EP0881305A1 - Process for manufacturing ferritic stainless steel thin strips and thin strips obtained - Google Patents

Abstract

The subject of the invention is a method of manufacturing ferritic stainless steel strips, according to which, directly from liquid metal, a strip of solidified between two cylinders close together with horizontal axes, internally cooled and rotating in opposite directions a ferritic stainless steel of the type containing at most 0.012% of carbon, at most 1% of manganese, at most 1% of silicon, at most 0.040% of phosphorus, at most 0.030% of sulfur and between 16 and 18% of chromium, characterized in that said strip is then cooled or allowed to cool while avoiding making it stay in the area of transformation of austenite into ferrite and carbides, in that the said strip is wound at a temperature between 600 ° C and the martensitic transformation temperature Ms, in that the wound strip is allowed to cool at a maximum speed of 300 ° C / h to a temperature between 200 ° C and room temperature, and in which is followed by a closed annealing of the said strip. A subject of the invention is also a strip of ferritic stainless steel of the type containing at most 0.012% of carbon, at most 1% of manganese, at most 1% of silicon, at most 0.040% of phosphorus, at most 0.030% of sulfur and between 16 and 18% chromium, characterized in that it is capable of being obtained by the preceding process. <IMAGE>

Description

The invention relates to the metallurgy of stainless steels. More specifically, it concerns the casting of ferritic stainless steels directly from liquid metal in the form of strips a few mm thick.

For several years, research has been conducted on the casting of tapes steel a few mm thick (10 mm maximum), directly from metal liquid, on so-called "continuous casting between cylinders" installations. These facilities mainly consist of two cylinders with horizontal axes, arranged side by side, having each an external surface that conducts heat strongly and is energetically cooled internally, and defining between them a casting space whose minimum width corresponds to the thickness of the strips that you want to pour. This casting space is closed laterally by two refractory walls applied against the ends of the cylinders. The cylinders are rotated in opposite directions and the casting space is supplied in liquid steel. Steel "skins" solidify against the surfaces of the cylinders and join at the "neck", that is to say at the level where the distance between the cylinders is minimum, to form a solidified strip which is continuously extracted from the installation. This band is then cools naturally or forcibly, before being coiled. The purpose of these research is to succeed in casting by this process steel strips of various shades, especially stainless steels.

In the most common casting conditions, where the strip coming out of the cylinders cools naturally in the open air, the winding of the strip occurs most often at a temperature of the order of 700 to 900 ° C., depending on its thickness and the casting speed. The winding temperature also depends, of course, on the distance between the cylinders and the winder. We then let the wound strip cool down naturally, before making it undergo metallurgical treatments comparable to those usually carried out on hot rolled strips made from slabs of conventional continuous casting.

The application of this casting process to ferritic stainless steels of the type AISI 430 standards, which typically contain 17% chromium, have shown that the bands thus obtained exhibited poor ductility. As a result, the most thin (with a thickness of the order of 2 to 3.5 mm) are excessively fragile and do not not support subsequent handling, carried out at room temperature, such as unwinding and shearing of the banks: during these operations, we note the appearance of cracks on the edges of the tapes, or even breaks in the tape when unwinding.

• la bande brute de coulée présente essentiellement une structure colonnaire à gros grains ferritiques (la taille moyenne du grain est supérieure à 300 µm dans l'épaisseur de la bande), qui est une conséquence directe de la succession d'une solidification rapide sur les cylindres et d'un séjour de la bande à haute température après qu'elle a quitté les cylindres, lorsqu'elle ne subit pas de refroidissement forcé :
• les grains ferritiques présentent une dureté élevée, due à leur sursaturation en éléments interstitiels (carbone et azote) ;
• la présence de martensite issue de la trempe de l'austénite présente à haute température.

This poor ductility is usually explained by several factors:

• the raw casting strip essentially has a columnar structure with large ferritic grains (the average grain size is greater than 300 μm in the thickness of the strip), which is a direct consequence of the succession of rapid solidification on the cylinders and of a stay of the strip at high temperature after it has left the cylinders, when it is not subjected to forced cooling:
• the ferritic grains have a high hardness, due to their supersaturation with interstitial elements (carbon and nitrogen);
• the presence of martensite from the quenching of austenite present at high temperature.

