EP0225263B1 - Sheet or strip of stainless ferritic steel, particularly for exhaust systems - Google Patents

Abstract

The invention concerns a strip or sheet of ferritic stainless steel of the following composition (% by weight): (C+N)<.060; Si<0.9; Mn<1; Cr=15 to 19; Mo<1; Ni<0.5; Ti<0.1; Cu<0.4; S<0.02; P<0.045; Zr=0.10 to 0.40, and 7(C+N)</=Zr</=7(C+N)+0.15; Nb=0.25 to 0.55, in non-combined form; Al=0.020 to 0.080; Fe=the balance, in which Al is essentially in solid solution. The process for the production of the sheet or strip comprises a final annealing operation which is carried out at between 980 DEG and 1020 DEG C., typically for 0.5 to 5 minutes at between 990 DEG and 1010 DEG C. The strip or sheet according to the invention is used for any application requiring an economic compromise in regard to ductility (sheet and welds), hot resistance and resistance to corrosion, for example in motor vehicle exhaust manifolds.

Description

The invention relates to the field of rolled products of stainless ferritic steel, and more particularly that of exhaust systems.

Statement of the problem

• - bonne ductilité des soudures sans métal d'apport pour la conformation en pièces et la fatigue;
• - bonne résistance au fluage à chaud (SAGTEST) ;
• - bonne résistance à l'oxydation à chaud, continue ou cyclique, aussi bien pour la pleine tôle que pour les soudures;
• - bonne résistance à la corrosion, en particulier vis-à-vis des projections d'eau salée provenant du salage des routes pendant l'hiver.

The manufacture and maintenance in service of exhaust systems, for example automobile exhaust manifolds and tubes, has a number of requirements which are difficult to meet simultaneously and economically:

• - good ductility of welds without filler metal for component shaping and fatigue;
• - good resistance to hot creep (SAGTEST);
• - good resistance to hot oxidation, continuous or cyclic, both for full sheet metal and for welds;
• - good resistance to corrosion, in particular vis-à-vis salt water projections from road salting during winter.

The Applicant has tried to improve the properties compromises obtained with sheets or strips of shades already known and more particularly with stainless ferritic grades stabilized with Nb and / or Zr, and this especially with regard to the creep resistance and to hot oxidation. Ferritic stainless grades are superior to austenitic grades because of their coefficient of thermal expansion.

Known state of the art

• - l'ordre de facilité de formation des nitrures et carbures est le suivant: «nitrure de Zr, carbure de Zr, carbure de Nb et nitrure de Nb», C et N sont donc piégés préférentiellement par Zr, et de façon plus stable que par Nb;
• - le zirconium dépassant de plus de 0,25% la quantité requise pour se combiner avec C et N entraîne une détérioration sensible de la ductilité et de la résistance à la corrosion;
• - le niobium en solution solide ne doit pas excéder 0,3%, sinon il conduit à une mauvaise ductilité des soudures, la fragilité pouvant être due à la formation d'un composé intermétallique Nb₂ (Fe,Cr)₃.

The patent US-A-4010049 relates to a ferritic stainless steel of composition: C 0.10% maximum; Cr 11 to 30%; Mo 3% max; Nb (columbium) 0.1% total to 0.3% in solid solution and not less than 7.7 x C% - (Zr% -6.5x N%); Zr 6.5 N% to 0.25% + (7.6% C + 6.5% N); Fe and residual impurities the balance. This document gives several indications on the role of Zr and Nb:

• - The order of ease of formation of nitrides and carbides is as follows: "Zr nitride, Zr carbide, Nb carbide and Nb nitride", C and N are therefore preferentially trapped by Zr, and more stably than by Nb;
• - the zirconium exceeding by more than 0.25% the quantity required to combine with C and N results in a significant deterioration of the ductility and of the corrosion resistance;
• - the niobium in solid solution must not exceed 0.3%, otherwise it leads to poor ductility of the welds, the brittleness possibly being due to the formation of an intermetallic compound Nb ₂ (Fe, Cr) ₃ .

