AUSTENITIC STAINLESS STEEL, PLATES FOR HEAT EXCHANGERS AND CHIMNEY PIPES MADE FROM THIS STEEL

Description

[0001] The present invention relates to the field of austenitic stainless steels. More particularly, it relates to austenitic stainless steels having a good compromise between high resistance to different types of corrosion, good formability and moderate cost obtained by limiting as much as possible the presence of expensive alloying elements such as Ni and Mo.

[0002] Preferred, but not exclusive, applications would be the manufacture of plates for heat exchangers, or chimney flue elements, which require both this excellent corrosion resistance, in particular at temperatures above ambient, and good formability. The prior art is described in JPH09279313A, EP3683324A1 and KR20180074322.

[0003] Among the most commonly used austenitic stainless steel grades is that known as X5CrNi189 (1.4301) according to standard EN10088-2, the standardized composition of which is given in weight percentages, as will be all the chemical element contents given in this text: 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 comparable to that designated "304" according to standard ASTM A240, with the difference that it limits Si to 0.75% and C to 0.08%.

[0004] Additions of Nb or Ti, of the order of 0.2% for example, can help to improve the corrosion resistance of welds, in that they lead to the formation of Nb or Ti carbides instead of Cr carbides, thus preserving the quantity of Cr in solution.

[0005] Another solution is to lower the carbon content in the steel, thus preventing the precipitation of chromium carbides during cooling, which deteriorates corrosion resistance. The low-carbon variant of the grade X5CrNi18-9 (1.4301) becomes X2CrNi18-9 (1.4307) according to EN 10088-2, and 304 becomes "304L" according to ASTM A240.

[0006] These grades have good corrosion resistance, but this may prove insufficient in particularly aggressive environments, for example marine environments, and chlorinated environments in general.

[0007] In such contexts where particularly high resistance to different types of corrosion is sought, grades of type X5CrNiMo17-12-2 according to EN 10088-2 and “316” according to ASTM A240, and those derived from them, are therefore often preferred to grade X5CrNi189 (1.4301) and 304 according to the same standards, respectively.

[0008] The usual standardized 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%. It is comparable to that of the grade designated by "316" in the ASTM A240 standard, with the difference that it limits Si to 0.75%, C to 0.08% and places chromium between 16 and 18%. Compared to X5CrNi189, we note a shift in the range of Cr contents towards slightly lower minimum and maximum values, a Ni content which, conversely, is most often higher, and above all the notable presence of Mo.

[0009] As with the 18% chromium and 9% nickel grades, even higher performance in terms of corrosion resistance in a chlorinated environment is obtained with the X2CrNiMo17-12-2 grade with the usual standardized composition: 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%. It therefore differs from the X5CrNiMo17-12-2 grade mainly by a lower maximum C content which contributes to giving it an even better resistance to intergranular corrosion in a chlorinated environment than that of X5CrNiMo17-12-2, due to the lower possibility of formation of carbides and carbonitrides of Cr. It is also more easily weldable. This grade is comparable to that designated by “316L” in the ASTM A240 standard.

[0010] X5CrNiMo17-12-2 grades and their known derivatives have the disadvantage of being more expensive than X5CrNiMo17-12-2 grades, due to their higher Ni contents and the significant presence of Mo. Also, the extraction of these elements from their ore is harmful to the environment. It would therefore be interesting to find suitable substitutes for them with a lower content of expensive alloying elements and a high ecological impact. This is the aim of the present invention.

[0011] To this end, the invention relates to an austenitic stainless steel, characterized in that its composition, in weight percentages, consists of: traces ≤ C ≤ 0.03%; 1.0% ≤ Mn ≤ 2.0%; 0.8% ≤ Si ≤ 2.0%; preferably 1.0% ≤ Si ≤ 1.5%; traces ≤ AI ≤ 0.06%; preferably traces ≤ AI ≤ 0.01%; traces ≤ P ≤ 0.045%; traces ≤ 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%; traces ≤ Sn ≤ 0.05%; traces ≤ Number ≤ 0.08%; traces ≤ V ≤ 0.15%; traces ≤ Ti ≤ 0.08%; traces ≤ Zr ≤ 0.08%; traces ≤ Co ≤ 1.0%; 0.02% ≤ Cu ≤ 0.6%; traces ≤ B ≤ 0.01%; traces ≤ W + Mo ≤ 0.8%; traces ≤ Pb ≤ 0.03%; traces ≤ N < 1000 ppm; traces ≤ O ≤ 0.01%; preferably traces ≤ O ≤ 0.005%; the remainder being iron and impurities resulting from the production.

[0012] Also described is an austenitic stainless steel, characterized in that its composition, in weight percentages, consists of: traces ≤ C ≤ 0.03%; 1.0% ≤ Mn ≤ 2.0%; 0.8% ≤ Si ≤ 2.0%; preferably 1.0% ≤ Si ≤ 1.5%; traces ≤ AI ≤ 0.06%; preferably traces ≤ AI ≤ 0.01%; traces ≤ P ≤ 0.045%; traces ≤ 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%; traces ≤ Sn ≤ 0.05%; traces ≤ Number ≤ 0.08%; traces ≤ V ≤ 0.15%; traces ≤ Ti ≤ 0.08%; 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 remainder being iron and impurities resulting from the production.

