limiting applications include the manufacture of heat exchanger plates or chimney duct elements, both of which require such excellent corrosion resistance and good formability, especially at temperatures exceeding ambient temperature.

[0003] One of the most commonly used austenitic stainless steel grades is the grade known as X5CrNi189(1.4301) according to standard EN10088-2, whose standardized composition is expressed in mass percentages, as with all chemical element content as described in the text of this invention: C≦0.07%; Si≦1.0%; Mn≦2.0%; P≦0.045%; S≦0.015%; N≦0.11%; Cr=17~19.5%; Ni=8~10.5%. This grade is equivalent to the grade known as "304" according to ASTM A240, differing in that the upper limit for Si is up to 0.75% and the upper limit for C is up to 0.08%.

[0004] Adding, for example, approximately 0.2% of Nb or Ti can contribute to improving the corrosion resistance of the joint by resulting in the formation of Nb carbides or Ti carbides instead of Cr carbides, thereby maintaining the amount of dissolved Cr.

[0005] Another solution involves reducing the carbon content in the steel, thereby avoiding the precipitation of chromium carbide during cooling, which reduces corrosion resistance. The low-carbon variant of X5CrNi18-9(1.4301) is X2CrNi18-9(1.4307) according to EN 10088-2, and 304 becomes "304L" according to ASTM A240.

[0006] While the above steel grades exhibit good corrosion resistance, they are generally insufficient, especially in erosive environments such as marine and chlorinated environments.

[0007] In situations where particularly high resistance to various types of corrosion is required, the X5CrNiMo17-12-2 steel grade according to EN 10088-2 and "316" according to ASTM A240, as well as steel grades derived therefrom, are often preferred over the X5CrNi189 steel grade (1.4301) and 304 according to the same standards.

[0008] The typical standard composition of X5CrNiMo17-12-2 is C≦0.07%;Si≦1.0%;Mn≦2.0%;P≦0.045%;S≦0.015%;N≦0.1%;Cr=16.5~18.5%;Mo=2.0~2.5%;Ni=10~13%. The composition is equivalent to the steel grade called "316" in standard ASTM A240, with the differences being that the upper limit of Si is up to 0.75%, the upper limit of C is up to 0.08%, and it incorporates 16~18% chromium. Compared to X5CrNi189, the Cr concentration range is slightly shifted to lower minimum and maximum values, while the Ni concentration is higher in most cases, and in particular, there is a large amount of Mo.

[0009] Similar to steel grades containing 18% chromium and 9% nickel, the X2CrNiMo17-12-2 steel grade with a standardized composition of C≦0.03%;Si≦1.0%;Mn≦2.0%;P≦0.045%;S≦0.015%;N≦0.1%;Cr=16.5~18.5%;Mo=2~2.5%;Ni=10~13% offers even higher performance in terms of corrosion resistance in chlorinated media. Therefore, the composition differs from X5CrNiMo17-12-2 mainly due to a lower maximum concentration of C, which contributes to achieving a composition with better intergranular corrosion resistance in chlorinated media than the X5CrNiMo17-12-2 composition, due to a lower possibility of Cr carbide and Cr carbonitride formation. The above steel grade is also easier to weld. Such a steel grade is equivalent to the steel grade called "316L" in ASTM A240. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] The X5CrNiMo17-12-2 steel grade and its known derivatives have the disadvantage of being more expensive than X5CrNiMo17-12-2 due to the higher concentration of Ni and the presence of a large amount of Mo. Furthermore, the extraction of such elements from ore is harmful to the environment. Therefore, it is of interest to find a suitable alternative to the aforementioned steel grade with lower concentrations of expensive and ecologically impactful alloying elements. The object of the present invention is to achieve this. [Means for solving the problem]

[0011] For this purpose, the subject of the present invention is a composition in mass percent, - Trace ≦C≦0.03%; - 1.0% ≤ Mn ≤ 2.0%; - 0.8% ≤ Si ≤ 2.0%; preferably 1.0% ≤ Si ≤ 1.5%; - Trace amount ≤ Al ≤ 0.06%; preferably trace amount ≤ Al ≤ 0.01%; - Trace ≦P≦0.045%; - Trace ≦S≦0.015%; - 8.0% ≤ Ni ≤ 12.0%; Preferably, 9.45% ≤ Ni ≤ 10.0%; - 17.5% ≤ Cr < 20.0%; - 0.4% ≤ Mo ≤ 0.8%; Preferably, 0.5% ≤ Mo ≤ 0.6%; - Trace amount ≤ Sn ≤ 0.05%; - Trace amount ≤ Nb ≤ 0.08%; - Trace amount ≤ V ≤ 0.15%; - Trace amount ≤ Ti ≤ 0.08%; - Trace amount ≤ Zr ≤ 0.08%; - Trace amount ≤ Co ≤ 1.0%; - 0.02% ≤ Cu ≤ 0.6%; - Trace amount ≤ B ≤ 0.01%; - Trace amount ≤ W + Mo ≤ 0.8%; - Trace amount ≤ Pb ≤ 0.03%; - Trace amount ≤ N < 1000 ppm; - Trace amount ≤ O ≤ 0.01%; Preferably, trace amount ≤ O ≤ 0.005%; The balance consists of iron and impurities generated during manufacturing, and it is an austenitic stainless steel.