In the communication presented to the Congress of the S.A.E. in Detroit, Michigan, on February 23-27, 1981 ("Influence of Columbium on the 870 ° C Creep Properties of 18% Chromium Ferritic Stainless Steels"), John N. JOHNSON studies the creep resistance to traction at 870 ° C of different 18% Cr ferritic stainless steels containing Ti + Nb and find that the creep results are improved by re-annealing the samples (already annealed in the factory) at temperatures ranging from 1040 to 1150 ° C, for example 30 min to 1095 ° C. The materials having contents of non-combined Nb of 0.3 to 0.6% comprise many intergranular precipitates based on Nb and the effect of the second annealing was to dissolve these precipitates and to increase the size of grain.

Two documents indicate the influence of an addition of Al. Patent application FR-A-2 463194 relates to ferritic steels with 1 to 20% Cr and Ti, Nb with 0.5 to 2% AI, in which a minimum AI content of 0.5% and preferably 0.75% is required to provide resistance to oxidation at elevated temperatures. Above 2%, AI has a detrimental effect on the weldability. In patent application J PA-82/146440, a ferritic stainless steel contains 0.08 to 0.5% AI and one or more of the elements B (2-50 ppm), Ti (0.005-0.4%), Nb (0.005-0.4%), V (0.005-0.4%) and Zr (0.005-0.4%), and AI results in structural changes at the various stages of sheet metal transformation with an increase in “ridging resistance” (resistance to the phenomenon of stringing or crumpling).

Statement of the invention

• (C+N) < 0,060; Si < 0,9; Mn < 1;
• Cr 15 à 19; Mo < 1;Ni < 0,5; Ti < 0,1;
• Cu < 0,4; S < 0,02; P < 0,045;
• Zr = 0,10 à 0,50 avec Zr compris entre 7 (C+N)
• - 0,1 et 7 (C+N) + 0,2
• Nb compris entre 0,25 et 0,55 si Zr > 7 (C+N) et compris entre 0,25 + 7 (C+N) - Zr et 0,55 + 7 (C+N) - Zr si Zr < 7 (C+N);
• AI 0,020 à 0,80; impuretés et Fe: le solde.

The subject of the invention is a sheet or strip of ferritic stainless steel, usually in the annealed state, the final annealing operation then being most often followed by a finishing pass and work hardening or “skinpass” producing an elongation of less than 1%, and intended in particular for the manufacture of manifolds and exhaust tubes. The composition of this sheet or strip is (% by weight):

• (C + N) <0.060; If <0.9; Mn <1;
• Cr 15 to 19; Mo <1; Ni <0.5; Ti <0.1;
• Cu <0.4; S <0.02; P <0.045;
• Zr = 0.10 to 0.50 with Zr between 7 (C + N)
• - 0.1 and 7 (C + N) + 0.2
• Nb between 0.25 and 0.55 if Zr> 7 (C + N) and between 0.25 + 7 (C + N) - Zr and 0.55 + 7 (C + N) - Zr if Zr < 7 (C + N);
• AI 0.020 to 0.80; impurities and Fe: the balance.

Zrest consumed by stabilization, that is to say by trapping C and N in the form of nitrides and carbides, up to approximately 7 (C + N)%. The free Zr is therefore limited to 0.2%, which makes it possible to avoid the drawbacks linked to the formation of eutectic compounds containing Fe ₃ Zr in the case where there is more than 0.25% of free Zr, these compounds resulting in a degradation of the mechanical characteristics, in particular of the ductility and of the creep resistance, and a reduction in the corrosion resistance, as indicated essentially by document US-A-4010049.

At this content at most equal to 0.2%, the free Zr has no significant direct influence on the resistance to oxidation.

The free or non-combined Nb is between 0.25 and 0.55%. The Nbtotal includes in addition to the free Nb an addition of 7 (C + N) -Zr, to compensate for the lack of stabilization by insufficient Zr, in the case where Zr is between 7 (C + N) - 0.1 and 7 (C + N).