[0013] Its average grain size can be between 11 and 6 ASTM.

[0014] The invention also relates to a plate for a heat exchanger, characterized in that it is made of this austenitic stainless steel.

[0015] The invention also relates to an element of a chimney flue, characterized in that it is made of this austenitic stainless steel.

[0016] As will be understood, the invention is based on a modification of the composition of the conventional grade X2CrNi18-9 by carefully balanced additions of Mo and Si, the Mo content remaining relatively low. These additions tend to bring the steel closer to the composition of X2CrNiMo17-12-2, due to the presence of Mo. But they do not correspond to a variant of this grade which would have been known until now, or which would have been obvious, in particular because the presence of Mo remains relatively moderate. This modification is thus not economically penalizing, and is nevertheless sufficient to, in combination with the Si content which may be higher than in X2CrNi18-9 and X2CrNiMo17-12-2, retain both mechanical properties and corrosion resistance at least as good as those of CrNiMo17-12-2.These properties are well suited to uses requiring both high resistance to different types of corrosion and good formability allowing the production of thin parts and parts with complex shapes, such as heat exchanger elements or chimney flues.

[0017] The inventors concluded that the following steel composition, expressed in % by weight, was best able to solve the above problems in terms of material cost, mechanical properties and corrosion performance.

[0018] The C content ranges from trace to 0.030%. This element is a highly gammagenic (austenitizing) element, and excessive C content would require compensation by adding expensive alphagenic (ferritizing) elements such as Cr or Mo. In addition, C is highly unfavorable to the intergranular corrosion content and greatly reduces the weldability of the grade.

[0019] The Mn content is between 1.0% and 2.0%. Mn ensures the stability of austenite by reducing its propensity to transform into martensite, under stress or thermally, and therefore increases its deformation capacity and reduces its work hardening, which is greatly appreciated when stamping heat exchanger plates. However, at high content it tends to reduce the corrosion resistance of the grade, and its content should be limited here to 2.0%.

[0020] The P content is at most 0.045%.

[0021] The S content is at most 0.015%.

[0022] S and P are extremely harmful elements for the corrosion resistance of stainless steel grades and also significantly reduce their mechanical strength and their capacity for hot deformation. Their contents should preferably be as low as possible, and in any case less than or equal to the limits cited.

[0023] The Si content is between 0.8% and 2.0%, and preferably between 1.0% and 1.5%. This element allows, according to the invention, when associated with a moderate Mo content, to significantly increase the corrosion resistance of the grade. It is also an important alphagenic (ferritizing) element, and its content must be limited to 2%, otherwise the grade would be unbalanced and the high Si content would have to be compensated by the presence of a gammagenic element, expensive like Ni or harmful like C.

[0024] Also, reducing the Mo content compared to the grades used previously, by replacing it with Si, allows to reduce the ecological impact of obtaining the necessary raw materials.

[0025] The Al content ranges from traces resulting from production to 0.06%. This element can be used by steelmakers as a deoxidizer. However, if poorly controlled, it can affect the inclusion cleanliness of the steel, and particularly the final appearance of the product's surface. It is also an alphagenic element whose excessive presence would require compensation by an expensive gammagenic element such as Ni or one that is detrimental to corrosion resistance properties, such as C. It is therefore important to limit its content to no more than 0.06%, and preferably no more than 0.01%.

[0026] Ni is a powerful gamma-generating element and increases the deformability and resilience of the steel grades considered. However, it is also relatively expensive and its content must bring a balance between the metallurgical stability of the grade and its cost. Thus, too low a Ni content (less than 8.0%) would lead to an unstable grade with the formation of martensite during deformation leading to a significant increase in mechanical strength (work hardening) and a decrease in elongation at break. However, too high a content would lead to an economically uncompetitive grade. According to the invention, the Ni content is between 8.0% and 12.0%, preferably between 9.45% and 10.0%.

[0027] Cr is the fundamental element for the production of stainless steel. Its content gives the steel most of its resistance to corrosion. For the applications covered by the invention and to give the steel its austenitic metallurgical state, it is necessary for the Cr to be between 17.5% and 20.0%.

[0028] The Mo content is between 0.4% and 0.8% and preferably between 0.5% and 0.6%. Mo is an element that increases corrosion resistance by strengthening the passive film that spontaneously forms on the surface of a stainless steel. According to the invention, the addition of Mo, carefully adjusted and associated with a precise range of Si contents, makes it possible to significantly increase the corrosion resistance properties of an austenitic steel without having to increase the Mo content to levels such as those present in the X2CrNiMo17-12-2 grade. The Mo content required by the invention must also take into account the possible presence of W, as will be discussed later.

[0029] The Sn content is limited between traces resulting from the elaboration and 0.05%, Sn strongly reducing the hot forgeability.