[0012] The composition is in mass percentage, - Trace amount ≤ C ≤ 0.03%; - 1.0% ≤ Mn ≤ 2.0%; - 0.8% ≤ Si ≤ 2.0%; Preferably, 1.0% ≤ Si ≤ 1.5%; - Trace amount ≤ Al ≤ 0.06%; Preferably, trace amount ≤ Al ≤ 0.01%; - Trace amount ≤ P ≤ 0.045%; - Trace amount ≤ S ≤ 0.015%; - 8.0% ≤ Ni ≤ 12.0%; Preferably, 9.45% ≤ Ni ≤ 10.0%; - 17.5% ≤ Cr ≤ 20.0%; - 0.4% ≤ Mo ≤ 0.8%; Preferably, 0.5% ≤ Mo ≤ 0.6%; - Trace amount ≤ Sn ≤ 0.05%; - Trace amount ≤ Nb ≤ 0.08%; - Trace amount ≤ V ≤ 0.15%; - Trace amount ≤ Ti ≤ 0.08%; - Trace ≦Zr≦0.08%; - Trace ≦Co≦1.0%; - Trace ≦B≦0.01%; - Trace amount≦W+Mo≦0.8%; - Trace ≦Pb≦0.03%; - Trace ≦N≦0.1%; - Trace ≦O≦0.01%; Also disclosed is an austenitic stainless steel characterized in that the remainder consists of iron and impurities generated during manufacturing.

[0013] Their average grain size can range from 11 to 6 ASTM.

[0014] A further subject of the present invention relates to a heat exchanger plate, characterized in that the plate is made of such austenitic stainless steel.

[0015] A further subject of the present invention relates to elements of a chimney duct, characterized by being made of such austenitic stainless steel.

[0016] Naturally, the present invention is based on modifying the composition of the classic steel grade X2CrNi18-9 by carefully balancing the addition of Mo and Si while keeping the Mo content relatively low. Such additions tend to bring the steel closer to the composition of X2CrNiMo17-12-2 due to the presence of Mo. However, this addition does not fall under the aforementioned subtle difference variations that were previously known or obvious, particularly because the presence of Mo remains relatively subdued. Thus, such modifications have no adverse economic impact and, in combination with Si concentrations that can be higher than those of X2CrNi18-9 and X2CrNiMo17-12-2, are sufficient to maintain both mechanical properties and corrosion resistance at least equivalent to those of CrNiMo17-12-2. Such properties are very suitable for applications that require both high resistance to various types of corrosion and good formability, such as in the manufacture of thin and complexly shaped parts like heat exchanger elements or chimney ducts.

[0017] The inventors concluded that the following steel compositions, expressed in mass percent, were best suited to solving the above problems related to material cost, mechanical properties, and corrosion resistance.

[0018] The concentration of carbon (C) ranges from trace amounts to 0.030%. C is a highly gamma-stable (austenitized) element, and excessive C concentrations necessitate the addition of expensive alpha-stable (ferriticized) elements such as Cr or Mo. Furthermore, C is highly undesirable in terms of intergranular corrosion resistance, significantly reducing the weldability of steel types.

[0019] The concentration of Mn is between 1.0% and 2.0%. Mn contributes to austenitic stability by reducing the tendency to convert to martensite under stress or heat, resulting in increased deformability and reduced strain hardening, which is greatly recognized in deep drawing of heat exchanger plates. However, high concentrations tend to reduce the corrosion resistance of the steel grade, and therefore, the concentration must be limited to 2.0% in this specification.

[0020] The maximum concentration of P is 0.045%.

[0021] The maximum concentration of S is 0.015%.

[0022] S and P are extremely detrimental elements to the corrosion resistance of stainless steel, and they also significantly reduce its mechanical strength and deformability at high temperatures. Their concentrations should, in principle, be as low as possible, and in all cases, they should be below the stated limits.

[0023] The Si concentration is between 0.8% and 2.0%, preferably between 1.0% and 1.5%. According to the present invention, when the element is combined with a moderate concentration of Mo, it significantly enhances the corrosion resistance of the steel grade. Si is also a highly alpha-stable (ferritic) element, and its concentration must be limited to 2%; otherwise, the steel grade will be unbalanced, and the high concentration of Si will have to be compensated for by the presence of gamma-stabilizing elements such as expensive Ni or harmful C.

[0024] Furthermore, the fact that replacing Mo with Si reduces the Mo concentration compared to steel grades used in the past mitigates the ecological impact of obtaining the necessary raw materials.

[0025] Al concentrations range from trace amounts to 0.06% due to manufacturing processes. Al can be used by steelmakers as an oxygen scavenger. However, insufficient control of Al can affect the overall cleanliness of the steel, particularly the final appearance of the product's surface. Al is also an alpha-stable element, and its excess requires correction with expensive gamma-stable elements such as Ni or elements that negatively affect corrosion resistance, such as C. Therefore, it is important to limit the Al concentration to a maximum of 0.06%, and preferably to a maximum of 0.01%.