It is known by JOHNSON (document cited above) that the free or uncombined Nb increases the creep resistance at the level of 0.3 to 0.6% when the tested samples have been annealed at least 1040 ° C. found, in annealing tests on sheets according to the invention of thickness 1 mm vertical, at 1040 ° C for 5 min and at 1150 ° C - 1 min, that it occurred with such high annealing temperatures unacceptable deformation of the sheet by creep. And it was observed that an annealing of 1000 ° C - 1 min led to good creep and ductility results for the sheets of the invention, and that generally an annealing at 1000 ± 10 ° C of duration between 0.5 and 5 min was suitable, which is much easier to use industrially than annealing at a temperature of at least 1040 ° C. As for the poor ductility of the welds reported by US-A-4010049, when Nb in solid solution (that is to say free or not combined) is in content greater than 0.3%, and this in the case of ferritic sheets stainless at 18% Cr with Nb + Zr, this drawback does not does not exist with the sheets of the invention whose TIG welds without filler metal are extremely ductile.

La perte de poids est ainsi diminuée de 26% par la présence de 0,020% d'AI et de 53% par la présence de 0,080% d'AI. La teneur en AI est limitée à 0,080% de façon à éviter, comme on en a l'expérience avec des aciers inoxydables à 17% Cr (type AISI 430), des crasses sur les cordons de soudure entraînant des oxydations irrégulières et des criques à la conformation, donc des corrosions plus rapides. Cet effet de crasses est important au niveau de 0,1% AI; mais si on veut aussi éviter ou limiter les inclusions d'alumine liées à la présence de trop d'aluminium, ces inclusions étant des sites d'initiation de piqûres à la suite des projections d'eau salée provenant du salage ou du dessalement des routes pendant l'hiver, il convient de rester en dessous de 0,05% d'AI comme il est fait dans la composition préférentielle indiquée plus loin.The total content of AI assayed corresponds essentially to AI in solid solution. Indeed, Zr has more affinity than AI for oxygen and there is little residual oxygen in the metal, so that there can be only very little AI in the form of alumina. Furthermore, the affinities of Zr and Nb for nitrogen and the greater affinity of AI for oxygen than for nitrogen means that aluminum nitride AIN does not form. The result, confirmed qualitatively by micrographic examinations, is that AI is in solid solution with the exception of a content at most equal to 0.003% and corresponding essentially to alumina. With a small addition of 0.020 to 0.080% AI, a surprising improvement in resistance to hot oxidation is obtained, linked to the role of aluminum in solid solution, whether it is continuous oxidation. with air between 800 and 1000 ⁰ C or alternating cyclic oxidation in full sheet metal or on welds. Thus, in continuous oxidation in air for 50 h, the limit temperatures corresponding to a weight loss of 200 g / m ² are 970 ° C for AI <0.002%, 1020 ° C for AI = 0.036% and 1070 ° C for AI = 0.090%. And in continuous oxidation at 1000 ° C for 50 h, the following weight losses are obtained:

Weight loss is thus reduced by 26% by the presence of 0.020% of AI and by 53% by the presence of 0.080% of AI. The AI content is limited to 0.080% so as to avoid, as has been the experience with 17% Cr stainless steels (type AISI 430), dross on the weld beads causing irregular oxidation and cracking conformation, therefore faster corrosion. This dross effect is significant at the 0.1% AI level; but if one also wants to avoid or limit the inclusions of alumina linked to the presence of too much aluminum, these inclusions being sites of initiation of stings following the projections of salt water coming from salting or desalination of roads during winter, it is advisable to remain below 0.05% of AI as it is done in the preferential composition indicated below.

We sought to explain this surprising improvement in the resistance to oxidation produced by such low aluminum contents. The study was made on sheet metal samples 1 mm thick, coming in particular from two flows at 16% Cr without Zr, N ° 101 and N ° 401, one at 0.6% Nb and 0.048% AI, and the other at 0.45% Nb without AI (AI <0.002%), continuously oxidized in air for 50 h at 900 ° C. In the case of the first flow containing Nb and AI, it was observed that the oxidized layer of thickness 10 μm was anchored to the sheet by small plates of typical unit dimensions 0.3 to 0.8 μm, containing alumina and in places of niobium, in the form of inclusions of Nb compound . This anchoring mechanism is quite different from the mechanism for forming an alumina layer specific to stainless ferritic grades with an AI content greater than 0.5%.

