EP0004819B1 - Process for the production of ferrous alloys with improved mechanical properties by the use of lanthanum, and ferrous alloys obtained by this process - Google Patents

Description

• The present invention relates generally to the use of lanthanum when developing ferrous alloys such as lamellar cast iron and / or spheroidal graphite, or steels.
• More specifically, the present invention relates to a process for the preparation of ferrous alloys making it possible to improve their mechanical properties thanks to the use of lanthanum, in particular in the form of inoculating alloys with a low cerium content or generally with a low content of Rare Earths (including cerium), that is to say with a lanthanum / Rare Earths (except lanthanum) weight ratio at least greater than 100/1. The invention also relates to alloys inoculating with lanthanum for implementing the method as well as the ferrous alloys obtained by the method of the invention.
• Furthermore, the method of the invention makes it possible to reduce or eliminate some of the defects of ferrous alloys such as pitting, cavities or shrinkage, carbides in spheroidal graphite cast irons to eliminate the presence of carbides in gray lamellar cast iron; improve the flowability, the rolling ability and / or reduce the anisotropy of steels.
• Pitting and cavities are two major defects affecting molded parts, in particular spheroidal graphite cast irons. These cavities are also called "shrinkage" and constitute the defect type B 221 of the international classification of foundry defects. The bites mentioned above are generally located under the skin of the part and are revealed by shot peening thereof and constitute the defect type B 123 of the international classification of foundry defects.
• Anisotropy constitutes a defect in steels which often have different mechanical properties in the longitudinal direction compared to the transverse direction, in particular in resilience.
• -C=3,3 à 3,8;
• -Si=1,8 à 3;
• -Mn=0,10 à 0,50;
• -P=,< à 0,05;
• -S=<, à 0,020.
Spheroidal graphite cast iron is prepared by adding magnesium to a base cast iron having the following composition (in percentage by weight):
• -C = 3.3 to 3.8;
• -If = 1.8 to 3;
• -Mn = 0.10 to 0.50;
• -P =, <0.05;
• -S = <, at 0.020.
• Magnesium is added either in the form of pure metal, or more frequently in the form of Fe-Si-Mg alloys. Some of these latter alloys contain cerium (0.2 to 0.4% of the alloy) which is intended to combat the possible effect of the elements Pb, Bi, As, all anti-nodulizing elements. The cast iron thus treated solidifies according to the two diagrams "Fe-GF ₃ " and "Fe-graphite".
• a) à une tendance à la solidification suivant le diagramme métastable Fe-CFe₃ qui donne naissance à des carbures.
• b) Ce type de solidification implique des surfusions notables. L'importance de ces surfusions est fonction du type de solidification dont une partie s'effectue suivant le diagramme "Fe-CFe₃" et dont l'autre partie s'effectue suivant le diagramme "Fe-graphite". Actuellement, le cycle de solidification n'est pas contrôlé par le processus d'élaboration.
• c) Une inoculation importante permet généralement de revenir au diagramme Fe-graphite, mais les résultats sont irréguliers, car fonction des modules de refroidissement des pièces moulées (ou des parties des pièces).
It should be noted that the introduction of magnesium into the cast iron leads to:
• a) a tendency to solidify according to the metastable Fe-CFe ₃ diagram which gives rise to carbides.
• b) This type of solidification involves notable supercooling. The importance of these supercoolings depends on the type of solidification, part of which is carried out according to the "Fe-CFe ₃ " diagram and the other part of which is carried out according to the "Fe-graphite" diagram. Currently, the solidification cycle is not controlled by the development process.
• c) A significant inoculation generally makes it possible to return to the Fe-graphite diagram, but the results are irregular, because it depends on the cooling modules of the molded parts (or parts of the parts).
• This process allows in gray lamellar cast iron to suppress the presence of carbides. Previous tests carried out on gray cast iron, or steel, using mischmetal (very variable mixture of 15 rare earth elements) or silicides of rare earths have given patchy results, discordant and not usable in practice industrial.
• For example, in US Pat. No. 3,125,442 Alexander, a process for the preparation of molded pieces of nodular cast iron is described, according to which a sufficient amount of lanthanum is added to a molten hypereutectic cast iron so that the molded piece obtained contains at least 0, 02% lanthanum, which allows the carbon or free graphite of the cast iron, normally in the form of flakes, to agglomerate, forming nodules. On the other hand, the lanthanum used is in fact a mixture of lanthanum with other Rare Earths of which cerium is a majority constituent.