[0026] Ni is a strong gamma-stable element that enhances the deformation performance and resilience of the steel grade under consideration. However, Ni is also relatively expensive, and its concentration must be balanced between the metallurgical stability of the steel grade and its cost. Therefore, concentrations of Ni that are too low (less than 8.0%) result in unstable steel grades due to the formation of martensite during deformation, which leads to a significant increase in mechanical strength (strain hardening) and a decrease in fracture elongation. However, concentrations that are too high result in steel grades that are not economically competitive. According to this invention, the concentration of Ni is between 8.0% and 12.0%, preferably between 9.45% and 10.0%.

[0027] Cr is a fundamental element for the manufacture of stainless steel. The concentration of Cr gives steel the majority of its corrosion resistance. For the applications targeted by this invention and to impart the austenite metallurgical state to the steel, the Cr content should be between 17.5% and 20.0%.

[0028] The concentration of Mo is between 0.4% and 0.8%, preferably between 0.5% and 0.6%. Mo is an element that enhances corrosion resistance by strengthening the passivation film that naturally forms on the surface of stainless steel. According to the present invention, the addition of Mo combined with Si in a carefully adjusted and precise concentration range significantly improves the corrosion resistance properties of austenitic steel without increasing the Mo concentration to levels such as those present in steel grade X2CrNiMo17-12-2. The Mo concentration required by the present invention must also take into account the potential presence of W, which will be discussed below.

[0029] The concentration of Sn is limited to trace amounts, ranging from 0.05%, resulting from manufacturing processes, and Sn strongly reduces hot forgeability.

[0030] The concentrations of Nb, Zr, and Ti range from trace amounts to 0.08% due to the manufacturing process. Due to the low concentration of C imposed by the present invention, such stabilizing elements against intergranular corrosion are not required herein. Preferably, the concentration of Nb is strictly less than 0.03%, and more preferably less than 0.02%.

[0031] The concentration of V (vitreous sulfate) ranges from trace amounts resulting from manufacturing to between 0.15%. V can be added to steel grades in moderate amounts to increase the solubility of N in austenite at high temperatures and to prevent the precipitation of chromium nitride. Preferably, the concentration of V is 0.03% or higher, and preferably 0.04% or higher, to improve forgeability.

[0032] The concentration of Co ranges from trace amounts resulting from manufacturing to between 1.0%. Although Co is a gamma-stable element and may consequently offer metallurgical advantages, it is excessively expensive and should be limited to 1.0% to avoid significantly increasing the cost of steel grades.

[0033] B is known to improve the forgeability and creep of steel. Its concentration ranges from trace amounts to 0.01% due to the manufacturing process.

[0034] W (water) is described in scientific literature as being used to enhance the corrosion resistance of steel grades in proportions equivalent to that of Mo (mo). However, W is an excessively expensive element, and its presence in large quantities significantly increases the cost of the steel grade. Therefore, W should be limited to a maximum value while satisfying the law Mo + W ≤ 0.8% depending on the proportion of Mo, and preferentially reduced to trace amounts resulting from manufacturing.

[0035] Cu should be present in the composition as an impurity resulting from the manufacturing process, at a maximum content of 0.6%, generally 0.5% or less, and more preferably less than 0.3%. The concentration of Cu should be at least 0.02%, or at least 0.10% depending on the manufacturing method.

[0036] The concentration of Pb ranges from trace amounts resulting from manufacturing to between 0.03%.

[0037] The concentration of N falls between 2.0% and 0.1 mass% (1000 ppm). Such concentrations prevent the degradation of mechanical properties that would otherwise be caused by higher concentrations. Preferably, the concentration of N remains at a maximum of 0.08% (800 ppm). The concentration of N is generally 0.03% (300 ppm) or higher.

[0038] The concentration of O is between trace amounts and 0.01%, and is preferably limited to the lowest possible concentration, in order to meet the overall cleanliness requirements according to the primary intended use.

[0039] Elements not mentioned are present only in trace amounts resulting from the manufacturing process. The term "trace amounts" should generally be understood to mean that the element is not intentionally added during manufacturing, or (as may be the case with other deoxidizing elements such as Al and Zr) is subsequently removed, for example by decanting any nonmetallic inclusions formed by the element, resulting in only very small amounts being present in the final steel.

[0040] It should be understood that the preferred ranges for various elements shown in the definition of steel according to the present invention are independent of each other. In other words, it is still in line with the present invention if the composition of steel is within the most common range defined above with respect to certain elements and within a preferred range with respect to other elements.

[0041] The average grain size can range from 11 to 6 ASTM. The 6 ASTM size is preferred in applications where complex shapes, such as heat exchanger plates, need to be manufactured by deep drawing, while the ASTM 11 size is preferred when the heat exchanger is brazed or welded by high-temperature diffusion welding. In this way, it is possible to impart mechanical strength to the heat exchanger after assembly operations, in accordance with the high pressures it will withstand during operation.

