EP2893049A1

EP2893049A1 - Ferritic stainless steel sheet, method for the production thereof, and use of same, especially in exhaust lines

Info

Publication number: EP2893049A1
Application number: EP12766456.3A
Authority: EP
Inventors: Pierre-Olivier Santacreu; Claudine MIRAVAL; Saghi SAEDLOU
Original assignee: Aperam Stainless France SA
Current assignee: Aperam Stainless France SA
Priority date: 2012-09-03
Filing date: 2012-09-03
Publication date: 2015-07-15
Anticipated expiration: 2032-09-03
Also published as: KR20150099706A; RU2015107432A; BR112015004633A2; IN2015DN01710A; RU2603519C2; CA2883538C; HUE052513T2; SI2893049T1; ES2831163T3; CN104903482A; US9873924B2; CA2883538A1; JP2015532681A; MX2015002716A; US20160115562A1; EP2893049B1; CN104903482B; WO2014033372A1

Abstract

The invention relates to a ferritic stainless steel sheet of the following composition expressed in weight percentages: trace ≤ C ≤ 0.03%; 0.2% ≤Mn ≤1%; 0.2 % ≤ Si ≤ 1%; trace ≤ S ≤ 0.01%; trace ≤ P ≤ 0.04%; 15% ≤ Cr ≤ 22%; trace ≤ Ni ≤ 0.5%; trace ≤ Mo ≤ 2%; trace ≤ Cu ≤ 0.5%; 0.160% ≤Ti ≤ 1%; 0.02% ≤ Al ≤ 1%; 0.2% ≤ Nb ≤ 1%; trace ≤ V ≤ 0.2%; 0.009% ≤ N ≤ 0.03%; trace ≤ Co ≤ 0.2%; trace ≤ Sn ≤ 0.05%; rare earths (REE) ≤ 0.1%; trace ≤ Zr ≤ 0.01%; the rest of the composition consisting of iron and inevitable impurities resulting from the processing thereof; the Al and rare earth (REE) contents satisfying the relation: Al + 30 x REE ≥ 0.15%; the Nb, C, N and Ti contents in % satisfying the relation: 1 / [Nb + (7/4) x Ti - 7 x (C + N)] ≤ 3; said sheet having an entirely recrystallised structure and an average ferritic grain size of between 25 and 65 µm. The invention also relates to a method for the production of such a ferritic stainless steel sheet, and to the use thereof for the production of parts involving shaping and welding, that are to be subjected to a periodic use temperature of between 50° C and 700° C and to a projection of a mixture of water, urea and ammonia.

Description

Ferritic stainless steel sheet, its manufacturing process, and its use, especially in exhaust lines

The invention relates to a ferritic stainless steel, its method of manufacture, and its use for the manufacture of mechanically welded parts subjected to high temperatures, such as elements of exhaust lines of internal combustion engines.

For certain applications of ferritic stainless steels, such as parts located in the hot parts of engine exhaust systems equipped with a urea or ammonia decontamination system (passenger cars, trucks, construction site, agricultural machinery, or maritime transport machinery) ensuring the reduction of nitrogen oxides, one simultaneously seeks:

good resistance to oxidation;

good mechanical strength at high temperature, namely the preservation of high mechanical characteristics and good resistance to creep and thermal fatigue;

- and good resistance to corrosion by urea, ammonia, their decomposition products.

Indeed, these parts are subjected to temperatures between 150 and 700 ° C, and a projection of a mixture of urea and water (typically 32.5% urea - 67.5% water ), or a mixture of ammonia and water, or pure ammonia. The decomposition products of urea and ammonia are also likely to degrade parts of the exhaust line.

The high temperature mechanical strength must also be adapted to the thermal cycles associated with the engine acceleration and deceleration phases. In addition, the metal must have good cold formability to be shaped by bending or hydroforming, as well as good weldability.

Different grades of ferritic stainless steels are available to meet the specific requirements of different areas of the exhaust line. Ferritic stainless steels containing 17% Cr stabilized with 0.14% titanium and 0.5% niobium (type EN 1 .4509, AISI 441) are thus known, allowing use up to 950 ° C.