• In any event, Alexander aims to replace magnesium with lanthanum as an agent which promotes the formation, in cast iron, of spheroidal graphite.
• This is not the aim sought in the present invention since lanthanum is introduced as an inoculating element in order to obtain the elimination of certain defects, which leads to a very low incorporation of lanthanum as mentioned below.
• On the other hand, we have already described in the review Metals Abstracts, volume 6, of February 1973, page 237, abstract n ° 320 172, the use of either small additions of lanthanum (0.01 to 0.2%) or aluminum (0.1 and 0.2%) to investigate their effect on the density of l steel with high manganese content.
• It is indicated that the addition of lanthanum gives rise to a significant increase in density, the optimal amount of incorporation being 0.02%. This effect is attributed to a degassing and refining effect.
• The addition of lanthanum according to the present invention is not intended to increase the density but simply to improve the flowability, the rolling ability and the isotropy of said steels and this is achieved by adding lanthanum as a post alloy -inoculant after prior deoxidation with aluminum.
• This prior deoxidation of aluminum goes against the teaching of this article which specifies in the last two lines that aluminum does not act as a deoxidizer and, therefore, according to this article, lanthanum is incorporated in the steel to act essentially as a deoxidizer.
• It cannot therefore have the effect obtained according to the process of the present invention which firstly carries out a deoxidation of the aluminum and then a post-inoculation with lanthanum in a small amount as explained below.
• In conclusion, the problem posed by the invention has not been resolved by the prior art.
• The present invention therefore aims to eliminate the aforementioned drawbacks and to provide a solution which makes it possible to reduce or eliminate certain defects in ferrous alloys such as pitting, cavities in spheroidal graphite cast iron, carbides in lamellar cast iron, anisotropy of steels, which is usable in industrial practice and which allows as much as possible to improve the mechanical properties of said ferrous alloys.
• The solution consists according to the invention in a process for the preparation of ferrous alloys characterized in that it comprises the incorporation, as an inoculating alloy or in the case of steels as a post-inoculating alloy after prior deoxidation with aluminum, d '' at least 0.0001% to 0.01% by weight of lanthanum to said ferrous alloy during its preparation, the lanthanum being incorporated either in the form of lanthanum-metal, or of a lanthanum compound or of a lanthanum alloy can be obtained by the use of lanthanum alone or combined with other Rare Earths (including cerium) provided that the lanthanum / Rare Earths (except lanthanum) weight ratio in the said alloy or lanthanum compound is at least higher at 100/1.
• According to a more preferred characteristic, it is possible to incorporate from about 0.001% by weight (or 10 ppm) to about 0.003% (or 30 ppm) by weight into the ferrous alloy during its production.
• According to another characteristic of the present invention, the lanthanum can be incorporated in the form of an alloy (s) with any metal capable of forming a homogeneous compound with the lanthanum, that is to say having a solubility diagram with the lanthanum alone or combined with other Rare Earths in a proportion of 0.01 to 90% by weight; or in the form of compounds such as chloride, fluoride, oxide obtained from lanthanides, or their mixtures, insofar as the weight ratio lanthanum / Rare Earths (except lanthanum) is greater than 100/1.
• In this regard, it can be noted that the addition of mischmetal (with a high proportion of cerium) in the steel modifies the nature of the sulphides by making them less harmful but does not improve the cleanliness of the steel which remains charged with a large amount of inclusions. The invention solves this problem.
• It may be noted that possibly in certain cases it is possible to use lanthanum in the form of a lanthanum pure metal preferably having a purity greater than 99%. The particularly preferred lanthanum inoculating alloys of the present invention are alloys based on Si-La-AI, La-Ni, La-Fe-Si, La-Fe-Mn, Si-Ca-Mg-La, La-Cr, Si-La-Mn and where the iron can constitute the balance. In the case where these lanthanum inoculating alloys contain other Rare Earths, including cerium, the above-mentioned lanthanum / Rare Earths ratio (except lanthanum) must in all cases be satisfied.
• According to the process of the present invention, certain defects in the cast iron are reduced or eliminated such as defects in the form of pits and cavities or shrinkage and the anisotropy of the steels is reduced, thereby making it possible to obtain ferrous alloys having better properties. mechanical.