[0042] The present invention will be better understood by reading the following description shown in conjunction with the attached drawings. [Brief explanation of the drawing]

[0043] [Figure 1] This figure shows the conventional yield strength Rp0.2 measured for the first series of various samples tested. [Figure 2] This figure shows the tensile strength Rm measured for the first series of various samples that were tested. [Figure 3] This figure shows the measured elongation at break A% for the first series of various samples that were tested. [Figure 4] This figure shows the pitting potential (Epit) of various tested steels, measured at 23°C in a 0.02M NaCl medium. [Figure 5] This figure shows the grain sizes of various steels tested at two different annealing temperatures. [Figure 6] This figure shows the results of measuring the conventional yield strength Rp0.2 for the same steel. [Figure 7] This figure shows the results of Rm measurement for the same steel. [Figure 8]This figure shows the results of measuring the fracture elongation A% for the same steel. [Figure 9] This figure shows the conventional yield strength Rp0.2 measured in tensile tests along three directions for two types of steel. [Figure 10] This figure shows the results of measuring the tensile strength RM in three directions for two types of steel. [Figure 11] This figure shows the results of measuring the fracture elongation A% in three directions for two types of steel. [Figure 12] This figure shows the limiting drawing ratio (LDR) for a reference steel. [Figure 13] This figure shows the limiting drawing ratio (LDR) for the steel according to the present invention. [Figure 14] This figure shows the effect of the salinity and temperature of the NaCl aqueous solution on the pitting corrosion resistance of two types of steel and one reference steel according to the present invention. [Figure 15] This figure shows the effect of PREN on pitting corrosion resistance for various steels being tested. [Figure 16] This figure shows current-voltage curves used to evaluate the sensitivity of two types of steel and one reference steel according to the present invention to uniform corrosion. [Figure 17] This figure shows the results of a droplet evaporation test to evaluate the stress corrosion resistance of two types of steel and three types of reference steel according to the present invention. [Figure 18] This figure shows the results of depassivation pH measurements for the steel according to the present invention and three types of reference steel. [Modes for carrying out the invention]

[0044] To find the proper balance between the concentrations of various elements Si, Mo, W, and Cu, which tended to have an a priori effect of improving the corrosion resistance of the classical X2CrNi18-9 steel type, either individually or in combination, due to their nobility or stoichiometry, the inventors first conducted comparative tests on steels of various compositions. The steels were used to distinguish the effect of each chemical element on various properties of such steel types, such as hot forgeability, austenitic stability, and corrosion resistance. Classical X2CrNiMo17-12-2 was also incorporated into the tests for comparison, and X2CrNiMo17-12-2 is Si-enriched, which, in conjunction with the presence of 2.0% Mo, appears at first glance to be a possible solution for improving the properties of such a basic steel type.

[0045] Table 1 summarizes the various compositions tested, expressed as mass percent. It should be understood that elements not listed in the table (such as those in other tables in the main text of this invention describing the composition of steel) are present in trace amounts resulting from the manufacturing process and have no metallurgical effect.

[0046] The names given to the various steel grades in Table 1 are not standardized, and the names are applied only within the specific framework of the text of this invention. They should be understood as corresponding to X5CrNi18-9 steel, 304 in the reference example, or X2CrNiMo17-12-2, 316L in the reference example, with a large amount of the element(s) mentioned in the name added to them, causing the steel to fall outside the standard range defining the composition of X5CrNi18-9 or X2CrNiMo17-12-2.

[0047] [Table 1]

[0048] Steel with the composition described in Table 1 was cast. A small ingot was obtained, and a 40 mm thick sample was taken from it. This sample was then hot-rolled to a thickness of 4 mm at 1150°C, then annealed at 1140-1120°C, and pickled. The ingot was then cold-rolled to a thickness of 1.5 mm, annealed at 1140-1120°C, and then forced-air-cooled and pickled.

[0049] Such preparation methods are entirely conventional in the preferred intended applications, particularly those listed, for the type of austenitic stainless steel that the present invention aims to replace.

[0050] The following conclusions can be drawn from this.

[0051] The average grain size of steel grade 304 and its derivatives ranges from 9.3 to 11.2 ASTM, as shown in Table 2, with typical 304 having the smallest average grain size (note that smaller grain sizes correspond to higher ASTM values). The addition of Si, and especially Mo or W, contributes to an increase in average grain size. Steel grade 316L has an average grain size of 9 ASTM, and the presence of 1.35% Si instead of 0.37% slightly increases the average grain size (8.7 ASTM) when all other factors are substantially equal.

[0052] [Table 2]

[0053] The mechanical properties that represent the formability of steel, namely the conventional yield strength Rp 0.2 Regarding the tensile strength Rm and the elongation at break A, the following findings were made, as can be seen in Figures 1-3.

[0054] Adding Mo alone to 304 at the tested proportions has no significant effect. The yield strength and tensile strength remain higher than those of 316L. The latter shows a fracture elongation that slightly exceeds that of 304, as well as that of the Mo and Si-enriched derivatives examined.

[0055] When added alone, W tends to degrade such properties.

[0056] The most significant effect, and more specifically on the elongation at break, is the addition of Si in combination with the addition of Mo at the proportions tested.

[0057] Figure 4 shows the pitting potential Vpit in an environment with 0.02 MNaCl at 23℃. 0.1 Regarding pitting corrosion resistance, as expressed by [the relevant factor], conventional 316L steel still exhibits better resistance than 304 steel. However, it should be noted that in both 304 and 316L, the addition of approximately 1.3% to 1.4% Si in 304 significantly increases the pitting potential. The best results in the tests were obtained for 304 steel containing 17.4% to 17.8% Cr with 0.5% Mo and 1.3% Si added. This result is even better than the results obtained for 316L containing 1.98% Mo and 16.4% Cr with 1.35% Si added.