To remedy this, we imagined performing on the coils, after their cooling, closed annealing at a temperature below the temperature (known as Ac1) of transformation of ferrite into austenite during reheating. Conventionally, this annealing is performed at around 800 ° C for at least 4 hours. We thus aim to precipitate carbides from the ferritic matrix, to transform martensite into ferrite and carbides, and to coalesce the chromium carbides in order to soften the metal. This treatment must allow a improvement of mechanical characteristics and ductility, despite the conservation of the columnar structure with large ferritic grains. However the tests carried out at scale have shown that this method is insufficient to obtain a strip of suitable ductility.

This persistent fragility of the strip is explained after annealing in isolation by the causes the raw strip, when wound, to undergo only a very slow cooling since its two faces are in contact with hot metal, and only its edges are in contact with ambient air and are free to radiate. This very slow cooling leads to an abundant precipitation of carbides from ferrite and the transformation of a part of the austenite made of ferrite and carbides, while the rest of the austenite forms martensite on cooling. Closed cup annealing completes the decomposition of the ferrite and carbide martensite, but it mainly contributes to the coalescence of large carbides in the form of continuous films. The fragility of the metal is, precisely, attributed to these large carbides whose size is of the order of 1 to 5 μm. They constitute seed sites for the ruptures which propagate by cleavage in the surrounding ferritic matrix: their effect harmful is added to that of the columnar structure with large grains.

As a result, various attempts have been made to develop a casting process between cylinders of thin strips of ferritic stainless steel having good ductility. They aimed to modify the nature of the precipitates formed during the strip cooling, or "breaking" the coarse coarse-grained structure ferritic.

In this regard, one can cite the document JP-A-62247029, which recommends a in-line cooling at a speed greater than or equal to 300 ° C / s between 1200 and 1000 ° C, followed by the winding which is carried out between 1000 and 700 ° C.

Document JP-A-5293595 recommends winding at one temperature from 700 to 200 ° C, while giving steel low carbon contents and nitrogen (0.030% or less) and a niobium content of 0.1 to 1% acting as a stabilizer.

Other documents propose to carry out an online hot rolling, which comes add to the previous analytical constraints on carbon and nitrogen, and can also be combine with niobium or nitrogen stabilization (see documents JP-A-2232317, JP-A-6220545, JP-A-8283845, JP-A-8295943).

Mention may also be made of document EP-A-0638653, which proposes, for a steel with 13-25% chromium, to impose a total of niobium, titanium, aluminum and vanadium contents from 0.05 to 1.0% , carbon and nitrogen contents of 0.030% maximum and a molybdenum content of 0.3 to 3%. The weight composition of the steel must, moreover, satisfy the condition "γp ≤ 0%" .γp is a criterion representative of the amount of austenite formed during precipitation. It is calculated by the formula: γp = 420 x% C + 470 x% N + 23 x% Ni + 9 x% Cu + 7 x% Mn - 11.5 x% Cr - 11.5 x% Si - 12 x% Mo - 23 x% V - 47 x% Nb - 49 x% Ti - 52 x% Al + 189.

In addition, it is necessary to perform a hot rolling of the strip in the range temperatures 1150-900 ° C with a reduction rate of 5 to 50%, then cool it to a speed less than or equal to 20 ° C / s or to maintain it in the temperature range 1150-950 ° C for at least 5 s, and finally to wind it at a lower temperature or equal to 700 ° C.

• des élaborations coûteuses et difficiles du métal liquide destiné à la coulée de la bande, si on veut obtenir les basses teneurs en carbone et azote nécessaires, voire le cas échéant les teneurs souhaitées en éléments stabilisants ;
• des traitements thermiques et thermomécaniques effectués sur la ligne de coulée au moyen d'installations lourdes (laminoir à chaud en ligne) ;
• et la réalisation de cycles thermiques complexes nécessitant également des installations spécialement adaptées pour obtenir les vitesses de refroidissement élevées ou les temps de maintien à haute température nécessaires.

To implement all these methods, it is therefore necessary to combine:

• costly and difficult preparations of the liquid metal intended for casting the strip, if it is desired to obtain the low carbon and nitrogen contents necessary, or even if necessary the desired contents of stabilizing elements;
• thermal and thermomechanical treatments carried out on the casting line by means of heavy equipment (in-line hot rolling mill);
• and carrying out complex thermal cycles also requiring installations specially adapted to obtain the high cooling rates or the high temperature holding times required.