In the case of the second casting without AI, there is no anchoring and, in glow discharge spectrographic examination, it has been verified that there was no Nb at the metal / oxidized layer interface.

It can therefore be concluded that, in the case of the sheets of the invention with Zr-Nb-AI, AI appears to intervene in conjunction with Nb to produce an anchoring favorable to the behavior of the oxidized layer and thus improve the resistance to corrosion. hot. In addition, in a series of alternating oxidation tests at 800 ° C., sheet metal samples according to the invention with Zr-Nb-AI showed beyond 350 h of test better resistance than sheet metal samples with Nb-AI without Zr having comparable non-combined Nb and AI contents, which seems to show that Zr has a role in this resistance to alternating or cyclic oxidation.

• (C+N) < 0,040; Si < 0,8; Cr 16 à 18;
• Mo < 0,3; Ni < 0,3; Ti < 0,05; S < 0,01;
• Zr = 0,10 à 0,40 avec Zr compris entre 7 (C+N) et 7 (C+N) + 0,15;
• Nb 0,30 à 0,52 et encore de préférence 0,33 à 0,50;

The constituent elements of the sheet or strip of the invention are taken individually or as a whole in the following preferred content ranges:

• (C + N) <0.040; If <0.8; Cr 16-18;
• Mo <0.3; Ni <0.3; Ti <0.05; S <0.01;
• Zr = 0.10 to 0.40 with Zr between 7 (C + N) and 7 (C + N) + 0.15;
• Nb 0.30 to 0.52 and more preferably 0.33 to 0.50;

AI 0.020 to 0.045 and more preferably 0.025 to 0.040.

The maximum contents of (C + N) and Zr can thus be lowered simultaneously, giving greater security for the ductility of the full sheet and welds as well as for corrosion resistance. Zr is then always in sufficient quantity for stabilization in the restricted sense, that is to say for the trapping of N and C in the form of nitrides and carbides. Nb is fully available for resistance to hot creep and included in the content ranges which give the minimum deflection to the SAGTEST tests at 850 ° C. AI can be included in increasingly narrow ranges of content, industrially feasible and representing an optimal compromise between resistance to hot oxidation and resistance to pitting corrosion.

In the delivery state, the sheet or strip according to the invention is in the annealed and optionally erected state, this annealed state typically corresponding to a treatment at 1000 ± 10 ° C for 0.5 to 5 min.

The invention also relates to the method of manufacturing a sheet or strip of ferritic stainless steel, in which, as is known, the hot rolled strip is annealed with a thickness of between 2.5 and 5 mm between 800 and 1000 ° C. under slightly oxidizing conditions, then it is shot and pickled, then it is cold rolled to the delivery thickness typically between 0.6 and 3 mm, with or without annealing and intermediate pickling, and it is annealed at the end of the parade, then it is subjected to a finishing pass or so-called "skin-pass" hardening producing an elongation of less than 1%, with possibly a final stripping. This process differs from the prior art in that the sheet or strip has the composition of the invention and in that the annealing, making it possible to obtain good hot creep results, is carried out between 980 and 1020 ° C. , and preferably between 990 and 1010 ° C for 0.5 to 5 min, or at a temperature and for a time giving an equivalent metallurgical state. This final annealing is typically carried out following a rolling producing an elongation of at least 100% from the previous annealing.

The test results which will follow, the figures and the tables which accompany them, will make it possible to illustrate and comment on the various aspects of the invention.

TESTS Test series N ° 1 - Hot creep tests

A number of laboratory castings of 25 kg each were carried out, the analyzes of which appear in Table 2 (Nb + AI castings) and in Table 3 (Zr + Nb + AI castings).

Other impurities were analyzed: W <0.003 - V = 0.02 to 0.06 - Sn <0.003 - Co 0.01 to 0.02 - Ti = 0.004 to 0.013 - Pb <0.002 - Ta <0.01 Se <0.002 - Mg <0.0002 - Ca = 0.0001 to 0.0003 - 0 = 0.0036 to 0.0172%.