• In this regard, the Applicant has discovered that the aforementioned defects of spheroidal graphite cast iron such as pits and cavities are due to the retention at various stages of a gas emitted during solidification. This gas seems to be a reducing gas because the walls of the cavities are smooth and not oxidized and one can think that it is either CO, or hydrogen, or a combination of the two.
• The appearance of this reducing gas (CO at least) would not be accidental, as claimed to date (oxidized raw material, oxidizing atmosphere, etc.) but systematic at certain stages of solidification, probably when the liquidus passes.
• - le cérium et le lanthane présentent une miscibilité totale dans le fer liquide;
• - la solubilité du cérium dans le fer à 600°C s'établit entre 0,35 et 0,40%. Cet élément forme alors des composés tels que Ce-Fe₅ (dur et cassant), Ce-Fe₂ etc.
• - le lanthane a par contre une faible solubilité dans le fer (pas de composés définis La-Fe).
By using the metallurgical and thermodynamic properties of each of the elements of the Rare Earths, the Applicant has highlighted that these are very specific and sometimes antagonistic. The plaintiff has shown that:
• - Cerium and lanthanum have total miscibility in liquid iron;
• - the solubility of cerium in iron at 600 ° C is established between 0.35 and 0.40%. This element then forms compounds such as Ce-Fe ₅ (hard and brittle), Ce-Fe ₂ etc.
• - on the other hand, lanthanum has a low solubility in iron (no defined compounds La-Fe).
• It follows from the above that the activity of Ce will be low since it is in the form of inter-metallic compounds while lanthanum will have a strong activity because it remains available for reactions with oxygen and sulfur .
• The use of lanthanum in the form of composite alloys (nodulizers, inoculants, desulphurizers) thus makes it possible to obtain a greater purification of the bath in oxygen and in sulfur which leads to a greater ferritization of the matrix and makes it possible to improve the mechanical characteristics of the ferrous alloys obtained.
• It should be noted that the presence of cerium, in relatively large quantities, that is to say already from about 1%, alone or in combination with other Rare Earths, except lanthanum, compared to the proportion lanthanum, as in the case of the mischmetal used before, practically does not make it possible to obtain the improvements obtained with the lanthanum according to the present invention having a low content of cerium because the Applicant has discovered that cerium has a harmful and antagonistic effect on lanthanum which manifests itself as soon as the cerium content is already around 1% compared to the proportion of lanthanum.
• Other objects, characteristics and advantages of the present invention will appear more clearly in the light of the explanatory description which will follow made with reference to the following examples given simply by way of illustration and which therefore cannot in any way limit the scope of the present invention. Examples 1 to 4 are illustrated by Figures 1 to 10 of the drawings.
• FIGS. 1 to 6 represent solidification curves of spheroidal graphite cast iron in which the temperature is mentioned on the ordinate while the time is mentioned on the abscissa. Figures 7 to 10 show your cavities or recesses obtained on parts prepared according to the prior art (Figures 7, 9 and 10) and according to the present invention (Figure 8).
• In the examples, the contents of the components are given as a percentage by weight.
Example 1
• -C=3, 68;
• -Si=2, 65;
• -Mn=0, 28;
• -S=0,013.
A basic cupola is produced using the following composition:
• -C = 3.68;
• -If = 2.65;
• -Mn = 0.28;
• -S = 0.013.
• This font is produced without inoculation and serves as a reference. The solidification curve obtained on a MECI crucible, with Cr-Ni couple for such a reference cast iron is represented in FIG. 1. This MECI crucible does not modify the solidification of the ingot and in particular ensures solidification quite comparable to that of a piece in a sand mold. The eutectic level is identifiable by an anomaly in the cooling curve which is characterized by a change in inflection of the recorded curve (see Figure 1).
• When incorporating into the above-mentioned cast iron, during its preparation, 0.3% by weight of Si-La-AI alloy (Si = 63%; La = 2.1%; AI = 1.45%, the rest being iron), that is to say 0.0063% by weight of lanthanum, or 63 ppm, molded parts are obtained which have no cavity. The solidification curve obtained, represented in FIG. 2, shows an elongation of the solidification interval of the order of 37% relative to the curve of FIG. 1 as well as an increase shift of 13 ° C in the temperature of the transformation stage. This displacement of the position of the eutectic bearing implies a transition to the Fe-graphite diagram and the lengthening of the solidification interval allows effective degassing which leads to the formation of the aforementioned healthy part.
Example 2
• -C=3, 40;
• -Si=2, 70;
• -Mn=0, 12;
• -S=0,01 0;