Ferritic stainless steels with a lower chromium content are also known, for example steels containing 12% Cr stabilized with 0.2% titanium (type EN 1 455 AISI 409) for maximum temperatures below 850 ° C. steels at 14% Cr stabilized with 0.5% niobium without titanium (type EN 1 .4595) for maximum temperatures below 900 ^. These have a high temperature behavior equivalent to that of previous grades, but a better fitness.

Finally, for the very high temperatures up to 1050 ° C., or for improved thermal fatigue resistance, a variant of the grade EN 1 .4521 AISI 444 at 19% Cr stabilized with 0.6% of niobium is known. and containing 1.8% molybdenum (see EP-A-1,818,422). However, despite their good mechanical properties in hot and oxidation in a conventional atmosphere of exhaust gas, the ferritic grades cited corrode excessively grain boundaries, in the presence of a projection of a mixture of water, water and water. urea and ammonia and for temperatures between 150 and 700 ^. This makes these steels insufficiently adapted to their use in exhaust systems equipped with urea or ammonia decontamination systems, as is often the case, for example, on diesel vehicles.

It has been noted, moreover, that the phenomena of intergranular corrosion by urea are aggravated when a stabilized or non-stabilized austenitic grade is used (types EN 1 .4301 AISI 304, EN 1 .4541 AISI 321 or EN 1 .4404 AISI 316L). Such nuances are therefore not a fully satisfactory solution to the problems encountered.

The present invention aims to solve the corrosion problems mentioned above. It aims in particular to make available to the users of engines equipped with a system for the removal of exhaust gases with urea or ammonia a ferritic stainless steel which has, compared to the known grades for this purpose, improved resistance to corrosion by a mixture of water, urea and ammonia. This steel must also maintain a good heat resistance, that is to say a high resistance to creep, thermal fatigue and oxidation at periodically varying operating temperatures of up to several hundred ', as well as a cold forming and welding ability equivalent to that of the EN 1 .4509 AISI 441 grade, ie guaranteeing a minimum elongation at break of 28% in tension, for mechanical characteristics in tension typically of 300 MPa for the yield strength Re and 490 MPa for the tensile strength Rm.

Finally, the mechanical strength of the welds of the exhaust line made with this steel must be excellent. For this purpose, the subject of the invention is a ferritic stainless steel sheet of composition, expressed in percentages by weight:

- traces <C <0.03%;

- 0.2% <Mn <1%;

- 0.2% <If <1%; - traces <S <0.01%;

- traces <P <0.04%;

- 15% <Cr <22%;

- traces <Ni <0.5%;

- traces <Mo <2%; - traces <Cu <0.5%;

0.160% <Ti <1%;

- 0.02% ≤ Al≤ 1%;

- 0.2% <Nb <1%;

- traces <V <0.2%; - 0.009% <N <0.03%; preferably between 0.010 and 0.020%;

- traces <Co <0.2%; - traces <Sn <0.05%;

- rare earths (REE) <0.1%;

traces <Zr <0.01%;

the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation; Al and rare earth (REE) contents satisfying the relationship:

Al + 30 x REE≥ 0.15%;

the contents of Nb, C, N and Ti in% satisfying the relation:

1 / [Nb + (7/4) x Ti - 7 x (C + N)] <3; said sheet having a completely recrystallized structure and an average ferritic grain size of between 25 and 65 μηι.

The invention also relates to two methods of manufacturing a ferritic stainless steel sheet of the above type.

According to a first method: - a steel having the above-mentioned composition is produced;

- Casting a half-product from this steel;

the semi-finished product is brought to a temperature greater than Ι ΟΟΟ'Ό and lower than 1250 ° C., and the semi-finished product is hot-rolled to obtain a hot-rolled sheet with a thickness of between 2.5 and 6 mm; said cold-rolled sheet is cold-rolled at a temperature below

300 'Ό, in a single step or in several steps separated by intermediate annealing;

a final annealing of the cold-rolled sheet is carried out at a temperature of between 1000 and 1100 ° C. and for a duration of between 10 seconds and 3 minutes to obtain a completely recrystallized structure with an average grain size of between 25 and 65 μηι.

