EP0613956A1 - Process for making gray cast iron for the surface-remelting treatment - Google Patents
Process for making gray cast iron for the surface-remelting treatment Download PDFInfo
- Publication number
- EP0613956A1 EP0613956A1 EP94400220A EP94400220A EP0613956A1 EP 0613956 A1 EP0613956 A1 EP 0613956A1 EP 94400220 A EP94400220 A EP 94400220A EP 94400220 A EP94400220 A EP 94400220A EP 0613956 A1 EP0613956 A1 EP 0613956A1
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- European Patent Office
- Prior art keywords
- cerium
- ppm
- oxygen
- inoculant
- cast iron
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D5/00—Heat treatments of cast-iron
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C1/00—Refining of pig-iron; Cast iron
- C21C1/08—Manufacture of cast-iron
Definitions
- the invention relates to a process for producing gray cast iron, for the production of parts intended to receive a surface reflow treatment.
- surface reflow is a treatment which consists in carrying out, using a torch, a point fusion on the surface of the part; this fusion is followed by a resolidification, and the surface to be treated is thus scanned so as to produce a surface hardening.
- This process is applied in particular to camshafts, the bearing surfaces of the cams of which are thus treated.
- parts intended for surface reflow must be produced from a cast iron with low oxygen content, the oxygen of which is uniformly distributed and bonded to elements giving stable compounds during reflow so as to avoid gas formation.
- the subject of the invention is therefore a process for the production of gray cast iron for the production of parts intended to receive a surface reflow treatment, having a low oxygen content and whose oxygen combined with other elements in a stable manner, does not give off gas during reflow; this process also comprising means at the level of the production of the cast iron to predict whether the cast parts will be able to receive the surface reflow treatment without revealing any defects at the end of the treatment.
- the process which is the subject of the invention is characterized in that it comprises, during the filling of the ladle, an operation for introducing an inoculant comprising mainly elements of the "rare earth” type, preferably cerium.
- cerium can be introduced in the form of a ferrosilicon loaded with cerium at a rate close to 8.8 percent.
- the method is also characterized by the fact that it comprises an operation for metering the oxygen contained in the pig iron and means for adapting the quantity of inoculant introduced to the quantity of oxygen detected.
- the method is also characterized by the fact that it includes an operation as a function of time of the variation in the level of cerium correlated with the level of oxygen present in the ladle so as to reject the contents of the ladle when the residual cerium level preferably becomes insufficient when this level is less than approximately 60 ppm.
- Figure 1 shows the level of cerium required for an oxygen content of the cast iron.
- Figures 2 and 3 show the results of the oxygen assays on a sample in two particular cases.
- FIG. 4 represents the variation over time of the rate of cerium in a ladle.
- the applicants have carried out a large number of tests by introducing rare earths into the molten iron.
- the tests were carried out mainly with cerium as an inoculating agent. They revealed a simple correlation between the quantities of oxygen contained in the cast iron and the quantities of inoculated cerium. This correlation makes it possible to determine the quantity of cerium to be inoculated in order to obtain parts without bites.
- a straight line d has been drawn on the graph, it separates the hatched circles and the white circles. This line passes through the following coordinates: 60 ppm of cerium for 75 ppm of oxygen 150 ppm of cerium for 135 ppm of oxygen.
- the quantity of cerium to be introduced into the pocket using the inoculant must therefore be greater, while remaining close to the quantity defined by the aforementioned line, depending on the oxygen level detected.
- Preferably and for safety there will be provided with respect to the right of an excess of cerium of the order of 15 to 20 ppm. Extreme conditions will also be avoided by limiting oneself to the range 30 to 125 ppm of cerium. The best conditions are those close to 80 ppm of oxygen and 60 ppm of cerium.
- Figures 2 and 3 represent analyzes carried out using a LECO TC 436 type oxygen metering device on samples taken from the ladles.
- Each of the two figures shows a first curve C1 representing the rise in temperature as a function of the time of the sample in the apparatus, and the second curve C2 represents the release of oxygen which occurs mainly in the vicinity of 2000 ° K.