[0058] W has virtually no effect on pitting potential. According to the tests, therefore its effect is completely separate from that of Mo.

[0059] The addition of copper has a negative effect because, assuming all other factors are equal, it lowers the pitting potential.

[0060] Steel grade 304, with moderate Mo content and Si content exceeding 1%, and the combination thereof, but with a higher Cr concentration, is thought to significantly improve the pitting corrosion resistance of austenitic stainless steel.

[0061] With respect to crevice corrosion, uniform corrosion, and stress corrosion, the addition of 0.5% Mo was found to be beneficial, regardless of the presence or absence of further Si, leading to performance equivalent to that of 316L.

[0062] Therefore, prior tests leading to the present invention suggest that a solution consisting of adding a small amount of Mo or a small amount of Mo and Si to conventional 304 steel may be a good alternative to the use of 316 or 316L steel, which is richer in Ni and M and therefore substantially more expensive than the steel of the present invention, and has a more significant ecological footprint and formability that is not necessarily optimal for the intended application, in terms of resistance to various types of corrosion, mechanical properties, and cost.

[0063] However, such excessive Si addition in conventional 304 steel tends to enhance mechanical properties, including hardness at high temperatures, in a way that is undesirable for the formability of the material. At a Si concentration of 1.35% combined with 0.5% Mo, the A4 temperature, which indicates the phase transformation from delta ferrite to austenite, was found to be too low, leading to crack formation at the edges of the product during hot rolling. Therefore, an optimized solution must be found.

[0064] To ensure the overall compatibility between the steel during hot rolling and the mechanical properties and corrosion resistance of the finished product, which are suitable for the intended applications of such steel, such as heat exchangers and chimney ducts, a balance of composition is required to obtain an appropriate A4 temperature, i.e., an A4 temperature exceeding the temperature reached before hot rolling.

[0065] Considering past trials, it was concluded that, for this purpose, it is desirable to make the following adjustments to the first attempt a priori: - Maintain significantly low concentrations of C to limit the risk of intergranular corrosion; - Since Nb is an expensive element and its advantage in intergranular corrosion resistance can also be obtained by reducing the concentration of C, do not add Nb, or at least do not add it in large quantities; - Maintaining sufficient levels of Cr and Mo to achieve the desired corrosion resistance; - Minimizing the addition of Ni to limit material costs; - Adjust the concentration of N to obtain good high-temperature ductility requiring an appropriate temperature A4; adjust the concentration of Si to increase the A4 temperature and decrease hardness while maintaining the synergistic effect of Mo and Si on corrosion resistance observed in the aforementioned preliminary tests.

[0066] For this purpose, a 50 kg ingot was cast, and its composition is shown in Table 3. Elements not listed in the table are impurities. Examples 11-14 are according to the present invention, and Example 15 is the reference 316 L. In Examples 11-14, therefore the Al concentration is a maximum of 0.06%, the Sn concentration a maximum of 0.05%, the Nb concentration a maximum of 0.08% (and less than 0.03%), the Ti concentration a maximum of 0.08%, the Zr concentration a maximum of 0.08%, the B concentration a maximum of 0.01%, the combined W and Mo concentration a maximum of 0.8%, and the Pb concentration a maximum of 0.03%. The examples according to the present invention differ from each other in terms of their Si content, ranging from approximately 1.3% to 1.0%. It should be noted that Mo is set uniformly at 0.5%, and N allows for the gradual increase of Si to be compensated for.

[0067] [Table 3]

[0068] Next, a sample measuring 150 x 100 x 25 mm was cut out. The sample was then hot-rolled to reduce its thickness from 25 mm to 2.8 mm.

[0069] Next, the first annealing was performed at 1100°C without any holding time, followed by etching, which resulted in overall recrystallization of the sample and an oxide-free surface.

[0070] Next, the sample is cold-rolled to a final thickness of 1 mm, which is the ideal thickness to ensure that the deep-drawing properties required for the application are actually obtained.

[0071] To obtain various average grain sizes, the final annealing operation was performed at temperatures of 1075°C and 1100°C.

[0072] As can be seen in Figure 5, the results obtained regarding grain size show very slight differences between samples at a given annealing temperature. The steel according to the present invention and 316L steel behave similarly. Annealing at 1075°C yields an average grain size of approximately 7.5–8 ASTM in all cases, and annealing at 1100°C yields an average grain size of approximately 8.5–9 ASTM in all cases, and in the case of the present invention, no effect of Si concentration is observed.

[0073] The average grain size of steel has a significant impact on its mechanical behavior, particularly its deep drawability. The smaller the ASTM grain size, the more deformable the material. Therefore, the ability to adjust the grain size to 6-11 ASTM is a major advantage in finding the right balance between the deformability required for deep drawing of parts and the mechanical strength needed for durability during use, especially in heat exchanger plates with complex shapes.

[0074] Tensile tests were also performed on the samples annealed at the two temperatures mentioned above, along the direction perpendicular to the rolling direction DL (in other words, along the transverse direction DT). Figure 6 shows the conventional yield strength Rp 0.2 Figure 7 shows the test results for tensile strength Rm, and Figure 8 shows the test results for elongation at break A%.