According to a second method: a steel having the composition mentioned above is produced;

- Casting a half-product from this steel;

the semi-finished product is brought to a temperature above Ι ΟΟΟ'Ό and lower than 1250 ° C., preferably between 1180 and 1200,, and the semi-finished product is hot-rolled to obtain a hot-rolled sheet of thickness between 2.5 and 6mm;

the hot-rolled sheet is annealed at a temperature of between 1000 and 1100 ° C. and for a period of between 30 seconds and 6 minutes;

said hot-rolled sheet is cold rolled at a temperature below 300 ° C. in a single step or in several steps separated by intermediate anneals;

a final annealing of the cold-rolled sheet at a temperature of between 1000 and 1100 ° C. and for a duration of between 10 seconds and 3 minutes is carried out in order to obtain a completely recrystallized structure with an average grain size of between 25 and 100.degree. and 65 micrometers. Preferably, in both processes, the hot rolling temperature is between 1180 and 1200 ° C.

Preferably, in both processes, the final annealing temperature is between 1050 and 1090 ^< €.

The invention also relates to the use of such a steel sheet for the manufacture of parts involving shaping and welding and intended to be subjected to a periodic operating temperature of between Ι δΟ 'Ό and 700 ^ and a projection of a mixture of water, urea and ammonia or a projection of urea or ammonia.

This may include engine exhaust system parts equipped with a catalytic system for reducing nitrogen oxides by injection of urea or ammonia.

As will be understood, the invention is based on the use of ferritic stainless steel sheets having the specified composition and structure, which the inventors have discovered are particularly well suited to solving the aforementioned technical problems. . The average grain size between 25 and 65 μηι is an important feature of the invention, and it is controlled both by the presence of nitrides and carbonitrides of titanium and niobium and by the final annealing performance temperature. .

Too small grain size hardens the metal, thus limiting its ability to shape, accelerates the diffusion of nitrogen from the decomposition of urea (since the density of grain boundary is greater than in the case of the invention), and reduces the creep resistance.

Conversely, a too large grain size decreases the resilience of the metal, especially in the welded zones (in particular Heat Affected Zones) and degrades the appearance of the parts after shaping (orange peel).

Obtaining the average grain size interval according to the invention avoids these disadvantages.

The invention will now be described in detail, with reference to the following figures: FIG. 1, which shows the thermal cycle to which the samples were subjected during the tests which will be described;

FIG. 2 which shows the sectional micrograph according to its thickness of the first 0.150 mm of a sample of a reference steel after a urea corrosion test; FIG. 3 which shows the sectional micrograph according to its thickness of the first 0.150 mm of a sample of a steel according to the invention after a urea corrosion test carried out under the same conditions as for the steel of the figure 2.

We will first justify the presence of the various chemical elements and their ranges of contents. All grades are given in percentages by weight. The carbon would be likely to increase the mechanical characteristics at high temperature, in particular the resistance to creep. However, because of its very low solubility in ferrite, the carbon tends to precipitate in the form of carbides M ₂₃ C ₆ or M ₇ C ₃ between 600 ~ C and about 900 ^, for example chromium carbides. This precipitation, usually located at the grain boundaries, can lead to a depletion of chromium in the vicinity of these joints, and thus to a sensitization of the metal to intergranular corrosion. This awareness can be found in particular in Heat Affected Zones (ZAC), which have been heated to very high temperatures during welding. The carbon content must therefore be low, ie limited to 0.03% to obtain a satisfactory resistance to intergranular corrosion and not to reduce the formability. In addition, the carbon content must satisfy a relationship with niobium, titanium and nitrogen, as will be explained later.

Manganese improves the adhesion of the oxide layer protecting the metal against corrosion when its content is greater than 0.2%. However, beyond 1%, the kinetics of hot oxidation becomes too fast and a less compact oxide layer develops, formed of spinel and chromine. The manganese content must therefore be contained between these two limits.

Like chromium, silicon is a very effective element for increasing the resistance to oxidation during thermal cycling. To fulfill this role, a minimum content of 0.2% is necessary. However, in order not to reduce the hot rolling and cold forming ability, the silicon content must be limited to 1%. Sulfur and phosphorus are important undesirable impurities because they decrease hot ductility and formability. In addition, phosphorus easily segregates at grain boundaries and decreases cohesion. In this respect, the sulfur and phosphorus contents must be less than or equal to 0.01% and 0.04%, respectively. These maximum levels are obtained by a careful choice of raw materials and / or by metallurgical treatments carried out on the liquid metal under development.