The total of the other impurities is thus clearly less than 0.3% and Fe constitutes the balance.

• - forgeage à chaud à épaisseur 14 mm,
• - rectification des 2 faces à l'épaisseur 12 mm,
• - laminage à chaud en épaisseur 3 mm, '
• - recuit 4 h à 800° C,
• - décapage,
• - laminage à froid à épaisseur 1 mm,
• - recuit à 1000° C soit 1 min, soit 5 min.

The main stages of the transformation into 1 mm thick sheet were:

• - 14 mm thick hot forging,
• - rectification of the 2 faces to the thickness 12 mm,
• - hot rolling in thickness 3 mm, '
• - annealing 4 h at 800 ° C,
• - pickling,
• - cold rolling 1 mm thick,
• - annealing at 1000 ° C either 1 min or 5 min.

Rectangular test specimens of 310 x 25 mm were cut from the sheets, and bent at 90 ° to 25 mm from one end. Then they were laid flat each on 2 supports of internal distance 254 mm and external distance 264 mm and they were subjected to continuous SAGTEST tests of creep under their own weight during 100 h at 850 ° C.

The graphs in Figures 1 and 2 show the arrows observed after 100 h at 850 ° C, and Tables 4 and 5 show the average arrows (means of 3 results) obtained for the test pieces in Table 2 and for those in Table 3 .

• - les éprouvettes au Nb+AI (tableau 4 et fig. 1 et 2) résistent d'autant mieux au fluage que la teneur en Nb libre est élevée, et le recuit préalable de 5 min à 1000° C donne, à même teneur en Nb libre, des résultats bien meilleurs que le recuit de 1 min à 1000° C, cela pour tout l'intervalle testé (0,1 à 0,54% de Nb libre);
• - les éprouvettes au Zr+Nb+AI (tableau 5 et fig.1 1 et 2) donnent des résultats bien meilleurs que les éprouvettes au Nb+AI avec le recuit de 1 min à 1000° C, et qui ne sont qu'un peu moins bons que ceux obtenus avec les éprouvettes semblables au Zr+Nb+AI recuites 5 min à 1000° C, spécialement entre 0,25% et 0,55% de Nb libre, intervalle dans lequel les résultats de flèches ne diffèrent que d'environ 0,3 à 0,7 cm. L'aptitude des coulées au Zr+Nb+AI selon l'invention à donner une relativement bonne résistance au fluage même après un recuit limité de la sorte (1 min à 1000° C) est un avantage industriel très important;
• - les éprouvettes au Zr+Nb+AI, qu'elles aient été recuites 1 ou 5 min à 1000° C, ont une résistance maximale au fluage à chaud (SAGTEST à 850° C - 100 h), c'est-à-dire une flèche minimale entre 0,30 et 0,52% de Nb libre ou non combiné, ou mieux entre 0,33 et 0,50% de Nb libre.

These results show three trends:

• - the Nb + AI test pieces (table 4 and fig. 1 and 2) resist creep the better the free Nb content is high, and the prior annealing of 5 min at 1000 ° C gives the same content in Nb free, much better results than annealing for 1 min at 1000 ° C, for the entire range tested (0.1 to 0.54% of Nb free);
• - Zr + Nb + AI test pieces (table 5 and fig. 1 1 and 2) give much better results than Nb + AI test pieces with annealing for 1 min at 1000 ° C, and which are only one not as good as those obtained with Zr + Nb + AI-like test tubes annealed for 5 min at 1000 ° C, especially between 0.25% and 0.55% of free Nb, the range in which the arrow results differ only d '' about 0.3 to 0.7 cm. The ability of the Zr + Nb + AI castings according to the invention to give a relatively good creep resistance even after such limited annealing (1 min at 1000 ° C.) is a very important industrial advantage;
• - Zr + Nb + AI test pieces, whether they have been annealed for 1 or 5 min at 1000 ° C, have maximum resistance to hot creep (SAGTEST at 850 ° C - 100 h), i.e. say a minimum deflection between 0.30 and 0.52% of free or uncombined Nb, or better still between 0.33 and 0.50% of free Nb.