[0075] From these, the following is clear: - Conventional yield strength Rp 0.2 And with respect to tensile strength Rm, the steel according to the present invention has a higher value than 316L for the same average grain size; the value tends to decrease with increasing Si and N concentrations; and even though the difference narrows at a Si concentration of about 1%, the steel according to the present invention is harder than 316L. - In all steels according to the present invention and 316L having the same average grain size, the elongation at break is very similar, differing only slightly from 8ASTM to 9ASTM.

[0076] Tensile tests were also performed on the same two examples 14 and 15 in three directions: the rolling direction DL, the transverse direction DT perpendicular to DL, and the 45° direction, i.e., along the bisectors of the other two directions. The tests in Figures 6-8 were only in the DT direction.

[0077] For all tests, the tested specimens, measuring 12.5 mm in length, 50 mm in width, and 1 mm in thickness, were given an average grain size of 8.6 ASTM in Example 14 of the present invention and 8.7 ASTM in Reference Example 15 of 316 L by final annealing at 1080°C without holding time. 0.2 The test results for Rm and A% are shown in Figures 9, 10, and 11, respectively.

[0078] Rp 0.2 With respect to and Rm, the two embodiments exhibit substantially similar behavior, with the deviation not exceeding 10 MPa in each measurement direction. The elongation at break A% is slightly higher than that of Embodiment 14 according to the present invention.

[0079] When calculating the plane isotropy coefficient Δr for the two embodiments from the stress-strain curves along the three directions, it can be seen that Δr corresponds to -0.286 for Embodiment 15 of 316L and -0.229 for Embodiment 14 of the present invention. The good mechanical properties of the steel according to the present invention, having high mechanical strength with large fracture strain and high isotropy, as well as resistance to various types of corrosion, make it a suitable substitute for the steel in applications of 316L where such properties are important.

[0080] The formability of a steel grade can be effectively characterized by its yield strength, but further testing is necessary to obtain a more accurate view, especially when deep drawing is intended. For this purpose, Erichsen tests and deep drawing tests were performed on Examples 14 and 15.

[0081] The Erichsen test aims to obtain the Erichsen index IE, which corresponds to the depth of deep drawing before crack initiation, in accordance with equibiaxial stress.

[0082] A punch with a constant diameter of 20 mm, a constant blank holder pressure of 1000 daN, brush-applied Molykote® lubricant, and a constant deep drawing speed of 5 mm / min were used. The thickness of the metal sheet tested was 1 mm.

[0083] Example 14 according to the present invention exhibits slightly better performance than Reference Example 15. The IE of Example 14 is 12 mm, while the IE of Example 15 is 11.5 mm.

[0084] The limiting aperture ratio (LDR) and susceptibility to delayed failure were also tested for Examples 14 and 15.

[0085] Theoretically, LDR corresponds to the ratio β of the maximum diameter of the blank before crack formation to the initial diameter of the punch.

[0086] The results are shown in Figures 12 and 13. The LDRs are very close in both embodiments, 2.22 in Example 14 according to the present invention (Figure 13) and 2.17 in Reference Example 15 (Figure 12). The LDRs of the embodiments according to the present invention are even slightly better than those of the reference steel 316L.

[0087] Regarding susceptibility to delayed fracture, at a β ratio of 2.12, no delayed fracture was observed in two examples, 14 and 15, for 2B finish (cold-rolled, non-bright annealed, pickled, and tempered).

[0088] For the two examples 14 and 15, the deformation due to elastic recovery after deep drawing was also evaluated. There were no significant differences in their behavior, which is consistent with the similarity of their yield strengths.

[0089] Corrosion resistance tests were also conducted on Examples 11 and 14 according to the present invention (containing 1.3% and 1.0% Si, respectively), and on Reference Example 15 of 316L steel.

[0090] Electrochemical tests were conducted on a deep drawing disc with a diameter of 15 mm polished in water with SiC paper having a particle size of 1200. Subsequently, the disc was degreased in an ultrasonic bath of acetone / ethanol, rinsed with distilled water, and aged by leaving it in ambient air for 24 hours.

[0091] Electrochemical corrosion tests were performed in distilled water for analysis and a solution of NaCl, and degassed with nitrogen and hydrogen. A saturated calomel electrode (SCE) was used as the reference electrode, and a platinum electrode was used as the counter electrode.

[0092] The pitting corrosion resistance is represented by the pitting potential E measured in mV / SCE for Samples 11, 14, and 15 in Table 3 in a degassed NaCl solution at pH 6.6. The sample is left at the free potential for 15 minutes, and then a potentiodynamic scan is performed at a constant scan rate (100 mV / min) until the intensity of 50 μA is reached to measure the potential E. Experiments were conducted at 23 °C and 50 °C in 0.02 M and 0.5 M NaCl solutions. The basic pitting probability Pi (cm) was measured as a function of the corrosion potential E. The results are shown in Figure 14. pit and the sample is left at the free potential for 15 minutes, and then the potential E pit is measured by performing a potentiodynamic scan at a constant scan rate (100 mV / min) until the intensity of 50 μA is reached. Experiments were conducted at 23 °C and 50 °C in 0.02 M and 0.5 M NaCl solutions. The corrosion potential E pit as a function of the basic pitting probability Pi (cm 2 ) was measured. The results are shown in Figure 14.