que l'on élabore au four électrique. On inocule lors de son élaboration 0,4% d'alliage inoculant habituellement utilisé par la fonderie ayant la composition suivante:
• -Si=70;
• -Ca=0,7;
• -AI=4;
• -Fe=le solde.
We use a basic font with the following composition:
• -C = 3.40;
• -If = 2.70;
• -Mn = 0.12;
• -S = 0.01 0;

that we make in the electric oven. 0.4% of the inoculating alloy usually used by the foundry having the following composition is inoculated during its preparation:
• -If = 70;
• -Ca = 0.7;
• -AI = 4;
• -Fe = the balance.
• The solidification curve obtained, using a MECI crucible, is shown in FIG. 3. The parts obtained have appearance defects such as cavities.
• -Si=63%;
• -La=2,1;
• -AI=1,45%;
• -Fe=le solde.
When incorporating, according to the present invention, during the preparation of this cast iron, 0.4% of an alloy having the following composition:
• -If = 63%;
• -La = 2.1;
• -AI = 1.45%;
• -Fe = the balance.
• That is to say 0.0084% lanthanum, or 84 ppm.
• The solidification curve shown in FIG. 4 is obtained, which shows an elongation of the solidification interval of the order of 30% and an increase in the temperature of the transformation plateau of the order of 10 ° C. The parts obtained are devoid of cavity.
Example 3
• -C=3,43%;
• -Si=2,62%;
• -Mn=0,18%;
• -S=0,011%.

que l'on élabore au four électrique.We use a font with the following basic composition:
• -C = 3.43%;
• -If = 2.62%;
• -Mn = 0.18%;
• -S = 0.011%.