- the sample shown in Figure 2 contains 53 ppm oxygen and the sample shown in Figure 3 contains 117 ppm oxygen.
- the metal containing more than 100 ppm of oxygen must be rejected, it gives camshafts which will exhibit pitting after surface reflow treatment.
- FIG. 4 represents the variation as a function of time of the level of cerium contained in the bag.
- a first curve t which represents the temperature of the liquid metal.
- the empty bag previously warmed up is not heated during or after filling.
- the curve there is a drop in temperature during filling, followed by a rise when the bag is full.
- the inoculant is introduced continuously during filling and the arrow F indicates the end of filling.
- the level of cerium measured at the end of filling exceeds 100 ppm. It can be seen, by observing the second curve C e, that this rate drops regularly, which corresponds to an evaporation of cerium from the liquid metal.
- the ladle it is preferable to use the ladle to pour parts before the cerium level drops below 60 ppm which corresponds to approximately fifteen minutes of use.
- solidification is rapid and the loss of cerium after casting is negligible.
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Manufacturing & Machinery (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Mechanical Engineering (AREA)
- Manufacture And Refinement Of Metals (AREA)
- Treatment Of Steel In Its Molten State (AREA)
Abstract
Description
L'invention concerne un procédé d'élaboration de fonte grise, pour la réalisation de pièces destinées à recevoir un traitement de refusion superficielle.The invention relates to a process for producing gray cast iron, for the production of parts intended to receive a surface reflow treatment.
On sait que la refusion superficielle est un traitement qui consiste à réaliser à l'aide d'une torche une fusion ponctuelle sur la surface de la pièce; cette fusion est suivie d'une resolidification, et on balaye ainsi la surface à traiter de manière à réaliser une trempe superficielle. Ce procédé est appliqué en particulier aux arbres à cames dont on traite ainsi les surfaces portantes des cames.It is known that surface reflow is a treatment which consists in carrying out, using a torch, a point fusion on the surface of the part; this fusion is followed by a resolidification, and the surface to be treated is thus scanned so as to produce a surface hardening. This process is applied in particular to camshafts, the bearing surfaces of the cams of which are thus treated.
On sait d'autre part que les arbres à cames ainsi traités présentent souvent des piqûres, ce qui occasionne un nombre important de rejets. Ces problèmes apparaissent de façon aléatoire à un stade très avancé de la finition des pièces.It is also known that the camshafts thus treated often exhibit pitting, which causes a significant number of rejections. These problems appear randomly at a very late stage in finishing the parts.
Il a été observé que les défauts des pièces traitées par refusion résultent de la teneur en gaz de la fonte, plus particulièrement de la teneur en oxygène. Pour remédier aux inconvénients précités, il faut réaliser des pièces destinées à la refusion superficielle à partir d'une fonte à faible teneur en oxygène, dont l'oxygène est uniformément réparti et lié à des éléments donnant des composés stables pendant la refusion de manière à éviter la formation de gaz.It has been observed that the defects of the parts treated by reflow result from the gas content of the cast iron, more particularly from the oxygen content. To overcome the aforementioned drawbacks, parts intended for surface reflow must be produced from a cast iron with low oxygen content, the oxygen of which is uniformly distributed and bonded to elements giving stable compounds during reflow so as to avoid gas formation.
On connaît des procédés très coûteux d'élaboration de fontes exemptes d'oxygène, comme par exemple l'élaboration sous vide à partir de matières premières sévèrement sélectionnées. De tels procédés utilisés dans l'industrie aéronautique ne conviennent pas pour la fabrication en grande série dans l'industrie automobile.Very expensive processes are known for producing oxygen-free cast irons, such as, for example, vacuum production from severely selected raw materials. Such processes used in the aeronautical industry are not suitable for mass production in the automotive industry.