[0030] The Nb, Zr and Ti contents are between traces resulting from the elaboration and 0.08%. These stabilizing elements with respect to intergranular corrosion are, here, not necessary due to the low C content which is imposed according to the invention. Preferably, the Nb content is strictly less than 0.03%, better still less than 0.02%.

[0031] The V content ranges from trace amounts resulting from processing to 0.15%. V increases the solubility of N in austenite at high temperatures, and can be added sparingly to the grade to avoid any precipitation of chromium nitrides. Preferably, the V content is greater than or equal to 0.03%, to improve forgeability, preferably greater than or equal to 0.04%.

[0032] The Co content ranges from trace amounts resulting from processing to 1.0%. Although Co is a gamma-generating element and therefore could have metallurgical advantages, it is excessively expensive and must be limited to 1.0% so as not to drastically reduce the cost of the grade.

[0033] B is known to increase the forgeability and creep of steels. Its content ranges from traces resulting from production to 0.01%.

[0034] W is described in the scientific literature as allowing the corrosion resistance of the grade to be increased in proportions equivalent to those of Mo. However, it is an excessively expensive element whose significant presence would drastically increase the cost of the grade. It must therefore be restricted to a maximum value depending on the proportion of Mo and respecting the law Mo + W ≤ 0.8%, and preferably reduced to the state of traces resulting from the development.

[0035] Cu is present in the composition as an impurity resulting from the production, in a content which must remain at most 0.6%, generally less than or equal to 0.5%, better still less than 0.3%. The Cu content is at least 0.02%, or, depending on the production process, at least 0.10%.

[0036] The Pb content ranges from traces resulting from production to 0.03%.

[0037] The N content is between traces and 0.1% (1000 ppm). Such a content prevents deterioration of mechanical properties that would be induced by higher contents. Preferably, the N content remains at most 0.08% (800 ppm). The N content is generally greater than or equal to 0.03% (300 ppm).

[0038] The O content is between traces and 0.01%, and preferably limited to as low a content as possible, in order to maintain inclusion cleanliness in line with the main applications targeted.

[0039] The elements not mentioned are present only in trace amounts resulting from the production process. This term "traces" must, in general, be understood to mean that these elements are not added deliberately during the production process, or that (which may be the case for Al and other deoxidizing elements such as Zr), they are subsequently eliminated, for example by decanting the non-metallic inclusions that they have formed, and are only found in a very marginal way in the final steel.

[0040] It should be understood that the preferred ranges relating to different elements, given in the definition of the steel according to the invention, are independent of each other. In other words, it would remain in accordance with the invention for the composition of the steel to be, for certain elements, in their most general range defined above, and for other elements in their preferred range.

[0041] The average grain size can be between 11 and 6 ASTM. The 6 ASTM size is preferred for applications in which complex geometries, such as exchanger plates, must be produced by stamping, and the 11 ASTM size is preferred in cases where the exchanger is brazed or diffusion welded at high temperatures. This ensures mechanical strength of the exchanger, after the assembly operation, which is adequate for the high pressures supported in service.

[0042] The invention will be better understood upon reading the following description, given with reference to the following appended figures: There figure 1 which shows the conventional elastic limit Rp 0.2 measured on a first series of different samples tested; The figure 2 which shows the tensile strength Rm measured on a first series of different samples tested; The figure 3which shows the elongation at break A% measured on a first series of different samples tested; The figure 4 which shows the pitting corrosion potential E pit of different tested steels, measured in a 0.02M NaCl medium at 23°C; The figure 5 which shows the grain sizes of various steels tested for two different annealing temperatures; The figure 6 which shows the results of measuring the conventional elastic limit Rp 0.2 for these same steels; The figure 7 which shows the results of measuring the tensile strength Rm for these same steels; The figure 8 which shows the results of measuring the elongation at break A% for these same steels; The figure 9 which shows the conventional elastic limit Rp 0.2 measured during tensile tests in three directions on two of these steels; The figure 10which shows the results of measuring the tensile strength Rm in three directions on two of these steels; The figure 11 which shows the results of measuring the elongation at break A% in three directions on two of these steels; The figure 12 And 13 which show, respectively for a reference steel and for a steel according to the invention, their limit drawing ratio LDR; The figure 14 which shows, for two steels according to the invention and a reference steel, the influence of the salinity and the temperature of an aqueous solution of NaCl on the resistance to pitting corrosion; The figure 15 which shows, for various steels tested, the influence of PREN on resistance to pitting corrosion; The figure 16 which shows, for two steels according to the invention and a reference steel, the intensity-voltage curves allowing the sensitivity of these steels to uniform corrosion to be assessed; The figure 17which shows, for two steels according to the invention and three reference steels, the results of drop-by-drop evaporation tests allowing their resistance to stress corrosion to be evaluated; figure 18 which shows the results of depassivation pH measurements for a steel according to the invention and three reference steels;

[0043] First, comparative tests were carried out on steels of various compositions, in order to find the right balance between the contents of the different elements Si, Mo, W, Cu which were a priori likely to have a positive effect on the corrosion resistance of the classic X2CrNi18-9 grade, by their nobility or their stoichiometry, alone or in combination. They made it possible to discriminate the respective influences of all these chemical elements on various properties of this grade, such as its hot forgeability, the stability of the austenite, its corrosion resistance, etc. A classic X2CrNiMo17-12-2 was also incorporated into the tests for comparison, and an X2CrNiMo17-12-2 enriched in Si which could appear, at first sight, as a possible solution to an improvement of the properties of this basic grade, in conjunction with the presence of 2.0% of Mo.