The appearance of the curves concerning the Zr + Nb + Al flows, different from those concerning the Nb + AI flows (fig. 1 and 2), is not completely explained by the classic considerations and observations of intergranular precipitation of intermediate phases iron-niobium and recrystallization.

The sheets according to the invention, with an uncombined Nb content of between 0.25 and 0.55%, and preferably with the intervals specified above, are thus indicated by their level of resistance to hot creep. In this regard, creep tests in tension at 800 ° C confirmed the SAGTEST tests.

Test series N ° 2- Ductility of welds

2.5 mm thick sheets were used which were annealed for 4 h at 800 ° C. from 4 of the previous castings. These sheets were made of full sheet welds (melting lines) of width 2 mm upside down in automatic TIG on a strip of groove width 10 mm, under pure argon, with 12 V-250 A and at speed 0.50 m / min. We then carried out successive bending tests of the welds, either in the transverse direction of the welds, or in the long direction: at 90 ° angle then at 180 ° on a radius of 5 mm, then on a radius of 2.5 mm, then a block (zero radius).

The results are shown in Table 6 below, "B" meaning "good" and "M" meaning "bad" (crack or crack). There are 4 tests per condition.

The welds of the sheets according to the invention all exhibit very good ductility, unlike sheets with Nb + AI, and this even in the case of casting No. 445 where the Zr content is slightly less than 7 (C + N) .

Test series N ° 3 - Continuous oxidation tests in air between 800 and 1050 ° C

The samples used come from 3 flows of 25 kg transformed according to the process indicated in connection with the test series N ° 1, the cold rolling being stopped at the thickness 1.5 mm and being followed by annealing under vacuum. from 1 h at 830 ° C. The analyzes of the 3 castings, at 0.4% Nb and with increasing AI content, are shown in Table 7. The samples are 20 x 30 mm plates cut out with a punch. in the annealed 1.5 mm sheets, then electrolytically polished in an aceto-perchloric bath (88-12) at room temperature for 5 min, then weighed in mg. Each oxidation test relates to 3 test pieces of the same type, with an additional test piece for metallographic examinations.

The hot air oxidation tests have a unit duration of 50 h, the air renewal is ensured by "chimney effect" by means of a Ø 6 mm hole made in the lower part of the oven. After the test, the oxides formed are removed by electrolytic pickling in a neutral medium, and it is the weight loss of the samples per unit area (in g / m ² ) which makes it possible to assess "a contrario" the resistance to hot oxidation. The results on the 3 test pieces of each test are well grouped in the case of continuous oxidation, and we have therefore given in this case only one result, average of the 3 individual results.

This series of tests No. 3 focused on oxidations at temperatures ranging from 50 ° C to 50 ° C between 800 ° C and 1050 ° C, the results appear in Table 8 and in Figure 3.

Considering Table 8 and Figure 3, it can be seen that AI in a content as low as 0.036% greatly improves the resistance to hot oxidation above 950 ° C. And, if we take for example as a limit 200 g / m ² in 50 h, it can be seen that the limit temperatures are staggered as has already been indicated in the description of the invention. As will be verified in series of tests No. 4, these observations on Nb flows apply to Z + Nb flows and in particular to flows according to the invention.

Test series N ° 4 - Continuous oxidation tests in air for 50 h at 1000 ° C

In this series, in addition to test pieces from the 3 previous flows, we tested test pieces of 2 Nb flows with respective AI contents of 0.525% and 1% and 2 flows according to the invention at 0.04% AI. (flows 201 and 202), the analyzes of which also appear in table 7. The results are given in table 9 and in FIG. 4. It can be seen that the points representative of the two flows according to the invention are placed correctly on the curve of weight loss plotted for Nb. The presence of aluminum leads to a loss of weight reduced by 50% by 0.04% AI, by 80% by 0.10% AI and which hardly changes beyond 0.3% AI, the weight loss then capped at 100 g / m ² , asymptote of the curve. The weight losses corresponding to the AI content limits of the steels of the invention are shown in Table 1 (description of the invention).

The AI contents are plotted on the graph in FIG. 4.