Chromium is an essential element for the stabilization of the ferritic phase and for the increase of the resistance to oxidation. In connection with the other elements present in the steel of the invention, its minimum content must be greater than or equal to 15% in order to obtain a ferritic structure at all operating temperatures and to obtain good resistance to corrosion. 'oxidation. Its maximum content must not, however, exceed 22%, otherwise the mechanical strength at room temperature may be excessively increased, which reduces the ability to shape, or promote embrittlement by demixing the ferrite around 475 ° C. Nickel is a gamma element that increases the ductility of steel. But in order to maintain a ferritic single-phase structure under all circumstances, its content must be less than or equal to 0.5%. Molybdenum improves resistance to pitting but reduces ductility and formability. This element is therefore not mandatory, and its content is limited to 2%.

Copper has a hot-curing effect that could be favorable. Present in excessive quantity, it nevertheless decreases ductility during hot rolling and weldability. As such, the copper content must be less than or equal to 0.5%.

Aluminum is an important element of the invention. Indeed, with or without rare earth elements (REE), it improves the resistance to corrosion by urea if one respects the formula Al + 30 x REE≥ 0,15%, and if also one realizes a stabilization of metal by titanium and niobium. The synergy between the elements Ti, Nb, Al and REE for the limitation of the diffusion at grain boundaries of nitrogen resulting, for example, from the decomposition of urea, is demonstrated by the experiments which will be described more far.

Furthermore, aluminum, associated or not with rare earths, greatly improves the mechanical strength of MIG / MAG welds (better ZAC performance). However, this improvement is observed only for ferritic chromo-forming stainless steel, that is to say containing less than 1% aluminum. On the other hand a content greater than 1% of aluminum strongly weakens the ferrite and greatly reduces its cold forming properties. The content is therefore limited to 1%. A minimum aluminum content of 0.020 is essential to the invention (while the REEs are not mandatory) to allow the control of the germination of TiN and therefore the grain size.

Niobium and titanium are also important elements of the invention. Usually, these elements can be used as stabilizing elements in ferritic stainless steels. Indeed, the phenomenon of sensitization to intergranular corrosion by chromium carbide formation, which has been mentioned above, can be avoided by the addition of elements forming carbonitrides very thermally stable.

In particular, titanium and nitrogen combine before the solidification of the liquid metal to form TiN; and in the solid state at 1100 ° C titanium carbides and carbonitrides are formed. In this way, the carbon and the nitrogen present in solid solution in the metal are reduced as much as possible during its use. Such presence at too high levels would reduce the corrosion resistance of the metal and harden it. To achieve this effect sufficiently, a minimum Ti content of 0.16% is required. It should be noted that usually the precipitation of TiN in the liquid metal is considered by steelmakers as a disadvantage in that it can lead to an accumulation of these precipitates on the walls of the nozzles of the casting vessels (pocket, continuous tundish) which may clog these nozzles. But TiN improves the structure that develops during solidification by helping to obtain an equiaxed rather than dendritic structure, and thus improve the final grain size homogeneity. In the case of the invention, it is considered that the advantages of this precipitation outweigh its disadvantages, which can be minimized by choosing casting conditions reducing the risk of plugging the nozzles.

Niobium combines with nitrogen and carbon in the solid state, and stabilizes the metal, just like titanium. Niobium thus stably fixes carbon and nitrogen. But niobium also combines with iron to form intermetallic compounds at the grain boundaries in the range 550 ° C-950 °, ie, Laves Fe ₂ Nb phases, which improves the creep resistance in this range. temperature. A minimum of 0.2% niobium content is required to obtain this property. The conditions for obtaining this improvement in creep resistance are also strongly related to the manufacturing method of the invention, in particular the annealing temperatures, and to an average grain size controlled and maintained within the limits of 25 to 65 μηι.

Finally, the experiment shows that when their titanium and niobium contents, associated with the carbon and nitrogen contents, respect the relation 1 / [Nb + (7/4) x Ti - 7x (C + N)] <3 Urea corrosion between 10 ° C and 700 ° C is greatly diminished. It is explained by the guarantee of having a quantity of Ti and Nb still free in the metal making it possible to help limit the diffusion at the grain boundaries of the nitrogen coming from the decomposition of the urea. This condition alone is however not sufficient, and the addition of aluminum or rare earths under the conditions mentioned elsewhere is necessary.