[0044] The different compositions tested are summarized in Table 1, expressed in % by weight. It should be understood that the elements not listed in the table (as in the other tables in this text describing steel compositions) are only present in trace form resulting from the production, without metallurgical influence.

[0045] The designations given to the different grades in Table 1 are not standardized; they only apply within the specific framework of this text and must be understood as corresponding to an X5CrNi18-9 steel, in the cases of reference to 304 or X2CrNiMo17-12-2 in the cases of reference to 316L to which the element(s) cited in the designation have been significantly added and which place it outside the standards governing the compositions of X5CrNi18-9 or X2CrNiMo17-12-2. Table 1 - Chemical compositions of shades intended to highlight the invention Example Designation C ppm Million % P % S ppm Are you % No % Cr % With % My % W % N ppm 1 Base 304 573 0.99 0.003 9 0.27 9.48 17.81 0.30 0.15 traces 389 2 304Mo 560 0.98 0.003 8 0.28 9.48 17.84 0.29 0.48 traces 412 3 304MoSi 566 0.97 0.003 7 1.34 9.49 17.80 0.30 0.50 traces 488 4 304MoSiCu 573 0.97 0.003 7 1.34 9.47 17.80 1.99 0.50 traces 621 5 Base 304 551 0.98 0.003 6 0.30 9.47 17.84 0.30 0.15 0,008 428 6 304W 539 0.97 0.003 6 0.29 9.48 17.57 0.30 0.15 0,35 421 7 304WSi 532 0.96 0.003 6 1.42 9.52 17.49 0.30 0.15 0,46 422 8 304WSiCu 508 0.96 0.003 5 1.42 9.51 17.53 2.00 0.15 0,45 428 9 316L base 211 1.27 0.003 9 0.37 10.79 16.42 0.29 1.98 traces 477 10 316LSi 229 1.26 0.003 7 1.35 10.82 16.39 0.29 1.98 traces 611

[0046] Castings of steels with the compositions listed in Table 1 were made. Small ingots were obtained, from which 40 mm thick samples were extracted, which were then hot rolled at 1150°C to a thickness of 4 mm, then annealed at 1140-1120°C and pickled. They were then cold rolled to a thickness of 1.5 mm, annealed at 1140-1120°C, then cooled in forced air and pickled.

[0047] This method of preparation is entirely conventional for austenitic stainless steels of the types which the invention aims to replace, in particular for the preferred applications envisaged which have been cited.

[0048] The following conclusions were drawn: The average grain sizes of 304 grades and their derivatives are 9.3 to 11.2 ASTM, as seen in Table 2, with ordinary 304 having the smallest average grain sizes (remember that a small grain size corresponds to a high ASTM value). The additions of Si, and especially Mo or W, contribute to increasing the average grain size. Grade 316L has an average grain size of 9 ASTM; the presence of 1.35% Si instead of 0.37%, all other things being substantially equal, slightly increases the average grain size (8.7 ASTM). Table 2 - Grain sizes obtained after cold rolling and annealing for the same thickness of 1.5 mm. Example Designation ASTM grain size 1 Base 304 11,2 2 304Mo 9,5 3 304MoSi 9,9 4 304MoSiCu 9,3 5 Base 304 10,8 6 304W 9,4 7 304WSi 10,2 8 304WSiCu 9,2 9 316L base 9,0 10 316LSi 8,7

[0049] Concerning the mechanical properties representative of the capacity of the steel to be formed, namely the conventional elastic limit Rp 0.2, the tensile strength Rm and the elongation at break A, the following observations were made, as seen in the figures 1 to 3 .

[0050] The addition of Mo alone to 304 in the proportions tested has no significant influence. The yield strength and tensile strength remain higher than those of 316L. The latter has a slightly higher elongation at break than 304 and its Mo- and Si-enriched derivatives studied.

[0051] Added W alone tends to reduce these properties.

[0052] It is above all the addition of Si, combined with that of Mo in the proportions examined, which has the most significant influence, in particular on the elongation at break.

[0053] Regarding the resistance to pitting corrosion expressed on the figure 4 by the pitting potential Epit 0.1 in a medium with 0.02M NaCl at 23°C, the classic 316L always have a better resistance than the 304. However, it is noted that for both the 304 and the 316L, the addition of approximately 1.3% to 1.4% of Si in the 304 allows to significantly increase the pitting potential. The best results of these tests are those of the 304 to which 0.5% of Mo and 1.3% of Si have been added and which contain 17.4 to 17.8% of Cr. They are even better than those obtained on a 316L with 1.98% of Mo and 16.4% of Cr, to which 1.35% of Si would have been added.