L'invention a donc pour objet un procédé pour l'élaboration d'une fonte grise pour la réalisation de pièces destinées à recevoir un traitement de refusion superficielle, ayant une faible teneur en oxygène et dont l'oxygène combiné à d'autres éléments de façon stable, ne donne pas de dégagement gazeux pendant la refusion; ce procédé comportant également des moyens au niveau de l'élaboration de la fonte pour prévoir si les pièces coulées seront aptes à recevoir le traitement de refusion superficielle sans faire apparaître de défauts en fin de traitement.The subject of the invention is therefore a process for the production of gray cast iron for the production of parts intended to receive a surface reflow treatment, having a low oxygen content and whose oxygen combined with other elements in a stable manner, does not give off gas during reflow; this process also comprising means at the level of the production of the cast iron to predict whether the cast parts will be able to receive the surface reflow treatment without revealing any defects at the end of the treatment.
Le procédé qui fait l'objet de l'invention se caractérise par le fait qu'il comporte pendant le remplissage de la poche de coulée, une opération d'introduction d'un inoculant comportant principalement des éléments du type "terre rare" préférentiellement du cérium. En particulier le cérium peut être introduit sous la forme d'un ferrosilicium chargé de cérium à un taux voisin de 8,8 pour cent.The process which is the subject of the invention is characterized in that it comprises, during the filling of the ladle, an operation for introducing an inoculant comprising mainly elements of the "rare earth" type, preferably cerium. In particular, cerium can be introduced in the form of a ferrosilicon loaded with cerium at a rate close to 8.8 percent.
Le procédé se caractérise aussi par le fait qu'il comporte une opération de dosage de l'oxygène contenu dans la fonte et des moyens pour adapter la quantité d'inoculant introduite à la quantité d'oxygène détectée.The method is also characterized by the fact that it comprises an operation for metering the oxygen contained in the pig iron and means for adapting the quantity of inoculant introduced to the quantity of oxygen detected.
Le procédé se caractérise également par le fait qu'il comporte une opération de suivi en fonction du temps de la variation du taux de cérium corrélé au taux d'oxygène présent dans la poche de coulée de manière à rejeter le contenu de la poche quand le taux de cérium résiduel devient insuffisant de préférence quand ce taux est inférieur à 60 ppm environ.The method is also characterized by the fact that it includes an operation as a function of time of the variation in the level of cerium correlated with the level of oxygen present in the ladle so as to reject the contents of the ladle when the residual cerium level preferably becomes insufficient when this level is less than approximately 60 ppm.
Le procédé sera mieux compris grâce à la description ci-dessous illustrée par les dessins annexés :The process will be better understood thanks to the description below illustrated by the accompanying drawings:
La Figure 1 représente le taux de cérium nécessaire pour une teneur en oxygène de la fonte.Figure 1 shows the level of cerium required for an oxygen content of the cast iron.
Les figures 2 et 3 représentent les résultats des dosages de l'oxygène sur un échantillon dans deux cas particuliers.Figures 2 and 3 show the results of the oxygen assays on a sample in two particular cases.
La figure 4 représente la variation au cours du temps du taux de cérium dans une poche de coulée.FIG. 4 represents the variation over time of the rate of cerium in a ladle.
On sait que les terres rares sont avides d'oxygène et par conséquent sont de nature à stabiliser l'oxygène dans la fonte en fusion.We know that rare earths are hungry for oxygen and therefore are likely to stabilize oxygen in molten iron.
Les demanderesses ont effectué un grand nombre d'essais en introduisant dans la fonte en fusion des terres rares. Les essais ont été réalisés principalement avec le cérium comme agent inoculant. Ils ont permis de découvrir une corrélation simple entre les quantités d'oxygène contenues dans la fonte et les quantités de cérium inoculé. Cette corrélation permet de déterminer la quantité de cérium à inoculer pour obtenir des pièces sans piqûres.The applicants have carried out a large number of tests by introducing rare earths into the molten iron. The tests were carried out mainly with cerium as an inoculating agent. They revealed a simple correlation between the quantities of oxygen contained in the cast iron and the quantities of inoculated cerium. This correlation makes it possible to determine the quantity of cerium to be inoculated in order to obtain parts without bites.