The object of the invention is to propose an economical mode of production of thin strips of ferritic stainless steel of type AISI 430 and related by casting between cylinders, which gives said strips sufficient ductility to allow operations of unwinding, shearing of the edges and cold processing (pickling, rolling ...) of occur without incidents such as tape breakage or the appearance of cracks on the banks. In order to achieve the economic objective, this process should not include steps requiring the addition of complex facilities to a casting machine between standard cylinders. Nor should it make it necessary to carry out a development of the liquid metal aimed at obtaining very low contents of elements such as carbon and nitrogen, as well as the addition of expensive alloying elements.

The subject of the invention is a method for manufacturing strips of stainless steel. ferritic, according to which, directly from liquid metal, it solidifies between two close cylinders with horizontal axes, internally cooled and turning in direction contrary, a strip of ferritic stainless steel of the type containing not more than 0.012% of carbon, at most 1% manganese, at most 1% silicon, at most 0.040% phosphorus, at most 0.030% sulfur and between 16 and 18% chromium, characterized in that it cools or then allows the said strip to cool while avoiding making it stay in the area of transformation of austenite into ferrite and carbides, in that one carries out the winding of said strip at a temperature between 600 ° C and the processing temperature martensitic Ms, in that the wound strip is allowed to cool at a maximum speed of 300 ° C / h up to a temperature between 200 ° C and room temperature, and in this that a closed annealing of the said strip is then carried out.

The subject of the invention is also a strip of ferritic stainless steel of the type containing at most 0.012% carbon, at most 1% manganese, at most 1% silicon, at plus 0.040% phosphorus, at most 0.030% sulfur and between 16 and 18% chromium, characterized in that it is capable of being obtained by the preceding process.

As will be understood, the invention consists, starting from a steel strip ferritic stainless steel of standard composition cast between cylinders, to cool and to wind said strip under special conditions, before subjecting it to a closed annealing. This treatment essentially aims to limit the formation of large carbides as much as possible weakeners. For that, it is necessary to limit the precipitation of carbides and to favor the transformation of austenite into martensite in the raw pouring stage, avoiding however that this transformation into martensite does not occur when the strip is not yet wound.

• la figure 1 qui situe sur un diagramme montrant les courbes de transformation au refroidissement de la nuance AISI 430 quatre exemples A, B, C, D de chemins thermiques suivis par la bande après sa sortie des cylindres de coulée, dont deux exemples C, D où elle subit un traitement selon l'invention ;
• la figure 2 qui montre un cliché en microscopie électronique en transmission sur lame mince d'une bande ayant suivi le chemin thermique A de la figure 1, puis un recuit vase clos ;
• la figure 3 qui montre un cliché en microscopie électronique en transmission sur lame mince d'une bande ayant, selon l'invention, suivi un chemin thermique intermédiaire entre les chemins C et D de la figure 1, puis un recuit vase clos.

The invention will be better understood on reading the description which follows, referring to the following appended figures:

• FIG. 1 which locates on a diagram showing the transformation curves on cooling of the grade AISI 430 four examples A, B, C, D of thermal paths followed by the strip after its exit from the casting rolls, including two examples C, D where she undergoes a treatment according to the invention;
• FIG. 2 which shows a picture in transmission electron microscopy on a thin blade of a strip having followed the thermal path A of FIG. 1, then annealing in a vacuum;
• FIG. 3 which shows a picture in electron microscopy in transmission on a thin blade of a strip having, according to the invention, followed an intermediate thermal path between paths C and D of FIG. 1, then annealing in a vacuum.

In the following of this description, we will reason on steels whose composition meets the usual criteria of AISI 430 standard on ferritic stainless steels standard, therefore containing at most 0.012% carbon, at most 1% manganese, at most 1% silicon, at most 0.040% phosphorus, at most 0.030% sulfur and between 16 and 18% of chrome. However, it goes without saying that the scope of the invention can be extended to steels containing, in addition, alloying elements not necessarily required by standards usual (for example stabilizers such as titanium, niobium, vanadium, aluminum, molybdenum), insofar as their contents are not high enough to interfere with the metallurgical processes which will be described and on which the invention is founded. In particular, the presence of these alloying elements should not modify the appearance transformation curves from the example in Figure 1 to the point that the paths thermal that the strip must follow, according to the invention, would no longer be accessible on a casting installation between cylinders.