Test series N ° 5 - Alternate oxidation tests

between 800 and 1000 ° C, in full sheet metal and supersoldering

In these alternating oxidation or cyclic oxidation tests, the test pieces prepared as described above are subjected to cycles each comprising: rapid heating, holding for 10 min at the test temperature, then air cooling and maintaining at room temperature or close to total duration 10 min. The duration of a test is 100 h during which 300 cycles are carried out giving an overall maintenance at the test temperature of 50 h.

We tested thus, with test temperatures staggered from 50 ° C to 50 ° C between 800 ° C and 1000 ° C, test pieces from casting with Nb + Al No 101 and casting according to the invention N _os 201 and 202, the analyzes of which appear in table 7. The tests relate to both full-sheet test pieces and test pieces containing welds, the latter being carried out as indicated with regard to the series of tests No. 2, and the right side of these welds then occupying% of the width of the test pieces.

• - les points représentatifs des éprouvettes pleine tôle des 3 coulées et ceux des éprouvettes avec soudure des coulées N^os 201 et 202 selon l'invention, appartenant au domaine hachuré (A);
• - les points représentatifs des éprouvettes avec soudure de la coulée N° 101, qui présente une anomalie de perte de poids (oxydation excessive dans ce test cyclique) entre 850 et 950° C et donne des pertes de poids relativement fortes pour 1000° C. Ces points sont compris dans le domaine hachuré (B).

The results obtained appear in Table 10 and in FIG. 5. The representative points of the minima and maxima of each group of 3 results are plotted in FIG. 5. There are two families of results:

• - the representative points of the full sheet test pieces of the 3 castings and those of the test pieces with welding of the castings N ^os 201 and 202 according to the invention, belonging to the hatched area (A);
• - the representative points of the test pieces with welding of casting N ° 101, which presents an anomaly of weight loss (excessive oxidation in this cyclic test) between 850 and 950 ° C and gives relatively strong weight losses for 1000 ° C. These points are included in the hatched area (B).

With the same AI content, the sheets of the invention are therefore distinguished from sheets of Nb without Zr in that their welds without filler metal have better resistance to alternating or cyclic oxidation in this temperature range (850 to 950 ° C) important for exhaust manifolds.

Test series N ° 6- Alternate oxidation tests of 500 h at 800 ° C

In this series of tests, alternating or cyclic oxidation tests were carried out for a total duration of 100 h, 250 h and 500 h with the cycles defined in test series No. 5. The tests focused on the flows Nos ^. 101, 102 and 201 (analyzes in Table 7): Nb, Zr, and Zr + Nb castings respectively according to the invention, having neighboring AI contents.

The results obtained, already mentioned in the description of the invention, are reported in Table 11 and in FIG. 6. The results of flows N ^os 101 and 201 for 100 h (at 800 ° C.) are already shown in the table 10. It is observed that the evolution of the weight loss as a function of the alternating or cyclic oxidation time is quite different for the 3 flows: flow 201, which gave the greatest weight losses at 100 h, gives well grouped weight losses and practically no longer evolving beyond 100 h to 250 h, while the flows N ° 101 and especially N ° 102 give results in strong increase with the data. Casting No. 201 according to the invention here outperforms casting Nos ^. 101 and 102 after approximately 350 hours of testing.

This behavior of the test specimens from casting No. 203, corresponding to a particular stability of the oxide layer in cyclic oxidation, seems to confirm that this stability, which seems to be linked to an anchoring phenomenon, does not only depend on the presence of aluminum. Compared with the behavior of the test specimens from flows Nos ^. 101 and 102, it seems to mean that the simultaneous presence of Zr and of N also plays a role.