[0054] W has no significant effect on the pitting potential. According to these tests, its effect is therefore completely distinct from that of Mo.

[0055] The addition of Cu is harmful, since it reduces the pitting potential, all other things being equal.

[0056] It then appears that the higher Cr content of grades 304, and its association with a moderate Mo content and an Si content greater than 1%, make it possible to significantly increase the resistance to pitting corrosion of austenitic stainless steels.

[0057] Concerning crevice corrosion, uniform corrosion and stress corrosion, again the addition of 0.5% Mo, with or without additional Si, proves beneficial and allows performance comparable to that of 316L to be obtained.

[0058] These preliminary tests to those leading to the present invention therefore suggested that the solution consisting of adding a little Mo or a little Mo and Si to a conventional type 304 steel could constitute, in terms of resistance to different types of corrosion, mechanical properties and cost, a good alternative to the use of type 316 or 316L steels, richer in Ni and Mo than the present invention and, therefore, significantly more expensive, having a more marked ecological footprint and whose formability is not always optimal for the intended applications.

[0059] However, too much Si addition in such a conventional 304 tended to increase the mechanical properties, including the hot hardness, in a way that was unfavorable to the formability of the material. It was found that for a Si content of 1.35% associated with 0.5% Mo, the A4 temperature, which marks the transformation of the delta ferrite phase to austenite, was too low and led, during hot rolling, to the formation of cracks on the edges of the product. An optimized solution therefore remained to be found.

[0060] A balancing of the composition which makes it possible to obtain a suitable A4 temperature, i.e. higher than the reheating temperature before hot rolling, is then necessary in order to ensure the integrity of the steel during hot rolling, and mechanical properties and corrosion resistance on the finished product compatible with the applications mainly envisaged for this steel: heat exchangers and chimney flues.

[0061] In light of previous trials, it was concluded that for this purpose it was desirable, a priori, to make the following adjustments to these first attempts: limit oneself to significantly low C contents, to limit the risk of intergranular corrosion; do not add Nb, at least significantly, as it is an expensive element whose advantages on resistance to intergranular corrosion can also be obtained by lowering the C content; maintain sufficient levels of Cr and Mo to ensure the desired corrosion resistance; minimize the addition of Ni to limit material costs; adjust the N content to achieve good hot ductility, requiring an adequate A4 temperature; adjust the Si content to increase the A4 temperature and decrease the hardness, while maintaining the synergy between Mo and Si on corrosion resistance, which was observed during the preliminary tests described.

[0062] For this purpose, 50 kg ingots were cast, the compositions of which are shown in Table 3. The elements which do not appear in this table are impurities. Examples 11 to 14 are according to the invention, Example 15 is the reference 316L. In Examples 11 to 14, the Al content is therefore at most 0.06%, the Sn content at most 0.05%, the Nb content at most 0.08% (even less than 0.03%), the Ti content at most 0.08%, the Zr content at most 0.08%, the B content at most 0.01%, the sum of the W and Mo contents remains at most 0.8% and the Pb content at most 0.03%. The examples according to the invention differ from each other very essentially in their Si content, which ranges from approximately 1.3% to 1.0%. It will be noted that the Mo is uniformly fixed at 0.5% and that N has made it possible to gradually compensate for the increase in Si. Table 3: Compositions of examples of steels that have been cast Example C % Million % Are you % No % Cr % With % My % V % Co % N ppm P % S ppm O ppm 11 0,022 0,97 1,32 9,57 17,93 0,30 0,50 0,084 0,2 991 0,027 8 63 12 0,022 0,97 1,22 9,58 17,98 0,31 0,50 0,085 0,2 815 0,027 8 47 13 0,024 0,96 1,12 9,53 17,99 0,30 0,50 0,085 0,2 692 0,027 8 51 14 0,023 0,96 1,02 9,59 17,95 0,30 0,50 0,085 0,2 662 0,027 7 62 15 0,020 1,24 0,39 10,06 16,56 0,31 2,04 0,085 0,2 558 0,025 8 69

[0063] Samples measuring 150 x 100 x 25 mm were then cut from the material and hot rolled to reduce their thickness from 25 to 2.8 mm.

[0064] A first annealing at 1100°C was then carried out without holding, followed by pickling, which resulted in total recrystallization of the samples and an oxide-free surface.

[0065] Then, cold rolling was carried out to a final thickness of 1 mm for these samples, which is an ideal thickness to ensure that the required drawing properties for the applications are obtained.

[0066] Final anneals were carried out at temperatures of 1075°C and 1100°C to achieve various average grain sizes.

[0067] As seen on the figure 5, the results obtained in terms of grain size are very little different from one sample to another for a given annealing temperature. The steels according to the invention and 316L behave in a similar way. Annealing at 1075°C leads in all cases to an average grain size of the order of 7.5 to 8 ASTM, and annealing at 1100°C leads in all cases to an average grain size of the order of 8.5 to 9 ASTM without, in the case of the invention, an influence of the Si content being clearly observed.