• carbone : 0,043% ;
• silicium : 0,24% ;
• soufre : 0,001% ;
• phosphore : 0,023% ;
• manganèse : 0,41% ;
• chrome : 16,36% ;
• nickel : 0,22% ;
• molybdène : 0,043% ;
• titane : 0,002% ;
• niobium : 0,004% ;
• cuivre : 0,042% ;
• aluminium : 0,002% ;
• vanadium : 0,064% ;
• azote : 0,033% ;
• oxygène : 0,0057% ;
• bore : moins de 0,001% ;

soit un total carbone + azote de 0,076% (ce qui est tout à fait habituel sur de telles nuances), un critère γp, calculé selon la formule habituelle citée plus haut, égal à 37,6% (ce qui n'est pas particulièrement bas, du fait notamment des relativement faibles teneurs en vanadium, molybdéne, titane et niobium, et une température Ac1 de transformation de la ferrite en austénite lors du réchauffage de 851°C. Cette dernière température est calculée au moyen de la formule classique : Ac1 = 35 x %Cr + 60 x %Mo + 73 x %Si + 170 x %Nb + 290 x %V + 620 x %Ti + 750 x %Al + 1400 x %B - 250 x %C - 280 x %N - 115 x %Ni - 66 x %Mn - 18 x %Cu + 310 The steels which were the subject of the tests, the results of which will be described and commented on in relation to FIGS. 1 to 3, had the following composition, expressed in percentages by weight:

• carbon: 0.043%;
• silicon: 0.24%;
• sulfur: 0.001%;
• phosphorus: 0.023%;
• manganese: 0.41%;
• chromium: 16.36%;
• nickel: 0.22%;
• molybdenum: 0.043%;
• titanium: 0.002%;
• niobium: 0.004%;
• copper: 0.042%;
• aluminum: 0.002%;
• vanadium: 0.064%;
• nitrogen: 0.033%;
• oxygen: 0.0057%;
• boron: less than 0.001%;

or a total carbon + nitrogen of 0.076% (which is quite usual on such grades), a criterion γp, calculated according to the usual formula cited above, equal to 37.6% (which is not particularly low, in particular due to the relatively low contents of vanadium, molybdenum, titanium and niobium, and a temperature Ac1 of transformation of the ferrite into austenite during reheating of 851 ° C. This last temperature is calculated by means of the conventional formula: Ac1 = 35 x% Cr + 60 x% Mo + 73 x% Si + 170 x% Nb + 290 x% V + 620 x% Ti + 750 x% Al + 1400 x% B - 250 x% C - 280 x% N - 115 x% Ni - 66 x% Mn - 18 x% Cu + 310

As explained above, when such a raw casting strip is wound around 700-900 ° C without having been forcedly cooled, then is allowed to cool naturally in a coil before undergoing closed annealing, the performances the ductility of the strip after this annealing is not satisfactory. The reason is that the slow cooling in the coil involves a passage of the metal in the precipitation range of chromium carbides of type Cr ₂₃ C ₆ from ferrite (precipitation which occurs at ferritic joints and at ferrite-austenite interfaces ), and especially in the area of decomposition of austenite into ferrite and chromium carbides of the Cr ₂₃ C _{6 type} . This mechanism promotes the growth of large embrittling carbides, and the closed annealing which follows accentuates the coalescence of large carbides in the form of continuous films. The transformation curves of Figure 1, valid for the AISI 430 grade considered, illustrate this phenomenon.

In this FIG. 1, the temperature Ac5 representative of the end of the transformation of the ferrite α into austenite γ during heating is reported, the temperature Ac1 at the start of this same transformation, and the temperatures Ms and Mf at the start and end from the transformation of austenite γ into martensite α 'on cooling. We have also reported curve 1 which delimits the temperature range where precipitation of chromium carbides of the Cr ₂₃ C _{6 type takes place} at the ferritic joints and at the ferrite-austenite interfaces, as well as curve 2 which delimits the zone of beginning transformation of austenite into ferrite and chromium carbides. Also reported are four examples A, B, C, D of heat treatments which the casting strip is subjected to after it leaves the cylinders, two of which (C and D) are representative of the invention.