However, it is also necessary to limit the additions of niobium and titanium. When at least one of the contents of niobium and titanium is greater than 1% by weight, the hardening obtained is too important, the steel is less easily deformable and recrystallization after cold rolling is more difficult.

Zirconium would have a stabilizing role close to that of titanium, but is not used deliberately in the invention. Its content is less than 0.01%, and therefore must remain of the order of a residual impurity. An addition of Zr would be expensive, and especially harmful, because the zirconium carbonitrides, by their shape and their large size, strongly reduce the resilience of the metal. Vanadium is a very poor stabilizer in the context of the invention given the low stability of vanadium carbonitrides at high temperature. On the other hand, it improves the ductility of the welds. However, at medium temperatures in a nitrogen atmosphere it promotes the nitriding of the metal surface by diffusion of nitrogen. The content is limited to 0.2%, given the intended application.

Like carbon, nitrogen increases the mechanical characteristics. However, nitrogen tends to precipitate at grain boundaries as nitrides, thus reducing corrosion resistance. In order to limit problems of sensitization to intergranular corrosion, the nitrogen content must be less than or equal to 0.03%. In addition the nitrogen content must satisfy the previous relationship binding Ti, Nb, C and N. A minimum of 0.009% nitrogen, however, is necessary for the invention, because it ensures the presence of TiN precipitates, and also the good recrystallization of the cold rolled strip during the final annealing operation to obtain a grain of average size less than 65 microns. A content between 0.010% and 0.020%, for example 0.013%, may be recommended.

Cobalt is a hot-curing element that degrades formability. For this purpose its content must be limited to 0.2% by weight.

In order to avoid hot forgeability problems, the tin content must be less than or equal to 0.05%. REE rare earths include a combination of elements such as cerium and lanthanum, among others, and are known to improve the adhesion of oxide layers that make the steel resistant to corrosion. It has also been shown that the rare earths improve the resistance to intergranular corrosion by urea between 150 ° C. and 700 ° C., as in the case of the aluminum already described, and while respecting the relation Al + 30 × REE≥ 0, 15%. In synergy with aluminum and stabilizers, REEs help to limit the diffusion of nitrogen. However, the rare earth content must not exceed 0.1%. Beyond this content, the elaboration of the metal would be made difficult because of the reactions of the REEs with the refractories coating the ladle. These reactions would lead to the notable formation of REE oxides which would degrade the inclusivity cleanliness of the steel. In addition, the effectiveness of the ESPs is sufficient for the grades proposed, and going beyond this would only unnecessarily increase the cost of development because of the high cost of the ESP, and also the accelerated wear of the refractories that this would entail. .

The sheet according to the invention can in particular be obtained by the following method: a steel having the above composition is produced;

- Casting a half-product from this steel;

the semi-finished product is carried at a temperature above 1000 ° C. and below 1250 ° C., preferably between 1180 and 1200 ° C., and the semi-finished product is hot-rolled to obtain a hot-rolled sheet of thickness between 2.5 and 6mm;

said hot-rolled sheet is cold-rolled, at a temperature between ambient and 300 ° C., in a single step or in several steps separated by intermediate anneals; it should be understood that the term "step" denotes here a cold rolling comprising either a single pass or a succession of several passes (for example five passes) which are not separated by any intermediate annealing; one can consider, for example, a cold rolling sequence comprising a first series of five passes, then an intermediate annealing, then a second sequence of five passes; typically (these data, which are customary for conventional methods of manufacturing ferritic stainless steel sheets, are not limiting for the definition of the invention), the intermediate anneals separating the steps are carried out between 950 and 1100 * 0 for 30 sec to 6 min;

a final annealing of the cold-rolled sheet is carried out at a temperature of between 1000 and 1100 ° C., preferably between 1050 ° and 1090 ° C., and for a period of between 10 seconds and 3 minutes, in order to obtain a completely recrystallized structure with average grain size between 25 and 65 μηι.

Alternatively, an annealing step can be added between hot rolling and cold rolling. This annealing takes place between 1000 and 1100 ° C for a period of 30 s to 6 min.