[0068] The average grain size of steel has a significant impact on its mechanical behavior, particularly its drawing capacity. The smaller the ASTM grain size, the more deformable the material. Therefore, for the intended applications, particularly for heat exchanger plates with complex geometries, the flexibility of adjusting the grain size between 6 and 11 ASTM is a major advantage in establishing a fair compromise between the deformation capacity required for drawing the part and the mechanical strength required for its service life.

[0069] Tensile tests were also carried out in the direction perpendicular to the rolling direction DL (i.e., in the transverse direction DT) on the samples annealed at the two above-mentioned temperatures. The results of these tests are reported on the figure 6for the conventional elastic limit Rp 0.2, 7 for the tensile strength Rm and 8 for the elongation at break A%.

[0070] It emerges that: Concerning the conventional elastic limit Rp 0.2 and the tensile strength Rm, the steels according to the invention have higher values, at equal average grain size, than 316L; these values ​​tend to decrease with the Si and N contents; the steels according to the invention are harder than 316L, even if the difference is reduced for an Si content of approximately 1%; The elongation at break is fairly comparable for all the steels according to the invention and for 316L, at identical average grain size, and it varies fairly little when going from 8 ASTM to 9 ASTM.

[0071] Tensile tests were also carried out in three directions on these same two examples 14 and 15: the rolling direction DL, the transverse direction DT perpendicular to DL and the 45° direction, i.e. the bisector of the other two directions. It should be remembered that the tests of the figures 6 to 8 only concerned the DT management.

[0072] For all these tests, the 12.5 mm long, 50 mm wide and 1 mm thick samples tested were given, using a final annealing at 1080°C without holding, an average grain size of 8.6 ASTM for example 14 according to the invention and 8.7 ASTM for the reference example 15 in 316L. The results of these tests on Rp 0.2, Rm and A% are respectively visible on the figure 9 , 10 And 11 .

[0073] Concerning Rp 0.2 and Rm, the two examples behave in roughly the same way, with deviations not exceeding 10 MPa for each measurement direction. The elongation at break A% is slightly higher for example 14 according to the invention.

[0074] If the planar isotropy coefficient Δr of the two examples is calculated from the tensile curves in the three directions, it is found to be equal to -0.286 for example 15 of 316L and to -0.229 for example 14 according to the invention. The good mechanical properties of the steel according to the invention, in that they have significant mechanical strengths associated with large deformations at break and high isotropy, therefore make it a good substitute for applications of 316L for which these properties are important, as is resistance to various types of corrosion.

[0075] The formability of a grade can usefully be characterized by the elastic limit, but additional tests must be carried out to get a more precise idea, particularly with a view to stamping. For this purpose, examples 14 and 15 were subjected to an Erichsen test and a deep drawing test.

[0076] The Erichsen test aims to obtain the Erichsen index IE which corresponds to the depth of the stamping before the appearance of a crack, according to an equibiaxial stress.

[0077] A punch with a constant diameter of 20 mm was used, a constant blank holding pressure of 1000 daN, Molykote ® lubricant spread with a brush, and a constant stamping speed of 5 mm / min. The thickness of the sheet metal tested was 1 mm.

[0078] Example 14 according to the invention performs slightly better than reference example 15: the IE of example 14 is 12 mm, that of example 15 is 11.5 mm.

[0079] The Limiting Drawing Ratio (LDR) and delayed breakage sensitivity of Examples 14 and 15 were also examined.

[0080] The LDR theoretically corresponds to the ratio β between the maximum diameter of the blank before cracking and the initial diameter of the punch

[0081] The results are visible on the figure 12 And 13 The LDRs are very similar for the two examples: 2.22 for example 14 according to the invention ( figure 13 ) And 2 , 17 for reference example 15 ( figure 12 ). The LDR of the example steel according to the invention is even slightly better than that of the reference 316L steel.

[0082] Concerning the sensitivity to delayed breakage, for a β ratio of 2.12 no delayed breakage was observed, on both examples 14 and 15, in the case of a 2B finish (cold rolled product, non-bright annealed, pickled and skin-passed).

[0083] The deformation due to elastic return after stamping was also evaluated for both examples 14 and 15. No notable difference is observed in their respective behaviors, which is consistent with the similarity of their elastic limits.

[0084] Corrosion resistance tests were also carried out on examples 11 and 14 according to the invention (respectively at 1.3% and 1.0% Si) and on reference example 15 in 316L steel.

[0085] Electrochemical tests were carried out on 15 mm diameter stamped discs, polished underwater with 1200 grit SiC paper. Then, they were degreased in an acetone / ethanol ultrasonic bath, rinsed with distilled water, and left to age for 24 h in ambient air.

[0086] Electrochemical corrosion tests were performed in a solution of distilled water and analytical grade NaCl, deaerated with nitrogen and hydrogen. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum electrode as the counter electrode.