Treatment A consists, in accordance with the prior art described above, of allow the strip to cool naturally in the open air after it leaves the casting cylinders, and wind it at around 800 ° C, while it is in the area of precipitation of chromium carbides at ferritic joints and at ferrite-austenite interfaces. This winding causes, as we said, a considerable slowing down of the cooling of the band, which is then forced to stay long in the area of transformation of austenite into ferrite and chromium carbides, before ending up at ambient temperature.

Treatment B consists in letting the strip cool naturally in the open air, letting it reach room temperature without winding it. The band does not stay in the zone of transformation of austenite into ferrite and chromium carbides, but it undergoes a significant martensitic transformation between Ms and Mf temperatures. We'll see why such treatment cannot be included in the invention.

Treatment C, representative of the invention, consists in first leaving the strip to cool naturally, without being coiled, so that it does not stay in the area transforming austenite into ferrite and chromium carbides, and not winding only at a temperature of approximately 600 ° C. During the cooling of the strip wound, this ends up substantially joining the final thermal path of treatment A.

Treatment D, also representative of the invention, is in principle identical to treatment C, but the winding of the strip takes place only at a temperature of 300 ° C approximately. This temperature remains imperatively higher than Ms (which depends on the chemical composition of the steel), and during the cooling of the coil we prevent the band from staying in the area where the martensitic transformation would take place very importantly. Its final thermal path joins those of treatments A and C.

The snapshot in Figure 2 shows a portion of a sample of a strip of reference which followed the thermal path A of FIG. 1 (therefore a winding at 800 ° C) for be brought in coiled form at room temperature, then undergone closed cup annealing under usual conditions, namely a stay at approximately 800 ° C. for 6 hours. The strip has the chemical composition specified above and a thickness of 3 mm. We observe that the majority of the sample is made up of large ferritic grains 3. Zones 4 comprising small ferritic grains resulting from the transformation of martensite α 'during the closed annealing represents only a minority fraction of the sample. We notice especially the presence, within the structure, of continuous films of chromium carbides 5. These carbide films result from the fact that, at first, the slow cooling of the strip wound in the zone of transformation of austenite into ferrite and carbides a caused a strong precipitation of carbides, and in a second time, the annealing vase clos has accentuated the coalescence of these carbides. As we will see, the presence of these films carbides is a cause of the poor ductility of the metal.

The snapshot in Figure 3 shows a portion of a sample of a strip according to the invention (of the same composition and thickness as that of FIG. 2) which followed a intermediate thermal path between paths C and D in Figure 1 to the room temperature (the tape was wound at 500 ° C), then underwent closed cup annealing identical to that undergone by the reference sample of FIG. 2. We observe that the large ferritic grains 3 are still present, but only areas with small ferritic grains 6 resulting from the transformation of α 'martensite are in greater proportion. The fact to have caused the band to rapidly cross the precipitation domain of carbides and nitrides and to have made him avoid the domain of transformation of austenite into ferrite and carbides first led to a limited precipitation of fine carbides in the ferrite (which is inevitable, given the speed of their precipitation). In addition, important austenite ranges, richer in carbon and nitrogen than ferrite, which then transformed into martensite. During the closed cup annealing which followed, fine carbides were precipitated within ferrite, and martensite decomposed into ferrite and fine carbides distributed much more evenly than in the reference sample in Figure 2. We no longer observe continuous films of coalesced carbides, but at most discontinuous strings 7 of small carbides (less than 0.5 µm) at the borders between the large ferritic grains and the zones with small ferritic grains strewn with carbides. These small carbides are much less sensitive to crack initiation than films of the reference sample. The notable appearance of small grain areas ferritics during closed annealing is due to the relaxation of the stresses stored during of the formation of martensite, which gives rise to a phenomenon of restoration. These beaches small ferritic grains are much more ductile than the coarse-grained matrix ferritics, and allow to imitate the brittleness of the metal, in particular by slowing the propagation of cracks by cleavage.