We will now describe a series of experiments demonstrating the interest of the invention. Laboratory flows have been investigated and the chemical analyzes given in Table 1.

Table 1: Analyzes of laboratory flows

The cast samples were processed according to the following method.

By hot rolling, the metal, which is initially in the form of a 20 mm thick sheet, is brought to a temperature of 1200 ° C. and is hot rolled in 6 passes to a thickness of 2. , 5 mm. According to a variant of the process according to the invention, a first annealing of the hot-rolled strip can then be carried out at Ι ΟδΟ 'Ό with keeping 1 min 30 sec of the sample at this temperature. The examples according to the invention Nos. 1 to 11 and some reference examples (Nos. 12 and 19) were treated with and without this first annealing, and it was possible to verify that they had, in both cases, very similar final properties. The execution of this first annealing makes it possible to obtain a slight improvement in the formability, but for the attainment of the typical objectives of the invention, it is the conditions of the final annealing that are the sole determining factors, in combination with the other essential characteristics. of the process and, of course, the composition of the steel. The results presented in Tables 2 and 3 correspond to those observed on the samples which have not undergone the first annealing of the variant which has just been described.

After blasting and pickling, the metal is cold rolled at room temperature, about 20 ° C, in five passes, to a thickness of 1 mm.

The metal is annealed at Ι ΟδΟ 'Ό with a hold of 1 min 30 sec at this temperature, then stripped. Metal coupons from each casting are subjected to the test procedure A and are then analyzed according to the analysis procedure B which will be described.

The urea corrosion phenomenon is revealed by the following test procedure A.

The sample is sprayed with a mixture containing 32.5% urea, and 67.5% water (flow rate: 0.17 ml / min), and simultaneously undergoes a thermal cycle between 200 and 800 O, with a triangular signal 120 sec period as shown in Figure 1 by the curve 1. The rise in temperature from 200 to 600 'Ό lasts 40 sec, then the cooling starts as soon as the temperature of 600 ° C is reached and continues until 200 ^ for 80 sec. According to the analysis procedure B, after 300 hours of testing, a section of the sample is made by the micro-chainsaw. Electrolytic copper plating of the sample is carried out before coating in a solution of CuSO ₄ at 210 g / l and H ₂ SO ₄ at 30 ml / l; the imposed current density is 0.07 A / cm ² for 5 minutes, then 0.14 A / cm 2 for 1 minute. This procedure is considered optimal for obtaining good coppering. Electrolytic etching is carried out in a solution of 5% oxalic acid for 15s to 20%. The imposed current density is 60 mA / cm ² . This procedure B reveals two areas corroded by urea observed under the microscope at magnification x 1000.

Two examples are presented and processed:

FIG. 2 shows the first 0.150 mm according to the thickness of the sample corresponding to the reference sample No. 28 of Table 1; - Figure 3 shows the first 0.150 mm according to the thickness of the sample corresponding to the sample according to the invention No. 2 of Table 1, a portion is, in addition, magnified.

These samples are characterized, as can be seen in Figures 2 and 3:

- the presence on their surface of a copper deposit 2, which would, of course, be absent from an industrial product;

- By a homogeneous zone 3 intended to be in contact with the atmosphere, and which consists of a mixture of oxides and nitrides with a maximum thickness of 30 μηι obtained after procedures A and B.

- By an intergranular corrosion zone 4 located under the previous layer 3 in the metal, and containing chromium nitride precipitates; the thickness of the intergranular corrosion zone is measured over the entire length of the section (3 cm); the average of the maximum values is carried out and gives the value retained as the thickness of the intergranular corrosion zone of the sample; this can reach 90 μηι when the process according to the invention is not used, and is reduced to a few μηι in the case of the invention, as will be seen; the objective of the invention is to achieve a thickness of the intergranular corrosion zone of less than 7 μηι under the test conditions mentioned, to be assured of not suffering unacceptable damage to the surface of the metal due to fatigue or acid corrosion by the condensates, when used in an exhaust line. Below this zone of intergranular corrosion, the metal is not affected. _{l f} _r ana _r enc _{y y} ana _i

The mechanical strength of the welds was evaluated by a tensile test at 300 'Ό. Two samples of the same casting are welded by the MIG / MAG process with a 430LNb wire under the following conditions: 98.5% argon, 1.5% oxygen, voltage: 26 V wire speed: 10m / min, amperage: 250 A, welding speed: 160 5 cm / min, energy: 2.5 kJ / cm (welding procedure C). The result is judged all the more satisfactory as the ratio between the mechanical strength for the welded specimen and the unwelded specimen is close to 100%.