[0093] It can be seen that the three samples have very close results to each other under the same experimental conditions. In particular, the change from a 0.02 M NaCl solution to a 0.5 M NaCl solution and from a temperature of 23 °C to a temperature of 50 °C has the same effect regardless of the composition of the samples being examined. Figure 14 reflects the above findings by showing the average pitting potential E pit and its standard deviation σ for each sample as a function of the NaCl concentration and temperature of the solution.

[0094] The positive effect of the addition of Si on the pitting corrosion resistance of 304-type stainless steel is thus confirmed. With the presence of only 1.0% Si, the results obtained are still very close to those of 316L. The pitting corrosion resistance is further slightly improved in the test at 23 °C by 1.3% Si used in combination with 0.5% Mo, while remaining equivalent at 50 °C.

[0095] For comparison, the same tests were performed on conventional 304 stainless steel, manufactured through industrial production, with a composition of 18.1% Cr, 0.29% Cu, 1.12% Mn, 0.29% Mo, 8% Ni, 0.42% Si, 0.049% C, and 0.052% N, with the remaining elements present in trace amounts, and on 316L steel, with a composition of 17% Cr, 0.27% Cu, 1.44% Mn, 2.02% Mo, 10% Ni, 0.33% Si, 0.022% C, and 0.035% N, with the remaining elements present in trace amounts. In industrial 304 and 0.02M NaCl solution, E pit It was found that the voltage could decrease to 490 mV at 23°C and to 390 mV at 50°C (Figure 15). In a 0.5 M NaCl solution, the same values ​​were 300 mV and 180 mV, respectively. Therefore, compared to ordinary 304 steel, the pitting corrosion resistance is significantly improved when Mo and Si are added according to the present invention, and the results obtained are competitive with those obtained for 316L even at 50°C, but the material cost is lower.

[0096] The inventors also took into account PREN (Pitting Equivalent Number), a classic concept intended to predict the susceptibility of stainless steel to pitting corrosion. PREN can be considered equivalent to %Cr + 3.3 × %Mo + 16 × %N. When exposed to a 0.02M or 0.5M NaCl environment at 23°C, an equivalent PREN is obtained by adding Mo and Si to conventional 304 steel according to the present invention. pit0.1 Figure 15 shows that the increase is estimated to be approximately 100–150 mV. The increase is milder in the 50°C test (50–100 mV at 23°C for 0.5 M NaCl, which is not as significant), but nevertheless it is still interesting under the most challenging conditions encountered during the test. The above, taken in isolation, incidentally indicates that PREN is not a sufficient criterion for predicting the sensitivity of stainless steel to corrosion resistance with very high precision.

[0097] In the study of uniform corrosion, the passivation layer was first removed from three samples 11, 14, and 15, and from 304 samples derived from industrial production. The residual potential V was then applied to a degassed solution of 2M sulfuric acid at a pH lower than the depassivation pH (Phd). corr The composition was obtained beforehand by immersion for 15 minutes. Potential polarization tests were performed at a scan speed of 10 mV / min, from -750 mV / SCE to 1800 mV / SCE. Current / voltage curves were determined. The curves are shown in Figure 16.

[0098] The curves are very similar across the three samples. In particular, the peak current I is even higher when the uniform corrosion of the metal is rapid. crit This clearly shows that the three samples tested were substantially identical, with the 1.3% Si sample showing 0.25 mA / cm². 2 For a 1.0% Si sample, the reading was 0.26 mA / cm². 2 In 316 samples, the reading was 0.20 mA / cm². 2 In 304 samples derived from industrial production, the reading was 0.23 mA / cm². 2 That is the case.

[0099] The conclusion drawn from this is that adding 1.3% or 1.0% Si to AISI 304 stainless steel containing 0.5% Mo yields the same results in terms of uniform corrosion resistance, and this is not significantly inferior to the uniform corrosion resistance of AISI 316L.

[0100] Stress corrosion resistance was evaluated by a droplet evaporation test according to ISO 15324. That is: - Use a 0.1 M NaCl aqueous solution at 23°C; - Spray 10 drops / minute onto the part from a drop height of 1 cm; - Heat the metal sample to 120°C during each drop-by-drop test; - Bend the sample into a U-shape according to ASTM G30 standard; - Apply a mirror finish to the parts.

[0101] The test measures the time it takes for cracks to appear in the sample. Three tests are performed for each type of steel sample. The results are shown in Figure 17.

[0102] Sample 11 of 304 with 0.5% Mo and 1.3% Si added showed a fairly wide dispersion in test results, with cracking occurring between 46 and 172 hours. Sample 15 of 304 with 0.5% Mo and 1.0% Si added showed a narrower dispersion, between 46 and 72 hours. Sample 16 of 316L showed cracking after 48 to 90 hours.

[0103] The conclusion from this is that, apart from the uncertainties of typical experiments, there is no clear difference between the steel according to the present invention and 316L in terms of stress corrosion resistance.

[0104] For comparison, samples of industrial AISI 304L steel (which differs from conventional 304 in that it has better corrosion resistance due to a lower maximum concentration of C and therefore a priori lower risk of Cr carbide formation) and industrial 316L steel were tested. The results are also shown in Figure 17. Industrial 304L has relatively low stress corrosion resistance, with a time to crack initiation of 22-26 hours. Industrial 316L has a time to crack initiation of 42-48 hours, which is equivalent to the time observed for the best laboratory samples of the same steel type and for 304 steel enriched with Mo and Si according to the present invention. The more consistent and comprehensive cleanliness of the industrial samples compared to the laboratory samples can explain the lower variability in the measurement results for the same material.