The ductilities of the bands obtained by the reference method and by the method according to the invention were evaluated by impact bending tests on Charpy test pieces with "V" notch, during which their resilience was evaluated by measuring the energy absorbed at 20 ° C by the samples. The tests were carried out on strip samples taken before and after the closed annealing. Their results are set out in the following table 1

It can be seen that the winding temperature has no influence on the ductility at 20 ° C. of the raw casting strip, which has not yet undergone closed annealing. This ductility is very poor, and it is not improved by closed cup annealing in the case of the reference strip, hot wound. As we have seen on the picture in Figure 2, the closed cup annealing in this reference case was powerless to promote a structure of the metal matrix and a distribution of carbides favorable to good ductility. On the other hand, the ductility of the wound strip under the conditions recommended by the invention could be considerably improved by closed cup annealing, and brought to a very satisfactory level. Experience shows, in fact, that a resilience of the order of 30 to 40 J / cm ² is sufficient so that cold treatments (unwinding, shearing of the edges in particular) can be carried out without damage to the strip.

The fact of having prevented the wound strip from crossing the transformation zone of the austenite made of ferrite and carbides led, during the cooling of the strip, to the formation of fine carbides in ferrite, whose morphology and distribution are significantly more favorable for obtaining, after closed annealing, fine carbides and regularly distributed. These are therefore much less troublesome for the ductility of the strip as the continuous films of carbides observed on the reference sample. The matrix metal obtained after cooling the wound strip at low temperature, which is richer in martensite, is also more favorable to good ductility of the strip final, because the closed annealing acts effectively on the martensite to decompose it mainly small grain ferrite.

Another test representative of the ductility of these same bands after the closed cup annealing was carried out. It consists of performing alternating bends at 90 ° from a test piece whose edges are rough or have been machined. Bending corresponds to an operation consisting in bending the sample at 90 °, then bringing it back to its initial straight configuration. The number of folds that can be performed before the sample breaks or has cracks in the fold zone is evaluated. The following table 2 groups together the average of the results of these experiments:

A number of folds equal to 0 means that the strip does not even support being folded only once before the first cracks or pure breakout appear and simple. Here again, it is clear that the strip which has been produced in accordance with the invention is is much better than the reference band, for the reasons that have been given previously.

In summary, the first fundamental idea of the invention is to impose on the band leaving the cylinders a cooling path which makes it possible to limit the precipitation carbides, especially avoiding those which could come from the decomposition of austenite and which could coalesce into large continuous films during vase annealing closed. The second idea is to promote, at the same stage of development, the transformation from austenite to martensite so as to obtain as much fine-grained ferrite as possible during closed annealing. These conditions are met if we limit the time spent by the strip cast in the precipitation range of carbides and nitrides from ferrite, and especially if it is avoided to stay in the field of the transformation of austenite in ferrite and carbides. In practice, the fulfillment of these conditions on AISI grades 430 and those related to it requires that the tape is wound at 600 ° C or less to prevent the strip from staying in the processing area of austenite made of ferrite and carbides while it is coiled. Depending on the conditions casting conditions such as strip thickness, casting speed and distance separating the rolls and the winder, these conditions can be fulfilled by a simple natural air cooling of the strip, or may require the use of a forced belt cooling installation, for example by means of a projection a cooling fluid such as water or a water-air mixture. We consider that imposing on the strip a cooling rate greater than or equal to 10 ° C / s between its exit from the cylinders and the time it reaches the temperature of 600 ° C from which can take place the winding generally provides the desired results.

However, the formation of martensite when the strip cools is controlled so that it does not itself become harmful. First, it is imperative to prevent martensite from forming before winding, as it would cause high risk of tape breakage during winding. For this, it is necessary that the winding is carried out at a temperature higher than the Ms transformation temperature from austenite to martensite, about 300 ° C. On the other hand, too rapid cooling of the coil (above 300 ° C / h) would lead to excessive formation of martensite very tough. This would make the tape too fragile to support handling without incident. of the coil preceding the annealing. The processing example B in FIG. 1 is representative faults which could lead to too rapid cooling of the strip: the absence winding resulted in an average cooling rate of around 1000 ° C / h. After this cooling, the strip had a hardness of 192 Hv, which is too high, so that the reference strip following path A had a hardness of 155 Hv. Groups according to the invention having undergone an intermediate treatment between paths C and D have hardnesses of the order of 180 Hv. It must be considered that the wound strip should not cool at a speed greater than 300 ° C / h. In practice, this condition is generally satisfied on industrial-size installations when no action is taken to accelerate the cooling of the coils (a cooling speed natural to air of the order of 100 ° C / h is usually observed).

On the other hand, to obtain good results, it is necessary to wait for annealing. in isolation that the wound strip has cooled sufficiently for the transformations had the time to be fulfilled, in particular the transformation of the austenite into martensite. In practice, closed annealing should be carried out on a coil, the temperature is initially between ambient and 200 ° C. It is typically performed at a temperature of 800-850 ° C for at least 4 hours.