[0087] Pitting corrosion resistance is expressed by the pitting corrosion potential E pit , measured in mV / SCE on samples 11, 14 and 15 of Table 3 in a deaerated NaCl solution at pH 6.6 by leaving the sample at free potential for 15 min, then performing a potentiodynamic scan at a constant scan rate (100 mV / min) until an intensity of 50 µA was reached at which the potential E pit was measured. The experiments were carried out in 0.02M and 0.5M NaCl solutions, at 23°C and 50°C. The elementary pitting probability Pi in cm 2< was measured as a function of the corrosion potential E pit . The results are shown in fig.14 .

[0088] It turns out that the three samples present very similar results to each other, under identical experimental conditions. In particular, the changes from a 0.02M NaCl solution to a 0.5M NaCl solution and from a temperature of 23°C to a temperature of 50°C have the same influences regardless of the composition of the sample considered. figure 14 translates this observation, by showing the average pitting corrosion potentials E pit and their standard deviations σ for each sample, as a function of the NaCl solution concentration and its temperature.

[0089] This confirms the positive effect of Si addition on pitting corrosion resistance in type 304 stainless steel. With the presence of only 1.0% Si, the results obtained remain very comparable to those of 316L. With 1.3% Si in conjunction with 0.5% Mo, the pitting corrosion resistance is even slightly improved for tests at 23°C, while remaining equivalent at 50°C.

[0090] For comparison, identical tests were carried out on a conventional 304 type stainless steel from 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, 0.052% N and the remaining elements in trace amounts, and on a 316L type steel with a composition of 17% Cr, 0.27% Cu, 1.44% Mn, 2.02% Mo, 10% Ni, 0.33% Si, 0.022% C, 0.035% N and the remaining elements in trace amounts. It was found ( figure 15) that for this industrial 304 and a 0.02M NaCl solution, E pit can drop to 490 mV at 23°C and 390 mV at 50°C. For a 0.5M NaCl solution, these same values ​​are respectively 300 mV and 180 mV. There is therefore a very significant improvement in the resistance to pitting corrosion in the case of an addition of Mo and Si according to the invention, compared to a usual 304, and competitive results are achieved with those obtained on 316L, including at 50°C, with, however, a lower material cost.

[0091] We also took into account the PREN (Pitting Resistance Equivalent Number) which is a classic concept aimed at predicting the sensitivity of a stainless steel to pitting corrosion. The PREN can be taken equal to %Cr + 3.3x%Mo + 16x%N. We see on the figure 15that, at equal PREN, the gain on E pit0.1 provided by the addition of Mo and Si according to the invention to a conventional 304 is estimated at around 100 to 150 mV, in the case of exposure to 0.02M or 0.5M NaCl at 23°C. The gain is more moderate for tests at 50°C (not very significant at 23°C, 50 to 100 mV for 0.5M NaCl) but nevertheless remains interesting for the most difficult conditions encountered during the tests. This shows, in passing, that the PREN is not, on its own, a sufficiently discriminating criterion to very precisely predict the sensitivity of a stainless steel to corrosion resistance.

[0092] For the study of uniform corrosion, the passive layer was first removed from the three samples 11, 14, 15 and from the 304 sample from industrial production whose composition was given earlier, by immersion in a deaerated 2M sulfuric acid solution at a pH lower than the depassivation pH (pHd), for 15 min at the resting potential V corr . Potentiodynamic polarization tests were carried out at a scanning rate of 10 mV / min, from -750 mV / SCE to 1800 mv / SCE. The current / voltage curves were determined. They are shown on the figure 16 .

[0093] They are very similar for the three samples. In particular, it emerges that the peak intensity I crit , which is higher as the uniform corrosion of the metal is rapid, is substantially identical for the three samples tested: 0.25 mA / cm 2< for the sample with 1.3% Si, 0.26 mA / cm 2< for the sample with 1.0% Si and 0.20 mA / cm 2< for the 316 sample and 0.23 mA / cm 2< for the 304 sample from industrial production.

[0094] It is concluded that the addition of 1.3% or 1.0% Si to an AISI 304 stainless steel also containing 0.5% Mo provides identical results in terms of uniform corrosion resistance, and that this is not significantly deteriorated compared to that of an AISI 316L.

[0095] Concerning the resistance to stress corrosion, this was evaluated by a drop-by-drop evaporation test according to ISO 15324, namely: Use of a 0.1M aqueous NaCl solution at 23°C; Projection onto the part of 10 drops / min with a drop height of 1 cm; Heating of the metal sample to 120°C during dripping; U-bending of the sample carried out according to the ASTM G30 standard; Imposing a mirror finish on the part.

[0096] During the test, the time after which cracking of the sample is observed is measured. Three tests are carried out for each example of grade. Their results are visible on the figure 17 .

[0097] Samples 11 of 304 with 0.5% Mo and 1.3% Si additives show a fairly large dispersion in their test results: between 46 and 172 h before cracking. Samples 15 of 304 with 0.5% Mo and 1.0% Si additives have a smaller dispersion, between 46 and 72 h. Samples 16 of 316L show cracking after 48 to 90 h.

[0098] It is concluded that, apart from the usual experimental uncertainties, there is no obvious difference between the steels according to the invention and 316L, from the point of view of resistance to stress corrosion.