that we make in the electric oven.
• During this elaboration, 0.4% of the inoculating alloy usually used in foundries mentioned in Example 2 is incorporated into this melting. The cooling curve shown in FIG. 5 is obtained, which was recorded on special crucibles. "tellurium-S" electronite. These crucibles are coated with a carburetor coating and provide cooling in the only metastable diagram "Fe-CFe ₃ ". Although less representative of the practical solidification of the parts, this type of crucible makes it possible to obtain a well marked eutectic bearing which makes it easier to compare the different lengths of the eutectic bearings.
• According to the process of the present invention, 0.4% of the lanthanum alloy, substantially free of cerium, mentioned in Example 2, is incorporated during the production of this cast iron. The cooling curve shown is obtained in FIG. 6 which shows an increase in the length of the transformation stage of the order of 260% and an increase in the transformation temperature of approximately 10 ° C. compared to that of FIG. 5.
• The parts cast with the alloy of the present invention are practically sound, the weights have only a slight dendritic shrinkage while the parts cast according to the prior method have cavities and pits.
• In order to compare the improvement in the mechanical properties obtained by the process of the present invention, tensile test pieces were prepared which were tested, the results obtained are mentioned in Table 1 below:

  
• The significant gains obtained in the elongation and the resilience confirm the influence of the ferritic structure on the mechanical characteristics.
Example 4
• From a base cast such that C = 3.65; Si-2.65; Mn = 0.08; S = 0.010 and prepared within the framework of a special process of nodulization of the cast iron in the mold (inmold process), the following two alloys were used in order to highlight the action of lanthanum on the ability to cavities ( "shrinkage") of the cast iron cast by this process. These two alloys were made from a Fe-S-Mg mother alloy:

  
• -Ce=49%;
• -La=20%;
• -Solde=autres Terres Rares.
The mischmetal used has the following composition:
• -Ce = 49%;
• -La = 20%;
• -Sale = other Rare Earths.
• The parts obtained, by incorporating 1% of the alloys 1 and 2, are shown in section, respectively in Figures 7 and 8. From Figures 7 and 8, it can be seen that the alloy 2 according to the present invention allows to obtain weights having only a primary dendritic recess while the counterweight prepared with the anterior mischmetal has a large recess or recess. It should be noted that the lanthanum / Rare Earth ratio in mischmetal is equal to 0.25. This ratio according to the present invention must be, as mentioned above, at least equal to 2, preferably at least equal to 10 and more preferably at least equal to 100.
• Mechanical tests were carried out on blocks for test pieces obtained after incorporating alloy 1 or alloy 2 and are summarized in Table II below:

  
• The results obtained confirm the happy influence of lanthanum on the structure (ferritization) and on the compactness of the castings.
• Alliage 3: Si=48,2%; Ca=0,58%; Mg=5,8%; Ce=1% (mischmétal 2%); Fecde reste.
• Le mischmétal utilisé présente la composition mentionnée précédemment pour l'alliage 1.
• Alliage 4: identique à l'alliage 3 sauf que l'on emploie 0,50% de cérium apporté sous forme de Fe-Ce au lieu de Mischmétal.
In order to confirm the specific action of lanthanum and to highlight the antagonistic action of cerium, two complementary tests were carried out in which 1% of the following two alloys are incorporated into the cast, respectively:
• Alloy 3: Si = 48.2%; Ca = 0.58%; Mg = 5.8%; Ce = 1% (mischmetal 2%); Fecde stays.
• The mischmetal used has the composition mentioned above for alloy 1.
• Alloy 4: identical to alloy 3 except that 0.50% of cerium supplied in the form of Fe-Ce is used instead of Mischmetal.
• The parts obtained with incorporation of alloys 3 and 4 are shown, respectively, in FIGS. 9 and 10. It can be seen that there is no reduction in the size of the cavities even in the case of alloy 3 for which has a final content of La = 0.4% which shows that the presence of cerium in an amount greater than 1% by weight relative to the weight of lanthanum inhibits the beneficial effect of lanthanum.
Example 5
• Si=75;
• Ca=3;
• AI=4;
• Fe=le solde.
During the preparation of a hyperpperectic cast iron at approximately 1310 ° C., 0.5% by weight of inoculating alloy usually used in foundries having the following composition (A) is inoculated in a manner known per se.
• If = 75;
• Ca = 3;
• AI = 4;
• Fe = the balance.
• A cast iron is obtained having the composition mentioned in Table III with the physical characteristics also mentioned in Table III.
• Si=₇₅;
• Ca=3;
• AI=4;
• La=0,5;

soit 25.10^-4% en poids de lanthane ou 25 ppm, on obtient une fonte ayant la composition et les caractéristiques physiques mentionnées également au Tableau III. On peut constater que le nombre des sphéroides de graphite obtenus est bien plus nombreux et les effets de la trempe (zone carburée) moindres avec l'utilisation de l'alliage inoculant selon la présente invention par rapport à l'alliage inoculant connu, et ceci de manière inattendue.