Compared to other existing processes aimed at improving the ductility of the strips of ferritic stainless steel containing about 17% chromium, the process according to the invention has the advantage of not requiring any particular and costly adaptations of the shade such as the incorporation of stabilizers and / or the lowering of carbon and nitrogen contents to unusually low levels. It can be run on a casting machine continuous between cylinders which does not need to be fitted with a rolling mill hot strip coming out of the cylinders. Nor does it require adaptations specific stages of the manufacturing cycle after casting (closed cup annealing, shearing of banks, stripping ...). The only modification to a casting installation between standard cylinders that its installation is likely to require is the possible addition of a belt cooling device under the cylinders. Such a device which could be very simple design, would ensure that the tape never stays in the area of transformation of austenite into ferrite and carbides and that the winding takes place always at 600 ° C or less, whatever the casting speed and the thickness of the strip, and even if the winder is located relatively close to the rolls (which can be a contrario desirable for the casting of other types of steels).

esprit de l invention d appliquer le procédé précédemment décrit à des bandes coulées entre cylindres qui subissent un laminage à chaud sous les cylindres, lorsque par ailleurs les conditions requises sur le refroidissement et le bobinage de la bande sont remplies. On peut désirer effectuer un tel laminage à chaud pour améliorer la santé interne de la bande en refermant ses éventuelles porosités, et pour améliorer sa qualité de surface. De plus, un laminage à chaud, effectué à des températures de 900 à 1150°C avec un taux de réduction d au moins 5%, a un effet bénéfique sur la ductilité de la bande dont l expérience montre qu il se cumule avec l effet du procédé selon l invention, sans qu il soit nécessaire de respecter les conditions analytiques très strictes exposées dans le document EP-A-0638653 déjà cité. On peut ainsi obtenir des ductilités de la bande plus élevées que celles que permettraient d atteindre la seule application d un laminage à chaud ou la seule application de la version de base du procédé selon l invention.He lives in the

spirit of the invention of applying the method described above to strips cast between rolls which undergo hot rolling under the rolls, when, moreover, the conditions required for the cooling and the winding of the strip are fulfilled. It may be desired to carry out such hot rolling to improve the internal health of the strip by closing its possible porosities, and to improve its surface quality. In addition, hot rolling, performed at temperatures from 900 to 1150 ° C with a reduction rate of at least 5%, has a beneficial effect on the ductility of the strip, the l experience shows that it is cumulative with l effect of the process according to l invention without it is necessary to comply with the very strict analytical conditions set out in the document EP-A-0638653 already cited. It is thus possible to obtain higher ductilities of the strip than those which d reach the only application of hot rolling or the only application of the basic version of the process according to the invention.

• carbone: 0,040% ;
• silicium: 0,23% ;
• soufre: 0,001% ;
• phosphore: 0,024% ;
• manganèse: 0,40% ;
• chrome: 16,50% ;
• nickel: 0,57% ;
• molybdène: 0,030% ;
• titane: 0,002% ;
• niobium: 0,001% ;
• cuivre: 0,060% ;
• aluminium: 0,003% ;
• vanadium: 0,060% ;
• azote: 0,042% ;
• oxygène: 0,0090% ;
• bore: moins de 0,001%.

As a example, we carried out tests on a tape steel d thickness 2.7 mm poured between cylinders, of composition (expressed in weight percentages):

• carbon: 0.040%;
• silicon: 0.23%;
• sulfur: 0.001%;
• phosphorus: 0.024%;
• manganese: 0.40%;
• chromium: 16.50%;
• nickel: 0.57%;
• molybdenum: 0.030%;
• titanium: 0.002%;
• niobium: 0.001%;
• copper: 0.060%;
• aluminum: 0.003%;
• vanadium: 0.060%;
• nitrogen: 0.042%;
• oxygen: 0.0090%;
• boron: less than 0.001%.

This composition corresponds to a γp criterion of 46.5% and at a temperature Ac1 826 ° C.