On voit sur la figure 1 un graphique avec en abscisse un taux de cérium exprimé en ppm (parties par million) et en ordonnée un taux d'oxygène en ppm. Un grand nombre d'essais ont été effectués et dans chaque cas, les teneurs en oxygène et en cérium ont été mesurées sur des échantillons prélevés dans la poche avant la coulée. Les résultats de ces mesures sont représentés sur le graphique, à chaque essai correspond un point représenté par un cercle. Les arbres à cames qui ont été réalisés ont suivi un traitement de refusion superficielle. Certains ont présenté des piqûres, ils correspondent aux cercles du graphique qui sont hachurés. Les cercles blancs correspondent aux arbres à cames qui, après traitement, se sont relevés satisfaisants.We see in Figure 1 a graph with a rate of cerium expressed on the abscissa in ppm (parts per million) and an ordinate an oxygen rate in ppm. A large number of tests were carried out and in each case, the oxygen and cerium contents were measured on samples taken from the bag before casting. The results of these measurements are represented on the graph, each point corresponds to a point represented by a circle. The camshafts which have been produced have undergone a surface reflow treatment. Some have presented pits, they correspond to the circles of the graph which are hatched. The white circles correspond to the camshafts which, after treatment, are found to be satisfactory.
Une droite d a été tracée sur le graphique, elle sépare les cercles hachurés et les cercles blancs. Cette droite passe par les coordonnées suivantes:
60 ppm de cérium pour 75 ppm d'oxygène
150 ppm de cérium pour 135 ppm d'oxygène.A straight line d has been drawn on the graph, it separates the hatched circles and the white circles. This line passes through the following coordinates:
60 ppm of cerium for 75 ppm of oxygen
150 ppm of cerium for 135 ppm of oxygen.
La quantité de cérium à introduire dans la poche à l'aide de l'inoculant doit donc être supérieure, tout en restant voisine de la quantité définie par la droite précitée, en fonction du taux d'oxygène détecté. De préférence et par sécurité, on prévoira par rapport à la droite d un excès de cérium de l'ordre de 15 à 20 ppm On évitera également les conditions extrêmes en se limitant à la fourchette 30 à 125 ppm de cérium. Les meilleures conditions sont celles voisines de 80 ppm d'oxygène et de 60 ppm de cérium.The quantity of cerium to be introduced into the pocket using the inoculant must therefore be greater, while remaining close to the quantity defined by the aforementioned line, depending on the oxygen level detected. Preferably and for safety, there will be provided with respect to the right of an excess of cerium of the order of 15 to 20 ppm. Extreme conditions will also be avoided by limiting oneself to the range 30 to 125 ppm of cerium. The best conditions are those close to 80 ppm of oxygen and 60 ppm of cerium.
Il a été constaté, au cours de nombreux essais, que l'inoculant doit être introduit de façon continue pendant le remplissage de la poche de telle sorte qu'à aucun instant le taux de cérium ne soit ni insuffisant ni excessif.It has been noted, during numerous tests, that the inoculant must be introduced continuously during the filling of the bag so that at any time the level of cerium is neither insufficient nor excessive.
Les figures 2 et 3 représentent des analyses effectuées à l'aide d'un doseur d'oxygène type LECO TC 436 sur des échantillons prélevés dans les poches de coulée. Chacune des deux figures montre une première courbe C₁ représentant la montée en température en fonction du temps de l'échantillon dans l'appareil, et la deuxième courbe C₂ représente le dégagement d'oxygène qui se produit principalement au voisinage de 2000°K.Figures 2 and 3 represent analyzes carried out using a LECO TC 436 type oxygen metering device on samples taken from the ladles. Each of the two figures shows a first curve C₁ representing the rise in temperature as a function of the time of the sample in the apparatus, and the second curve C₂ represents the release of oxygen which occurs mainly in the vicinity of 2000 ° K.