By inoculating 0.5% by weight of an inoculating alloy according to the present invention, having the following composition (B):
• If = ₇₅ ;
• Ca = 3;
• AI = 4;
• La = 0.5;

or 25.10 ^-4 % by weight of lanthanum or 25 ppm, a cast iron is obtained having the composition and the physical characteristics also mentioned in Table III. It can be seen that the number of graphite spheroids obtained is much more numerous and the effects of quenching (carburetted area) less with the use of the inoculating alloy according to the present invention compared to the known inoculating alloy, and this unexpectedly.

• In the case of steels, lanthanum can solve the problems associated with the deoxidation of steel. In this regard, to make the best use of the desulphurizing properties of lanthanum, it is important to deoxidize the steel beforehand in a conventional manner, for example by deoxidation prior to the furnace by adding 0.8 to 1% by weight of aluminum than the is complete by a ladle deoxidation using lanthanum rate in the aforementioned interval, that is to say in an amount preferably between 10- ⁴ and 10-% ^2% or 1 to 100 ppm, and preferably 1-10 to 30 ppm.
• Thus, due to the small amount of lanthanum added, there are very few inclusions, these being well distributed, thus eliminating the viscosities of the inclusions, which results in a very fluid steel bath, solidification, and lastly to a very clean steel. In addition, the almost complete desulphurization of the steel likewise reduces the surface tension of the latter and leads to an improvement in its flowability.
• These observations are confirmed by the following example.
Example 6
• We want to prepare a steel with the following chemical composition:

  
• For this, from a steel of conventional composition, a casting of 13570 kg is carried out in an oven to which 0.07% of carbon and 0.15% of Mn are added.
• To achieve refinement of the oxygen content, a deoxidation is carried out beforehand in the oven, according to the traditional method, by the addition of approximately 0.8% of aluminum. After the addition of aluminum, a sample of steel is taken directly from the furnace, the following composition being obtained:

  
• The crystallographic analysis shows that this steel contains macro-inclusions of aluminate and silicate and micro-sulfides.
• According to the present invention, after the above-mentioned oven deoxidation with aluminum, a pocket deoxidation is carried out by adding 27 kg of a silico-lanthanum alloy comprising 45% of Si, 0.5% of La, the remainder being iron, an addition of about 0.20% of the alloy lanthanum which corresponds to an addition of about 10- ^3% lanthanum or about 10 ppm.
• A sample of steel is taken from the pocket after the deoxidation with the lanthanum inoculating alloy of the present invention and a steel is obtained having the following composition:

  
• The crystallographic analysis of this steel shows that a steel is obtained comprising aluminate and silicate micro-inclusions by obtaining refractory globules of small average diameter of the order of 1 to 2 microns and in limited number.
• Furthermore, the lanthanum according to the present invention in alloy with other metals, including with Rare Earths in so far as the above-mentioned lanthanum / Rare Earths ratio is respected, gives the possibility, in the course of the deoxidation kinetics, desulfurization, denitriding and dehydration, to provide and obtain the number of inclusions of the desired size and composition for the steel applications that it is desired to produce, which constitutes a particularly remarkable industrial result.
• Thus the addition of lanthanum, under the conditions of the present invention, makes it possible to reduce the anisotropy of the steels and thereby improve the ratio of the longitudinal and transverse resilience obtained.
• It should generally be noted that lanthanum is found in the ferrous alloy in the form of compounds such as oxides, and / or sulfides, and / or nitrides, and / or hydrides, and / or carbides forming non-annoying inclusions in ferrous alloys.
• In addition, during the development of the ferrous alloy, if the cast iron or steel settles well, one must have 70% of the lanthanum compounds formed which go up in the slag. Thus, in general, less than 30% of lanthanum compounds are found in the ferrous alloy obtained.
• Advantageously, lanthanum is incorporated into the ferrous alloy, during its preparation in the form of an inoculating alloy having the following composition (percentage by weight):