In l absence of hot rolling, when the winding of the strip is carried out at 800 ° C (in accordance with treatment A of FIG. 1) before the closed annealing, the strip does not support a single bending on sheared edges and the rupture occurs immediately . In the case of winding at 670 ° C, the strip only supports a single fold on sheared edges. But if the winding is carried out at 500 ° C according to the method of l invention, the strip can support 4 folds on sheared edges. These tests therefore confirm those of the example illustrated in Figures 1 to 3.

When, moreover, said strip undergoes hot rolling at a temperature of 1000 ° C. with a reduction rate of its thickness equal to 30%, a winding carried out at 500 ° C. according to the invention provides the strip with energy absorbed at 20 ° C. (after closed cup annealing) of 160 J / cm ² , for conditions of test similar to those of the tests in table 1 above. By comparison, if the winding is carried out at 800 ° C, the energy absorbed at 20 ° C is only 100 J / cm ² .

• une structure colonnaire à gros grains ferritiques coexistant avec de nombreuses zones à petits grains ferritiques parsemés de carbures ;
• l'absence de films continus de gros carbures, remplacés par des chapelets de petits carbures discontinus, présents aux frontières entre les gros grains ferritiques et les zones à petits grains ferritiques ;
• dans le cas, selon la version de base de l invention, où on n a pas procédé à un laminage à chaud de la bande avant son bobinage, l'absence des structures dénotant classiquement qu'on a procédé à un tel laminage à chaud ;
• et, généralement, l'absence de teneurs significatives en éléments stabilisants tels que le niobium, le vanadium, le titane, l'aluminium, le molybdène ; comme on l'a dit, de tels éléments peuvent éventuellement être présents pour diverses raisons, mais ils n'exercent pas d'influence notable sur la ductilité de la bande.

The bands capable of being produced by the method according to the invention differ from the bands of the prior art essentially in that they combine

• a columnar structure with large ferritic grains coexisting with numerous zones with small ferritic grains dotted with carbides;
• the absence of continuous films of large carbides, replaced by strings of small discontinuous carbides, present at the borders between the large ferritic grains and the zones with small ferritic grains;
• in the case, depending on the basic version of the invention, where n has not carried out a hot rolling of the strip before its winding, the absence of the structures conventionally indicating that such a hot rolling has been carried out;
• and, generally, the absence of significant contents of stabilizing elements such as niobium, vanadium, titanium, aluminum, molybdenum; as has been said, such elements may possibly be present for various reasons, but they do not exert a notable influence on the ductility of the strip.

Their good ductility makes these strips able to undergo without damage the usual metallurgical operations which will transform them into finished products usable by a customer, including cold rolling.

Claims (6)

Method for manufacturing thin strips of thick ferritic stainless steel less than 10 mm, according to which, directly from liquid metal, it solidifies between two reciprocating cylinders with horizontal axes, internally cooled and turning in opposite directions contrary, a strip of ferritic stainless steel of the type containing not more than 0.012% of carbon, at most 1% manganese, at most 1% silicon, at most 0.040% phosphorus, at most 0.030% sulfur and between 16 and 18% chromium, characterized in that it cools or the strip is then allowed to cool while avoiding making it stay in the area of transformation of austenite into ferrite and carbides, in that one carries out the winding of said strip at a temperature between 600 ° C and the processing temperature martensitic Ms, in that the wound strip is allowed to cool at a maximum speed of 300 ° C / h up to a temperature between 200 ° C and room temperature, and in this that a closed annealing of the said strip is then carried out. Method according to claim 1, characterized in that said closed annealing is carried out at a temperature of 800 to 850 ° C for at least 4 hours. Method according to claim 1 or 2, characterized in that it avoids doing stay the strip in the field of transformation of austenite into ferrite and carbides in giving it a cooling speed greater than or equal to at least 10 ° C / s between the when the solidified strip leaves the cylinders and when it reaches temperature 600 ° C. Method according to claim 3, characterized in that the said speed is imparted cooling the said strip by spraying the surface of the strip with a fluid cooling. Process according to

one of claims 1 to 4, characterized in that moreover, a hot rolling of the strip is carried out prior to its winding, at a temperature between 900 and 1150 ° C. and with a reduction rate of 1 strip thickness of at least 5%. Ferritic stainless steel strip of the type containing not more than 0.012% carbon, at most 1% manganese, at most 1% silicon, at most 0.040% phosphorus, at most 0.030% sulfur and between 16 and 18% chromium, characterized in that it is capable of being obtained by the method according to one of claims 1 to 5.

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