L'échantillon représenté par la figure 2 contient 53 ppm d'oxygène et l'échantillon représenté par la figure 3 contient 117 ppm d'oxygène.The sample shown in Figure 2 contains 53 ppm oxygen and the sample shown in Figure 3 contains 117 ppm oxygen.
Le métal contenant plus de 100 ppm d'oxygène doit être rejeté, il donne des arbres à cames qui présenteront des piqûres après traitement de refusion superficielle.The metal containing more than 100 ppm of oxygen must be rejected, it gives camshafts which will exhibit pitting after surface reflow treatment.
La figure 4 représente la variation en fonction du temps du taux de cérium contenu dans la poche. On voit sur la figure une première courbe t qui représente la température du métal liquide. La poche vide préalablement mise en température n'est pas chauffée pendant ni après le remplissage. On constate, en observant la courbe, une chute de température pendant le remplissage, suivie d'une remontée quand la poche est pleine.FIG. 4 represents the variation as a function of time of the level of cerium contained in the bag. We see in the figure a first curve t which represents the temperature of the liquid metal. The empty bag previously warmed up is not heated during or after filling. By observing the curve, there is a drop in temperature during filling, followed by a rise when the bag is full.
L'inoculant est introduit en continu pendant le remplissage et la flèche F désigne la fin du remplissage. Le taux de cérium mesuré à la fin du remplissage dépasse 100 ppm. On voit, en observant la deuxième courbe Ce que ce taux baisse régulièrement ce qui correspond à une évaporation de cérium à partir du métal liquide.The inoculant is introduced continuously during filling and the arrow F indicates the end of filling. The level of cerium measured at the end of filling exceeds 100 ppm. It can be seen, by observing the second curve C e, that this rate drops regularly, which corresponds to an evaporation of cerium from the liquid metal.
Il est préférable d'utiliser la poche pour couler des pièces avant que le taux de cérium ne descende en dessous de 60 ppm ce qui correspond environ à quinze minutes d'utilisation. Quand la fonte est coulée dans les moules, la solidification est rapide et la perte de cérium après la coulée est négligeable.It is preferable to use the ladle to pour parts before the cerium level drops below 60 ppm which corresponds to approximately fifteen minutes of use. When the cast iron is poured into the molds, solidification is rapid and the loss of cerium after casting is negligible.
Pour optimiser le procédé, il est bon de sélectionner les matières premières qui servent à élaborer la fonte, afin d'avoir un taux de cérium modéré. Dans le cas de la fabrication d'arbres à cames pour l'industrie automobile, ces matières premières se composent généralement de chutes de tôles. Il est nécessaire d'effectuer des prélèvements de matières premières et de ne conserver que celles dont la teneur en oxygène est voisine de 80 ppm. Cette teneur en oxygène doit être suivie pendant l'élaboration de la fonte de manière à n'utiliser la fonte que lorsque la teneur est inférieure à 80 ppm.To optimize the process, it is good to select the raw materials which are used to elaborate the cast iron, in order to have a moderate level of cerium. In the case of the production of camshafts for the automotive industry, these raw materials generally consist of scrap sheet metal. It is necessary to take samples of raw materials and to keep only those whose oxygen content is close to 80 ppm. This oxygen content must be followed during the development of the cast iron so as to use the cast iron only when the content is less than 80 ppm.
De nombreux essais ont montré que les retours du procédé ne peuvent être recyclés pour la même utilisation. Il est impératif d'utiliser des matières premières qui n'ont pas suivi un traitement antérieur selon le procédé. Les composés de cérium que contiennent ces retours faussent les prévisions et perturbent la mise en oeuvre du procédé.Numerous tests have shown that the process returns cannot be recycled for the same use. It is imperative to use raw materials which have not followed a previous treatment according to the process. The cerium compounds contained in these returns distort the forecasts and disrupt the implementation of the process.
Bien entendu, le procédé selon l'invention ne se limite pas à l'exemple décrit, c'est ainsi qu'au lieu de cérium on pourrait utiliser comme inoculant une autre terre rare.Of course, the method according to the invention is not limited to the example described, this is how instead of cerium one could use as inoculant another rare earth.