Advantages of the invention

• a) bonne résistance au fluage à chaud, particulièrement pour 0,30 à 0,52% de Nb libre;
• b) cette résistance au fluage est obtenue à partir d'un état recuit industriellement avantageux, typiquement 1000 ± 10° C pendant 0,5 à 5 min;
• c) bonne résistance à l'oxydation continue à chaud, liée de façon surprenante à l'addition de AI en faible teneur, en conjonction avec Nb;
• d) stabilité particulière de la couche d'oxyde en oxydation cyclique à 800° C, liée à la présence simultanée de Zr et de Nb en même temps que AI en faible teneur;
• e) bon comportement des soudures sans métal d'apport en oxydation cyclique particulièrement aux alentours de 900° C, ce comportement restant voisin de celui de la pleine tôle;
• f) bonne ductilité des soudures sans métal d'apport;
• g) bonne résistance à la corrosion, dans des conditions correspondant à l'utilisation des collecteurs d'échappement automobile, grâce à la limitation de la teneur en AI.

The sheets of the invention thus have numerous advantages responding to the problem posed:

• a) good resistance to hot creep, particularly for 0.30 to 0.52% of free Nb;
• b) this creep resistance is obtained from an industrially advantageous annealed condition, typically 1000 ± 10 ° C for 0.5 to 5 min;
• c) good resistance to continuous hot oxidation, surprisingly linked to the addition of AI in low content, in conjunction with Nb;
• d) particular stability of the oxide layer in cyclic oxidation at 800 ° C, linked to the simultaneous presence of Zr and Nb at the same time as AI in low content;
• e) good behavior of welds without filler metal in cyclic oxidation, particularly around 900 ° C., this behavior remaining close to that of full sheet metal;
• f) good ductility of welds without filler metal;
• g) good corrosion resistance, under conditions corresponding to the use of automobile exhaust manifolds, thanks to the limitation of the AI content.

Applications

The strips or sheets of the invention, usually in the annealed state and in a thickness of 0.6 to 3 mm and most often 1.2 to 2.5 mm, are used for any application in which an economic compromise of ductility (sheet metal and welds), heat resistance (creep, continuous or cyclic air oxidation) and resistance to corrosion. Application to exhaust systems is particularly typical.

Claims (7)

1. Ferritic stainless steel strip or sheet which is intended in particular for the production of exhaust systems, characterised in that it is of the following composition (% by weight):

(C+N) < 0.060; Si < 0.9; Mn < 1;

Cr 15 to 19; Mo < 1; Ni < 0.5; Ti < 0.1; Cu < 0.4;

S < 0.02; P < 0.045; Zr = 0.010 to 0.50 with Zr between 7 (C+N) - 0.1 and 7 (C+N) + 0.2; Nb between 0.25 and 0.55 if Zr a 7 (C+N) and between 0.25 + 7 (C+N) - Zr and 0.55 + 7 (C+N) - Zr if Zr < 7 (C+N);

AI 0.020 to 0.080; other elements and iron: the balance,

and that AI is in solid solution therein except for an amount which is at most equal to 0.003%.

2. Strip or sheet according to claim 1, characterised in that it contains (% by weight):

C+N < 0.040; Si < 0.8; Cr 16 to 18; Mo < 0.3;

Ni < 0.3;Ti < 0.05; S < 0.01; Zr = 0.10 to 0.40, with Zr between 7 (C+N) and 7 (C+N) + 0.15; Nb 0.30 to 0.52; AI 0.020 to 0.045.

3. Strip or sheet according to claim 2, characterised in that it contains (% by weight): Nb 0.33 to 0.50 and AI 0.025 to 0.040.

4. Strip or sheet according to any one of claims 1 to 3, characterised in that it is in the condition of being annealed for 0.5 to 5 minutes at 1000 ± 10° C and dressed.

5. A process for the production of a strip or sheet of a composition in accordance with any one of claims 1 to 3, wherein the strip which is hot rolled, of a thickness of between 2.5 and 5 mm, is annealed at between 800 and 1000° C under substantially non-oxidising conditions, it is then shot- blasted and cleaned, it is then cold rolled to the thickness for delivery of between 0.6 and 3 mm, with or without intermediate annealing and cleaning operations, and it is subjected to final annealing in a moving mode and it is subjected to a finishing and cold working pass or "skin-pass" producing a degree of elongation of less than 1 %, characterised in that the final annealing operation is carried out at between 980 and 1020° C.

6. A process according to claim 5, characterised in that the final annealing operation is carried out at between 990 and 1010° C for a period of from 0.5 to 5 minutes.

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