[0105] The crevice corrosion resistance of two examples, 14 and 15, was also evaluated. A 2M NaCl solution with a pH of less than 3, adjusted by the addition of hydrochloric acid and maintained at 23°C, was used to simulate environments leading to crevice corrosion (low pH and high chloride ion concentration). The objective was to determine the pH at which the passivation layer of each sample is destroyed.

[0106] For this purpose, the sample was first cathode-polarized at -750 mV / SCE for 2 minutes, and then left to stand at its residual potential. Next, potential-dynamic measurements were started in the anodic direction at a scanning speed of 10 mV / min from -750 mV / SCE. Measurements were performed at various pH values ​​to determine the maximum intensity within the active region of the polarization curve. The results are shown in Figure 18.

[0107] It is clear from these two samples that they exhibit very similar behavior. The depassivation pH was 1–1.2 in both cases, which is advantageous as it is equivalent to the typical industrial range AISI 304 (1.7–2.3) and also equivalent to the typical industrial range AISI 316 (1.5–1.65).

[0108] Good crevice corrosion resistance is particularly desired in chimney ducts, for example, where they are in contact with flue gas condensate and are under assembly conditions that lead to the occurrence of such corrosion. In conclusion, the corrosion resistance of the two steel grades according to the present invention to various types of corrosion, with 0.5% Mo and 1.3% or 1.0% Si, as thoroughly tested, is not significantly lower than the corrosion resistance of conventional 316L.

[0109] In conclusion, it is thus supported that the coexistence of Si and Mo in the precise proportions according to the present invention in stainless steel with a composition close to that of X2CrNi189(1.4307), which is otherwise equivalent to AISI 304L, has beneficial effects on resistance to various types of corrosion and the formability of the steel. The properties required herein are very close to, or even exceed, those of 316L, which makes it possible for the steel of the invention to economically replace X2CrNiMo17-12-2, which is equivalent to AISI 316L, without metallurgical drawbacks in applications requiring such quality, such as the manufacture of heat exchanger plates and chimney ducts.

Claims

1. In mass percentage, - 0% < C ≤ 0.03%; - 0.96% ≤ Mn ≤ 2.0%; - 0.8% ≤ Si ≤ 2.0%; - 0% < Al ≤ 0.06%; - 0% < P ≤ 0.045%; - 0% < S ≤ 0.015%; - 8.0% ≤ Ni ≤ 12.0%; - 17.5% ≤ Cr < 20.0%; - 0.4% ≤ Mo ≤ 0.8%; - 0% < Sn ≤ 0.05%; - 0% < Nb ≤ 0.08%; - 0% < V ≤ 0.15%; - 0% < Ti ≤ 0.08%; - 0% < Zr ≤ 0.08%; - 0% < Co ≤ 1.0%; - 0.02% ≤ Cu ≤ 0.6%; - 0% < B ≤ 0.01%; - 0.4% ≤ W + Mo ≤ 0.8%; - 0% < Pb ≤ 0.03%; - 0%<N<1000ppm; - 0% < 0 ≤ 0.01%; Austenitic stainless steel characterized by being made of steel having a composition consisting of iron and impurities produced during manufacturing.

2. The austenitic stainless steel according to claim 1, characterized in that it has an average grain size that falls between 11 and 6 ASTM.

3. The austenitic stainless steel according to Claim 1, characterized in that 0% < Nb < 0.03%.

4. The austenitic stainless steel according to claim 3, characterized in that 0% < Nb < 0.02%.

5. An austenitic stainless steel according to any one of claims 1 to 4, characterized in that 0.03% ≤ V ≤ 0.15%.

6. The austenitic stainless steel according to claim 5, characterized in that 0.04% ≤ V ≤ 0.15%.

7. An austenitic stainless steel according to any one of claims 1 to 4, characterized in that 300 ppm ≤ N < 1000 ppm.

8. The austenitic stainless steel according to claim 7, characterized in that 300 ppm ≤ N < 800 ppm.

9. An austenitic stainless steel according to any one of claims 1 to 4, characterized in that 1.0% ≤ Si ≤ 1.5%.

10. The austenitic stainless steel according to any one of claims 1 to 4, characterized in that 0% < Al ≤ 0.01%.

11. An austenitic stainless steel according to any one of claims 1 to 4, characterized in that 9.45% ≤ Ni ≤ 10.0%.

12. An austenitic stainless steel according to any one of claims 1 to 4, characterized in that 0.5% ≤ Mo ≤ 0.6%.

13. An austenitic stainless steel according to any one of claims 1 to 4, characterized in that 0% < 0 ≤ 0.005%.

14. An austenitic stainless steel according to any one of claims 1 to 4, characterized by having an average grain size of 10 to 7 ASTM.

15. A heat exchanger plate characterized by being made of austenitic stainless steel as described in any one of claims 1 to 4.

16. An element for a chimney duct, characterized by being made of austenitic stainless steel as described in any one of claims 1 to 4.

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