The results of the tests performed on the various samples are shown in Table 2, which also specifies whether the tested samples meet three of the particular analytical conditions required by the invention (in which case the values are underlined).

Casting Size 0.15≤ 0.2≤ Nb 1 / [Nb + 7/4 Ti - Corrosion Mechanical strength grains (μπι) AI + 30REE 7 * (C + N)] ≤3 intergranular by welds at 300 ° C urea - thickness (% relative to base metal (μm))

1 27 0.272 0.370 2.10 2 90

2 35 0.207 0.400 2.00 3 90 o 3 49 0.317 0.400 2.04 2 85

4 21 1.772 0.600 1 .50 2 85

5 28 0.740 0.610 1 .44 2 95

6 62 1, 146 0.380 2.18 4 90 a)

ω

ω 7 45 0.177 0.630 1 .36 2 95 ω 8 55 1, 150 0.510 1 .24 3 95

9 48 0.167 0.420 1 .86 5 90

10 29 0.246 0.250 0.97 3 95

11 32 0.337 0.260 2.82 3 85

12 5Z 0.021 0.462 2.08 9 65

13 28 0.021 0.398 1 .77 9 50

14 21 0.027 0.650 1 .49 9 65

15 44 0.029 0.440 0.89 9 55

16 62 0.027 0.390 2.28 1 1 60

17 33 0.027 0.600 3.42 21 65

18 45 0.026 0.026 2.76 8 60

19 41 0.028 0.430 4.39 30 65

*

20 28 1, 244 0.001 2.55 15 60 w 21 46 1.717 0.015 3.39 16 60

22 55 1.214 0.180 7.84 40 55

23 36 0.040 0.016 2.08 13 55

24 26 0.039 0.010 1 .82 8 50

25 42 0.019 0.398 6.38 40 60

26 61 0.025 0.280 3.17 10 55

27 33 1, 213 0.025 12.35 42 60

28 56 0.031 0.003 7.97 80 65

29 44 0.028 0.023 1 .03 35 60 Table 2: Results of intergranular corrosion tests with urea and strength of welds at 300 'Ό

This table shows that, under equal treatment conditions, the simultaneous respect of three analytical conditions on the proposed analysis is necessary to guarantee an intergranular attack on a thickness of less than 7μηι:

- 1 / [Nb + 7/4 Ti - 7 ^* (C + N)] <3; Al + REE≥0.15%;

- Nb≥0.2%.

It also shows that the welds made on the castings according to the invention have mechanical strengths very comparable to those of the base metal, that is always greater than 80%. The mechanical strength of the welds present in the components of the exhaust line, in particular when they are obtained by the MIG / MAG process, is therefore improved by the invention.

Furthermore a minimum content of 0.2% Nb is a condition to improve the creep resistance and limit the deformation of the parts during their use at high temperature.

For all the samples according to the invention, the tensile mechanical characteristics found are equivalent to that of a 1 .4509. In particular, it has been verified that the elongation at break A is always greater than 28%. Additional experiments conducted in particular on samples of casting No. 2 which meets the compositional conditions according to the invention have demonstrated that obtaining the fully recrystallized structure and the size of grains prescribed are, moreover, essential for the satisfaction of the requirements of the invention. Their results are summarized in Table 3. Size Temperature Al + Nb 1 / [Nb + 7 / 4Ti - Corrosion Mean annealing strength 30 ^* REE (%) 7 ^* (C + N)] intergranular welds at 300 ° C final grain (° C) (% ) by urea, (% of that

(μη) depth (μηι) of the base metal)

35 1070 0.207 0.4 2 3 90

5,900 0.207 0.4 2 1 1 90

200 1 150 0.207 0.4 2 2 70

Table 3: Depth of intergranular corrosion by urea and mechanical strength of welds according to the average grain size of a sample

Thus, from Table 3, it can be seen that the grain size obtained on the product after the final annealing is a fundamental characteristic for the simultaneous obtaining of all the properties concerned. A grain size too small (5μηι in the example cited) leads to intergranular corrosion by urea which extends over too great a depth. Too large a grain size (200 μηι in the example cited) makes it possible to maintain a sufficiently low sensitivity to intergranular corrosion, but it is then the mechanical strength of the welds that becomes unsatisfactory.