  
• The steels prepared by the process of the present invention can be in particular structural steels, special steels, stainless steels, molding or rolling but are not limited to these.

Claims (13)

1. A method for obtaining ferrous alloys allowing reducing or removing certain defects in the said ferrous alloys, such as pinholes and cavities in spheroidal graphite cast-iron, carbides in flake-graphite grey cast-iron; improving the castability, rollability and isotropy of steel; and improving the mechanical properties of the said ferrous alloys, characterized in that it comprises the incorporation, as inoculating alloy or, in the case of steel, as an alloy for post-inoculation subsequent to deoxidizing by means of aluminium, of at least, by weight, 0.0001 to 0.01% lanthanum into said ferrous alloy during its production, said lanthanum being incorporated in the form of either metal lanthanum or of a lanthanum compound or of a lanthanum alloy obtained by using lanthanum either alone or associated with other rare earths (including cerium) provided the ratio, by weight, of said lanthanum to said rare earths (excluding lanthanum) in the said alloy or lanthanum compound is at least higher than 100/1.

2. A method according to claim 1, characterized in that it comprises the incorporation of, by weight, 0.001 to 0.003% lanthanum into the said ferrous alloy during its production.

3. A method according to claim 1 or 2, characterized in that a spheroidal graphite cast-iron is obtained from a basic cast-iron having the following composition by weight:

into which lanthanum is incorporated in the aforementioned proportions during its production.

4. A method according to claims 1 to 3, characterized in that, in the case of steel, before the incorporation of lanthanum, there is added, by weight, from 0.8 to 1% aluminium.

5. A method according to the foregoing claims, characterized in that lanthanum is added in the form of alloys with any metal capable of forming a homogeneous compound with lanthanum, i.e., presenting a solubility diagram with lanthanum alone or associated with other rare earths (including cerium), in a proportion, by weight, of 0.01 to 90% lanthanum.

6. A method according to claims 1 to 4, characterized in that lanthanum is incorporated in the form of a compound such as a chloride, a fluoride, an oxide obtained from lanthanides, or of a mixture of said compounds.

7. A method according to one of claims 1 to 4, characterized in that lanthanum is incorporated in the form of metal lanthanum having a purity greater than 99%.

8. A method according to claim 5, characterized in that lanthanum is incorporated in the form of an alloy based on Si-La-AI, La-Ni, La-Fe-Mn, Si-Ca-Mg-La, La-Cr, Si-La-Mn, with iron constituting the balance.

9. A method according to claim 8, characterized in that lanthanum is incorporated in the form of an alloy having the following composition (weight percent):

10. A method according to claim 8, characterized in that lanthanum is incorporated in the form of an alloy having the following composition (weight percent):

11. An inoculating or post-inoculating alloy containing lanthanum, characterized in that it has the following composition (weight percent):

the weight ratio of lanthanum to rare earths (excluding lanthanum) in the said alloy being at least higher than 100/1, with iron constituting the balance.

12. An inoculating or post-inoculating alloy containing lanthanum, characterized in that it has the following composition (weight percent):

the weight ratio of lanthanum to rare earths (excluding lanthanum) being at least higher than 100/1, with iron constituting the balance.

13. An inoculating or post-inoculating alloy according to claim 11 or 12, characterized in that it contains only lanthanum as a rare earth element.

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