[0099] For comparison, tests were carried out on industrial samples of AISI 304L industrial steel (distinguished from conventional 304 by a lower maximum C content, therefore a priori by better corrosion resistance due to the lower risk of Cr carbide formation) and industrial 316L. Their results are also visible on the figure 17. Industrial 304L has relatively modest resistance to stress corrosion, with a time before cracking of 22 to 26 hours. Industrial 316L has a time before cracking of 42 to 48 hours, therefore comparable to what is observed on the best laboratory samples of this same grade and the 304 enriched with Mo and Si according to the invention. A more regular inclusion cleanliness of the industrial samples compared to that of the laboratory samples may explain the lower dispersion of the measurement results concerning them.

[0100] The crevice corrosion resistance of both examples 14 and 15 was also evaluated. A simulation of an environment conducive to this type of corrosion (low pH and high chloride ion concentration) was carried out using a 2M NaCl solution at a pH below 3 adjusted by the addition of hydrochloric acid, maintained at 23°C. The aim was, for each sample, to determine the pH that would destroy its passivation layer.

[0101] For this purpose, the samples were first subjected to cathodic polarization at -750 mV / SCE for 2 min and then left at their resting potential. Potentiodynamic measurements were then started at a scanning rate of 10 mV / min in the anodic direction from -750 mV / SCE. The measurements were carried out at different pH values ​​in order to determine the maximum intensity in the active region of the polarization curves. Their results can be seen on the figure 18 .

[0102] It turns out that the two samples have, again, very similar behaviors. The depassivation pH is, in both cases, between 1 and 1.2, which is a range of values ​​that compares favorably with that of ordinary industrial AISI 304 (1.7-2.3), and also with that of ordinary industrial 316 (1.5-1.65).

[0103] Good resistance to crevice corrosion is particularly sought after, for example, for chimney flues which are in contact with combustion flue gas condensates and in assembly conditions conducive to such corrosion occurring. At the end of all these results, the resistance to the different types of corrosion of the two grades according to the invention with 0.5% Mo and 1.3% or 1.0% Si which have been thoroughly tested is therefore not significantly inferior to that of conventional 316L.

[0104] In conclusion, it is therefore confirmed that the joint presence of Si and Mo, in the precise proportions according to the invention, in a stainless steel whose composition, in other respects, is close to that of X2CrNi189 (1.4307), comparable to AISI 304L, has beneficial effects on the properties of resistance to various types of corrosion and on the capacity of the steel to be formed. The properties sought here are very similar, or even superior, to those of 316L, and this allows the steel according to the invention to be substituted economically, and without metallurgical drawbacks, for an X2CrNiMo17-12-2, comparable to AISI 316L, for uses which require these qualities, such as the manufacture of heat exchanger plates and chimney flues.

Claims

1. Austenitic stainless-steel, characterized in that the composition thereof, in weight percentages, consists of: - traces ≤ C ≤ 0.03%; - 1.0% ≤ Mn ≤ 2.0%; - 0.8% ≤ Si ≤ 2.0% ; preferentially 1.0% ≤ Si ≤ 1.5%; - traces ≤ Al ≤ 0.06%; preferentially traces ≤ Al ≤ 0.01%; - traces ≤ P ≤ 0.045%; - traces ≤ S ≤ 0.015%; - 8.0% ≤ Ni ≤ 12.0% ; preferentially 9.45% ≤ Ni ≤ 10.0%; - 17.5% ≤ Cr < 20.0% ; - 0.4% ≤ Mo ≤ 0.8% ; preferentially 0.5% ≤ Mo ≤ 0.6%; - traces ≤ Sn ≤ 0.05%; - traces ≤ Nb ≤ 0.08%; - traces ≤ V ≤ 0.15%; - traces ≤ Ti ≤ 0.08%; - traces ≤ Zr ≤ 0.08% ; - traces ≤ Co ≤ 1.0%; - 0.02% ≤ Cu ≤ 0.6%; - traces ≤ B ≤ 0.01%; - traces ≤ W + Mo ≤ 0.8%; - traces ≤ Pb ≤ 0.03%; - traces ≤ N < 1000 ppm; - traces ≤ O ≤ 0.01%; preferentially traces ≤ O ≤ 0.005%; the rest being iron and impurities resulting from the production.

2. The austenitic stainless-steel according to claim 1, characterized in that the average grain size thereof is comprised between 11 and 6 ASTM, and preferentially between 10 and 7 ASTM.

3. The austenitic stainless-steel according to any of claims 1 and 2, characterized in that traces ≤ Nb < 0.03%.

4. The austenitic stainless-steel according to claim 3 characterized in that traces ≤ Nb < 0.02%.

5. The austenitic stainless-steel according to any 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. The austenitic stainless-steel according to any of claims 1 to 6, 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. A heat exchanger plate, characterized in that same is made of austenitic stainless-steel according to one of claims 1 to 8.

10. An element of a chimney duct, characterized in that same is made of an austenitic stainless-steel according to one of claims 1 to 8.