Claims (9)
caractérisé en ce qu'il comporte une opération de dosage de l'oxygène contenu dans la fonte, et des moyens pour adapter la quantité d'inoculant introduite à la quantité d'oxygène détectée.Process for the production of gray cast iron, for the production of parts intended to receive a surface reflow treatment comprising, during the filling of the ladle, an operation for introducing an inoculant comprising mainly cerium,
characterized in that it comprises an operation for metering the oxygen contained in the cast iron, and means for adapting the quantity of inoculant introduced to the quantity of oxygen detected.
60 ppm de cérium pour 75 ppm d'oxygène
150 ppm de cérium pour 135 ppm d'oxygène.Process according to Claim 2, characterized in that the quantity of cerium introduced into the cast iron using the inoculant increases with the quantity of oxygen present in the cast iron and must be greater while remaining close to the quantity defined by a straight line passing through the following coordinates:
60 ppm of cerium for 75 ppm of oxygen
150 ppm of cerium for 135 ppm of oxygen.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR9301757 | 1993-02-17 | ||
| FR9301757A FR2701717B1 (en) | 1993-02-17 | 1993-02-17 | Process for producing gray cast iron for surface reflow treatment. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| EP0613956A1 true EP0613956A1 (en) | 1994-09-07 |
Family
ID=9444126
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| EP94400220A Ceased EP0613956A1 (en) | 1993-02-17 | 1994-02-02 | Process for making gray cast iron for the surface-remelting treatment |
Country Status (2)
| Country | Link |
|---|---|
| EP (1) | EP0613956A1 (en) |
| FR (1) | FR2701717B1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002081758A1 (en) * | 2001-04-04 | 2002-10-17 | Foseco International Limited | Improved inoculant |
Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DD74110A (en) * | ||||
| GB1223694A (en) * | 1967-12-07 | 1971-03-03 | Foseco Int | Inoculation of grey cast iron |
| EP0151244A1 (en) * | 1983-12-02 | 1985-08-14 | Georg Fischer Aktiengesellschaft | Grey cast iron alloy, process for its manufacture and uses thereof |
| EP0161624A2 (en) * | 1984-05-07 | 1985-11-21 | Toyota Jidosha Kabushiki Kaisha | Method of producing a camshaft |
| US4681642A (en) * | 1985-11-20 | 1987-07-21 | Toyota Jidosha Kabushiki Kaisha | Cast iron making by cerium addition |
| EP0325810A1 (en) * | 1988-01-23 | 1989-08-02 | SKW Trostberg Aktiengesellschaft | Process for making spheroidal graphite cast iron |
-
1993
- 1993-02-17 FR FR9301757A patent/FR2701717B1/en not_active Expired - Fee Related
-
1994
- 1994-02-02 EP EP94400220A patent/EP0613956A1/en not_active Ceased
Patent Citations (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DD74110A (en) * | ||||
| GB1223694A (en) * | 1967-12-07 | 1971-03-03 | Foseco Int | Inoculation of grey cast iron |
| EP0151244A1 (en) * | 1983-12-02 | 1985-08-14 | Georg Fischer Aktiengesellschaft | Grey cast iron alloy, process for its manufacture and uses thereof |
| EP0161624A2 (en) * | 1984-05-07 | 1985-11-21 | Toyota Jidosha Kabushiki Kaisha | Method of producing a camshaft |
| US4681642A (en) * | 1985-11-20 | 1987-07-21 | Toyota Jidosha Kabushiki Kaisha | Cast iron making by cerium addition |
| EP0325810A1 (en) * | 1988-01-23 | 1989-08-02 | SKW Trostberg Aktiengesellschaft | Process for making spheroidal graphite cast iron |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2002081758A1 (en) * | 2001-04-04 | 2002-10-17 | Foseco International Limited | Improved inoculant |
Also Published As
| Publication number | Publication date |
|---|---|
| FR2701717A1 (en) | 1994-08-26 |
| FR2701717B1 (en) | 1995-03-31 |
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