It should also be noted that during the implementation of the method according to the invention, it is possible, without departing from the scope of the invention, to perform one or more stripping of the sheet, following thermal and thermomechanical treatments performed. at higher or lower temperatures (hot rolling, annealing) if they have been carried out in an oxidizing atmosphere such as air, and have therefore led to the formation of an undesirable layer of scale on the surface of the sheet . We have seen that such stripping was practiced during the development of the examples above. This calamine formation can be limited or avoided when the thermal or thermomechanical treatment is carried out in a neutral or reducing atmosphere, as is well known. The properties for which the sheet according to the invention is particularly advantageous are not affected by the execution or not of such stripping.

Claims

1.- Ferritic stainless steel sheet composition, expressed in percentages by weight: traces <C <0.03%; 0.2% <Mn <1%; 0.2% <If <1%; traces <S <0.01%; traces <P <0.04%; 15% <Cr <22%; traces <Ni <0.5%; traces <Mo <2%; traces <Cu <0.5%; 0.160% <Ti <1%; 0.02% <Al <1%; 0.2% <Nb <1%; traces <V <0.2%;

0.009% <N <0.03%; preferably between 0.010% and 0.020%; traces <Co <0.2%; traces <Sn <0.05%; rare earth (REE) <0.1%; traces <Zr <0.01%; the rest of the composition consisting of iron and unavoidable impurities resulting from the elaboration; Al and rare earth (REE) contents satisfying the relationship: AI + 30 x REE≥0.15%; the contents of Nb, C, N and Ti in% satisfying the relationship: 1 / [Nb + (7/4) x Ti - 7 x (C + N)] <3; said sheet having a completely recrystallized structure and an average ferritic grain size of between 25 and 65 μηι.

2. A method of manufacturing a ferritic stainless steel sheet characterized in that:

a steel having the composition according to claim 1 is produced;

- Casting a half-product from this steel; - the half-product is brought to a temperature greater than Ι ΟΟΟ'Ό and less than

1250 ° C., and the semi-finished product is hot rolled to obtain a hot-rolled sheet with a thickness of between 2.5 and 6 mm;

said hot-rolled sheet is cold-rolled, at a temperature between ambient and 300 ° C., in a single step or in several steps separated by intermediate anneals;

3. A method of manufacturing a ferritic stainless steel sheet characterized in that:

a steel having the composition according to claim 1 is produced;

- Casting a half-product from this steel;

the semi-finished product is brought to a temperature greater than Ι ΟΟΟ'Ό and lower than 1250 ° C., and the semi-finished product is hot-rolled to obtain a hot-rolled sheet with a thickness of between 2.5 and 6 mm;

the hot-rolled sheet is annealed at a temperature of between 1000 and 1100 ° C. and for a period of between 30 seconds and 6 minutes; said hot-rolled sheet is cold rolled at a temperature below 300 ° C. in a single step or in several steps separated by intermediate anneals;

a final annealing of the cold-rolled sheet at a temperature of between 1000 and 1100 ° C. and for a duration of between 10 seconds and 3 minutes is carried out in order to obtain a completely recrystallized structure with an average grain size of between 25 and 100.degree. and 65 micrometers.

4. A process according to claim 2 or 3, characterized in that the hot rolling temperature is 1,180 to 1,200 ° C.

5.- Method according to one of claims 2 to 4, characterized in that the temperature of the final annealing is between 1050 and 1090 ° C.

6. - Use of a steel sheet manufactured by the method according to one of claims 2 to 5 for the manufacture of parts involving shaping and welding and intended to be subjected to a periodic operating temperature included between Ι δΟ'Ό and 700 ° C and a projection of a mixture of water, urea and ammonia or a projection of urea or ammonia.

7. - Use according to claim 6, characterized in that said parts are parts of the exhaust lines of combustion engines equipped with a catalytic system for reducing nitrogen oxides by injection of urea or ammonia.